Van Allen Probes
Van Allen Probes / former RBSP Mission
On November 9, 2012, NASA renamed the former RBSP (Radiation Belt Storm Probes) Mission to Van Allen Probes, in honor of the late James Van Allen, the head of the physics department at the University of Iowa, who discovered the radiation belts encircling Earth in 1958. During his career, Van Allen was the principal investigator for scientific investigations on 24 Earth satellites and planetary missions, beginning with the first successful American satellite, Explorer I, and continuing with Pioneer 10 and Pioneer 11. 1) 2)
The Radiation Belt Storm Probes mission is part of NASA's LWS (Living With a Star) Geospace program to explore fundamental processes that operate throughout the solar system, in particular those that generate hazardous space weather effects near the Earth and phenomena that could affect solar system exploration.
Background: Earth's radiation belts are often referred to as the "Van Allen Belts" due to their discovery by James Van Allen and his team at the University of Iowa in 1958. The radiation belts were discovered during the flight of the very first American satellite. Van Allen and colleagues had installed a Geiger-Müller tube on Explorer 1 to detect cosmic rays, and as the satellite made its eccentric orbit around the Earth, the readings periodically went off the top of the counter's scale. It happened again during the flight of Explorer 3 several months later. Several followup missions proved that the space around Earth was not empty, but instead enriched with electrons, protons, and energy created by interactions between Earth's magnetic field (or magnetosphere), the solar wind, and (occasionally) cosmic rays arriving from beyond the solar system (Ref. 85).
The prime goal of the RBSP mission is to understand the sun's influence on the Earth and near-Earth space by studying the planet's radiation belts on various scales of space and time. The LWS Geospace program will launch two spacecraft, the Radiation Belt Storm Probes, to discover the fundamental physics underlying the source, loss, and transport processes that govern the radiation belts. Observations from the two spacecraft will enable the development of empirical and physics-based models for the radiation belts. The empirical models will be used by engineers to design radiation-hardened spacecraft, while the physics-based models will be used by forecasters to predict geomagnetic storms and alert both astronauts and spacecraft operators to potential hazards. The knowledge gained from the mission will be applicable to particle acceleration processes occurring throughout the plasma universe. 3) 4) 5) 6) 7) 8) 9) 10)
The RBSP mission seeks to resolve decades-old scientific mysteries of how these particles become energized to such high levels, and how the radiation belts vary so dramatically with changing conditions on the sun.
The mission's science objectives are to:
• Discover which processes, singly or in combination, accelerate and transport radiation belt electrons and ions and under what conditions.
• Understand and quantify the loss of radiation belt electrons and determine the balance between competing acceleration and loss processes.
• Understand how the radiation belts change in the context of geomagnetic storms.
Figure 1: A cutaway model of the radiation belts with the 2 RBSP satellites flying through them (image credit: NASA) 11)
Legend to Figure 1: The radiation belts are two donut-shaped regions encircling Earth, where high-energy particles, mostly electrons and ions, are trapped by Earth's magnetic field. This radiation is a kind of "weather" in space, analogous to weather on Earth, and can affect the performance and reliability of our technologies, and pose a threat to astronauts and spacecraft.
The inner belt extends from about 1000 to 8000 miles above Earth's equator. The outer belt extends from about 12,000 to 25,000 miles. This graphic also shows other satellites near the region of trapped radiation.
The instruments on the two RBSP spacecraft will provide the measurements needed to characterize and quantify the processes that produce relativistic ions and electrons.
In July 2000, NASA/GSFC was designated as the NASA Lead Center for the LWS program by the Office of Space Science (OSS), of NASA Headquarters. As Lead Center, GSFC has primary responsibility for managing the implementation of the LWS program and its associated projects.
In 2006, JHU/APL (Johns Hopkins University/Applied Physics Laboratory) of Laurel, MD was awarded a NASA contract to design, built, integrate and operate the twin probes mission. Following a successful confirmation review in late 2008, NASA has given the JHU/APL the go-ahead to continue development of the RBSP mission. APL will build and operate the twin probes that will study the radiation belts surrounding Earth, with a primary mission of two years. 12) 13) 14)
The construction phase of the two RBSP spacecraft started in January 2010 following a three-day CDR (Critical Design Review) in December 2009.
The RBSP mission is composed of two minisatellites with identical sets of instruments to measure charged particle populations, fields, and waves in the inner magnetosphere. The spacecraft are designed to be sun-pointed spinners in near-equatorial elliptical orbits with apogees inside the geosynchronous orbit. Near-equatorial orbits and spin orientation were chosen in order to maximize the coverage of the equatorial pitch angle distributions.
Figure 2: Artist's view of the RBSP spacecraft constellation (image credit: JHU/APL)
Spin axis orientation: Both RBSP spacecraft are stable inertial spinners (nominally at 5 rpm), with the spin axes pointed generally in the direction of the sun. Since the spacecraft spin axes stay inertially fixed, but the Earth moves about the sun nearly 1º/day, it is necessary to maneuver the spin axis of each spacecraft by an average 1º/day to maintain general sun pointing. Attitude maneuvers, however, are scheduled to occur only every 21 days, during which the RA (Right Ascension) of the Sun will increase by approximately 21º. To accommodate this solar motion, the attitude maneuvers will reposition the spin axes approximately 10.5º east (+RA) of the sun. For the next 21 days, the sun will march eastward (in a +RA direction), passing by the spin axes on day 11, and reaching a point 10.5º east of the spin axes on day 21. The process is then repeated throughout the duration of the mission. 15)
Additionally, there is a seasonal north-south bias of the spin axes. When the sun is in the northern hemisphere (March 21 – Sept 23), the spin axes are offset 17º south of the ecliptic plane, while when the sun is in the southern hemisphere (September 23 – March 21), the spin axes are offset 17º north of the ecliptic plane. Figure 3 shows the attitude control concept for both the in-plane and out-of-plane components.
The mass of the two minisatellites is < 1500 kg. The mission duration is 2 years (expandable to 4 years).
Table 1: Overview of basic spacecraft requirements (Ref. 13)
Figure 4: Illustration of the RBSP spacecraft (image credit: JHU/APL)
The spacecraft structure consists of a primary load-bearing central cylinder and aluminum honeycomb decks for mounting instruments and spacecraft components. The two observatories are held together by a RUAG-supplied Lightband low-shock separation system. This same separation system is also used between the stacked observatories and the launch vehicle. 16)
The RBSP spacecraft has a single string fault tolerant architecture. Critical single string spacecraft components use un-switched power and have the ability to be power cycled (or "off-pulsed") in the event that a radiation induced failure causes a fault that requires removal of power. Critical boxes can be off-pulsed individually or as a group. Both software and hardware command loss timers are part of this "off-pulse" architecture and result in a power cycle of spacecraft electronics if a specific command is not received for a defined duration.
The spacecraft block diagram is shown in Figure 8. The avionics for the system are contained in the IEM (Integrated Electronics Module). The IEM consists of five cards connected across a common backplane. A 32-bit PCI bus, clocked at 16.5 MHz, connects the single board computer (SBC), solid-state recorder (SSR) and Spacecraft Interface Card (SCIF) for flow of commands and telemetry. The SBC is a BAE RAD750 based design clocked at 33 MHz for 50 MIPS (nominal),with 16 MB of SRAM, 4 MB of EEPROM and 64 kB of PROM. The SSR contains 16 Gbit of SDRAM memory with EDAC and hardware scrubbing. The selected SDRAM has a low upset rate (even in the RBSP environment); the few SDRAM errors that are expected will be corrected by SSR EDAC.
The SC IF card contains a custom FPGA design that implements interface logic and thruster control. The board also houses the spacecraft precision oscillator which is used for generation of MET (Mission Elapsed Time). The DC-DC converter card provides regulated secondary voltages derived from the s/c primary power bus and implements the box off-pulse capability. The telemetry card gathers temperature, analog and discrete data and is connected to the SC IF card via an internal I2C bus.
The IEM handles both commands and telemetry data flow to each instrument via 115.2 kbaud UART (Universal Asynchronous Receive Transmit) links. The UARTs are synchronized to the spacecraft timekeeping system via a 1PPS (One Pulse-per-Second) interface. Commands to the instruments and other S/C bus components are sent out via two sequenced transmission buffers with the delay from 1PPS dependent upon the prior command buffer usage.
Figure 5: The IEM is shown installed on spacecraft A (image credit: JHU/APL)
Figure 6: Block diagram of the IEM (image credit: JHU/APL)
An electrically-isolated function within the IEM is the HWCLT (Hardware Command-Loss Timer) utilized as part of the fault management system. This is a discrete, logic-based circuit that maintains a count-down between successive "reset" pulses from the ground. If the HWCLT is not "reset" by a specific command sent from the ground within 3.58 days, a logic pulse is sent to the PDU which initiates a PDU sequence to off-pulse the PDU and then the IEM and XCVR (Transceiver). As with all off-pulse implementations, there are multiple levels of protection on this action including an inhibit feature within the PDU itself and two physical interfaces to each box being off-pulsed. The 3.58 day duration is set based upon other fault management mitigation events such as a software based command loss-timer and specific actions initiated through the ground.
C&DH (Command and Data Handling): The C&DH subsystem is resident in the flight computer. The C&DH software is a set of functional applications and libraries, which were implemented to be used with the core Flight Executive (cFE) software developed by NASA/GSFC and the VxWorks operating system. The cFE software provides standard services with a standard API (Application Programmer's Interface). The RBSP C&DH software is composed of seventeen (17) unique applications and eight (8) libraries. Each of these applications and libraries has been designed and implemented to perform a specific set of functions. Having a single common design and single common code base had the advantages of improved software review, testing and maintenance, and in the end reduced the total software development effort, producing a quality C&DH flight software system. As RBSP is the first mission developed by APL using this architecture, it is expected that future cFE application software development at APL will benefit from this application framework. 17)
EPS (Electrical Power Subsystem): Each spacecraft utilizes a DET (Direct Energy Transfer) power system topology. The power bus voltage varies with the 8-cell Li-ion battery voltage. The EPS consists of the PSE (Power System Electronics), BME (Battery Management Electronics), SAJB (Solar Array Junction Box), the 50 Ah Li-ion battery, and four deployed solar array panels. A simplified block diagram of the power system is shown in Figure 7.
The PSE consist of a single fault tolerant sixteen stage sequential analog voltage control shunt regulator with maximum battery current limit. The loads are connected to the single 8-cell, 50Ah Li-ion battery via the PDU (Power Distribution Unit). The nominal bus voltage is 30 V and can vary between 24 and 32 V depending on the state of charge and temperature of the battery. Each battery cell can be by-passed by a bypass switch that is activated by ground command to remove a single cell from the battery in case of a pending cell failure. In the case where by-pass switch activation has occurred, the corresponding bus voltage range becomes 21 to 28 V.
The primary battery charge control method is CC/CV (Constant Current followed by a Constant Voltage) taper charge. The battery is charged at a high rate, limited to C/5, where C is the battery capacity, using the available S/A power that is not used by the loads until the battery SOC (State-of-Charge) reaches 60%. The on board coulometer then reduces the battery charge current to C/10. The battery maximum voltage is controlled to preset safe levels via voltage (V) limits that are implemented in the single fault tolerant voltage regulator. Whenever the battery voltage reaches the V limit, the V control loop will force the charge current to taper.
The BME consists of an interface board and a cell shunt board. Each battery cell has a parallel connected analog shunt used during the mission to balance the end of charge voltage of each Li-Ion battery cell. Each cell shunt is limited to 0.75A maximum current bypassed around the cell in order to limit the amount of power dissipated in the BME. The BME contains eight relays which allow the battery cells to be disconnected from cell shunts to limit leakage current during ground operations or whenever the BME sis not powered. During safe mode operation, the current controller and BME are not powered and the system relies on the single fault tolerant voltage limit regulator. The average spacecraft load power during flight is expected to be 277 W.
Solar array: The RBSP solar array consists of four deployed panels with a total active area of 3.2 m2. Each panel is approximately 0.739 m wide and 1.26 m long. The panel substrates are 25.4mm thick aluminum honeycomb with composite face sheets. The panel front cell side is insulated with Kapton, co-cured with the graphite fiber face sheet. The back face sheet is not painted. Three different solar cell sizes are used to maximize the cell packing density. Each panel contains 12 strings of 24 series connected 28.3 cm2 solar cells. The boomless panels (two panels of the four) contain an additional two strings of 22 series connected 26.62cm2 solar cells and four strings of 22 series connected 11.5 cm2 solar cells. The boom panels (two panels of the four) contain an additional string of 22 series connected 26.62cm2 solar cells and two strings of 22 series connected 11.5cm2 solar cells. The solar cells are triple junction cells with minimum efficiency of 28.5% [BTJ (2nd Generation Triple-Junction)], from EMCORE Photovoltaics. The cover glass on each cell is 0.5 mm thick cerium-doped microsheet, from Qioptiq with Indium tin oxide coating.
The PDU (Power Distribution Unit) provides switched, unswitched, and pulsed power to the spacecraft components. The PDU receives primary power from the PSE and has a serial UART command/telemetry interface with the IEM. The PDU box is a modular slice design. Each slice consists of a printed circuit board housed in a mechanical frame, and the slices stack and bolt together. The slices are electrically connected using internal rigid-flex connectors for signals. A wiring harness external to the box is used for power connections. A solid 350 mil thick aluminum chassis and solid 150 mil aluminum radiation shields (located in thinned areas of the PDU chassis) mitigate the effects of radiation on the electronics parts, allowing the PDU to function nominally in a high radiation environment.
ADCS (Attitude Determination and Control Subsystem): The science investigation requires a post-processed attitude knowledge of ≤ 3º (3σ). Both spacecraft are nominally sun-pointing and spin stabilized. To maintain the observatory's sun sensors within the operational range and to ensure sufficient power system margins, the total off-pointing angle between each observatory and the Sun-Earth line is required to remain within the range of 15-27º. Furthermore, each observatory is required to operate within the spin range of 4-6 rpm during normal operations, and to maintain the spin rate within 0.25 rpm once the nominal spin rate is established.
Attitude determination is accomplished by two means. The flux-gate magnetometer sensor is the primary sensor for determining definitive observatory attitude. The data from this sensor is not processed on-board, so additional sensing is required to allow for autonomous attitude sense for spacecraft health and safety. This on-board attitude determination is accomplished via two sun sensors. These sensors provide coarse spacecraft attitude information that is sufficient for autonomous spacecraft health and safety. This data is also sent to the ground where it is used in the estimation of definitive spacecraft attitude that is used for science processing. Each Observatory also includes two passive nutation dampers that help maintain a stable spacecraft attitude and damp out any "wobble" after propulsive maneuvers.
The propulsion system is a simple monopropellant blowdown system. Three Inconel tanks store the 56 kg of hydrazine propellant onboard, and feed the eight 0.9 N thrusters. The position and orientation of these eight thrusters allow for spin up and spin down about the primary spin axis, positive and negative precession about the spinplane axes, and velocity change toward and away from the sun. These thrusters are the only active attitude control mechanisms on board the spacecraft, and provide the full set of capability required to maintain the spacecraft attitude and spin rate and perform orbit corrections, collision avoidance maneuvers as required, and the final de-orbit burn at the end of the mission.
The spacecraft also includes diagnostic instrumentation – termed the ERM (Engineering Radiation Monitor) – to monitor the in-situ radiation environment. The long-term health and operability of the spacecraft electronics and materials are directly affected by the total incident radiation dose. The ERM will measure this incident dose, and will provide a means for correlating upsets in spacecraft electronics with the environment present at that time. This monitor will also allow refinement of the standard total dose curves that are traditionally used for the design of spacecraft that operate in the Earth's radiation belts.
