MinXSS (Miniature X-ray Solar Spectrometer) nanosatellite
MinXSS is a 3U CubeSat solar physics mission of the CU (University of Colorado) at Boulder, designed to better understand the energy distribution of solar flare SXR (Soft X-ray) emissions and its impact on the Earth's ITM (Ionosphere, Thermosphere, and Mesosphere). This project started in 2011 and heavily involves in particular graduate student team members with scientists and engineers at the Laboratory for Atmospheric and Space Physics (LASP), located on the east campus at CU. 1) 2) 3)
The peak solar energy in the SXR is expected to be emitted near 2 nm; however, only limited spectral measurements are available near that wavelength to verify this expectation. Energy from SXR radiation is deposited mostly in the ionospheric E-region, in the altitude range of ~80 to ~150 km, but the precise altitude is strongly dependent on the SXR spectrum because of the steep slope and structure of the photoionization cross sections of atmospheric gases in this wavelength range.
Despite many decades of solar SXR observations, almost all have been broadband measurements with insufficient spectral resolution to fully understand the varying contributions of emission lines amongst the underlying thermal and non-thermal continua. The MinXSS mission will improve the understanding of how highly variable solar X-rays affect the ITM, advance our knowledge of flare energetics in the SXR, and provide new spectral observations of the solar SXR near the maximum of solar cycle 24.
The educational objective of MinXSS is to train students as the next generation STEM ( Science, Technology, Engineering, and Mathematics) workforce. The students will learn – through formal university classes and with hands-on experience – about scientific instrumentation, satellite technology, and science data analysis and modeling techniques.
Measurement requirements: To address the MinXSS science objective, measurements of the solar SXR irradiance (full-disk, not imaging) with spectral resolution better than 1 nm and with accuracy better than 30%. While simple in concept, the technology to do this in the 1-5 nm range has traditionally been difficult. Grazing-incidence grating spectrometers are only effective longwards of 5-10 nm (e.g., SDO/EVE measures down to 6 nm with 0.1 nm resolution. Bragg crystal spectrometers, often used in the 1960s and 1970s for solar SXR measurements, have extremely high spectral resolution but a very narrow range of about 1 nm, and they are large and heavy instruments. Solid-state (semiconductor) photon-counting detectors work very well for obtaining HXR (Hard X-ray) and gamma ray spectra shorter than 0.5 nm (e.g. RHESSI measures up to 0.4 nm.
SDD (Silicon Drift Detectors) are the novel technology that enable new solar spectral measurements over a wider range in the SXR. The Amptek X123 SDD is a commercial off-the-shelf (COTS) instrument that the project acquired; it was flown on a NASA sounding rocket in 2012 as part of the project's calibration program for the SDO/EVE instrument.
The MinXSS mission is most relevant for the NSF Division of Atmospheric and Geospace Sciences (AGS). In particular, MinXSS directly supports goal number 1 for the NSF Geospace Aeronomy CEDAR(Coupling, Energetics, and Dynamics of Atmospheric Regions) program: to study the "dynamics and energetics of the upper atmosphere, with particular emphasis on the hard to observe region between 80 and 150 km."
The MinXSS project began as a graduate student project in the Aerospace Engineering Sciences (AES) department at CU and ran through the Spring 2014 semester, with an average of 11 graduate students each semester. The AES department supported the first year of the project, which focused on development of the CubeSat Card Cage design and assembly. The second year was supported by the National Science Foundation (NSF), which enabled the design and manufacture of the first flight prototype. With NSF support in 2013 and subsequent NASA funding in 2014–2016, three MinXSS units have been fabricated (Figure 1): a prototype and two FMs (Flight Models). The flight prototype model included the structure, CDH (Command and Data Handling) custom board, EPS (Electrical Power System) custom board, custom motherboard, custom battery pack, and plastic 3D printed prototypes of the secondary instrument housing and antenna deployment module. The prototype MinXSS has been valuable for early testing and fit checks, and as extra unit for developing flight software in parallel with other build activities. 4) 5)
Figure 1: Photos of the MinXSS family (left to right): prototype unit, FM1 (Flight Model 1), and FM2 (image credit: LASP/University of Colorado)
In the current design, MinXSS requires approximately 6 W of orbit-averaged power to fulfill the mission goals. A 3U body-mounted (fixed) solar panel can provide, at most, 7 W of power with triple-junction solar cells, so a deployable solar panel is needed. The team has performed extensive trade studies on numerous solar panel configurations, and has decided to use a double deployed 3U panel array along with a fixed panel to provide about 20 W of power while on the dayside of the orbit, which will provide sufficient orbit-averaged power for any orbit and can adequately handle the peak power demands including charging the battery during the dayside (Ref. 2).
