ERG (Exploration of energization and Radiation in Geospace) /Arase
ERG is a Japanese (JAXA/ISAS) STP (Solar Terrestrial Physics) minisatellite mission into geospace focused on the formation of the radiation belts associated with magnetic storms. The aim is to elucidate acceleration and loss mechanisms of relativistic particles in the inner magnetosphere during space storms. The relativistic electrons of the Van Allen radiation belts often cause satellite malfunctions and anomaly, and therefore it is important to understand the flux loss and enhancement mechanisms. Moreover, the forecast of the large flux enhancement of the relativistic electrons is one of the key issues of the modern space weather study. 1) 2) 3) 4)
The ERG satellite will observe directly the plasma/particle distribution for wide energy range and the electric/magnetic field for the wide frequency range, which are essential to understand the particle acceleration. A newly developed wave-particle interaction analyzer will also be installed on the spacecraft.
The ERG project consists of not only the SPRINT-B/ERG satellite team, but also of a ground network team and of an integrated data analysis team. Moreover, the science coordination team and the project science center work for the project management.
Note: SPRINT (Small space science Platform for Rapid INvestigation and Test) is the name of a minisatellite platform of JAXA/ISAS.
Figure 1: Schematic picture of the SPRINT-B/ERG exploration region (image credit: JAXA/ISAS)
Comprehensive observations for plasma/particles, fields and waves near the magnetic equator are important for understanding the cross energy coupling for relativistic electron accelerations and dynamics of space storms (Figure 1).
The region of outer space near the Earth, known as geospace, is populated by a large volume of very high-energy electrons and ions trapped in the Van Allen radiation belts by the Earth's magnetic field. These energetic highly charged particles cause a variety of problems such as the malfunctioning of the computers on satellites and undesirable charging of equipment, or radiation exposure to astronauts. In space storms, the outer belt electrons decrease significantly during the main phase, then recover to, and often increase over, pre-storm levels during the recovery phase. During huge magnetic storms, the radiation belts are largely deformed, and large flux enhancement are observed in the slot region and the inner belt. 5)
Two possible mechanisms have been proposed for the acceleration of relativistic electrons. One is the external source process via quasi-adiabatic acceleration. In this process, the energy of electrons increases due to the conservation of their first and second adiabatic invariants, when electrons are transported from the plasma sheet to the inner magnetosphere. This process has been modeled as the stochastic radial diffusion process, which is a fundamental transportation mode of energetic electrons. The ULF (Ultra-Low Frequency) pulsations called "Pc5" with periods of a few minutes have been considered a main driver for the radial transportation via drift-resonance with electrons. Another candidate is termed the internal acceleration process. It has been suggested that resonant interactions by whistler mode waves cause relativistic electron acceleration inside the radiation belts. The free energy for generating whistler mode waves is the temperature anisotropy of electrons of tens of keV , and subsequent wave-particle interactions including nonlinear process will generate chorus waves that accelerate relativistic electrons of the outer belt. Figure 2 summarizes the transport/acceleration mechanisms in the L vs. energy diagram of the inner magnetosphere. L-value describes the set of magnetic field lines which cross the Earth's magnetic equator at a number of Earth-radii equal to the L-value.
Figure 2: Diagram of cross-energy coupling of the inner magnetosphere (image credit: JAXA/ISAS)
The spacecraft uses the SPRINT bus; it is spin stabilized with 7.5 rpm, pointing toward the sun. The spacecraft has a launch mass of about 350 kg and a power generation of ~750 W.
Figure 4 depicts system block diagram. Bold lines in the figure stand for SpaceWire network. There are three different SpaceWire networks within the ERG satellite system. The first one is for SMS (Satellite Management Subsystem). A SMU (Satellite Management Unit), a DR (Data Recorder), a TCIM (Telemetry and Command Interface Module), an AOCP (Attitude and Orbit Control Processor), a PCU (Power Control Unit), a HCE (Heater Control Unit), and a MDP-E (Mission Data Processor Electronics) are connected through a SWR1 (SpaceWire network Router 1, Ref. 5).