Fault protection: Each spacecraft also includes robust APA (Fault Protection and Autonomy) systems that work together to maintain the overall health and safety of the flight segment. Because the spacecraft includes limited hardware redundancy, the FPA systems are of particular importance on RBSP. The architecture uses a layered response approach to maximize science data collection in the event of a fault. The system protects against the extended loss of communications by way of both software and hardware command loss timers. It also monitors the sun angle of a given observatory, and can safe the system and notify the ground in case of an exceedance of the minimum or maximum sun angle. Similarly, the FPA system monitors the bus voltage and battery state of charge, and can safe the system in case of a LVS (Low Voltage Sense) or a LBSOC (Low Battery State of Charge) where the bus voltage or battery state of charge drop below a minimum preset level.
The system also monitors the current condition and the health of spacecraft components, and it has the ability to individually off-pulse the primary unswitched loads (the IEM, the PDU and the transceiver) to restore those systems to a known startup configuration and presumably to clear any faults that my be present. In addition to monitoring and managing spacecraft bus health and safety, the system can also monitor instrument currents and heartbeats and can power off instruments in the case of a fault, and they can also individually power off the instruments based on a turn off request generated by that instrument.
All parts used in the RBSP observatory were specified to survive a total ionizing dose of 34 krad (Si) [23 krad (Si) for the IEM] without parametric or functional failure. This value is based on a 2-year (plus 75 day) life, with a RDM (Radiation Design Margin) factor of 2, and a nominal shield depth of 350 mils (6.3 mm) [500 mils (12.7 mm) for the IEM] of aluminum.
Lastly, the fault protection system manages the separation sequence after launch, deploying the solar arrays and powering on the RF downlink (uplink is enabled by default at launch).
Figure 8: Block diagram of the RBSP spacecraft (image credit: JHU/APL, revision Oct. 28, 2011, Ref. 13)
Figure 9: RBSP spacecraft layout with side panels in a non-flight "open" orientation (image credit: JHU/APL)
Figure 10: Photo of the RBSP-A spacecraft in Nov. 2011 during testing on the solar arrays (image credit: JHU/APL) 18)
Table 2: Overview of RBSP spacecraft resources (Ref. 13)
Figure 11: Technicians at the Astrotech payload processing facility prepare the RBSP spacecraft for encapsulation in the payload fairing (image credit: NASA)
Figure 12: Orbital configuration of the RBSP observatory showing the instrument FOVs (image credit: JHU/APL)
Launch: The two identical RBSP spacecraft were launched on August 30, 2012 (UTC) from the Cape Canaveral Air Force Station (launch complex 41) in Florida. The launch provider was ULA (United Launch Alliance), using an Atlas-V 401 launch vehicle. 19) 20) 21)
The probes were released from the rocket's Centaur upper stage one at a time and sent off into different orbits, kicking off the two-year mission to study Earth's radiation belts. The RBSP-A spacecraft separated from the Atlas rocket's Centaur booster 1 hour, 18 minutes, 52 seconds after launch. The second spacecraft, RBSP-B, separated 12 minutes, 14 seconds later.
After deployment of RBSP-A, the Atlas-V will raise its own apogee to approximately 30540 km and then deploy RBSP-B. This slight difference in apogee altitude values will cause RBSP-A to lap RBSP-B approximately once every 75 days (201 & 200 orbits respectively).
Orbit: Near-equatorial HEO (Highly Elliptical Orbit, almost like GTO), perigee ~ 620 km, apogee ~ 5.8 RE (~ 30,500 km), inclination = 10º, period ~9 hours.
Figure 13: Illustration of the two RBSP spacecraft in their orbit (image credit: JHU/APL)
Figure 14: RBSP mission design (image credit: JHU/APL)
The RF communications system contains a single transceiver, an 8 W SSPA (Solid-State Power Amplifier), a diplexer, and two broadbeam, near-hemispherical antennas. The system provides S-band uplink, downlink and radiometric tracking capability. It supports both ½ turbo and convolutional encoding, and it uses coherent downlink to allow for Doppler navigation.
The mission uses the S-band only version of the Frontier Radio, built on JHU/APL's SDR (Software Defined Radio) architecture. The Frontier Radio is based on coherent transceiver (XCVR) technology and is compatible with NASA's STRS (Space Telecommunications Radio System) architecture. The downlink data rate is up to 2 Mbit/s (QPSK modulation). In addition, highly accurate coherent Doppler data is needed for spacecraft navigation (the orbit determination process uses the Doppler data). 22) 23)
Due to its (nominally) 5 rpm spin, the spacecraft is designed with the two antennas' boresights parallel to the spin axis to ensure uninterrupted telecommunications while spinning. Each antenna provides sufficient gain (–4 dBic minimum) from its boresight to 70º from boresight.
• Two antennas: S-band conical bifilar helix, circular polarization, broadbeam
Figure 15: The RBSP mission requires broadbeam antenna coverage from boresight to 70º for each antenna. The antenna is shown on the right, with and without a radome (image credit: JHU/APL)
• RF routing: procured power divider & diplexer (L-3 Communications Corporation)
• SSPA (Solid-State Power Amplifier): 8 W, S-band power amplifier. SSPA is using commercially available devices. The use of packaged devices allowed for rapid development and design verification, while meeting the efficiency and output power requirements of the SSPA. The SSPA comprises a three-stage microwave amplifier and power converter in a common aluminum chassis.
The block diagram in Figure 16 shows the RF amplifier line up, power conditioner and telemetry interfaces. The SGA-4386 is a SiGe HBT (Heterojunction Bipolar Transistor) amplifier. The SZA-2044 is an InGaP (HBT) amplifier. The TGA2924-SG is a partially matched HFET (Heterostructure Field Effect Transistor) amplifier. An isolator is used at the output of the SSPA to protect the power amplifier during testing as well as to ensure stable power amplifier performance. Conditioned power is supplied to the SSPA by an internal EPC (Electronic Power Converter). The EPC supplies the necessary ±5 V and +9 Volt DC power and provides the power-on and power-off sequencing of the voltages needed to prevent damage to the SSPA. All secondary currents and voltages are sensed and amplified as analog voltages and converted to digital values by the PRIO (Power Remote Input Output) chip.
The power amplifier and power converter are housed in a single aluminum chassis. The chassis and cover provide 9mm thick aluminum shielding for all six sides of the enclosure to mitigate the radiation environment. The power amplifiers were fabricated on Rogers 4350B material. The use of a single board for these lines eliminates the need for tight tolerances and alignment of separate circuit carriers to the adjoining amplifier stages. The power converter consists of two converter bricks and associated circuitry on a 4-layer Polyimide circuit board.
Table 3: SSPA design summary
• Frontier Radio: DC/DC, DSP (Digital Signal Processor), receiver, exciter slices.
Frontier Radio (Demonstration Payload):
The Frontier Radio is a low-power, low-mass, modular SDR (Software Defined Radio) platform designed for communications, navigation, radio science, and sensor applications- the first radio with full software implementation. The objective is to demonstrate a TRL-6 (Technology Readiness Level) of 6 and to be qualified for a spaceflight mission. 24) 25) 26)
Figure 17: Block diagram of the single-string RF communications subsystem (image credit: JHU/APL)
Table 4: High level design constraints for the DSP (Digital Signal Processor) system, including hardware, firmware, and software 27)
Hardware: The radio level constraints focused the DSP system design effort on specific paths. The first set of trades focused on the hardware necessary to process the communication formats listed in Table 4. The demodulation scheme for even these simple phase modulation formats costs 350 to 450 MOPS, which is derived from the original New Horizons FPGA design. These operations do not include state machines or overhead control logic, they are mostly simple 32 bit operations on the main data stream. 10 to 20 MOPS (Million Operations Per Second) are necessary for controlling the radio hardware and communicating with the spacecraft, and (if a lookup table based modulation and encoding architecture is used) the transmission path costs 3-5 OPS per transmitted symbol (300-500 MOPS for the radio goal data rates). There was no space grade standard processor that could come close to meeting these MOPS requirements while fitting within the receive only power consumption specification. The TI C6701 DSP processor came close to these capabilities, but the radiation reliability, SEE (Single Event Effect) rate, on this device did not meet the requirements. There was no single commercial solution available.
Processor combinations with a FPGA were investigated as another option, but were again ruled out due to total power consumption or the board area required to create multiple high efficiency core voltages. During the investigation process the math operations themselves were looked at more closely. Nearly all of the high rate operations were additions or subtractions for the CIC (Cascaded Integrator-Comb) filter based receive channel, and base 2 finite field arithmetic for the transmit channel. Most of the difficult operations, multiplication and division, were carried out at a much lower rate, <5 MOPS, to create control loop filters or gain elements. The remaining operations were low rate basic state machines and mathematics for hardware control and communication with the spacecraft interface.
Very quickly it became clear that the ideal solution was a front end FPGA for filtering and demodulation, a low power DSP processor running in real time to close control loops, and a low rate RISC processor to handle abstraction of the radio hardware and firmware up to the spacecraft interface. At that time FPGA gate count had risen to the point where it became possible to imbed all of these technologies into one FPGA device. Designing and implementing a system of this level of complexity for a new radio had significant cost and schedule risk. The benefits of limiting the DSP design to one device, thereby reducing board area and interconnect I/O power, prevailed in this trade.
The next hardware trade was the selection of the target FPGA device, and the main contenders were the Actel RTAX and Xilinx Virtex-4 device series. SRAM based FPGA devices had exceptional processing capabilities due to their larger gate count and faster master clock rate, plus they are completely reconfigurable which makes them attractive for an SDR (Software Defined Radio). The main problems with this choice were reliability in radiation and power consumption. The former issue fed the latter because of triple voting and the need for an external device to continually refresh the configuration memory. The SRAM device also required an array of non-volatile memories for the configuration file which would use up valuable board space resources. The Actel RTAX device family was finally selected because it allowed the entire DSP system to meet all of the radio high level design constraints listed in Table 4.
Firmware platform: The first work done on the Frontier Radio firmware architecture was to create a platform to which all of the other firmware and hardware elements would be connected. This effort was the selection of the low rate RISC processor and its interfaces. At that time, there were only a few processor IP cores available from Actel, but a wide selection in the greater FPGA community. Most of these processors fell into the three main categories of too small, too large, and too unique. The small processors were 8 to 16 bit, and most were based upon the 8051 architecture.
Manipulating the 32 and 64 bit fixed point numbers of the existing New Horizons firmware design would have been difficult for these processors, as well as controlling the >24 bit address space. Processors like the LEON3 offered extensive capability, but performed a significant number of unnecessary functions like IEEE-754 floating point mathematics. These designs would have forced the firmware to grow into, and possibly beyond, the RTAX40009 device size. Most of the other processor IP designs made significant use of specific Xilinx or Altera devices resources that were not available in the RTAX design. Many of these custom processor designs were also unique, like the SCIP (Scalable Configurable Instrument Processor) developed at JHU/APL for Actel FPGA based space instrument control. This is a stack based processor that only had an existing compiler for the Forth language. These unique processors would have taken too long to modify for the RTAX family, space applications, or to connect to the rest of the radio. They would have also taken too long to develop a programming flow.
The final selection choice was an instantiation of the MIPS3000 architecture. This particular design did not include a floating point unit or any other unwanted peripherals, which was desirable for the current radio requirements and allowed for upgrades to add these capabilities in the future. The most favorable aspect of the MIPS3000 was the wide variety of software tools available to program the device.
Once the general purpose processor was chosen the firmware architecture began to take shape. Each processing element, the MIPS processor being one of them, was named a "core", and all of the cores were connected to a standard microprocessor bus. The cores that control access to the external memory banks and the spacecraft interface were placed directly on the main bus of the MIPS processor. All of the other cores were placed on a separate APB (Advanced Peripheral Bus), which was connected to the MIPS bus via a custom controller core. Separation of the MIPS bus from the APB was done very early, before specifications were complete on each of the cores. The unknown requirements (bus clock rates, latency, and activity) for the new DSP cores were the main reason for this separation. All high rate communication between cores would be carried out with custom core to core buses.
The separation of the core bus connections helped the design and development process in several ways. Foremost, the design allowed the design team to develop the general purpose processing platform (MIPS bus) separate from the DSP section. A simple APB to UART controller was built to support DSP core development and testing through a Matlab interface. The Matlab low level control tool is still in use today for debugging or detailed testing. In parallel, a firmware and software team was preparing the basic software development tools, planning out memory maps, and outlining the first radio software framework. Placing these two development efforts in series would have broken the project schedule. The other main benefit of the APB is its ability to handle variable response latency gracefully, which is good for real-time processors (the DSP cores) that cannot respond immediately to read/write requests. The APB controller isolates the MIPS from these variable delays, and prevents individual polling routines, and handshaking or interrupt circuitry. It does this by off loading two main burst read/write operations from the MIPS processor and bus.
Firmware DSP Cores: The goal for each of the DSP Core designs was to be as programmable, software defined, as possible while not exceeding the capacity of a RTAX2000 FPGA. The MIPS processor platform itself was allocated ~40% of the FPGA, including the MIPS processor, memory controller, spacecraft interface, APB controller, clock management, and the hardware interfaces. The allocation was derived fairly easily due to reuse of existing designs, as well as the straight forward nature of these circuits. This left the DSP Cores with about 6,000 flip-flops and 11,000 4-input logic cells. At this early stage reusable designs already existed for Convolutional & Turbo encoding (including framing), critical command decoding, demodulation, and individual control loop filters. Given the size of these designs and the FPGA gate area remaining, several key decisions were made about each DSP core. The functionalities, boundaries, and names for each of the cores were mapped out first. Then trades were made on each core concerning software based flexibility, realistic requirements, and FPGA resources. The resulting firmware architecture has remained the same since its creation in 2007, and is shown in Figure 18.
The final step in the firmware process was to break each core down one step further into distinct "blocks" that could each be individually specified, designed, implemented, and tested within approximately one month of time. This was done for three main reasons; modularity, team work, and incremental testing. Modularity is important for STRS compliance, and its use has great benefits in reducing new development cost. This allows one functional element within each core (e.g. CIC filter, ALU, or encoder core circuit) to be removed, inserted, modified, or completely redesigned while having minimal effects on the surrounding circuitry. Modularity also allows for automated customizations at the time the firmware is built for a particular implementation.
The Frontier Radio is the focal point of the RF telecommunication system. It receives an S-band uplink for spacecraft commanding, generates a coherent S-band downlink for telemetry and science data (either convolutional or turbo coded) and navigation, and interfaces with the C&DH (Command and Data Handling) subsystem of the spacecraft.
The radio is a low-power, low-mass, modular SDR consisting of four slices: power converter, receiver, exciter, and DSP (Digital Signal Processing) slice. The DSP slice processes both the uplink and downlink signals and handles mode and status communications with the C&DH subsystem. The exciter slice receives baseband data from the DSP slice and modulates it onto a high-quality S-band carrier to generate the downlink. The receiver slice acquires the uplink signal from the ground station and downconverts it to an IF (Intermediate Frequency) that is passed to the DSP slice. The receiver slice also contains an OCXO (Oven Controlled Crystal Oscillator) which is used to generate various clock signals required for the radio. The DSP digitizes the IF signal and further filters and processes it in firmware within an Actel RTAX-2000 FPGA (Field Programmable Gate Array). The power converter slice generates secondary voltages from the main 30 V spacecraft bus (Figure 18).
The Frontier Radio platform was designed for flexibility. Uplink and downlink frequencies, line coding, error correction coding, control loops, and many other features are all programmable. For the RBSP implementation, these features have been determined by the mission requirements and have been hardcoded to maximize reliability in the harsh radiation environment that is anticipated. However, the underlying flexibility of the architecture allowed quick iterations of the design during prototype development.