The structure of the nanosatellite is partitioned into three basic blocks (Figure 2): 1U for the instrument (COTS Amptek X123 spectrometer), 1.5U for system electronics, and 0.5U for the ADCS (Attitude Determination and Control Subsystem ). There are two solar arrays to provide 20 W when in sunlight; a body-mounted solar panel (3U surface area: 10 cm x 34 cm) fixed to the solar-oriented side, and a deployed double panel (6U surface area: 20 cm x 34 cm).
The deployable UHF antenna, UHF communication, and EPS (Electrical Power System) are inherited from the CSSWE (Colorado Student Space Weather Experiment), an NSF nanosatellite at CU, which was launched on Sept. 13, 2012.
The ADCS will maintain the spacecraft orientation such that the solar panels and instruments are always sun-pointed, and the radiator (on the opposite side from the deployable solar array) is oriented roughly towards zenith (Figure 3).
Figure 2: Mechanical model of MinXSS (image credit: CU)
Figure 3: Nominal orbital attitude configuration (image credit: CU)
Table 1: Hardware component TRL
ADCS (Attitude Determination and Control Subsystem): The MinXSS bus is a 3-axis-controlled nanosatellite to observe the solar SXR spectrum between 0.04 and 3 nm. Use of the XACT (fleXible ADCS Cubesat Technology) unit of BCT (Blue Canyon Technologies). XACT is a standalone 0.5U 3-axis stabilized ADCS unit, featuring a star tracker, coarse sun sensor, IMU (Inertial Measurement Unit), reaction wheels, and torque rods.
Table 2: Specification of the XACT performance
Figure 4: Blue Canyon's XACT Attitude Determination Control Subsystem (image credit: BCT)
Figure 5: Functional block diagram of XACT (image credit: CU)
BCT (Blue Canyon Technologies) has developed FlexCore, which is a highly-configurable ADCS that uses a core electronics box (based on the XACT CubeSat ADCS), combined with any of the various reaction wheel sizes in the BCT product line. The FlexCore electronics and software stays the same, regardless of the spacecraft. The wide range of reaction wheel and torque rod sizes supports spacecraft sizes from large CubeSats to 100s of kilograms. The stellar-based attitude determination and control provides accuracy of 0.002º RMS. Features of FlexCore include: multiple nano star trackers with integrated stray-light baffles; 3 or 4 low-jitter reaction wheels; 3 torque rods; GPS receiver; MEMS IMU; MEMS magnetometer; sun sensors; integrated processor and electronics; auto-generated flight software, including star identification, Kalman filter, momentum control, thruster control, and orbit propagation. The table-driven, auto-coded software is easily configured to support any mission, and is delivered to the user fully programmed. 6)
Figure 6: Functional block diagram of the MinXSS subsystems (image credit: CU)
CDH (Command and Data Handling subsystem): The CDH is designed and developed by students. The core processor is the microchip DSPic32 (40 MIPS 16 bit µcontroller). The core processor has a flight history on the SDO/EVE (Solar Dynamics Observatory/EUV Variability Experiment) rocket flight (June 23, 2012) of LASP (Laboratory for Atmospheric and Space Physics) at the University of Colorado. The processor runs on RTOS (slot architecture).