Figure 3: Artist's view of the deployed ERG spacecraft (image credit: JAXA/ISAS) 6)
The second network is for attitude and orbit control subsystem. Within the network, AOCP is connected to four ACIMs (Attitude Control Interface Modules), which are ACSSA, ACMDZ, ACANA, and ACVDI, through SWR2 (SpaceWire network Router 2). Each ACIM is connected to AOCS sensors and actuators, respectively. The last one is for mission network. In the mission network, each mission instrument has a SpaceWire Router, and consists ring-type network when considered in the aggregate system.
Figure 4: Block diagram of the ERG spacecraft (image credit: JAXA/ISAS)
Table 1 shows the specifications of the satellite. The transponder of the satellite has four different transmission rates, 4 k/64 k/0.5 M/1M bit/s, for the telemetry downlink. Since the slant range between the satellite and ground stations varies greatly due to its highly elliptic orbit, these transmission rates are selected in an appropriate manner to maximize the amount of telemetry data. The RCS (Reaction Control Subsystem) is used to increase the perigee altitude and to control attitude rate. For initial spin rate control and nutation control, using quartz rate sensors (MEMS gyro) are used to feedback the satellite attitude rate. Spin axis estimation is done on ground using data from SSAS (Spin-type Sun Aspect Sensor), GAS (Geomagnetic Aspect Sensor), and SSC (Small Star Scanner).
Table 1: System specification
Standardized bus and its modifications:
In the satellite bus module, the project adopted a spin-type standard bus for the small science satellite. By utilizing the same components applied in the first small science satellite SPRINT-A or "HISAKI", which is the extreme ultraviolet space telescope for remote observation of the planets launched in 2012, the ERG project fully enjoys the benefits such as:
- The bus components fabrication period and system integration period was shortened compared with conventional ISAS science satellites.
- Recurrent production and short development period enables satellite development at lower cost.
- Recurrent production improves reliability of the satellite.
The SPRINT-A was the three-axis control inertially-fixed attitude LEO satellite, and the ERG is the spin-stabilized Sun pointing attitude satellite with highly elliptic orbit. The major differences in the system requirements of the ERG from APRINT-A are as follows:
1) The attitude is spin-stabilized. The attitude of the satellite is estimated on ground, and the spin axis and spin rate control commands are planned on ground.
2) The mission orbit must be highly elliptic to cover inner and outer radiation belt.
3) After orbit insertion, the perigee altitude must be increased by ΔV maneuvers.
4) To measure electrically charged particles, magnetic field, and electromagnetic waves, static charge on the satellite surface material must be prevented, and EMC (Electromagnetic Compatibility) requirement is extremely severe to reduce noise on science data.
5) The satellite must tolerate severe environment such as high energy radiation in the outer radiation belt around apogee, and high flux AO (Atomic Oxygen) around perigee.
6) To reduce the mass of the satellite, single non-redundant bus components must be used except transponders. Instead, system level functional redundancies are preferred.
Although SPRINT-A and ERG both adopted the same standard bus components, the satellite system design was largely modified to meet these ERG specific requirements. ERG has two spin-type sun aspect sensors, a geomagnetic aspect sensor, and a newly developed spin-type star scanner for offline attitude determination. Hydrazine thruster system is adopted to the ERG for ΔV and spin control, while SPRINT-A did not have any thrusters. To meet the EMC requirements, double shields are applied to the harnesses, the EMC filters are added to the PCU. Black polyimide film is used as MLI (Multilayer Insulation) surface material to prevent static charge. Other thermal control materials are also selected very carefully after many radiation and AO flux tests.
Even though there are differences on the system design between SPRINT-A and ERG, a standard small science satellite bus design is adopted to the network architecture using SpaceWire, data handling components and software, communication subsystem, electrical power subsystem, and structures. While customized flexibly based on different requirements, the ERG satellite system has still its adequate quality utilizing advantage of the standard bus.