The two RBSP spacecraft will operate in different RF channels. Both channels adhere to coherent turnaround ratios of 240/221. The information required for these frequency assignments, along with acquisition control loops, calibrations, and other software needed to define the characteristics of the radio are referred to as the Radio Personality and are stored in memory within the radio.
The mission communications system has been designed to operate with the JHU/APL 18 m ground station (APL-18) in Laurel, Maryland, USA, as the primary ground station. Periodic additional contacts will be made over two USN (United Space Network) 13 m ground stations in South Point, Hawaii, USA, and Dongara, Australia. These ground stations provide additional coverage for launch and early operations, emergencies, and science downlink bandwidth when needed.
In addition, communications with NASA's TDRSS (Tracking and Data Relay Satellite System) are foreseen. Low rate downlink and uplink modes are required for TDRSS contacts. The radio microcode had to be optimized for both high and low bit rate performance. The downlink bit rate ranges from 1 kbit/s to 2 Mbit/s for this mission. The downlink bit rate is controlled completely within the radio and is determined by programmable prescaler and divider circuits within the FPGA. Although only six downlink rates are planned for use in the RBSP mission, these circuits allow the radio to be programmed to a variety of other bit rates below 2 Mbit/s. Above that rate, hardware specific to the RBSP design becomes a limiting factor. For this RBSP implementation, the uplink bit rates are 2000 bit/s and 125 bit/s. Other rates are available in the generic radio design.
Table 5: Frontier radio support for near-Earth applications (Ref. 25)
Figure 19: Configuration of the ground support equipment (image credit: JHU/APL)
Table 6: Key performance parameters of the RBSP Frontier Radio
- Simultaneous twopoint measurements by identical spacecraft in common orbits with a slow separation in phase, lapping one another 4-5 times/year.
- Covering the full range of local times in 2 years
- Apogee of ~ 5.8 RE to sample the outer belt and ring current
- Perigee of ~ 630 km to sample the inner belt.
The two RBSP spacecraft will operate entirely within the radiation belts throughout their mission. When intense space weather occurs and the density and energy of particles within the belts increases, the probes will not have the luxury of going into a safe mode, as many other spacecraft must do during storms. The spacecraft engineers must therefore design probes and instruments that are "hardened" to continue working even in the harshest conditions.
Figure 20: Photo of the Frontier Radio (image credit: JHU/APL) 28)
Operation of the Frontier Radio on the Van Allen Probes and future outlook:
The first two Frontier Radios provide a robust, high capability SDR with very low SWaP (Size, Weight, and Power). The targeted applications typically require highly sensitive communications and radio navigation modes, resulting in a baseline architecture that excels at meeting the performance requirements of a large variety of other spaceborne applications. The Van Allen Probes make use of this SDR in an S-band duplex configuration to support mission-critical spacecraft commanding, the return of spacecraft housekeeping telemetry and science data. A transponder function provides for two-way Doppler navigation of the spacecraft. 29)
An on-board CCD (Critical Command Decoder) provides a method for command reception without an external spacecraft processor when needed; this CCD could be used for spacecraft fault detection, radio reconfiguration, or in-flight software uploads should a particular mission require any of those functions. Other advanced features include support for a variety of coding formats (typical convolutional and Turbo codes) and an internal ovenized oscillator for improved timekeeping accuracy and RF performance such as low phase noise and implementation loss.
The highly modular architecture of the Frontier Radio facilitates low-cost, low-risk infusion of new technology or alternate configurations to meet new, unique, or future mission requirements. This is particularly useful for ORS (Operational Responsive Space) applications, where pre- and post-launch reconfiguration is an essential capability. Modularity is a theme carried throughout the hardware, firmware, and software architectures.
Alternate Frontier Radio configurations and modules have been developed that provide Ka-band (26, 32, or 38 GHz) transmit and receive capability, with symbol rates as high as 150 Msample/s and support for high order vector modulation schemes (e.g., 300 Mbit/s QPSK, 450 Mbit/s 8-PSK, and so forth). At the other extreme, an X-band duplex (with an optional dual-band X/Ka-band exciter) configuration supports data rates to below 10 bit/s and two-way ranging for deep space applications.
The SPP (Solar Probe Plus) mission of NASA, with the launch planned for 2018, is highly constrained in mass and power, thus is leveraging this deep space configuration of the Frontier Radio. The wide bandwidth capability of this SDR will enable the SPP mission to take advantage of substantially increased precision during ΔDOR (delta Differential One-way Ranging) operations; this is critical in simplifying the navigation solution for a spacecraft that will fly as fast as 200 km/s at closest approach to the Sun. SPP also requires the Ka-band downlink capability to close highly constrained communications links as the spacecraft optimizes pointing to keep its sun shade properly oriented (in contrast to optimizing for the communications link).
The advanced signal processing capabilities of this SDR can be used to enable new mission operations scenarios that could make or break the viability of a mission. The ability to implement very narrow bandwidth tracking algorithms and more exotic phaselocked-loop implementations (e.g. adaptive or high order loop filters) provide receive sensitivity that mirrors that of the ground receivers used for deep space missions. This further enables the use of advanced decoding algorithms with minimal implementation loss at deep space data rates (e.g. 7.8 bit/s). These features enable advanced capabilities such as future deep space relay or highly sensitive remote sensing (deep space, near earth, or airborne) with a very low SWaP impact. Software management within the SDR further facilitates more advanced operations that may require performance optimization for different operations scenarios or more autonomous signal acquisition, operation, or fault recovery. Software modifications within the current function set can be implemented and tested in as little as a few days to weeks, an essential capability for ORS applications.
The versatility and low SWaP of the existing Frontier Radio platform has proven useful in applying this SDR to remote sensing applications. More specifically, remote characterization of the ionosphere in concert with the DORIS system has many civilian and defense-related applications, and realizing a global system of these remote sensors becomes more economically feasible where hosted payload space and rides of opportunity are leveraged (Figure 21).
These situations are much more viable when the SWaP of the payload is minimized. Interest in this area has spawned recent development of the waveforms required to operate the Frontier Radio for ionosphericcharacterization applications. The very low SWaP of the existing platform sets this SDR apart from other high-reliability space-grade SDRs, making it a highly viable choice for hosted payload applications with long mission life. Fortunately, most of the signal processing algorithms required for this application are synonymous with the existing algorithms that are used for the radio communications and navigation functions. Therefore, a minimum development effort is required to support the new waveform. The wide variety of hosted payload opportunities provides for a moving target in terms of available accommodations and resources. The volume of the current Frontier Radio lends itself well to small satellites, in as small as 500 to 1000 cm3 for remote sensing configurations. The required volume depends on the accommodations provided by the host spacecraft. For example, a power converter and customized data interfaces would not be required for Cubesat rides where power regulation and conditioning are routinely provided and relatively standard data interfaces are utilized. In this configuration, power consumption is ~2.5 W for a 5 Vregulated input, which includes an internal ovenized precision oscillator.
Wide application of the Frontier Radio to a variety of small satellite applications is a natural fit, given the very low SWaP yet high capability of this SDR. The built-in processing capabilities (currently an embedded MIPS in FPGA) are more than sufficient for simple autonomous decision making required in some small satellites and probes. By leveraging ever-increasing available FPGA resources, higher capability processors, like the LEON3, can readily be embedded into the SDR's existing FPGA, alongside the DSP cores. This would provide the capability required for full-featured small satellites, for example, where guidance and control systems might provide 3-axis active pointing. Substantial SWaP savings can be realized in small satellites that leverage these options, essentially embedding multiple systems like communications/RF, command and data handling, and guidance and control within one FPGA. The RF functions within the SDR's FPGA could equally be ported to another FPGA on the spacecraft to minimize overall SWaP or improve redundancy and reliability. Highly integrated spacecraft systems like these have the power to substantially reduce the spacecraft SWaP (and associated hardware cost) and/or dramatically increase the available science/mission data for each launch. Further, this opens up the window to more advanced cooperative systems such as a mesh of small spacecraft or nodes that perform distributed sensing or form distributed RF apertures.
The Frontier SDR has been mission enabling for its low mass and power and high functionality and multi-band capability. These same advantages lead to its use on the SPP (Solar Probe Plus) mission and on a LEO mission needing high bandwidth, low power, and radiation tolerance. Cubesat and nanosat implementations are in the early stages of design, and are likely to bring the incorporation of multiple spacecraft functions into one central processor. High integration, or the ability to port firmware and software IP to different host processors within a spacecraft, is a game changing capability that will shape future missions and open up new opportunities. The existing SWaP and capability of the Frontier SDR provide an exceptional baseline for new small satellite missions, with substantially untapped processing capability for adding advanced new features (Ref. 29).
Status of the Frontier Radio:
• September 2016: The VAP (Van Allen Probes) mission successfully transitioned the Frontier Radio technology to TRL-9 in an S-band duplex configuration for Near-Earth applications (Frontier NE). The availability of Frontier Radio's Ka-band (26 GHz) capability provided motivation for NASA Headquarters to fund an effort to develop X-band duplexed deep space capability with a 32 GHz Ka-band downlink mode. This funding came at the same time as the VAP development and resulted in an X/X/Ka-band Frontier Radio in an identical form factor as the S/S/Ka-band unit. The same core hardware, firmware, and software design was preserved across both instantiations of the radio. A TRL-6 demonstration proved out several key enhancements: 30)
1) X-band uplink from 7.8125 bit/s to 1.3 Mbit/s
2) Deep space receive sensitivity, supporting carrier tracking loop bandwidths as low as 0.1 Hz (2BN)
3) X-band downlink from 10 bit/s to 75 Mbit/s
4) Ka-band (32 GHz) downlink up to 150 Msps (300 Mbit/s QPSK tested)
5) Two-way ranging and wideband ΔDOR capability.
- The successful VAP effort and TRL-6 X/X/Ka-band development efforts provided a deep space Frontier Radio (Frontier DS) with high heritage from the TRL-9 near-Earth unit. In the meantime, the NASA SPP (Solar Probe Plus) mission, a current program in development at JHU/APL to probe the corona of the Sun (planned launch in 2018), had begun looking for radio options with Ka-band capability that could meet its substantial mass constraints and high radiation tolerance requirements. These two requirements precluded the use of bulk and spot shielding methods to reduce the effective radiation exposure to the radio electronics. The low-SWaP and intrinsically high radiation tolerance of the Frontier Radio DS uniquely qualified it for the SPP application and resulted in the mission baselining this radio. As with VAP for the near-Earth radio, the SPP effort supported the maturation of the deep space radio enhancements, including the necessary compatibility testing with the DSN (Deep Space Network). Flight Frontier Radios for the SPP mission (Figure 22) have completed qualification as of August 2016 and will be integrated into the spacecraft during the remainder of 2016. 31)
- An X-band-only Frontier DS, largely based on the SPP design, is currently under contract via the University of Colorado Boulder LASP (Laboratory for Atmospheric and Space Physics) for delivery to the United Arab Emirates' Emirates Mars Mission, scheduled for launch in July 2020 for its Mars arrival in 2021.
- The NASA Europa program's current primary project, the Europa Clipper, is a joint mission between the JPL (Jet Propulsion Laboratory ) and JHU/APL with a planned launch in the early 2020s. The X/X/Ka-band Frontier DS, matured on the SPP mission, is the baseline radio for this mission. Further, the Europa Program is developing a Lander mission to the surface of the Jovian moon. This lander is baselining the Frontier XL, a cross-link radio currently in development. Due to the geometry and power constraints of the lander mission, direct-to-Earth communications are not possible for the lander spacecraft. The lander mission includes a carrier spacecraft ("Carrier") that will remain in close proximity the Europa. Both Carrier and Clipper (Clipper launching several years in advance) will carry XL packages as well as DS Frontier Radios to provide communication capabilities for the Lander.
- Frontier Radio Light: A further SWaP-reduced radio implementation (Frontier Radio LT) is under development that streamlines the design to a single board or slice and a 1W receive mode to more readily support infusion into 3U and larger CubeSat as well as microsatellites. 32) In addition, an enhanced processor board is under development that will complement the Frontier Radio LT for applications requiring significant in-flight processing; applications emphasizing wider instantaneous bandwidth, radar, and signals intelligence will make use of the processor complement to the radio. The enhanced processor board will also facilitate shared processor resources on microsatellites, allowing multiple subsystems such as command and data handling, guidance and control, communications, and payload functions to share hardware resources in a highly efficient manner. This provides a method for substantial reductions in spacecraft SWaP, or a commensurate increase in payload capability. An extension of this development is currently under consideration for use in the proposed ESA/NASA AIDA (Asteroid Impact & Deflection Assessment) mission's DART (Double Asteroid Redirection Test) spacecraft.
- In summary, key technology infusion efforts such as the TADT (Telemetry Aided Doppler Tracking) navigation technique and the New Horizons low power digital receiver paved the way for JHU/APL's low-power space-qualified radios. NASA funded the core development efforts that gave rise to the Frontier Radio. The VAP mission brought this technology to flight, maturing the design and undergoing multiple radio and packaging qualification efforts. The Frontier Radio is currently flight-qualified in the near-Earth and deep space configurations. A number of enhancements are also under way for the Frontier Radio, emphasizing further reduction in SWaP and increased bandwidth and capability. The highly modular architecture of the Frontier Radio allows all of these enhancements to come at an incremental development cost, substantially minimizing NRE (Non-Recurring Engineering) costs and development risk to new missions (Ref. 30).
• Fall 2014: Each Van Allen Probes spacecraft's telecommunications system includes an S-band version of the Frontier Radio, a solid-state power amplifier, RF routing components, and dual low-gain antennas. This mission marks the first flight of the Frontier Radio, which is baselined for use in the upcoming Solar Probe Plus (launch 2018) and Europa Clipper missions (launch planned in 2022). 33)
• The NASA Van Allen Probes mission continues to provide new insights into the Earth's radiation belts and their interactions with the Sun. The telecommunications system, designed to operate in a very harsh environment, has been enhanced to enable even greater data throughput capacity, enabling more science and reducing contact time costs to the project. The communication system should continue to exceed pre-launch requirements throughout the mission lifetime.
- Overall health: Internal voltages, DC power consumption, RF uplink and downlink power levels, oscillator frequencies, error counters, and link characteristics are trended on a quarterly basis, and both Van Allen Probes spacecraft telecommunications systems are performing within expectations with the exception of an unexpected slow degradation (within mission requirements) of the SSPAs. The rate of the degradation indicates the mission will continue to meet its requirements throughout any conceivable extended mission.
- Increasing usable antenna fields of view: Initially, the fields of view for the spacecraft antennas for all communications links were from boresight to 70º in order to minimize spacecraft scattering and interferometric effects. This forces an "exclusion band" about the spacecraft during which no communications could take place. As shown in Figure 23, this cuts out a large amount of potential ground contact opportunities for downlinking science data.
- The 70º cutoff of antenna use was originally based on computed spacecraft antenna patterns from worst-case modeling of the interferometric effects. Once in flight, actual characterization of antenna performance in the exclusion zone was performed to determine if any extra contact opportunities could be realized. To assess the antenna performance, both uplink and downlink were characterized during APL-18 contacts through eight passes through the exclusion zone (four passes each for spacecraft A and B).
Figure 23: Antenna line-of-sight angle to in-view ground stations. Each point represents 15 minutes during the nominal mission. 0º and 180º correspond to boresights of the top and bottom antennas respectively (image credit: JHU/APL)
- The uplink was characterized by recording the received signal strength at the Frontier Radio receiver for both spacecraft, as well as sending and confirming receipt of no-op commands at 125 bit/s at roughly 1.25 s intervals. The command capability was assessed by subtracting the received commands from the sent commands over each 5 s interval, shown for spacecraft B in Figure 24.