Figure 7: Photo of the CDH subsystem board design (image credit: CU)
EPS (Electrical Power Subsystem): The nanosatellite uses 28 PV triple junction solar cells distributed among its components. A BCR (Battery Charge Regulator) is used for the battery assembly. Two Li-ion batteries have a storage capacity of 14.8 Whr. The battery cells are connected in series to supply between 6 and 8.5 V (nominally 7.4 V) to the power system.
The project modified the DET (Direct Energy Transfer)EPS design that was inherited from the CSSWE CubeSat to include an additional specially selected resistor to create a PPPT (Pseudo-Peak Power Tracking) system. The extra resistor was chosen to prevent a rapid voltage drop from the solar cells when the battery attempts to draw a large current, namely when the battery state of charge is relatively low right as the spacecraft exits the orbit eclipse.
In the CSSWE and MinXSS EPS design, the output of the solar panels power 8.6 V regulators that then provide regulated 8.5 V power directly to the battery and system. In this DET design, the batteries will charge up to 8.5 V, and there are no supporting electronics required to control the battery charging process. In reality, this simple approach only provides about 50% of the power intended from the solar panels when the battery capacity is low. In particular, when the battery needs more power input (high current) for charging, the high current draw from the solar cells results in much lower voltage, following the standard solar cell current-voltage (I-V) curve. When the solar panel output voltage goes below the minimum input voltage level of the 8.6 V regulator, the regulator turns off. Consequently, the current drops and the solar panel output voltage increases, and the 8.6 V regulator turns back on. This results in a high-frequency on-off regulator oscillation that had the EPS 8.6 V regulators on for only about 50% of the time during the early part of the orbit dayside during mission simulations. The MinXSS solar panels were designed for 80% of peak efficiency at EOL, but the 50% decrease in power was an unacceptable power loss for the nominal power budget.
The solution for MinXSS, without having to redesign or rebuild the EPS board, was to replace the sense resistor on the output of the solar panel regulator with a larger resistance so that the effective current draw out of the solar panel would be limited and thus would not cause
Figure 8: A simplified circuit diagram of PPPT used for the MinXSS EPS (image credit: CU)
One disadvantage to the PPPT implementation is that there is additional heating of the EPS board because of the larger resistance; however, this extra heating peaks right after exiting eclipse, the precise time when temperatures are cooler and heating is desired anyway. For example, the power loss (heating) in the PPPT current-limiting resistor is estimated to be 2.6 W when the battery voltage is at its lowest value of 7.1 V, decreasing to 0.93 W when the battery voltage is at 7.5 V, and reduces to less than 0.1 W once the battery voltage is above 8.0 V. The primary caveat in the PPPT design is that resistor tuning must be done a priori, and is fixed, whereas maximum PPT (MPPT) systems can tune resistance in real time to maintain the maximum power point on the solar cell I-V curve. The trade studies performed for CSSWE and MinXSS resulted in the selection of a custom DET EPS due to the simplicity of design. Both teams were unaware of the consequential loss of power generation at the time of the original designs. The advantage of the PPPT circuit is that it is only minimally more complex than DET, adding little risk for a large benefit.
Figure 9: MinXSS electrical block diagram (image credit: CU)
COMM (Communications subsystem): A half-duplex communication scheme is used with two deployable steel-tape monopole antennas connected together with a 180º hybrid for RF communications. This dual monopole configuration has been tested, and exhibits a gain pattern very close to that of a dipole. A GMSK (Gaussian Minimum Shift Keying) modulation chip has been selected to be the heart of the communications board, the SX1231.
The COMM microcontroller (µC) interfaces the C&DH subsystem with a GMSK modem and sends all data received by the C&DH to the GMSK modem to be modulated and transmitted to the GN (Ground Network). The nanosatellite uses a MSP430 microcontroller for the COMM and C&DH subsystems, which enables a common programming environment and reduces interface complexity.
The transceiver operates in the UHF band at 433 MHz (amateur band), which allows cooperation with any number of amateur ground stations across the globe. The transmission data rate is 9.6 kbit/ in downlink and uplink. 7) 8)
The ground station antenna is a M2 MCP436CP30 cross polarized 42 element yagi antenna with a gain of 14.15 dBdc.