Mission system: The thermal and structural design is independent from the bus system, and the interface are defined cleanly. Two sets of mission data processors / data recorders (MDP/MDR) installed in the MDP-E function as gateways to the satellite management network. Figure 5 shows details of the mission SpaceWire ring-type network. In the network, each instrument has a CPU board which has a functionality of the SpaceWire router and the data processor. This architecture enables high speed data sharing within the mission components for integrated data analysis and system level redundancy.
Figure 5: ERG mission SpaceWire network configuration (image credit: JAXA/ISAS)
Development of ERG: Figure 6 shows the project schedule. After SDR (System Development Requirements), BBMs (Bread Board Models) and EMs (Engineering Models) of the mission components were designed and tested. A MTM (Mechanical Test Model) and a TTM (Thermal Test Model) of the mission module structure were also designed and tested to confirm the environmental conditions. The first system AIT (Assembly, Integration, and Testing) was conducted in early 2015 using the flight models and the engineering models. The final AIT was conducted from October 2015 to September 2016.
Figure 6: ERG project development (image credit: JAXA/ISAS)
Figure 7: The ERG satellite in the clean room of the M rocket launch site (image credit: JAXA) 7)
Launch: The SPRINT-B/ERG spacecraft of JAXA was launched on December 20, 2016 (11:00 UTC). The launch vehicle was Epsilon-2 and the launch site was USC (Uchinoura Space Center), Japan. The launch vehicle flew as planned, and at approximately 13 minutes and 27 seconds after liftoff, the separation of ERG was confirmed. The nominal mission life is planned to be longer than 1 year. 8) 9) 10) 11)
Orbit: HEO (Highly-elliptical Earth Orbit), perigee ~ 440 km, apogee ~33,000 km), inclination ~ 32º, the period is ~ 570 minutes.
Geospace is the region of outer space near the Earth. The radiation belt called the "Van Allen radiation belt" lies within the geospace, and the belt captures a huge volume of highly charged energy particles that exceed mega electron volts (MeV). The ERG project aims at elucidating how highly charged electrons have been born while they generate and vanish repeatedly along with space storms caused by the disturbance of solar wind caused by space storms, and how space storms are developed.
Figure 8: Artist's rendition of the ERG spacecraft in geospace (image credit: JAXA/ISAS)
• As of the summer of 2017, the ERG satellite is working quite well as designed, and preliminary mission data are being obtained (Ref. 5).
• On March 24, 2017, the ERG project finished the commissioning phase and started the science operation phase. Figure 9 shows the operation image of the ERG in a highly elliptic orbit. The ERG is mainly operated using ground stations at Uchinoura (34 m and 20 m diameter), Katsuura (20 m diameter), and Okinawa (18 m diameter) because large antenna dishes are required for high speed communication around apogee. Science data is recorded continuously to the mission data recorder. In addition, huge burst mission data are recorded when space storms or other space events happen. Thus, most of the long pass window around apogee above Japan is used to downlink mission data with high speed (1 Mbit/s or 0.5 Mbit/s) S-band. On the other hand, satellite ranging for orbit determination is also important, since the altitude of the perigee is so low that orbit perturbations are not negligible due to the air drag. Ranging operations are mainly conducted around the perigee with low speed transmissions (4 kbit/s or 64 kbit/s) in S-band.
- In order to synchronize science data among multiple instruments, the so-called IP (Index Pulse) is distributed from AOCP. An IP is normally generated from a Sun detection pulse generated by the SSAS per every one spin. But when the satellite is in the shadow of the Earth, the IP generation mode must be changed to an auto IP mode.
Figure 9: Operation image of the ERG mission in highly elliptic orbit (image credit: JAXA/ISAS)
- Sun direction moves approximately 1 degree per day due to the Earth revolution about the Sun. Thus, the spin axis must be controlled to track the Sun. During the science operation phase, the RCS is not used for this spin axis control operation. Instead, MTQs (Magnetic Torquers) are used in order not to excite the vibration or liberation by flexible structures such as wire antennas and extensible masts. Furthermore, because the orbit is highly elliptic, the MTQ control is effective only when the altitude of the satellite is lower than 3,000 km, where Earth's magnetic field is enough strong. On the other hands, driving MTQs makes magnetic noise to the science instruments, so that drive duration must be minimized and demagnetization operation is required when MTQ is used.