- Nominally, the antennas would not be used between mast angles of 70º and 110º. However, the data showed a viable RF uplink with a new exclusion zone between 85º and 95º, which would significantly increase the realizable uplink contacts.
Figure 24: Uplink command rejections in the antenna exclusion zone for Spacecraft B. The noise on the curves are a result of the 5 s binning of the data and single uplink commands straddling bins. Spacecraft A showed a slightly better performance (image credit: JHU/APL)
- The downlink was similarly characterized by recording the received signal strength at the APL-18 receiver and logging the received downlink FER (Frame Error Rate) using a 125 kbit/s downlink. The data were again binned in 5 s intervals, and the FER results are shown in Figure 25. Again, the data indicate a similar coverage as in the uplink, with a viable downlink all the way to 85º from boresight.
- Because CFDP (CCSDS File Delivery Protocol) is used only with APL-18 and not the USN stations or TDRSS, use of the expanded antenna fields of view is only used while downlinking to APL-18. That way, any antenna null that may exist in the exclusion zone that was not characterized during the test would not have any impact on data throughput in the event retransmissions are needed. This has served to increase the data return over the APL-18 station resulting in less reliance on the more costly commercial stations.
• The RF system is sized to enable downlinking of at least 6.7 Gbit of data per day per spacecraft, including realtime housekeeping telemetry and adequate margins, during the operational phase of the mission. This data requirement and the available ground contact durations flow requirements on how to size the RF communications system. To meet data return requirements, given the constraints of the mission system, the RF subsystem must be able to provide data rates up to 2 Mbit/s. The data rate changes during a ground contact as the link parameters change due to the changing spacecraft ranges and angles to the ground stations. 34)
Figure 26: Ground station available coverage at apogee. The yellow lines indicate the spacecraft's ground track. (image credit: JHU/APL)
• Post-launch commissioning activities were driven by the requirement to verify both spacecraft's communication systems over multiple ground networks, including JHU/APL's own 18-m ground station, the Universal Space Network, and TDRSS. Enhanced science data downlink volume was enabled by expanding the usable field of view of the spacecrafts' antennas once in-flight calibrations of the antenna patterns were completed, as well as reducing downlink link margins to a bare minimum when downlinking via APL's 18-m dish, where the CFDP (CCSDS File Delivery Protocol) is used to guarantee file delivery. Radiation drove some of the hardware design; the radios have experienced several predicted fault conditions at the predicted rates, and have reacted autonomously as designed to minimize impact to the science downlink.
• March 27, 2017: NASA's Van Allen Probes act as space detectives, to help study the complex particle interactions that occur in these rings, known as the Van Allen radiation belts. Recently, the spacecraft were in just the right place, at just the right time, to catch an event caused by the fallout of a geomagnetic storm as it happened. They spotted a sudden rise in particles zooming in from the far side of the planet, improving our understanding of how particles travel in near-Earth space. 35)
- The twin Van Allen Probe spacecraft orbit one behind the other, investigating clues in a way a single spacecraft never could. On one typical day, as the first instrument traveled around Earth, it spotted nothing unusual, but the second, following just an hour later, observed an increase in oxygen particles speeding around Earth's dayside — the side nearest the sun. Where did these particles come from? How had they become so energized?
Figure 27: The twin Van Allen Probes orbit one behind the other, investigating clues in a way a single spacecraft never could. In this model, the trailing spacecraft saw an increase in injected oxygen particles (blue), which was unobserved by the first. The increase in particles was due to a geomagnetic storm front that moved across the path of the orbit after the first spacecraft passed (image credit: NASA/GSFC, Mike Henderson/Joy Ng, Producer)
- Scientists scoured the clues to figure out what was happening. With the help of computer models, they deduced that the particles had originated on the night side of Earth before being energized and accelerated through interactions with Earth's magnetic field. As the particles journeyed around Earth, the lighter hydrogen particles were lost in collisions with the atmosphere, leaving an oxygen-rich plasma. The findings were presented in a recent paper in Geophysical Review Letters. 36)
- Data from the Van Allen Probes HOPE (Helium, Oxygen, Proton, and Electron) spectrometers reveal hitherto unresolved spatial structure and dynamics in ion populations. Complex regions of O+ dominance, at energies from a few eV to >10 keV, are observed throughout the magnetosphere. Isolated regions on the dayside that are rich in energetic O+ might easily be interpreted as strong energization of ionospheric plasma.
• March 15, 2017: Earth's radiation belts, two doughnut-shaped regions of charged particles encircling our planet, were discovered more than 50 years ago, but their behavior is still not completely understood. Now, new observations from NASA's Van Allen Probes mission show that the fastest, most energetic electrons in the inner radiation belt are not present as much of the time as previously thought. The results are presented in a paper in the Journal of Geophysical Research and show that there typically isn't as much radiation in the inner belt as previously assumed — good news for spacecraft flying in the region. 37) 38)
- Past space missions have not been able to distinguish electrons from high-energy protons in the inner radiation belt. But by using a special instrument, MagEIS (Magnetic Electron and Ion Spectrometer) on the Van Allen Probes, the scientists could look at the particles separately for the first time. What they found was surprising —there are usually none of these super-fast electrons, known as relativistic electrons, in the inner belt, contrary to what scientists expected.
- "We've known for a long time that there are these really energetic protons in there, which can contaminate the measurements, but we've never had a good way to remove them from the measurements until now," said Seth Claudepierre, lead author and Van Allen Probes scientist at the Aerospace Corporation in El Segundo, California.
- Of the two radiation belts, scientists have long understood the outer belt to be the rowdy one. During intense geomagnetic storms, when charged particles from the sun hurtle across the solar system, the outer radiation belt pulsates dramatically, growing and shrinking in response to the pressure of the solar particles and magnetic field. Meanwhile, the inner belt maintains a steady position above Earth's surface. The new results, however, show the composition of the inner belt isn't as constant as scientists had assumed.
- Ordinarily, the inner belt is composed of high-energy protons and low-energy electrons. However, after a very strong geomagnetic storm in June 2015, relativistic electrons were pushed deep into the inner belt.
- The findings were visible because of the way MagEIS was designed. The instrument creates its own internal magnetic field, which allows it to sort particles based on their charge and energy. By separating the electrons from the protons, the scientists could understand which particles were contributing to the population of particles in the inner belt.
- "When we carefully process the data and remove the contamination, we can see things that we've never been able to see before," said Claudepierre. "These results are totally changing the way we think about the radiation belt at these energies."
Figure 28: During a strong geomagnetic storm, electrons at relativistic energies, which are usually only found in the outer radiation belt, are pushed in close to Earth and populate the inner belt. While the electrons in the slot region quickly decay, the inner belt electrons can remain for many months (image credit: NASA/GSFC, Mary Pat Hrybyk-Keith)
- Given the rarity of the storms, which can inject relativistic electrons into the inner belt, the scientists now understand there to typically be lower levels of radiation there — a result that has implications for spacecraft flying in the region. Knowing exactly how much radiation is present may enable scientists and engineers to design lighter and cheaper satellites tailored to withstand the less intense radiation levels they'll encounter.
- In addition to providing a new outlook on spacecraft design, the findings open a new realm for scientists to study next. "This opens up the possibility of doing science that previously was not possible," said Shri Kanekal, Van Allen Probes deputy mission scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, not involved with the study. "For example, we can now investigate under what circumstances these electrons penetrate the inner region and see if more intense geomagnetic storms give electrons that are more intense or more energetic."
• Sept. 28, 2016: The dipole configuration of the Earth's magnetic field allows for the trapping of highly energetic particles, which form the radiation belts. Although significant advances have been made in understanding the acceleration mechanisms in the radiation belts, the loss processes remain poorly understood. Unique observations on 17 January 2013 provide detailed information throughout the belts on the energy spectrum and pitch angle (angle between the velocity of a particle and the magnetic field) distribution of electrons up to ultra-relativistic energies. The study team shows that although relativistic electrons are enhanced, ultra-relativistic electrons become depleted and distributions of particles show very clear telltale signatures of electromagnetic ion cyclotron wave-induced loss. Comparisons between observations and modelling of the evolution of the electron flux and pitch angle show that electromagnetic ion cyclotron waves provide the dominant loss mechanism at ultra-relativistic energies and produce a profound dropout of the ultra-relativistic radiation belt fluxes. 39)
- Clear differences were observed in the dynamics at different energies, that is, the 1.02 MeV electrons are accelerated, while EMIC (Electromagnetic Ion Cyclotron) wave-induced precipitation depletes the 4.2 MeV electron population. The narrowing of the pitch angle distribution at 4.2MeV is consistent with EMIC wave scattering, providing additional confirmation of the EMIC wave-induced loss.
- During a moderate storm on 17 January (Figure 29 a,b shows the index of geomagnetic activity Kp inferred from the fluctuations of magnetic field on the ground), flux of relativistic electrons at During a moderate storm on 17 January (Figure 29 a,b shows the index of geomagnetic activity Kp inferred from the fluctuations of magnetic field on the ground), flux of relativistic electrons at 1.02 MeV increased (Figure 29 c). The combination of local acceleration and inward radial diffusion moved the inner boundary of the outer belt to lower radial distances (Figure 29 c). The short-lived dropout of fluxes is associated with the reversible changes. As clearly seen in the observations after the adiabatic dropout (Figure 29), fluxes return to pre-storm values.
- Ultra-relativistic electrons at 4.2 MeV (Figure 29 d) show a very different evolution, which is a net decrease in flux. This loss of ultra-relativistic electrons is not produced by the loss to the magnetopause, as the multi-MeV electron belt is located deep inside the outer zone, below 4.5 RE, whereas the magnetopause for this event was compressed down to 7.1 RE according to an empirical model and the variation of the global magnetic field in the inner magnetosphere was not significantly large (Figure 30).
Figure 29: Observations of radial profiles of electron fluxes and pitch angle distributions during the 17 January 2013 storm (a, b), image credit: Magnetic field study team, Ref. 39)
Legend to Figure 29: Evolution of the index of geomagnetic activity Kp as a function of day of January 2013. Observations of electron flux at 85º equatorial pitch angle as a function of radial distance and day at (c) 1.02 and (d) 4.20 MeV energy by the MagEIS and REPT instruments on the Van Allen Probes spacecraft. Observations of electron flux at L* =3.9 as a function of equatorial pitch angle and day by MagEIS and REPT for (e) 1.02 and (f) 4.20 MeV electrons. The radial profiles of fluxes show that over the course of the storm at 1.02 MeV electron fluxes increase in the heart of the belt (c), while at 4.2 MeV electrons drop out near L=4 (d). Pitch angle distributions show narrowing at 4.2 MeV (f) that is not observed at 1.02 MeV (e).
Figure 30: Observations of EMIC wave activity on 17 January 2013 (image credit: Magnetic field study team, Ref. 39)
Legend to Figure 30: Observations of EMIC wave activity on 17 January 2013 from a latitudinal array of ground-based search-coil magnetometer stations in Finland, arranged from highest to lowest L. (a–d). The spectrograms show the observed wave power as a function of frequency and UT (Universal Time) on 17 January 2013 with the MLT (Magnetic Local Time) of the stations also indicated. EMIC waves were observed for several hours and the peak wave intensity was observed at Oulu station at L=4.5. The white line is the estimated helium gyrofrequency and the magenta line is the estimated oxygen gyrofrequency at the equatorial crossing of the field line passing through the station. These lines bound the wave observations at Oulu, indicating that the source region for the waves was likely located in the heart of the radiation belts. 40)
• August 15,2016: Planet Earth is nestled in the center of two immense, concentric doughnuts of powerful radiation: the Van Allen radiation belts, which harbor swarms of charged particles that are trapped by Earth's magnetic field. On March 17, 2015, an interplanetary shock – a shockwave created by the driving force of a CME (Coronal Mass Ejection) from the sun – struck Earth's magnetic field, called the magnetosphere, triggering the greatest geomagnetic storm of the preceding decade. And NASA's Van Allen Probes were there to watch the effects on the radiation belts. 41) 42)
- One of the most common forms of space weather, a geomagnetic storm describes any event in which the magnetosphere is suddenly, temporarily disturbed. Such an event can also lead to change in the radiation belts surrounding Earth, but researchers have seldom been able to observe what happens. But on the day of the March 2015 geomagnetic storm, one of the Van Allen Probes was orbiting right through the belts, providing unprecedentedly high-resolution data from a rarely witnessed phenomenon.
- The spacecraft measured a sudden pulse of electrons energized to extreme speeds – nearly as fast as the speed of light – as the shock slammed the outer radiation belt. This population of electrons was short-lived, and their energy dissipated within minutes. But five days later, long after other processes from the storm had died down, the Van Allen Probes detected an increased number of even higher energy electrons. Such an increase so much later is a testament to the unique energization processes following the storm.
- "The shock injected – meaning it pushed – electrons from outer regions of the magnetosphere deep inside the belt, and in that process, the electrons gained energy," said Shri Kanekal, the deputy mission scientist for the Van Allen Probes at Goddard and the leading author of a paper on these results.
- Researchers can now incorporate this example into what they already know about how electrons behave in the belts, in order to try to understand what happened in this case – and better map out the space weather processes there. There are multiple ways electrons in the radiation belts can be energized or accelerated: radially, locally or by way of a shock. In radial acceleration, electrons are carried by low-frequency waves towards Earth. Local acceleration describes the process of electrons gaining energy from relatively higher frequency waves as the electrons orbit Earth. And finally, during shock acceleration, a strong interplanetary shock compresses the magnetosphere suddenly, creating large electric fields that rapidly energize electrons.
- Scientists study the different processes to understand what role each process plays in energizing particles in the magnetosphere. Perhaps these mechanisms occur in combination, or maybe just one at a time. Answering this question remains a major goal in the study of radiation belts – a difficult task considering the serendipitous nature of the data collection, particularly in regard to shock acceleration. - Additionally, the degree of electron energization depends on the process that energizes them. One can liken the process of shock acceleration, as observed by the Van Allen Probe, to pushing a swing. "Think of ‘pushing' as the phenomenon that's increasing the energy," Kanekal said. "The more you push a swing, the higher it goes." And the faster electrons will move after a shock.
- The March 2015 geomagnetic storm was one of the strongest yet of the decade, but it pales in comparison to some earlier storms. A storm during March 1991 was so strong that it produced long-lived, energized electrons that remained within the radiation belts for multiple years. With luck, the Van Allen Probes may be in the right position in their orbit to observe the radiation belt response to more geomagnetic storms in the future. As scientists gather data from different events, they can compare and contrast them, ultimately helping to create robust models of the little-understood processes occurring in these giant belts.
• July 2016: The Van Allen Probes have traversed the radiation belts for the past 3 years and performed successfully with minimal support required. The observatories have returned more than twice the expected science data. These science data have enabled major new discoveries about the radiation belts, including a temporary third radiation belt, acceleration energy within the belts themselves, and a nearly impenetrable barrier that prevents the fastest and most energetic electrons from reaching Earth. Preparations are under way to define new science objectives, orbit variations, and spacecraft configuration changes that will provide enhanced science data for an extended mission. The Van Allen Probes have demonstrated that they are well designed to support continued operations in the Van Allen radiation belts and have the capacity to make further contributions to studies of the Earth–Sun system. 43) 44)
• June 20, 2016: Earth's magnetosphere, the region of space dominated by Earth's magnetic field, protects our planet from the harsh battering of the solar wind. Like a protective shield, the magnetosphere absorbs and deflects plasma from the solar wind which originates from the Sun. When conditions are right, beautiful dancing auroral displays are generated. But when the solar wind is most violent, extreme space weather storms can create intense radiation in the Van Allen belts and drive electrical currents which can damage terrestrial electrical power grids. Earth could then be at risk for up to trillions of dollars of damage. 45) 46)
- A new discovery led by researchers at the University of Alberta shows for the first time how the puzzling third Van Allen radiation belt is created by a "space tsunami." Intense so-called ULF (Ultra-Low Frequency) plasma waves, which are excited on the scale of the whole magnetosphere, transport the outer part of the belt radiation harmlessly into interplanetary space and create the previously unexplained feature of the third belt. "Remarkably, we observed huge plasma waves," says Ian Mann, physics professor at the University of Alberta, lead author on the study and former Canada Research Chair in Space Physics. "Rather like a space tsunami, they slosh the radiation belts around and very rapidly wash away the outer part of the belt, explaining the structure of the enigmatic third radiation belt."