Figure 10: Block diagram of the COMM subsystem (image credit: CU-Boulder)
The MinXSS nanosatellite has a mass of ~4 kg and a power consumption of 7.3/18.1 W(average/peak).
• In the summer of 2015, FM-1 has completed environmental testing and is ready for delivery and launch later in 2015, with deployment expected in early 2016. FM-2 has been built and integrated and is now ready for environmental testing in late 2015 and launch in mid-2016.
Launch: The MinXSS nanosatellite was launched as a secondary payload on December 6, 2015 (21:44:57 UTC) on the Cygnus Orbital ATK CRS-4 mission with the ULA Atlas-V-401 vehicle to the ISS (International Space Station). The launch site was SLC-41 (Space Launch Complex-41) at CCAFS (Cape Canaveral Air Force Station), FL. 9) 10) 11)
Note: The Atlas-V vehicle of ULA will provide a launch service for the Orbital ATK Cygnus CRS-4 cargo vehicle to the ISS as a replacement rocket for the Orbital Sciences Antares rocket which is grounded following its catastrophic Oct. 28 explosion on the Orb-3 mission from NASA Wallops. — The Orb-4 launch also marks ULA's first launch to the ISS. It may be followed by another Cygnus launch atop an Atlas V in 2016 as Orbital works to bring the Antares back into service. 12)
- Flock-2e, 12 3U CubeSats of Planet Labs
- CADRE, a 3U CubeSat technology mission of the University of Michigan
- MinXSS-1, a 3U CubeSat solar physics mission of the CU (University of Colorado) at Boulder
- NODES (Network & Operation Demonstration Satellite), two 1.5 CubeSats of NASA
- STMSat-1 (Saint Thomas More School Satellite), Arlington, VA, 1U CubeSat.
- SIMPL (Satlet Initial-Mission Proofs and Lessons), a microsatellite of NanoRacks. SIMPL is a modular, hyper integrated satellite designed to provide complete satellite functionality in a nanosatellite scale. It will be the first NanoRacks microsatellite deployed from the space station and the first propulsion-capable satellite deployed from the NanoRacks MicroSat Deployer, known as Kaber. The commercial deployer system aims to address the growing market of customers wanting to deploy microsatellites from the ISS orbit.
Orbit: Near-circular orbit, altitude of ~400 km, inclination =51.6º.
MinXSS FM-1 will be deployed from the NRCSD (NanoRacks CubeSat Deployer) on the ISS in February 2016, where it will have an expected 5–12 month orbital lifetime, dependent on atmospheric conditions. MinXSS FM-2 is being planned for a higher altitude, longer mission in a sun-synchronous polar orbit via a launch on the Skybox Minotaur C launch in 2016.
Figure 11: Artist's view of the MinXSS spacecraft orbit (image credit: CU-Boulder)
• February 21, 2017: The MinXSS-1 continues at full operational status. Science data are still being collected and operations are all automated. The altitude is down to 338 km but so far the ADCS has had no problem dumping drag-induced momentum. JSpOc anticipates reentry in late May. 13)
Figure 12: Tracking of the orbital altitude decay of MinXSS-1 (image credit: CU/LASP)
Figure 13: MinXSS-1 Level 0C solar spectra downlinked (image credit: CU/LASP)
• November 16, 2016 marked 6 months on orbit for MinXSS-1 (deployed from the ISS on May 16, 2016). Operations are continuing nominally and we're still collecting science data. The project team has been tracking the altitude decay of the spacecraft and projecting how much longer the mission may last. The bottom line: a few more months, probably. 14)
- The plot of Figure 14 shows the actual MinXSS-1 altitude in black and an exponential fit to that data in red. The team uses that fit to extrapolate to 300 km (where the mission will likely be over as the spacecraft burns up in the atmosphere) and 0 km, when you can be absolutely sure it's over.