- The ERG satellite is now in good condition, and obtaining excellent science data. A campaign observation from the end of March to the beginning of May 2017 was conducted and the data are being provided to scientists for detailed analyses.
Figure 10: Operational phase definition in ERG (image credit: JAXA/ISAS)
• February 2017: In the commissioning phase, the project conducted subsystem functions and performance checkout, initial start-up of mission instruments including high voltage power supply. In order to avoid electrical discharge in the insufficient vacuum condition by outgassing, the project had to wait at least one month before turning high voltage ON. After all mission components were started-up and initial mission data was confirmed, we conducted operation
• January 23, 2017: JAXA confirmed the completion of LEOP (Launch and Early Orbit Phase) of the ERG (Exploration of energization and Radiation in Geospace)/Arase mission. It results from a sequence of significant tasks that occurred as anticipated: perigee up maneuvers and extension of its wire antennae and masts. 12)
- Arase is currently in good condition and is scheduled to enter into a two-month commissioning phase, then followed by a science operations phase.
- The critical operation phase is divided into three detailed phase. In the first phase, the ERG completed spin-down and active nutation control, SAP deployment, and Sun acquisition. Figure 11 shows the ground track of the ERG. The first apogee pass was over the Southern Hemisphere, and the pass duration at the Santiago Ground Station was nearly 9 hours. The second apogee pass was operated at the ground stations in Japan. But the pass duration was still nearly 9 hours so that the project could have enough time to check the satellite status. During this period operation team confirmed communication link establishment, power and thermal budget, and determined its orbit. It took almost three days.
Figure 11: Ground track during critical operations phase (image credit: JAXA/ISAS)
• The first signals were received in the Santiago Ground Station, the Republic of Chile at 20:37 UTC on Dec. 20, 2016. ERG's solar array paddles have been deployed as planned. Also, ERG has completed the attitude control based on the sun acquisition. JAXA has nicknamed ERG "Arase" for the following reasons: 13) 14)
- ERG starts a new journey to the Van Allen radiation belts, located in the Earth's inner magnetosphere, where energetic charged particles are trapped. "Arase", a Japanese word for a river raging with rough white water, is a fitting description for the journey that lies ahead of ERG.
- The Arase River runs in Kimotsuki, Kagoshima, where JAXA's Uchinoura Space Center is located. The Arase River has a local folktale of bird's beautiful singing. Since ERG observes "chorus" *, it conveys the significance well.
Legend: * Chorus is a plasma wave generated in the magnetic equator of Earth's magnetosphere. Its frequency band ranges within several kHz. Audibly converted, Chorus sounds much like bird's singing.
Sensor/experiment complement: (PPE, PWE, MGF, S-WPIA)
There are various kinds of plasma waves in the inner magnetosphere. Whistler mode chorus waves and the ion Bernstein mode waves will be important for non-adiabatic acceleration to generate relativistic electrons. EMIC (Electromagnetic Ion Cyclotron) waves, that are generated from ring current ions, will work for rapid pitch angle scattering of relativistic electrons. Whistler mode hiss waves inside plasmapause work for the pitch angle scattering of electrons.
The PWE (Plasma Wave and Electric Field) instrument can observe the frequency spectrum and wave-form of these plasma waves. The MHD (Magnetohydrodynamics) pulsations with ~5 min periods are a driver for adiabatic acceleration by radial diffusion, which can be observed by the PWE (Plasma Wave Experiment) instrument as well as the MGF (Magnetic Field Experiment) instrument. Thermal plasma density that is important information for wave-particle interactions is determined from cutoff-frequency of the upper-hybrid resonance waves. The onboard measurement of the thermal plasma density will be developed for the ERG satellite. 15)
Figure 12: Science payload system on ERG (image credit: JAXA/ISAS)
PPE (Plasma and Particle Experiment):
The PWE instrument observes electric fields at the frequency range from DC to 10 MHz as wells as the magnetic field at the frequency range from a few Hz to 20 kHz. The electric field is measured by two pairs of wire dipole antennas, and its length is about 30 m tip-to-tip. The high-frequency magnetic field is measured by the two orthogonal search coils.