- Mann is co-investigator on the NASA Van Allen Probes mission. One of his team's main objectives is to model the process by which plasma waves in the magnetosphere control the dynamics of the intense relativistic particles in the Van Allen belts—with one of the goals of the Van Allen Probes mission being to develop sufficient understanding to reach the point of predictability. The appearance of the third Van Allen belt, one of the first major discoveries of the Van Allen Probes era, had continued to puzzle scientists with ever increasingly complex explanation models being developed. However, the explanation announced today shows that once the effects of these huge ULF waves are included, everything falls into place. "We have discovered a very elegant explanation for the dynamics of the third belt," says Mann. "Our results show a remarkable simplicity in belt response once the dominant processes are accurately specified."
Figure 31: This is an illustration to explain the dynamics of the ultra-relativistic third Van Allen radiation belt (image credit: Andy Kale)
• May 19, 2016: New findings based on a year's worth of observations from NASA's Van Allen Probes have revealed that the ring current – an electrical current carried by energetic ions that encircles our planet – behaves in a much different way than previously understood. — The ring current has long been thought to wax and wane over time, but the new observations show that this is true of only some of the particles, while other particles are present consistently. Using data gathered by the Radiation Belt Storm Probes Ion Composition Experiment, or RBSPICE, on one of the Van Allen Probes, researchers have determined that the high-energy protons in the ring current change in a completely different way from the current's low-energy protons. Such information can help adjust our understanding and models of the ring current – which is a key part of the space environment around Earth that can affect our satellites. 47) 48)
Figure 32: During periods when there are no geomagnetic storms affecting the area around Earth (left image), high-energy protons (with energy of hundreds of thousands of electronvolts, or keV; shown here in orange) carry a substantial electrical current that encircles the planet, also known as the ring current. During periods when geomagnetic storms affect Earth (right), new low-energy protons (with energy of tens of thousands of electronvolts, or keV; shown here in magenta) enter the near-Earth region, enhancing the pre-existing ring current (image credit: JHU/APL)
- "We study the ring current because, for one thing, it drives a global system of electrical currents both in space and on Earth's surface, which during intense geomagnetic storms can cause severe damages to our technological systems," said lead author of the study Matina Gkioulidou, a space physicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. "It also modifies the magnetic field in near-Earth space, which in turn controls the motion of the radiation belt particles that surround our planet. That means that understanding the dynamics of the ring current really matters in helping us understand how radiation belts evolve as well."
- The ring current lies at a distance of approximately 10,000 to 60,000 km from Earth. The ring current was hypothesized in the early 20th century to explain observed global decreases in the Earth's surface magnetic field, which can be measured by ground magnetometers. Such changes of the ground magnetic field are described by what's called the Sym-H index.
- "Previously, the state of the ring current had been inferred from the variations of the Sym-H index, but as it turns out, those variations represent the dynamics of only the low-energy protons," said Gkioulidou. "When we looked at the high-energy proton data from the RBSPICE instrument, however, we saw that they were behaving in a very different way, and the two populations told very different stories about the ring current."
- The Van Allen Probes, launched in 2012, offer scientists the first chance in recent history to continuously monitor the ring current with instruments that can observe ions with an extremely wide range of energies. The RBSPICE instrument has captured detailed data of all types of these energetic ions for several years. "We needed to have an instrument that measures the broad energy range of the particles that carry the ring current, within the ring current itself, for a long period of time," Gkioulidou said. A period of one year from one of the probes was used for the team's research.
- "After looking at one year of continuous ion data it became clear to us that there is a substantial, persistent ring current around the Earth even during non-storm times, which is carried by high-energy protons. During geomagnetic storms, the enhancement of the ring current is due to new, low-energy protons entering the near-Earth region. So trying to predict the storm-time ring current enhancement while ignoring the substantial pre-existing current is like trying to describe an elephant after seeing only its feet," Gkioulidou said.
• March 23, 2016: During their prime mission, the Van Allen Probes verified and quantified previously suggested energization processes, discovered new energization mechanisms, revealed the critical importance of dynamic plasma injections into the innermost magnetosphere, and used uniquely capable instruments to unveil inner radiation belt features that were all but invisible to previous sensors. 49)
- Now, through an extended mission that began 1 November 2015, the Van Allen Probes will advance understanding of the dynamics of near-Earth particle radiation. The overarching objective of this extended mission is to quantify the mechanisms governing Earth's radiation belt and ring current environment as the solar cycle transitions from solar maximum through the declining phase.
- The Van Allen Probes mission extends beyond the practical considerations of the hazards of Earth's space environment. Twentieth century observations of space and astrophysical systems throughout the solar system and out into the observable universe show the universality of processes that generate intense particle radiation within magnetized environments such as Earth's. Earth's radiation belts are a unique natural laboratory for developing our understanding of the particle energization processes that operate across the universe.
- Effects of the Solar Cycle Decline: The sunspot number reached a peak in April 2014. From historical measurements, we can expect that radiation belt activity will keep intensifying with the decline of the solar cycle: The biggest radiation belt enhancements during geomagnetic storms of two previous solar cycles occurred in their declining phase. As the solar cycle wanes, high-speed solar wind streams become more prominent compared to the solar coronal mass ejections that tend to prevail during solar maximum. Not surprisingly, the two biggest geomagnetic storms of this decade occurred last year, on 17 March and 21 June 2015.
- The local time positions of the apogees of the Van Allen Probes' orbits drift westward and complete a full circle around Earth over a period of about 2 years (Figure 33). By the end of the extended mission (roughly June 2019), the Van Allen Probes will be the first inner magnetospheric mission to circle Earth four times, enabling us to quantify how the relative role of various acceleration and loss mechanisms changes with the decline of the solar cycle.
- Understanding Local Particle Energization: Particle acceleration mechanisms have been a key focus of the Van Allen Probes mission. The probes have provided the first definitive evidence that, at times, local particle acceleration within the heart of the radiation belts dominates over other processes that invoke transport and adiabatic compression of particle population from distant regions. The local acceleration is attributed to quasilinear particle interactions with electromagnetic waves called "whistler waves". Whistler waves transfer energy from copious low-energy particles to sparse high-energy particles.
- At the same time, the probes have also discovered highly unexpected nonlinear wave structures in the heart of the radiation belt. Such structures can rapidly energize very low energy (~10 electron volts) electrons up to intermediate energies (~100 keV), thereby providing a seed population for subsequent acceleration to radiation belt MeV energies by the whistler waves. The probes have also observed whistler waves with unusually large amplitudes that are likely to more rapidly accelerate keV particles to MeV energies with nonlinear processes. A key theme of the probes' extended mission aims to sort out the relative importance of quasilinear and nonlinear interactions for the buildup of radiation belt intensities.
- To determine the relative importance of nonlinear interactions, we need to measure the evolution of the wave fields and particle distributions along field lines. In the extended mission, Van Allen Probes will provide two unique opportunities for such measurements. First, by adjusting the orbital phase of one spacecraft slightly with respect to the other, we can roughly align them along the same magnetic field line and thus sample field-aligned evolution of particles and wave fields.
- Coordinating with Japan's ERG (Exploration of Energization and Radiation in Geospace) spacecraft, planned for launch in December 2016, will afford us the second opportunity to sample wave interactions simultaneously at different magnetic latitudes. ERG, by design, will sample at higher magnetic latitudes than the probes. Using three-point measurements from ERG and the probes will provide a more global view of wave-particle interactions at different magnetic latitudes, important for quantifying nonlinear effects.
- Investigating Particle Loss: Defining particle loss mechanisms is critical to understanding dynamic variability of the radiation belt intensities. The Van Allen Probes and the associated BARREL (Balloon Array for Radiation-Belt Relativistic Electron Losses) have conducted joint experiments for quantifying particle precipitation, which is the scattering of particles from radiation belts into the atmosphere. The BARREL mission launched multiple high-altitude balloons to measure precipitation of relativistic electrons into the atmosphere along field lines that map to the radiation belts. Within the belts, the Van Allen Probes measure the plasma waves that drive these losses.
- Exceedingly close correlations have been observed between the so-called whistler-mode "hiss" waves and electron precipitation modulations, suggesting that losses capable of depleting radiation belt intensities can happen globally on time scales as short as 1 to 20 minutes.
Figure 33: As their orbits precess around Earth, the probes sample various acceleration and loss mechanisms that sculpt global distributions of radiation belts and ring current populations. The figure shows the intensity (marked with color) of electrons accelerated to 4.5 MeV along the spacecraft orbit projected onto the equatorial plane (in the solar magnetic coordinate system), image credit: NASA, JHU/APL
• January 19, 2016: About 1000 km from Earth's surface is the first of two donut-shaped electron swarms, known as the Van Allen Belts, or the radiation belts. Understanding the shape and size of the belts, which can shrink and swell in response to incoming radiation from the sun, is crucial for protecting the technology in space. The harsh radiation isn't good for satellites' health, so scientists wish to know just which orbits could be jeopardized in different situations. 50)
- Since the 1950s, when scientists first began forming a picture of these rings of energetic particles, our understanding of their shape has largely remained unchanged — a small, inner belt, a largely-empty space known as the slot region, and then the outer belt, which is dominated by electrons and which is the larger and more dynamic of the two. But a new study of data from NASA's Van Allen Probes reveals that the story may not be so simple. "The shape of the belts is actually quite different depending on what type of electron you're looking at," said Geoff Reeves from Los Alamos National Laboratory and the New Mexico Consortium in Los Alamos, New Mexico, lead author on the study published on Dec. 28, 2015, in the Journal of Geophysical Research. "Electrons at different energy levels are distributed differently in these regions." 51)
Figure 34: The traditional idea of the radiation belts includes a larger, more dynamic outer belt and a smaller, more stable inner belt with an empty slot region separating the two. However, a new study based on data from NASA's Van Allen Probes shows that all three regions — the inner belt, slot region and outer belt — can appear different depending on the energy of electrons considered and general conditions in the magnetosphere (image credit: NASA/GSFC, Duberstein)
- Rather than the classic picture of the radiation belts — small inner belt, empty slot region and larger outer belt — this new analysis reveals that the shape can vary from a single, continuous belt with no slot region, to a larger inner belt with a smaller outer belt, to no inner belt at all. Many of the differences are accounted for by considering electrons at different energy levels separately.
- The researchers found that the inner belt — the smaller belt in the classic picture of the belts — is much larger than the outer belt when observing electrons with low energies, while the outer belt is larger when observing electrons at higher energies. At the very highest energies, the inner belt structure is missing completely. So, depending on what one focuses on, the radiation belts can appear to have very different structures simultaneously.
- These structures are further altered by geomagnetic storms. When fast-moving magnetic material from the sun — in the form of high-speed solar wind streams or coronal mass ejections — collide with Earth's magnetic field, they send it oscillating, creating a geomagnetic storm. Geomagnetic storms can increase or decrease the number of energetic electrons in the radiation belts temporarily, though the belts return to their normal configuration after a time.
Figure 35: At the highest electron energies measured — above 1 MeV — researchers saw electrons in the outer belt only (image credit: NASA/GSFC, Duberstein)
- These storm-driven electron increases and decreases are currently unpredictable, without a clear pattern showing what type or strength of storm will yield what outcomes. There's a saying in the space physics community: if you've seen one geomagnetic storm, you've seen one geomagnetic storm. As it turns out, those observations have largely been based on electrons at only a few energy levels.
- "When we look across a broad range of energies, we start to see some consistencies in storm dynamics," said Reeves. "The electron response at different energy levels differs in the details, but there is some common behavior. For example, we found that electrons fade from the slot regions quickly after a geomagnetic storm, but the location of the slot region depends on the energy of the electrons."
- Often, the outer electron belt expands inwards toward the inner belt during geomagnetic storms, completely filling in the slot region with lower-energy electrons and forming one huge radiation belt. At lower energies, the slot forms further from Earth, producing an inner belt that is bigger than the outer belt. At higher energies, the slot forms closer to Earth, reversing the comparative sizes.
- The twin Van Allen Probes satellites expand the range of energetic electron data we can capture. In addition to studying the extremely high-energy electrons — carrying millions of electron volts — that had been studied before, the Van Allen Probes can capture information on lower-energy electrons that contain only a few thousand electron volts. Additionally, the spacecraft measure radiation belt electrons at a greater number of distinct energies than was previously possible. "Previous instruments would only measure five or ten energy levels at a time," said Reeves. "But the Van Allen Probes measure hundreds."
- Measuring the flux of electrons at these lower energies has proved difficult in the past because of the presence of protons in the radiation belt regions closest to Earth. These protons shoot through particle detectors, creating a noisy background from which the true electron measurements needed to be picked out. But the higher-resolution Van Allen Probes data found that these lower-energy electrons circulate much closer to Earth than previously thought. "Despite the proton noise, the Van Allen Probes can unambiguously identify the energies of the electrons it's measuring," said Reeves.
- Precise observations like this, from hundreds of energy levels, rather than just a few, will allow scientists to create a more precise and rigorous model of what, exactly, is going on in the radiation belts, both during geomagnetic storms and during periods of relative calm. "You can always tweak a few parameters of your theory to get it to match observations at two or three energy levels," said Reeves. "But having observations at hundreds of energies constrain the theories you can match to observations."
Figure 36: The radiation belts look much different at the lowest electron energy levels measured, about 0.1 MeV. Here, the inner belt is much larger than in the traditional picture, expanding into the region that has long been considered part of the empty slot region. The outer belt is diminished and doesn't expand as far in these lower electron energies (image credit: NASA/GSFC, Duberstein)
Figure 37: During geomagnetic storms, the empty region between the two belts can fill in completely with lower-energy electrons. Traditionally, scientists thought this slot region filled in only during the most extreme geomagnetic storms happening about once every 10 years. However, new data shows it's not uncommon for lower-energy electrons — up to 0.8 MeV — to fill this space during almost all geomagnetic storms (image credit: NASA/GSFC, Duberstein)
• October 2015: The two year primary mission consisted of two spin stabilized spacecraft in highly eccentric Earth orbits. The spacecraft provide insight into the dynamics of Earth's radiation belts by measuring the relevant in-situ environment (magnetic and electric fields) and key parameters of energetic particles and ions. The two spacecraft have slightly different orbital periods that cause one spacecraft to lap the other approximately four times per year. The difference in orbital elements resulted in an offset in the natural precession rate induced by Earth oblateness, causing the lines of apsides (or petals) of the two orbits to deviate. The project, which is currently in a bridge phase, considered several extended mission trajectory options that alter the rate of petal separation and lapping rate in order to study new PSGs (Prioritized Science Goals) for the first extended mission. The limiting factor on the various options is fuel consumption. 52)
- The metric used to evaluate the degree of petal separation is the difference in solar phase of apsides angles of the two spacecraft, which describes the orbit orientation with respect to the sun. The solar phase of apsides angle is defined as the angle between the line of apsides (pointing towards apogee) projected into the ecliptic plane and the Earth-Sun line. Based on the current orbital configuration (the difference in apogee altitude between the probes is 140 km), the delta in solar phase of apsides is increasing at 0.8º/year. One of the options being considered is increasing the altitude difference by 300%, which would cause the delta in solar phase of apsides to increase at 3.6º/ year. The maneuvers that yield the chosen extended mission orbit configuration are currently planned for June 2015. This is the preferred date because the attitudes of the spacecraft are sun-pointed and the local solar time at apogee will be dusk. Apogee local solar times at dawn or dusk enable the most efficient maneuver executions as the inertially fixed thrust vector is aligned with the velocity vector at either apses. This paper will detail the design and impact of all the trajectory options being considered for extended mission.