Figure 14: Tracking of the orbital altitude decay of MinXSS-1 (image credit: CU/LASP)
• November 6, 2016: So far, the MinXSS CubeSat has observed more than 40 C-class and seven M-class solar flares. The largest flares observed by the spacecraft to date occurred on July 23, 2016 with an M5.0 flare that peaked at 2:11 GMT, an M7.6 flare that peaked at about 5:16 GMT, and an M5.5 flare that peaked a few minutes later. 15)
- Besides conducting solar flare observations, MinXSS also serves as a technology demonstrator of the Blue Canyon Technologies XACT attitude determination and control system (ADCS), one of the only commercially available 3-axis ADCSs for CubeSats.
• July 1, 2016: The MinXSS nanosatellite was deployed successfully from the ISS on May 16, 2016 and its commissioning was completed on June 9, 2016 — and the nanosatellite status is now in support of normal science operations. The primary goal for MinXSS is to study the solar soft X-ray (SXR) spectra to help understand coronal heating and energetic events, such as solar flares, and to further understand how the solar SXR variability impacts Earth's upper atmosphere. The MinXSS spacecraft and instruments are healthy and are routinely observing the Sun every orbit. This MinXSS- 1 mission is expected to be 6-12 months long due to starting at the ISS altitude of about 400 km, but MinXSS-2, scheduled to launch this fall into an orbit at 500 km altitude, could have a 5-year mission. The Sun has been relatively quiet in June; nonetheless, MinXSS-1 has observed five flares with the largest being the C6.5 flare on June 11, 2016. 16)
- The MinXSS-1 mission is also verifying new space technology. One is the Amptek X123 SDD for solar SXR spectral measurements in the 0.5-30 keV range (0.04-2.5 nm) with 0.15 keV energy resolution. Comparison of the X123 spectra integrated over the GOES XRS-B 1- 8 A0 band is the primary topic of this MinXSS Science Nugget. In addition, the MinXSS mission is the first flight of the BCT (Blue Canyon Technologies) ADCS (Attitude Determination and Control Subsystem) that is designed for 3-axis control of nanosatellites with 10 arc sec precision. Another new flight technology is the SPS (Solar Position Sensor) using the GOES-R EXIS custom electrometer; the SPS has validated the BCT ADCS solar pointing precision to be about 8 arcsec with an uncertainty of 2 arcsec.
- The MinXSS solar SXR spectral irradiance measurements are important for filling the spectral gap between the EUV (Extreme Ultraviolet) spectral irradiance measurements by SDO (Solar Dynamics Observatory) EVE (Extreme ultraviolet Variability Experiment) and the hard X-ray (HXR) spectral measurements by RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager). The SXR range is of particular interest for flares that are expected to have peak increases near 1 keV (1.2 nm). The rest of the discussion concerns the comparison of these early X123 measurements to the GOES XRS (X-Ray Sensor) broad band measurement.
- The GOES XRS-B 1-8 A0 band measurement has been the reference for classifying X-ray flare magnitude since the 1970s, and yet, the previous GOES XRS instruments have not had a full spectral calibration. For consistency of flare magnitude, the GOES XRS results have not been adjusted from the original level over the many different GOES missions. NOAA's analysis of the GOES XRS data indicates that the XRS-B 1-8 A0 band probably needs to be divided by 0.7 to be on a more accurate irradiance scale and that the XRS-A 0.5-4 A0 band probably needs to be divided by 0.85. We apply these scaling factors for the following comparison with MinXSS SXR measurements.
- The solar observations by MinXSS X123 can provide a spectral calibration for the GOES XRS because MinXSS instruments had pre-flight calibrations at NIST with an accuracy of about 10%. We also note that the next generation GOES-R XRS has also had pre-flight NIST calibrations, and the first GOES-R series satellite launch is planned for this fall. The X123 solar SXR spectral irradiance, which are considered preliminary until its Level 1 product is released, is integrated over the 1-8 A0 range (1.55-12.4 keV) to obtain "X123-B" irradiance in units of W/m2 and then compared to GOES XRS-B irradiance values that have already been scaled by the NOAA 0.7 factor. Table 3 lists these comparisons for the pre-flare and flare peak irradiances from five events. Figure 15 shows the X123 solar SXR spectra for the C6.5 flare on June 11, 2016 as an example for the data used to make Table 3.