Instrument: PPE consist of four electron sensors (LEP-e, MEP-e, HEP-e, and XEP-e) and two ion sensors (LEP-i, and MEP-i). The PPE electron sensors can measure electrons from 10 eV to 20 MeV, while ion sensors can measure ions from 12 eV/q to 180 keV/q with mass discrimination. The energy ranges of each detector are designed to overlap each other, which can provide a seamless energy spectrum. 16) 17) 18)
• LEP-e (Low-energy particle experiments - electron analyzer). The LEP-e instrument is an analyzer which enables the ERG satellite to observe the energy distribution and density of low energy electrons surrounding the Earth. The LEP-e instrument was designed and developed at the Institute of Space and Plasma Sciences, NCKU (National Cheng Kung University), Taiwan. 19)
• MEP-e (Medium-energy particle experiments - electron analyzer)
• HEP-e (High-energy electron experiments)
• XEP-e (Extremely high-energy electron experiments)
The HEP-e and XEP-e instruments mainly observe relativistic electrons of the radiation belts,; these instruments are essential to derive the phase space density profile. On the other hand, the LEP-e and MEP-e instruments observe the hot electrons that are a free energy source for plasma waves. Since the anisotropies of the distribution function should be a free energy of the plasma waves, observations of the distribution function is important to clarify how the plasma waves are generated inside the radiation belts.
The measurement of particles at the energy range of tens of keV is very difficult in the radiation belts. However, newly developed technologies to remove the background contamination can be applied in PPE, making detailed observations of these high energy (tens keV) electrons possible.
The ion observation instrument, LEP-i and MEP-i, will observe several ion species in the inner magnetosphere. Although the same contamination problems will exist as for the electron observations, in particular at tens of keV energies, the new technology introduced will realize the observation of ions up to 180 keV/q in the radiation belts.
• LEP-i (Low-energy particle experiments - ion mass analyzer)
• MEP-i (Medium-energy particle experiments - ion mass analyzer)
These ion observation data will be used to study the evolution of ring current ions; the ion observations with mass discrimination are essential to study the composition of the ring current particles that come from both, the solar wind and the ionosphere.
PWE (Plasma Wave and Electric Field):
The objective of the PWE instrument is to observe electric fields at the frequency range from DC to 10 MHz as well as the magnetic field at the frequency range from a few Hz to 20 kHz. The electric field is measured by two pairs of wire dipole antennas, which are ~ 30 m in length (tip-to-tip). The high-frequency magnetic field is measured by the two orthogonal search coils.
There are various kinds of plasma waves in the inner magnetosphere. The Whistler mode chorus waves and the ion Bernstein mode waves will be important for non-adiabatic acceleration to generate relativistic electrons. EMIC (Electromagnetic Ion Cyclotron) waves that are generated from ring current ions will work for rapid pitch angle scattering of relativistic electrons. Whistler mode hiss waves inside of the plasmapause work for the pitch angle scattering of electrons.
The PWE instrument can observe the frequency spectrum and wave form of these plasma waves. The MHD (Magnetohydrodynamic) pulsations with ~5 min periods are a driver for adiabatic acceleration by radial diffusion, which can be observed by the PWE instrument as well as the MGF instrument. Thermal plasma density that is important information for wave-particle interactions is determined from the cutoff-frequency of the upper-hybrid resonance waves. The onboard measurement of the thermal plasma density will be developed for the ERG satellite.
MGF (Measurement of Geomagnetic Field):
The goal of the MGF instrument is to observe the ambient magnetic field as well as the MHD pulsations. A fluxgate sensor with the boom is used for the measurements. The observations of the ambient magnetic field are required for the knowledge of the ambient plasma environment around the ERG satellite. The plasma distribution function and pitch angle distribution is obtained using the ambient magnetic field. The local cyclotron frequency is also determined from the MGF measurement. 20)
The MGF instrument observes the MHD pulsations and the EMIC waves in parallel to the PWE instrument. Since the ring current evolution produces distortions of the ambient magnetic field, and its distortion affects the particle distribution and trajectories in the inner magnetosphere, accurate measurements of the magnetic field deviation from the intrinsic magnetic field are important to evaluate the ring current effect. The MGF instrument can measure such deviations of the magnetic fields during space storms.