- An advantage to accelerating the petal separation by means of further deviating the relative semi-major axes would be a faster lapping rate. This is considered advantageous by the Van Allen Probes Science Team as it allows targeting a specific geomagnetic relative position during each close approach event, specifically, enabling the spacecraft to spend a period of time near the same Earth magnetic field line and take very unique measurements. Two such close approach events have already taken place to demonstrate the scientific value of these unique magnetic field measurements. The most recent of these close approach experiments was conducted in April 2015. A maneuver was executed on Van Allen Probe B on April 3, 2015 to place the two spacecraft along the same Earth magnetic field line at the close approach on April 9, 2015. The 22 cm/s maneuver resulted in the spacecraft being within a kilometer of the same magnetic field line. Figure 38 shows the difference in geomagnetic position from the same electric field line of the two spacecraft after execution of the trajectory correction maneuver.
- The Mission Design and Navigation team has explored several extended mission trajectory options. In order to balance both extended mission Prioritized Science Goals and mission lifetime, the SWG (Science Working Group) ultimately selected reducing the RBSPA apogee altitude by 75 km and increasing the RBSPB apogee altitude by 75 km. Preliminary maneuver planning and design has been completed and analysis has shown mission objectives will be achieved. The preliminary maneuver parameters will be iterated using the latest navigation solution the week of the maneuver.
• On August 30, 2015, three years after NASA's Van Allen Probes were launched from the Cape Canaveral Air Force Station in Florida, the twin spacecraft continue to push the boundaries of what is known about the space above our world.
• July 2015: The Van Allen Probes mission is approved by NASA to continue planning against the current budget guidelines. Any changes to the guidelines will be handled through the budget formulation process. The Van Allen Probes mission will be invited to the 2017 Heliophysics Senior Review. 53)
- The Van Allen Probes two spacecraft mission operates from a low inclination orbit in the inner magnetosphere. Each spacecraft carries an extensive suite of in situ particle instruments spanning a wide range of energies, together with electric and magnetic field sensors, to examine the wave-particle interactions which control the acceleration transport and loss of energetic particles in the Earth's ring current and radiation belts. The mission completed its two year prime phase on October 31, 2014, having met all level 1 requirements. Following the declaration of Mission Success on March 26, 2014, the mission was granted a bridging phase to align the program with the schedule for this Senior Review. This is the mission's first proposal for extended phase operations.
• June 2015: The 2015 Heliophysics Senior Review panel undertook a review of 15 missions currently in operation in April 2015. The panel found that all the missions continue to produce science that is highly valuable to the scientific community and that they are an excellent investment by the public that funds them. 54)
- Van Allen Probes Overview of the Science Plan: The Van Allen Probes team describes a focused science plan targeting the understanding of the dynamics of the inner magnetosphere and radiation belts for the extended phase operations of the two NASA Van Allen Probes. The proposal outlines three PSGs (Prioritized Science Goals) addressing high profile and high impact science targets which build upon and exploit the already impressive discoveries obtained using mission data thus far. As defined in the proposal these PSGs focus on developing understanding of:
6) The relative roles of local verses transport mechanisms of particle acceleration and the role that nonlinear mechanisms play in the acceleration process.
7) The relative importance of precipitation and magnetopause losses of energetic particles in the inner magnetosphere and provide more definitive information about the causes and consequences of the precipitation.
8) The relative roles of global-scale transport processes and mesoscale dynamic injections in the inner magnetosphere and their relative roles in the production of geo-effective waves.
The satellites and instrument suites are all in excellent health, with sufficient fuel for optimization of orbits during the extended mission operations and for maintaining the required sun-pointing alignment of the spacecraft spin axis. The team proposes a careful and fuel-efficient approach to maximizing scientific returns from optimized probe orbits, including separating the probes further in azimuth for the diagnosis of wave and shock propagation interactions. The proposed change in apogee will increase the probe lapping rate thereby increasing the frequency of close conjunctions. The team will also minimize the distances of closest approach along the same field line through careful along-orbit phasing. Overall, the operations planning will enable new scientific discoveries in the new mission configuration utilizing new data collection and burst modes and increased data bandwidth to the ground.
The upcoming declining phase of the solar cycle is also expected to provide new and additional information about the solar cycle dependence of inner magnetosphere dynamics, especially from the expected changing conditions from CMES to CIR solar wind drivers arising from solar transequatorial coronal holes in the transition from solar maximum to the declining phase. The unusual solar asymmetry in the dynamics of coronal holes and active regions in the northern and southern hemispheres of the Sun during this solar cycle maximum also represent important and unusual conditions which may change the way the radiation belts respond during the solar cycle. Overall, the team presented a truly compelling science case for the extended phase operation of a mission which has already delivered a fantastic science return from data of uniquely high quality, cleanliness and energy resolution.
The team also proposes clear and significant interactions with other mission assets in the HSO (Heliophysics System Observatory), as well as with ground-based arrays during the extended mission. Moreover, coordination with a number of upcoming cube satellite missions which are scheduled to fly during this period will provide critical new information about particle precipitation into the atmosphere. Despite the expected availability of precipitation data from such cube satellite missions, however, the panel also noted the teams stated desire to seek "more definitive information about the causes and consequences of precipitation." Since the loss cone is too small to resolve in Van Allen Probe data, particle dynamics related to the transition from trapped trajectories to entry into the loss cone cannot be measured directly. The team does propose to use modeling of the dynamical variations of particle distributions during wave-particle interactions to infer the causative action of scattering loss into the atmosphere. However, as the team note in their proposal: "For precipitation losses, we need more opportunities than those afforded by the two 1-month BARREL campaigns to obtain coordinated near-equatorial and precipitating flux." The panel is supportive of meeting this need during extended mission; however, the panel noted that flights like those of the BARREL balloon campaigns which were available during the prime mission phase are not currently base-lined for the extended mission.
Finally, the expected launch of the Japanese ERG (Exploration of energization and Radiation in Geospace) satellite in the summer of 2016 provides further opportunities not only for multi-point characterization of inner magnetosphere dynamics, but will provide important complimentary measurements from higher magnetic latitudes.
- Van Allen Probes Science Strengths: Strong synergy with other HSO assets, especially those operating in geospace (e.g. MMS and THEMIS), as well as with those monitoring both solar activity (e.g., STEREO, SDO and Hinode) and the incident solar wind (ACE and Wind). The Van Allen Probes orbit enables data collection from a key region which serves as a pathway for energy transport and a reservoir for energy storage in energetic particle populations in the inner magnetosphere. The ultimate loss of these trapped energetic particles through precipitation into the atmosphere also represents a key element of the energy budget for ionospheric and atmospheric dynamics.
- Van Allen Probes value to the HSO (Heliophysics Great Observatory): The Van Allen Probes mission is very evidently driving new scientific discoveries from the assets in the HSO. There are clear connections and strong synergies with important supporting HSO observations such as those from ACE and Wind (and DSCOVR) in the upstream solar wind, those monitoring of the driving conditions at the Sun (e.g., STEREO, SDO, Hinode), as well as those making in situ observations elsewhere in the magnetosphere such as with THEMIS and MMS. The Van Allen Probes mission focuses on the scientific processes which control the transport, acceleration, storage and loss of energetic particles trapped in the ring current and radiation belts, which are strongly coupled to different plasma and particle populations in the magnetosphere. Thus concurrent observations of, for example, the plasma sheet, which can act as the source particle population for the radiation belts and ring current, are highly valuable at system level. Furthermore, other HSO assets in elliptical orbits offer additional high-cadence passes through the inner magnetosphere, thereby providing not only higher temporal cadence cuts through the inner magnetosphere but also additional MLT (Mesosphere Lower Thermosphere) coverage. Similarly, tail and magnetopause dynamics may control the generation and entry of source populations, and shocks and waves which propagate through the system and influence energetic particle dynamics. All of these benefit from the availability of multi-point measurements from the geospace fleet of satellites in the HSO. Finally, precipitation of energetic particles into the atmosphere can affect ionospheric and atmospheric dynamics, for example through their impacts on NOx and HOx pathways. Despite the importance of the Van Allen Probes within the HSO network, and its value for monitoring the energy flow though the solar-terrestrial system, the Van Allen Probes will have their greatest impact through driving new scientific discoveries relating to radiation belt science within the HSO.
- Van Allen Probes spacecraft / instrument health and status: Both Van Allen Probes remain extremely healthy and all instruments are operating nominally. There are sufficient fuel reserves for the proposed maneuvers during the extended mission as well as to maintain the spacecraft spin axis pointing towards the Sun. This is the first extended mission for the Van Allen Probes, following the successful completion of their prime mission, and the data from the mission continues to deliver scientific discovery of the highest quality with a payload functioning as it did immediately after launch.
- Van Allen Probes Data Operations (accessibility, quality control, archiving): The data from the mission are of extremely high quality, and the high-energy particle data is exceptionally clean and of very high energy resolution representing probably the best radiation belt data ever collected. The extensive scientific output from the mission is testimony to the high caliber of the data, and the quality control within the team is exemplary. Following the successful commissioning and calibration of the instruments in the prime mission, the Van Allen Probes team has applied considerable effort to deriving the best possible calibration and cross-calibration of the instruments. This included cross calibration between the probes, as well as inter-calibration between the particle instruments across energy boundaries, especially between MagEIS (Magnetic Electron Ion Spectrometer) and REPT (Relativistic Electron Proton Telescope).
The data are being made widely and openly available from all of the instruments through a broad range of platforms, including from the mission instrument team web sites, through CDAWeb (Coordinated Data Analysis Web) at SPDF (Space Physics Data Facility), and increasingly through SPEDAS (Space Physics Environment Data Analysis Software) and the relevant VxOs, etc. As a result, the data are being widely used to drive scientific discovery.
- Van Allen Probes Overall Assessment and Findings: The panel noted the great impact and exceptional scientific returns derived from the Van Allen Probes mission in its prime mission phase. The panel further noted the compelling science proposed for the extended mission phase, which both builds on the extensive scientific discoveries from the prime mission and which utilize new and optimized orbital configurations including new data and burst modes and benefit from increased data rates to the ground. Together with existing and new assets in the HSO, as well as through coordinated operations with a number of upcoming cube satellites and the Japanese ERG mission the panel finds the proposal compelling and expects a significant ongoing level of high impact science returns from the mission.
• Feb. 19, 2015: On Oct. 8, 2013, an explosion on the sun's surface sent a supersonic blast wave of solar wind out into space. This shockwave tore past Mercury and Venus, blitzing by the moon before streaming toward Earth. The shockwave struck a massive blow to the Earth's magnetic field, setting off a magnetized sound pulse around the planet. NASA's Van Allen Probes, twin spacecraft orbiting within the radiation belts deep inside the Earth's magnetic field, captured the effects of the solar shockwave just before and after it struck. 55)
- Now scientists at MIT's Haystack Observatory, the University of Colorado, and elsewhere have analyzed the probes' data, and observed a sudden and dramatic effect in the shockwave's aftermath: The resulting magnetosonic pulse, lasting just 60 seconds, reverberated through the Earth's radiation belts, accelerating certain particles to ultrahigh energies.
- "These are very lightweight particles, but they are ultrarelativistic, killer electrons — electrons that can go right through a satellite," says John Foster of MIT's Haystack Observatory. "These particles are accelerated, and their number goes up by a factor of 10, in just one minute. We were able to see this entire process taking place, and it's exciting: We see something that, in terms of the radiation belt, is really quick."
- The findings represent the first time the effects of a solar shockwave on Earth's radiation belts have been observed in detail from beginning to end. Foster and his colleagues have published their results in the Journal of Geophysical Research. 56)
Figure 39: Earth's magnetosphere is depicted with the high-energy particles of the Van Allen radiation belts (shown in red) and various processes responsible for accelerating these particles to relativistic energies indicated. The effects of an interplanetary shock penetrate deep into this system, energizing electrons to ultra-relativistic energies in a matter of seconds (image credit: NASA)
• Nov. 26, 2014: The Van Allen radiation belts have been found to contain a nearly impenetrable barrier that prevents the fastest, most energetic electrons from reaching Earth. The belts can wax and wane in response to incoming energy from the sun, sometimes swelling up enough to expose satellites in low-Earth orbit to damaging radiation. The discovery of the drain that acts as a barrier within the belts was made using NASA's Van Allen Probes. This barrier for the ultra-fast electrons is a remarkable feature of the belts, according to Dan Baker of the University of Colorado and the lead author of the Nature article (Ref. 58). - For the first time, the team of scientists was able to study this phenomenon, because there were never such accurate measurements of these high-energy electrons done before. Understanding what gives the radiation belts their shape and what can affect the way they swell or shrink helps scientists predict the onset of those changes. Such predictions can help scientists protect satellites in the area from the radiation. 57)
There is a region of dense cold plasma around the Earth known as the plasmasphere, the outer boundary of which is called the plasmapause. The two-belt radiation structure was explained as arising from strong electron interactions with plasmaspheric hiss just inside the plasmapause boundary, with the inner edge of the outer radiation zone corresponding to the minimum plasmapause location. Recent observations have revealed unexpected radiation belt morphology, especially at ultrarelativistic kinetic energies (> 5 MeV). 58)
- The data from the newly discovered barrier of the Van Allen Probes show that the inner edge of the outer belt is, in fact, highly pronounced (Figure 40). For the fastest, highest-energy electrons, this edge is a sharp boundary that, under normal circumstances, the electrons simply cannot penetrate.
- The radiation belts are not the only particle structures surrounding Earth. A giant cloud of relatively cool, charged particles called the plasmasphere, fills the outermost region of Earth's atmosphere, beginning at ~1000 km in altitude and extending partially into the outer Van Allen belt (Figure 41). The particles at the outer boundary of the plasmasphere cause particles in the outer radiation belt to scatter, removing them from the belt.
- This scattering effect is fairly weak and might not be enough to keep the electrons at the boundary in place, except for a quirk of geometry: The radiation belt electrons move incredibly quickly, but not toward Earth. Instead, they move in giant loops around Earth. The Van Allen Probes data show that in the direction toward Earth, the most energetic electrons have very little motion at all – just a gentle, slow drift that occurs over the course of months. This is a movement so slow and weak that it can be rebuffed by the scattering caused by the plasmasphere .
- This also helps explain why – under extreme conditions, when an especially strong solar wind or a giant solar eruption, such as a coronal mass ejection, sends clouds of material into near-Earth space – the electrons from the outer belt can be pushed into the usually-empty slot region between the belts (Ref. 57) .
Figure 41: A cloud of cold, charged gas around Earth, called the plasmasphere and seen here in purple, interacts with the particles in Earth's radiation belts — shown in grey— to create an impenetrable barrier that blocks the fastest electrons from moving in closer to our planet (image credit: NASA/GSFC)
• On November 1, 2014, the Van Allen Probes completed their primary two year mission and are currently operating in a bridge phase. During the Senior Review proposal process with NASA headquarters, new PSGs (Prioritized Science Goals) were identified for the first extended mission. The project considered several extended mission trajectory options that alter the orbital parameters in order to satisfy the new PSGs. The limiting factor on the various options was fuel consumption, and the selected option balanced the science goals and mission lifetime.