Table 3: Comparison of X123 solar SXR spectra integrated over 1-8 A0 range to the GOES XRS-B band. The flare peak result is at a time when X123 data are available and is not always at the GOES flare maximum. The irradiance values are in units of W/m2. The GOES XRS-B irradiance values have already been scaled up by 0.7 calibration factor.
- The average ratio of X123 integrated irradiance to GOES XRS-B 1-8 A0 irradiance is 1.03 with standard deviation of 0.07 for the flare peak comparison and is consistent with the comparison of the rocket X123 measurements to GOES by Caspi, Woods, & Warren (2015). However, the X123/GOES ratio for the pre-flare comparison has a downward trend for the lower irradiance values as illustrated in Figure 16. Because the data processing for the GOES XRS irradiance assumes a fixed solar spectrum, it is not too surprising to find that the ratio has a trend for the pre-flare comparison whose spectra can be described by plasma at cooler temperatures. On the other hand, the flare peak spectra, except for the B4.4 LDE (Long Duration Event), are more consistently at the same plasma temperature of 4-6 MK and thus have similar ratio values. This result appears to confirm that the NOAA recommended 0.7 calibration factor is appropriate for C-class flares, and possibly also for larger flares if their plasma temperatures are also about 4-6 MK. We do underscore that these results are preliminary and that this comparison is with only ten SXR spectra.
- This result also emphasizes the importance of processing broad band measurements with appropriate SXR spectra to obtain more accurate irradiance values. The new MinXSS solar SXR spectra can benefit the analysis of GOES XRS data and also several other broad band SXR photometers: XPS on TIMED-SEE and SORCE, ESP on SDO-EVE, SOHO-SEM, SNOE-SXP, and PROBA 2-LYRA.
Figure 15: Comparison of X123 solar SXR spectral irradiance to the GOES XRS-B 1-8 A0 band. The X123 spectra are the pre-flare (green) and flare peak (black) spectra for the C6.5 flare on June 11, 2016. The GOES XRS-B irradiance values have already been scaled up by the NOAA recommended 0.7 calibration factor. The "X123-B" bars are for irradiance of the X123 spectra integrated over the 1- 8 A0 (XRS-B) range. The irradiance numbers listed are in units of W/m2 for direct comparison to the GOES irradiance values; the X123 spectral irradiance units of photons/sec/cm2 were converted to W/m2 before doing the integration.
• June 30, 2016: In its first ever flight, the XACT (fleXible ADCS CubeSat Technology) attitude control system from BCT (Blue Canyon Technologies) is providing high-performance guidance, navigation, and control to the orbiting MinXSS (Miniature X-ray Solar Spectrometer) nanosatellite. 17)
- The XACT unit provides state-of-the-art attitude control for small spacecraft like MinXSS. On-orbit telemetry from the XACT unit and independent spacecraft instrumentation indicates a 1σ cross-axis pointing error of < 8 arcseconds, which is less than the width of the Empire State Building as viewed from Los Angeles .
- BCT has developed a range of high-performance attitude control system products that can support Cubesats to ESPA-class and beyond. By leveraging high reuse of flight-proven electronics and software, and high-volume manufacturing, the XACT, XACT-Core, and FlexCore attitude control systems provide a cost-effective solution for the growing small satellite market(Ref. 6).
• On May 16, 2016, the MinXSS nanosatellite was deployed from the ISS to begin its journey into space (NanoRacks deployment services). The NASA-funded MinXSS , built and operated by University of Colorado Boulder students and faculty at the LASP (Laboratory for Atmospheric and Space Physics) and the AES (Aerospace Engineering Sciences Department), studies emissions from the sun that can affect our communications systems. 18) 19)
- MinXSS will operate for up to 12 months. The CubeSat observes soft X-rays from the sun, which can disrupt Earth's upper atmosphere and hamper radio and GPS signals traveling through the region. The intensity of the soft x-ray emissions emitted from the sun is continuously changing over a large range – with peak emission levels occurring during large eruptions on the sun called solar flares.