S-WPIA (Software-Wave Particle Interaction Analyzer):
In order to measure the wave-particle interactions, i.e. the energy conversion process between plasma/particles and waves, the newly developed S-WPIA system will be installed in the ERG satellite. The vector cross-product of the particle velocity and the electric field velocity should be equal to the time derivative of the kinetic energy of particles; the positive E v means the acceleration of particles by waves, while the negative E v means the growth of waves. Therefore, the relative phase between the electric field and the velocity of particles determines the direction of the energy flow.
Using the particle data from MEP-e/HEP-e and the wave form data from the PPE, the S-WPIA system can calculate the relative phase between the waves and the particles for each particle. This will be the first observation to identify directly the wave-particle interaction process in space; hence, the cross-energy coupling process can be obtained via the wave-particle interaction process. 21)
ERG ground network observations & integrated studies/simulation
Observations from the ground will play a role to complement the ERG satellite observations. The global SuperDARN HF radar networks, magnetometer networks, optical image networks, riometer (relative ionospheric opacity meter)/VLF observation groups join the ERG project. These ground network observations provide the global variation of the electric field, magnetic field, current system, and the plasma/particle distribution. Moreover, the riometer/aurora observations will show, when and where the precipitation of energetic particles takes place by the wave-particle interactions. In this way, these ground observations are the remote-sensing tool to observe the global dynamics of the geospace.
The ERG project has been designed to closely collaborate satellite observations with ground-based observations and simulations and resolve the issues. Namely, the ERG project is organized into the following teams: the satellite observation team, the ground-based network observation team, and the integrated data analysis/simulation team (Figure 13). The science coordination team and the science center also work with project management. About 100 researchers from about 30 Japanese and overseas institutes/universities participate in the ERG project. 22)
The ERG Science Center is operated by JAXA/ISAS and ISEE (Institute for Space-Earth Environmental Research) at Nagoya University. 23)
Figure 13: Close collaborations between the three teams (image credit: JAXA/ISAS)
To understand the various types of data from the satellite and the ground observations, an integrated analysis is essential in the use of the many data sets to gain a science output. Both, global simulation and micro-process simulation such as wave-particle interactions, are important to understand the physical process through quantitative comparisons with observations. As one of the activities of the integrated studies/simulation study group, the Solar-Terrestrial Environment Laboratory of Nagoya University, Japan starts the project GEMSIS (Geospace Environment Modeling System for Integrated Studies), involving the new physical model of the ring current and radiation belts as shown in Figure 14. The electric potential at the sub-auroral latitude has also been developed.
Figure 14: Snapshot of GEMSIS-Ring Current model (left) and GEMSIS-Radiation Belt model (right), image credit: JAXA/ISAS
Legend to Figure 14: Left: Three-dimensional visualization of the current structure obtained from the GEMSIS-RC (ring current) model developed by the GEMSIS-Magnetosphere team. This new simulation code is unique in the sense that enables us to solve time evolution of ions and fields including ULF waves in the inner magnetosphere self-consistently to investigate plasma and electromagnetic field variation during geospace storms. — Right: An example of the spatial distribution of relativistic electrons in the outer radiation belt obtained from the GEMSIS-RB (radiation belt) model. The model precisely solves relativistic electron trajectories in arbitrary electric and magnetic field models of the inner magnetosphere.
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9) "Launch of the Second Epsilon Launch Vehicle (Epsilon-2) with the Exploration of energization and Radiation in Geospace (ERG) on Board," JAXA Press Release, Nov. 15, 2016, URL: http://global.jaxa.jp/press/2016/11/20161115_epsilon2.html
12) "Completion of Exploration of energization and Radiation in Geospace Arase(ERG)'s Critical Operations Phase," JAXA Press Release, Jan. 23, 2017, URL: http://global.jaxa.jp/press/2017/01/20170123_arase.html
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).