• On August 30, 2014, the Van Allen Probes were 2 years on orbit - observing the sun's influence on our planet and near-Earth space. The probes, shortly after launch in August 2012, discovered a third radiation belt around Earth when only two had previously been detected. The operation of the Probes is nominal. 59) 60)
Figure 42: This image was created using data from the Relativistic Electron-Proton Telescopes on NASA's twin Van Allen Probes. It shows the emergence of a new third transient radiation belt. The new belt is seen as the middle orange and red arc of the three seen on each side of the Earth (image credit: APL, NASA)
• On March 26, 2014, NASA declared the Van Allen Probes mission – designed to explore and unlock the mysteries of Earth's radiation belts – an official success. This certification comes just one year, six months, and 27 days into the two-year primary mission of the twin spacecraft, which orbit Earth roughly every nine hours. 61)
• March 19, 2014: Scientists have discovered a new, persistent structure in Earth's inner radiation belt using data from the twin NASA Van Allen Probes spacecraft. Most surprisingly, this structure is produced by the slow rotation of Earth, previously considered incapable of affecting the motion of radiation belt particles, which have velocities approaching the speed of light. 62) 63)
Data from the RBSPICE (Radiation Belt Storm Probes Ion Composition Experiment) instrument on board each of the twin spacecraft orbiting Earth revealed that the highly energized population of electrons of the inner radiation belt is organized into very structured patterns that resemble slanted zebra stripes. Scientists had previously believed that increased solar wind activity was the primary force behind any structures in our planet's radiation belts. These zebra stripes were shown to be visible even during low solar wind activity, which prompted a search for a new physical mechanism of their generation. That quest led to the surprising discovery that the stripes are caused by rotation of Earth. The findings are reported in the March 20, 2014 issue of the journal Nature. 64)
It is because of the unprecedented high energy and temporal resolution of the energetic particle experiment, RBSPICE, that the team now understands that the inner belt electrons are, in fact, always organized in zebra patterns. The modeling clearly identifies Earth's rotation as the mechanism creating these patterns. Aleksandr Ukhorskiy of JHU/APL, the Co-PI of RBSPICE remarked: "It is truly humbling, as a theoretician, to see how quickly new data can change our understanding of physical properties."
Because of the tilt in Earth's magnetic field axis, the planet's rotation generates an oscillating, weak electric field that permeates through the entire inner radiation belt. If the inner belt electron populations are viewed as a viscous fluid, these global oscillations slowly stretch and fold that fluid, much like taffy is stretched and folded in a candy store machine. This stretching and folding process results in the striped pattern observed across the entire inner electron belt, extending from above Earth's atmosphere in the region from ~800 - 13,000 km above the planet's surface.
Figure 43: 'Zebra stripes' in the inner radiation belt: An example of energetic electron spectra, measured on June 18, 2013 by RBSPICE during low solar activity. The striped, banded pattern is caused by the rotation of the Earth, previously thought to have no effect on the highly energetic particles of the radiation belt (image credit: A. Ukhorskiy, JHUAPL)
Figure 44: Rotation-driven disturbance (image credit: A. Ukhorskiy, JHUAPL)
Legend to Figure 45: a-g, Examples of energetic electron spectra measured by RBSPICE A, averaged over 45 s, in the inner radiation belt during quiet times (a–c) and during geomagnetic storms (e–g), parameterized by the 9-h averaged geomagnetic activity.
• March 7, 2014: Using data from NASA's Van Allen Probes, researchers have tested and improved a model to help forecast what's happening in the radiation environment of near-Earth space — a place seething with fast-moving particles and a space weather system that varies in response to incoming energy and particles from the sun. 65)
Now, two sets of related research published in the Geophysical Research Letters improve on these goals. By combining new data from the Van Allen Probes with a high-powered computer model, the new research provides a robust way to simulate events in the Van Allen belts. The recent work centers around using Van Allen Probes data to improve a three-dimensional model created by scientists at LANL (Los Alamos National Laboratory), called DREAM3D (Dynamic Radiation Environment Assimilation Model in 3 Dimensions). Until now the model relied heavily on the averaged data from the CRRES (Combined Release and Radiation Effects Satellite) mission. 66)
The new technique provides for gathering real-time global measurements of chorus waves, which are crucial in providing energy to electrons in the radiation belts. The team compared Van Allen Probes data of chorus wave behavior in the belts to data from the NOAA/POES weather satellite series in LEO. Using this data and some other historical examples, they correlated the low-energy electrons falling out of the belts to what was happening directly in the belts. Once the relationship between the chorus waves and the precipitating electrons could be established, the research team could use the POES satellite constellation - which has quite a few satellites orbiting Earth and get really good coverage of the electrons coming out of the belts.
The second paper describes a process of augmenting the DREAM3D model with data from the chorus wave technique, from the Van Allen Probes, and from NASA's Advanced Composition Explorer, or ACE, which measures particles from the solar wind. Los Alamos researchers compared simulations from their model – which now was able to incorporate real-time information for the first time – to a solar storm from October 2012. 67)
• In research published Dec. 19, 2013 in Nature, lead author Richard Thorne and colleagues report on high-resolution measurements, made by the Van Allen Probes, of high-energy electrons during a geomagnetic storm of Oct. 9, 2012, which they have analyzed together with a data-driven global wave model. Their analysis reveals that linear, stochastic scattering by intense, natural very low-frequency radio waves known as "chorus" in the Earth's upper atmosphere can account for the observed relativistic electron build-up. 68) 69)
Such electrons in the Earth's outer radiation belt can exhibit pronounced increases in intensity, in response to activity on the sun, and changes in the solar wind — but the dominant physical mechanisms responsible for radiation belt electron acceleration has remained unresolved for decades.
Two primary candidates for electron acceleration exist, one external and one internal. From outside the belts, a theoretical process known as "inward radial diffusive transport" has been developed; from within the belts, scientists hypothesize that the electrons are undergoing strong local acceleration from very low frequency plasma waves. And controversies exist as to the very nature of the wave acceleration: Is it "stochastic" – that is, a linear and diffusive process – or is it non-linear and coherent?
Legend to Figure 46: The top panel shows electron fluxes before (left) and after (right) a geomagnetic storm. The injection of low-energy plasma sheet electrons into the inner magnetosphere (1) causes chorus wave excitation in the low-density region outside the cold plasmasphere (2). Local energy diffusion associated with wave scattering leads to the development of strongly enhanced phase space density just outside the plasmapause (3). Subsequently, radial diffusion can redistribute the accelerated electrons inwards or outwards from the developing peak (4).
The successful point-by-point inter comparison of radiation belt features observed by the Van Allen Probes with the predictions of the state of the art model developed by Richard Thorne and his group at UCLA dramatically demonstrates the significance of in situ particle acceleration within the Earth's radiation belts.
• December 2013: Although the Earth's Van Allen radiation belts were discovered over 50 years ago, the dominant processes responsible for relativistic electron acceleration, transport and loss remain poorly understood. Evidence is presented for the action of coherent acceleration due to resonance with ultra-low frequency waves on a planetary scale. Data from the , launch July 25, 1990, and the Van Allen Probes mission (launch Aug. 12, 2012), with supporting modelling, collectively show coherent ultra-low frequency interactions which high energy resolution data reveals are far more common than either previously thought or observed. The observed modulations and energy-dependent spatial structure indicate a mode of action analogous to a geophysical synchrotron; this new mode of response represents a significant shift in known Van Allen radiation belt dynamics and structure. These periodic collisionless betatron acceleration processes also have applications in understanding the dynamics of, and periodic electromagnetic emissions from, distant plasma-astrophysical systems. 70)
The latest discovery uses measurements, taken by a UNH-led ECT (Energetic Particle, Composition, and Thermal Plasma) instrument on board the Van Allen Probes twin spacecraft, reveal that the high-energy particles populating the radiation belts can be accelerated to nearly the speed of light. This mode of action is analogous to that of a particle accelerator like the Large Hadron Collider. However, in this case, the Earth's vast magnetic field, or magnetosphere, which contains the Van Allen belts, revs up drifting electrons to ever-higher speeds as they circle the planet from west to east. 71)
The recent finding comes on the heels of a related discovery — also made by the UNH-led EPIC (Energetic Particles and Ion Composition Experiment) instrument suite on CRRES in GTO — showing similar particle acceleration, but on a microscopic rather than a planetary scale. - Now, having the twin spacecraft in orbit with the Van Allen Probes and making simultaneous measurements in different regions of nearby space is a key part of the mission, allows the scientists to look at data separated in both space and time.
Figure 47: Artist's conception of NASA's Van Allen Probes twin spacecraft (image credit: Andy Kale, University of Alberta)
• July 2013: A team of scientists, led by the Los Alamos National Laboratory in New Mexico and involving the University of Colorado Boulder, have discovered a massive particle accelerator in the heart of one of the harshest regions of near-Earth space, a region of super-energetic, charged particles surrounding the globe, called the Van Allen radiation belts. Scientists knew that something in space accelerated particles in the radiation belts to more than 99 % of the speed of light, but they didn't know what that something was. New results from NASA's Van Allen Probes now show, that the acceleration energy comes from within the belts themselves. Particles inside the belts are sped up by local kicks of energy, buffeting the particles to ever faster speeds, much like a perfectly timed push on a moving swing. 72) 73)
The discovery that the particles are accelerated by a local energy source is akin to the discovery that hurricanes grow from a local energy source, such as a region of warm ocean water. In the case of the radiation belts, the source is a region of intense electromagnetic waves, tapping energy from other particles located in the same region. Knowing the location of the acceleration will help scientists improve space weather predictions, because changes in the radiation belts can be risky for satellites near Earth.
Figure 48: Artist's rendition of the Van Allen belts surrounding Earth - and an energy-acceleration graph that local particles experience in the belts (image credit: NASA, G. Reeves/M. Henderson)
• June 2013: In new NASA-funded research, the radiation belt group in the UCLA Department of Atmospheric and Oceanic Sciences explained the development of this third belt and its decay over a period of slightly more than four weeks. - By performing a "quantitative treatment of the scattering of relativistic electrons by electromagnetic whistler-mode waves inside the dense plasmasphere," the investigators were able to account for the "distinctively slow decay of the injected relativistic electron flux" and demonstrate why this unusual third radiation belt is observed only at energies above 2 MeV. 74)
• Feb. 2013: NASA's Van Allen Probes mission has discovered a previously unknown third radiation belt around Earth, revealing the existence of unexpected structures and processes within these hazardous regions of space. Previous observations of Earth's Van Allen belts have long documented two distinct regions of trapped radiation surrounding our planet. - The REPT (Relativistic Electron Proton Telescope) instruments aboard the twin Van Allen Probes quickly revealed to scientists the existence of this new, transient, third radiation belt. This discovery shows the dynamic and variable nature of the radiation belts and improves our understanding of how they respond to solar activity. 75) 76) 77)
Legend to Figure 49: Two giant swaths of radiation, known as the Van Allen Belts, surrounding Earth were discovered in 1958. In 2012, observations from the Van Allen Probes showed that a third belt can sometimes appear. The radiation is shown here in yellow, with green representing the spaces between the belts.
• On Oct. 28, 2012, the Van Allen Probes/RBSP mission completed their 60-day commissioning phase, and began their two-year primary science mission. 78)
• In early September of 2012, scientists with the newly launched Van Allen Probes got permission to turn on one of their instruments after only three days in space instead of waiting for weeks (until the completion of the commissioning phase), as planned. They wanted to turn on the REPT (Relativistic Electron Proton Telescope) so that its observations would overlap with another mission, SAMPEX (Solar, Anomalous, and Magnetospheric Particle Explorer), that was soon going to de-orbit and reenter Earth's atmosphere. 79)
Due to a stroke of luck, the dynamic nature of space weather provided the formation of the third radiation belt. This phenomenon was previously unknown. The scientist watched – in disbelief – while their data showed the extra belt forming, then suddenly disappear, like it had been cut away with a knife. They have not yet seen a recurrence of a third belt.
What happened is that shortly before the REPT instrument was turned on, solar activity on the sun had sent energy toward Earth that caused the radiation belts to swell. The energetic particles then settled into a new configuration, showing an extra, third belt extending out into space. — By the fifth day REPT was on, the project was able to plot out the observations and watch the formation of a third radiation belt (Figure 50). The third belt persisted beautifully, day after day, for four weeks - when a CME from the sun destroyed the third belt. The observations were made from institutions including; LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado, Boulder, CO; NASA/GSFC (Goddard Space Flight Center), Greenbelt, MD; LANL (Los Alamos National Laboratory) in Los Alamos, N.M.; and the Institute for the Study of Earth, Oceans, and Space at the University of New Hampshire in Durham (Ref. 75).
Figure 50: Energetic electron data gathered by the REPT instruments from Sept. 1, 2012 to Oct. 4, 2012 (horizontal axis), image credit: LASP, NASA (Ref. 11)
Legend to Figure 50: This graph shows three discrete energy channels (measured in MeV). The third belt region (in yellow) and second slot (in green) are highlighted, and exist up until a CME (Coronal Mass Ejection) destroys them on Oct. 1, 2012. The vertical axis in each is L*, effectively the distance in Earth radii at which a magnetic field line crosses the magnetic equatorial plane.
• After deployment, RBSP entered a 60 day commissioning phase of operations, where all of the spacecrafts' systems and instruments are activated, monitored, and made ready for the two-year primary science mission. 80) 81)
• The geometry of spacecraft separation from the launch vehicle forced a flurry of communications activity in the first few hours of in-flight life of the spacecraft. Orbit insertion occurred at perigee altitude, where both spacecraft exhibit a large tangential velocity component with respect to the Earth ground stations. The slightly different orbits required that separation from the launch vehicle be accomplished at different times for proper orbit injection. This caused the first AOS (Acquisition of Signal ) with Spacecraft A to occur late in the available first pass over the USN Hawaii station with AOS of Spacecraft B mostly unrecoverable until reaching the lower elevation available to the next available ground station, APL-18 (Ref. 33).
Due to the multiple communications-related activities across both spacecraft and multiple ground networks, careful coordination was required to ensure neither spacecraft's instructions were misinterpreted for the other's. Independent voice network channels for both spacecraft to each ground network were employed, with a dedicated telecommunications engineer leading each spacecraft's communications configuration, and one lead telecommunications engineer overseeing the overall communications commissioning effort. The communications commissioning was successfully completed in the first day after launch; nonetheless, a rotating team of telecom engineers staffed the Mission Operations Center alongside Operations personnel for the first three days.
Sensor complement: (ECT, EMFISIS, EFW, RBSPICE, RPS)
In the summer of 2006, NASA selected four university teams to provide experiments and supporting hardware for the RBSP spacecraft to study the near-Earth space radiation environment of the inner magnetosphere. The instruments are being developed by Boston University, University of Iowa, University of Minnesota, New Jersey Institute of Technology. In addition, NASA signed a memorandum of agreement with NRO (National Reconnaissance Office) to provide a fifth science investigation. 82) 83) 84)
Each RBSP spacecraft carries five instruments or instrument suites to perform in situ measurements of the ions and electrons, electric and magnetic fields, and electric and magnetic waves in the radiation belts.