Figure 17: Astronaut Tim Peake on board the International Space Station captured this image of a CubeSat deployment on May 16, 2016. The bottom-most CubeSat is the NASA-funded MinXSS CubeSat, which observes soft X-rays from the sun—such X-rays can disturb the ionosphere and thereby hamper radio and GPS signals. The other nanosatellite is CADRE of the University of Michigan (image credit: ESA, NASA)
• March 21, 2016: So far MinXSS nanosatellite has not been deployed. The project is experiencing delays. The current best estimate is the first week of April. The NRCSD (NanoRacks CubeSat Deployer) will be used for the MinXSS deployment. The ground station computers have been upgraded from the previous CSSWE mission. 20)
Sensor complement: (X123)
Background: There is a rich history of solar SXR (solar Soft X-Ray) spectral observations over the past three decades, but with a significant gap of spectrally resolved measurements in the 0.4–6 nm range.1 There were many new discoveries about solar flares during the 1980s using solar SXR spectral measurements from the DoD STP P78-1 mission (launch in 1979), NASA's SMM (Solar Maximum Mission, launch 1980), and the JAXA Hinotori satellites. For example, Doschek provides results about flare temperatures, electron densities, and elemental abundances for some flares during these missions. 21)
A review of flare observations from Yohkoh and the CGRO (Compton Gamma Ray Observatory), for the hard (higher energy) X-ray (HXR) range, is provided by Sterling et al.22) These earlier missions laid a solid foundation for studies of flare physics and flare spectral variability that the RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager) and the SDO (Solar Dynamics Observatory) continue today for the HXR and EUV ranges, respectively. 23) 24) With solar flare spectral variability expected to peak near 2 nm, in a range not currently observed by any spectrometer, MinXSS measurements of the solar SXR irradiance will provide a more complete understanding of flare variability in conjunction with measurements from RHESSI and SDO EUV Variability Experiment (EVE).25) 26)
There are also nearly four decades of broadband (5-10 nm wide) SXR measurements that do not provide spectrally resolved measurements. The very limited spectral information from these broadband measurements cannot quantify the specific spectral energy distribution, nor directly quantify the varying contributions of emission lines (bound-bound) amongst the thermal radiative recombination (free-bound) and thermal and non-thermal bremsstrahlung (free-free) continua. These broadband measurements include, among others, the two GOES XRS (X-Ray Sensor) channels covering a combined band of 1.6–25 keV (0.05–0.8 nm) and the even broader band of 0.2–12 keV (0.1–7 nm) from several missions, including the Yohkoh SXT (Soft X-ray Telescope, 1991–2001), Student Nitric Oxide Experiment (SNOE, 1998–2002), Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED, 2002–present), the Solar Radiation and Climate Experiment (SORCE, 2003–present), and SDO (2010–present). 27) 28) 29) 30) 31)
Broadband measurements of solar SXR instruments have helped to resolve an outstanding difference between ionospheric models and measurements, such as the electron density from the Haystack Observatory incoherent scatter radar at Millstone Hill. In particular, the SNOE solar measurements were able to resolve the factor-of-4 difference between models and measurements because the SNOE data indicated much more SXR irradiance than had been previously thought. 32)
Additional broadband SXR measurements have been made since then; however, differences still remain in understanding solar SXR spectral distribution and atmospheric photoelectron flux. While smaller, these discrepancies are still as large as a factor of 2 at some wavelengths, as shown in Figure 18; the lack of spectral resolution in the SXR range is thought to be the culprit for most of these disagreements. For example, Peterson et al. show that discrepancy between photoelectron measurements and models were significantly improved with new EUV spectral measurements down to 6 nm, and we anticipate further improvement with new solar SXR spectral measurements and atmospheric modeling with data from MinXSS due to its ability to measure all wavelengths in its spectral range simultaneously and with the relatively high spectral resolution of 0.15 keV FWHM.14
Figure 18: Solar 0.1-7 nm irradiance currently measured by broadband SXR photometers onboard NASA's SORCE and SDO satellites (image credit: University of Colorado)
X123 (X-ray Spectrometer)
The science objective of MinXSS is to better understand the solar irradiance energy distribution of solar flare SXR (Soft X-Ray) emission and its impact on Earth's ITM (Ionosphere, Thermosphere, and Mesosphere). Energy from SXR radiation is deposited mostly in the ionospheric E-region, from ~80 to ~150 km, but the altitude is strongly dependent on the SXR spectrum. This wavelength dependence is due to the steep slope and structure of the photoionization cross sections of atmospheric gases in this wavelength range. The main reason that Earth's atmospheric cross section changes so dramatically in this range is due to the K-edges of O at 0.53 keV (2.3 nm) and of N at 0.4 keV (3.1 nm). Figure 19 shows two different solar SXR spectra. Although they have the same 0.1-7 nm integrated irradiance values, their peak energy deposition near the mesopause has a separation of about 5 km. This separation is considered significant because it is about one scale height near 100 km, it is critical to E-region electrodynamics, and the mesopause, the coldest region of the atmosphere, is a critical transition between the middle and upper atmosphere.
Figure 19: (Left) Two examples of CHIANTI model solar spectra at 0.01 nm resolution, scaled to have identical 0.1-7 nm integrated energy flux: a Sun with bright but non-flaring active regions (green), and a solar flare (red). (Right) Because the atmospheric cross sections decrease with wavelength in the 0-7 nm range, the different spectral distributions deposit the same amount of energy very differently in Earth's mesosphere and lower thermosphere: the flare spectrum is deposited at much lower altitudes. The peak height difference is ~5 km, which is comparable to the ~5.8 km scale height at the mesopause (dashed line), image credit: University of Colorado
Instrument: The MinXSS science measurements will be achieved using the commercially available X123-SDD (Silicon Drift Detector) advanced X-ray spectrometer of Amptek which has an active area of 25 mm2, an effective Si thickness of 0.5 mm, an 8 µm thick Be (beryllium) filter on the detector vacuum housing, an active 2 stage TEC (Thermoelectric Cooler) on the detector, and sophisticated MCA (Multichannel Analyzer) detector electronics. This commercially available handheld X-ray spectrometer was recently flown on a sounding rocket and demonstrated its ability to operate in space.
Key to achieving our science goals is accurate pointing control and knowledge. With the wide field of view of the X123 spectrometer (±4°), the pointing requirements for MinXSS are only 2° (3σ) accuracy and 0.05° (3σ) knowledge.
Features of the X123 science instrument: 33)
• Observes the soft X-ray solar spectrum from <1 keV (>12 Å) to >5 keV (<2.5 Å)
- Resolution: < 0.5 keV
- Accuracy: < 30%
- Measures in a 'gap' of current knowledge.
• Complete package: silicon-drift detector, thermoelectric cooler, preamp, digital pulse processor and power supplies
• Photon counter, pulse pile-up rejection, multi-channel analyzer (creates spectra).
Figure 20: Photo of the X123 X-ray spectrometer (image credit: Amptek)
Figure 21: Illustration of the detector system (image credit: Amptek)
Background: The Amptek X123 (X-ray Spectrometer), a payload flown on the NASA SDO/EVE (Solar Dynamics Observatory/Extreme ultraviolet Variability Experiment) calibration sounding rocket in June 2012 (an underflight of the SDO satellite mission), will be flown again in October 2013; it is the same one that is being integrated into MinXSS. This X123 instrument has also been taken to the SURF (Synchrotron Ultraviolet Radiation Facility) of NIST (National Institute of Standards and Technology) for calibration. Hence, the primary science instrument onboard MinXSS will be fully calibrated and proven in flight. 34)
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20) Information provided by James Mason of CU-Boulder.
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34) Information provided by James Mason, PhD candidate at the University of Colorado/LASP, Boulder, CO.
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).