Table 7: Science instrument accommodation parameters
Figure 51: Spacecraft configuration with instrument fields of view (FOV) from the observatory aft perspective (image credit: JHU/APL, Ref. 43)
Figure 52: The particle experiments of the RBSP mission (image credit: NASA, Ref. 3)
Figure 53: Artist's conception shows the Van Allen radiation belts (green), which are two doughnut-shaped (torus) regions full of high-energy particles that fill the near-space around Earth (image credit: NASA)
Legend to Figure 53: The blue and red lines between and around the belts depict the north and south polarity of the planet's magnetic field. The inner belt, a blend of protons and electrons, can reach down as low as 1,000 km in altitude. The outer belt, comprised mainly of energetic electrons, can swell to as much as 60,000 km above Earth's surface. Both rings extend to roughly ±65º in north and south latitude. 85)
ECT (Energetic Particle, Composition, and Thermal Plasma Suite):
The ECT instrumentation is being developed at Boston University, Boston MA, PI: Harlan E. Spence. The overall objective of ECT is to determine the spatial, temporal, and pitch angle distributions of electrons and ions over a broad and continuous range of energies. The coordinated ECT particle measurements, analyzed in combination with fields and waves observations and state-of-the-art theory and modeling, are necessary yet sufficient for understanding the acceleration, global distribution, and variability of radiation belt electrons and ions, key science objectives of the LWS program and the RBSP mission. 86)
- Determine the physical processes that produce radiation belt enhancements
- Determine the dominant mechanisms for relativistic electron loss
- Determine how the inner magnetospheric plasma environment controls radiation belt acceleration and loss
- Develop empirical and physical models for understanding and predicting radiation belt space weather effects.
The ECT suite consists of three highly-coordinated instruments (MagEIS, HOPE, and REPT) that cover comprehensively the full electron and ion spectra from one eV to 10's of MeV with sufficient energy resolution, pitch angle coverage and resolution, and with composition measurements in the critical energy range up to 50 keV, and also from a few to 50 MeV/nucleon. All three instruments are based on measurement techniques proven in the radiation belts, optimized to provide unambiguous separation of ions and electrons and clean energy responses even in the presence of extreme penetrating background environments.
• MagEIS (Magnetic Electron Ion Spectrometer): MagEIS uses magnetic focusing and pulse height analysis to provide the cleanest possible energetic electron measurements over the critical energy range of 30 keV to 4 MeV for electrons and 20 keV to 1 MeV for ions. Magnetic focusing is accomplished by magnets being used to deflect particles towards the senors to distinguish them from background radiation. MagEIS will provide the cleanest measurements of radiation belt electrons to date. A total of four MagEIS Instruments are carried aboard each spacecraft, each covering a particular part of the energy spectrum and a large range of pitch angles.
• HOPE (Helium Oxygen Proton Electron): HOPE uses an electrostatic top-hat analyzer and time-gated coincidence detectors to measure electrons, protons, and helium and oxygen ions with energies from less than or equal to 20 eV or spacecraft potential (whichever is greater) to greater than or equal to 45 keV while rejecting penetrating backgrounds. Particles that are being measured by HOPE exist throughout the solar system and play a significant role in Van Allen Belt Dynamics because these slower particles generate electromagnetic waves which can affect higher energy particles trapped in the belts. The HOPE device is being provided by LANL (Los Alamos National Laboratory). 89)
• REPT (Relativistic Electron Proton Telescope): The REPT instrument measures protons from 17 to >100 MeV with 30% energy resolution, and electrons from 1.6 to 18.9 MeV with 25% energy resolution. It consists of a stack of high-performance silicon solid-state detectors in a telescope configuration, a collimation aperture, and a thick case surrounding the detector stack to shield the sensors from penetrating radiation and bremsstrahlung. The signal from the detectors is collected by the charge-sensitive amplifiers, which drive the pulse-shaping circuits. REPT does all of its processing in a field-programmable gate array located on a digital board. The low- and high-voltage power boards provide the needed voltages to support all electronics. It is on continuously in one operational mode, where it provides energy bins, pulse heights, and space weather packets, as well as housekeeping telemetry.
Figure 54: Illustration of the MagEIS (left) and HOPE (right) instruments (image credit: JHU/APL)
Figure 55: Illustration of the REPT instrument (image credit: JHU/APL)
The ECT Science Team is distributed across eight funded US institutions (The Aerospace Corporation, Boston University, Dartmouth College, Los Alamos National Laboratory (LANL), MIT, Southwest Research Institute, University of California Los Angeles, and University of Colorado), one unfunded US Government partner agency (NOAA/SEC) and three unfunded international collaborators (University of Alberta, British Antarctic Survey, and CERT-ONERA). The Science Team comprises radiation belt community leaders in areas of: instrument design and operation; science data analysis and management; theory and modeling; and space weather/radiation belt applications.
Figure 56: Overview of the ECT science team (image credit: BU)
EMFISIS (Electric and Magnetic Field Instrument Suite and Integrated Science)
The EMFISIS instrumentation is being developed at UI (University of Iowa), Iowa City, PI: Craig A. Kletzing. The objective of the investigation focuses on the important role played by magnetic fields and plasma waves in the processes of radiation belt particle acceleration and loss. EMFISIS offers the opportunity to understand the origin of important magnetospheric plasma waves as well as the evolution of the magnetic field that defines the basic coordinate system controlling the structure of the radiation belts and the storm-time ring current. 90)
The science goals are: 91)
- Differentiate among competing processes affecting the acceleration and transport of radiation particles
- Differentiate among competing processes affecting the precipitation and loss of radiation belt particles
- Quantify the relative contribution of adiabatic and non-adiabatic processes on energetic particles
- Understand the effects of the ring current and other storm phenomena on radiation electrons and ions
- Understand how and why the ring current and associated phenomena vary during storms.
The EMFISIS instrument suite consists of the following components:
• MAG (Tri-axial Magnetometer): MAG is a tri-axial fluxgate magnetometer: Vector B, DC-15 Hz, 0.1 nT accuracy, three sensors on rigid boom.
• Waves components:
- Magnetic field - vector B: 10 Hz - 12 kHz, sensitivity: 3 x 10-11 nT 2Hz-1 @ 1 kHz, 3 sensors on rigid boom
- Electric field - spin-plane E: 10 Hz- 12 kHz (vector), 10 kHz-400 kHz (single channel), sensitivity: 3 x 10-17 V 2m-2Hz-1 @ 1 kHz, shares booms with EFW (Electric Fields and Waves) instrument.
Figure 57: Boom-mounted components of EMFISIS (image credit: JHU/APL)
EFW (Electric Field and Waves Suite)
The EFW instrumentation is being developed at UMN (University of Minnesota), Minneapolis, PI: John R. Wygant. The EFW objective is to study the electric fields in near-Earth space that energize radiation particles and modify the structure of the inner magnetosphere.
This investigation consists of a set of four spin-plane electric field (E-field) antennae and a set of two spin-axis stacer (tubular, extendable) booms. The investigation will provide understanding of the electric fields associated with particle energization, scattering and transport, and the role of the large-scale convection electric field in modifying the structure of the inner magnetosphere.
- Energization by the large-scale convection E-field
- Energization by substorm injection fronts propagating in from the magnetotail
- Radial diffusion of energetic particles mediated by ULF (Ultra-Low Frequency) magnetohydrodynamic (MHD) waves
- Transport and energization by intense magnetosonic waves generated by interplanetary shock impacts upon the magnetosphere
- Coherent and stochastic acceleration and scattering of particles by small-scale, large-amplitude plasma structures, turbulence and waves (electromagnetic and electrostatic ion cyclotron waves, kinetic Alfven waves, solitary waves, electron phase space holes, zero frequency turbulence).
Figure 58: Components of the EFW instrument suite (image credit: JHU/APL)
Key measurements of EFW:
- Spin plane component of E at DC - 12 Hz (0.05 mV/m accuracy)
- Spin axis component of E at DC - 12 Hz (~3 mV/m accuracy)
- E- and B-field spectra for nearly-parallel and nearly-perpendicular to B components between 1 Hz and 12 kHz at 6-second cadence
- Spacecraft potential estimate covering cold plasma densities of 0.1 to ~100 cm-3 at 1 second cadence
- Burst recordings of high-frequency E- and B-field waveforms, as well as individual sensor potentials for interferometric analyses.
The EFW instrument measures the three dimensional electric field and cold plasma density estimates from the spacecraft potential all over a frequency range from DC to ~500 kHz. Measurements from the spatially separated spacecraft will provide information on azimuthal and radial spatial scales and propagation velocities of large scale structures. The spin plane electric field vector is obtained from spherical sensors at the ends of two pair of orthogonal booms with tip-to-tip separations of 80 and 100 m. The spin axis measurement is obtained from a pair of stace booms with a tip-to-tip separation of ~12 m. The electric field below 12 Hz is telemetered continuously while higher time resolution is obtained from a programmable burst memory with a maximum sampling rate for six quantities of ~ 30,000 samples/s. 94)
The high time resolution data includes interferometric timing measurements between individual probes at the ends of the booms which provide information on small scale structures and phase velocities. DC magnetic fields from the fluxgate magnetometer and wave magnetic fields from the search coil, both associated with the University of Iowa instrument are input into the EFW instrument for processing in the burst memory and in the Digital Signal Processing Board (DSP). The DSP provides wave spectra and cross spectra of electric and magnetic field and cold plasma density fluctuations. The EFW instrument also provides wave electric field signals to the University of Iowa EMFISIS instrument.
RBSPICE (Radiation Belt Storm Probes Ion Composition Experiment)
The RBSPICE instrumentation is being developed at NJIT (New Jersey Institute of Technology), Newark, NJ, PI: Louis J. Lanzerotti. The objective of RBSPICE is to determine how space weather creates what is called the "storm-time ring current" around Earth and determine how that ring current supplies and supports the creation of radiation populations.
The geomagnetic field drives relativistic electron motion via time-dependent gradient-curvature drift. Thus, structural variations of the inner magnetospheric field due to storm-time ring current growth control transport and losses in the outer belt.
This investigation will accurately measure the ring current pressure distribution, which is needed to understand how the inner magnetosphere changes during geomagnetic storms and how that storm environment supplies and supports the acceleration and loss processes involved in creating and sustaining hazardous radiation particle populations.
The science objectives are: 95)
- Understand the effects of the ring current and other storm phenomena on radiation electrons and ions
- Understand how and why the ring current and associated phenomena vary during storms
- Support development and validation of specification models of the radiation belts for solar cycle time scales.
Measurement requirements: Hot plasma pressure must be derived to calculate the ring current contribution to storm-time magnetic fields. Thus, it is necessary to resolve the full energy spectrum of the ring current as well as its composition (H, He, O).
Figure 59: Photo of the RBSPICE instrument (image credit: JHU/APL)
The measurement quality is independent of the angle between the B-field and the spin axis.
- Ion composition energy range is low enough to determine the complete Ring Current energy density.
- High angle and energy resolution provide detailed pitch-angle and energy spectra: Δθ = 22.5º, ΔE/E = 0.1.
RPS (Relativistic Proton Spectrometer)
NASA entered into a partnership agreement with the NRO (National Reconnaissance Office) to provide the RPS instrument, PI: Clark Groves. - Note: the instrument is also referred to as PSBR (Proton Spectrometer Belt Research).
The objective of RPS is to measure the inner Van Allen belt protons with energies from 50 MeV to 2 GeV. Presently, the intensity of trapped protons with energies beyond about 150 MeV is not well known and thought to be underestimated in existing specification models. Such protons are known to pose a number of hazards to astronauts and spacecraft, including total ionizing dose, displacement damage, single event effects, and nuclear activation. This instrument will address a priority highly ranked by the scientific and technical community and will extend the measurement capability of this mission to a range beyond that originally planned. The project's goal is the development of a new standard radiation model for spacecraft design.
- Support development of a new AP9/AE9 standard radiation model for spacecraft design
- AFRL to develop and test model for RBSP data in general and RPS specifically
- AP9 (protons) and AE9 (electrons) will provide standardized worst-case specifications: dose rate; internal charging/deep dielectric charging; surface charging (most intense fluxes in keV electrons).
Figure 60: Illustration of the RPS instrument (image credit: JHU/APL)
RPS measures energy spectra and angular distributions of protons from 50 MeV to 2 GeV (expect full inner-zone spatial distributions with better-than-weekly cadence):
- Energetic protons responsible for total dose in MEO for shielding thickness over 200 mils aluminum
- Protons responsible for displacement damage
Telescope consists of 8 silicon detectors and a Cherenkov detector:
- Stacked Si detectors used for 50 MeV to ~400 MeV, incident angle constrained by 8-fold coincidence
- Cherenkov detector used for > 400 MeV
- Absolute flux accuracy: dJ/J ~10%
- Energy resolution: dE/E ~30% @ 50 MeV, to 100% @ 2 GeV
- Angular resolution: 30º instantaneous, 5º de-convolved.
Mission operations: The mission operations concept is designed for mostly unattended spacecraft operations, with distributed science operations. Because the spacecraft are spin-stabilized and nominally sun-pointing, they are inherently in a safe state and the need for constant monitoring is minimized. All critical activities – including commissioning activities and all propulsive maneuvers – are performed in contact with the ground, but nominal science operations are not constrained to occur during "staffed" periods of time. Similarly, the instrument operations are performed "offline" of the MOC (Mission Operations Center), and instrument commands are queued up at the MOC remotely from the SOCs (Science Operations Centers), and uploaded to the spacecraft during the next regular contact. This approach of unattended, decoupled operations greatly reduces the cost of the operational phase of the mission, and it has been successfully demonstrated at APL on the STEREO mission.
The MOC is located at APL, and it serves as the central hub through which all commands and telemetry flow. Figure 5 shows the distributed nature of the operations architecture. APL's SCF (Satellite Control Facility), with its 18 m antenna, serves as the primary ground station for the mission. Supplemental contacts using the NEN (Near Earth Network) will be used to ensure sufficient daily data download to maintain data volume margin on the spacecraft solid-state recorders. The spacecraft will also utilize the TDRSS system for communications shortly after launch and during early operations, and also rely on TDRSS for emergency communications (Ref. 16).
Figure 61: MOC and SOC architecture of the RBSP distributed ground system (image credit: JHU/APL)
The primary ground station is the 18 m antenna on the APL campus and is controlled by the Satellite Communications Facility. The 18 m dish supports the majority of contact time required to bring down the desired 5.9 Gbit of data per day per observatory. During periods when the orbital geometry does not allow that support, the Universal Space Network's South Point (Hawaii) and Dongara (Australia) stations are used. For navigation purposes, short contacts with the Dongara station are required on a weekly basis. The mission also uses communications with the Tracking and Data Relay Satellite System Space Network only for launch support, commissioning, and critical activities due to the 1 kbit/s downlink rate.
The mission operations functionality is broken down into planning and scheduling, real-time control, and performance assessment functions, with each of these functions feeding into the other to facilitate safe and efficient operations. The planning function is used for scheduling observatory activities. These activities include RF control for ground station contact, SSR (Solid State Recorder) control, eclipse entry and exit notifications, various housekeeping functions, and maneuvers. These activities, or events, comprise repeatable command sequences that may have various instantiations depending on the specific scheduling criteria. All events are classified as either routine or sporadic. Routine events are scheduled at regular intervals based on either time or contacts.
Figure 62: Ground station location and fields of view at apogee distance. Note that at closer distances, the coverage area diminishes; however, the closer distance enables a higher downlink rate for increased data volume throughput (image credit: JHU/APL)
The mission operations concept is based on a decoupled operations approach that is enabled by the mission itself and the observatory design. Because the observatories are spin-stabilized and nominally sun-pointing, they are inherently in a safe state. This minimizes the need for constant monitoring and allows for unattended operations. Figure 6 shows the distributed nature of the operations architecture. The relative observatory attitude remains constant, and the power system is sized to allow 100% duty cycle of the instruments. All critical activities—including commissioning activities and all propulsive maneuvers—are performed in contact with the ground. Nominal science operations are not constrained to occur during "staffed" periods of time.
Figure 63: Photo of the 18 m antenna in Laurel, MD (image credit: JHU/APL)
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