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Medium-energy particle experiments—electron analyzer (MEP-e) for the exploration of energization and radiation in geospace (ERG) mission
© The Author(s) 2018
Received: 4 July 2017
Accepted: 19 April 2018
Published: 2 May 2018
The exploration of energization and radiation in geospace (ERG) project is designed to explore the Earth’s radiation belt region, where relativistic-energy electrons, with energy of the order of MeV, are generated from considerably lower-energy source populations, such as solar wind electrons with energy of hundreds of eV and electrons from ionospheric sources with sub-eV energy (Miyoshi et al. 2017). The ERG spacecraft was launched from the Uchinoura space center in Kagoshima, Japan, at 11:00 UTC on December 20, 2016, and thereafter was nicknamed “Arase,” after a wild river near the launch site. The spacecraft altitude is 440 km in perigee and 32,000 km in apogee after the initial maneuvering, with an inclination of ~ 31°. For the extensive plasma measurements, the spacecraft is equipped with eight sensors for particles and fields and one software-type analyzer.
Using MEP-e, we obtain the velocity distribution functions of medium-energy electrons, providing key information regarding the local energization and pitch-angle scattering, as well as on the global dynamics. The main topics to be addressed with MEP-e are the (1) enhancement and decay of the electron ring current, which is the seed population for higher-energy electrons, (2) evolution of pitch-angle distributions during flux increase/decrease, and (3) energy transfer between electrons and electromagnetic waves via Landau/gyro-resonances. All observations contribute to the determination of the mechanisms of generation and loss of relativistic electrons in the radiation belt. This paper describes the measurement principle of MEP-e, presents the ground calibration results, and illustrates the in-flight performances.
Overview of the instrument
ERG is a sun-pointing spinning spacecraft, and the measurement of the MEP-e is in principle synchronized with the spacecraft spin. The spin is sectored by 32 (i.e., 32 spin-phase channels, 11.25° each), and the applied high voltage is swept through 16 steps in each spin phase for the energy scan. The time cadence for the data acquisition is adjusted at each spin, based on the previous spin period. The sequence is shown in Fig. 3c. Because the nominal spin period of the ERG is ~ 8 s, each SV (sweeping voltage) step is ~ 15.6 ms (= 8 s/32-spin-phase/16-SV-step).
Specification of the MEP-e
FWHM of the electrostatic analyzer response
16 steps per scan
Sensor field of view
360°(azimuth) × 3.5°(elevation)
~ 3.5°(azimuth) × 3.5°(elevation) per detector
Azimuthal gaps exist between detectors
Number of APDs
6.6 × 10−5 cm2 sr keV/keV per detector
APD efficiency not included
4 s for the 3-D distribution function
250 ms for one energy scan
15.6 ms for one energy step
~ 1.5 μs for the S-WPIA data
For nominal spacecraft spin (8 s)
ϕ318 mm × 395 mm
Including DC/DC converter efficiency
1.756 kB per one energy scan
Due to the resource of the FPGA, the parallel (simultaneous) signal handling is limited to two lines per ADC chip. This implies that, for a single ADC chip, the third or later signals within the dead time are ignored. Assuming that saturation becomes gradually significant at a signal rate of ~ 20 kHz per ADC chip (four signal lines), for which the average interval is five times the dead time, the corresponding count rate per azimuth channel and the energy differential flux are 5 kHz and ~ 108 keV/cm2 sr keV s, respectively. Above this flux level, the saturation (dead time) may occur.
The sensor FPGA also controls and monitors the high-voltage board, which consists of an SV output and four outputs for the APDs (APD-HV, four APDs per each output). The maximum output of the SV is ~ 5 kV corresponding to the measurement energy of ~ 87 keV. The maximum output of the APD-HV is -250 V, while the nominal value is around − 160 to 170 V. The gain of the APD (ratio of the output PH to the incident energy) depends on the temperature, and therefore, the temperature of the APDs is necessary to determine the incident electron energies. For this purpose, four temperature sensors are mounted on the APDv board. Two other temperature sensors are also installed on the sensor chassis, with two survival heaters shown in Fig. 4 (for the engineering purpose). One is on the side wall of the ESA, below the black kapton MLI shown in Fig. 2a. The other is in the electronics box. These heaters are controlled by the spacecraft bus heater system.
All electronics boards are powered by the PSU1 (digital 3.3 V) and PSU2 (± 12 V). Both of these power supply boards, as well as the CPU board, are common for ERG scientific instruments, except for the differences in the PSU2 output values. The PSU1 and PSU2 are powered by the bus power supply of nominally ~ 44 V.
The application software in the CPU interfaces with the sensor FPGA and the mission bus network. The sensor FPGA receives the spin pulse and associated time indicators from a CPU middleware for synchronization, as well as sensor commands, while it sends science data at every spin phase and HK data every 1 s to the CPU. The software acquires the data via the middleware, edits the obtained data, and sends them to the mission data processor. SpaceWire is used for this communication between the sensor FPGA and CPU and between the CPUs of the mission instruments. The CPU of the MEP-e is nominally connected to those of the HEP and LEP-i and one of the mission data processor/recorders.
Despite their scientific importance, medium-energy electrons occur in an energy range gap between low-energy plasma sensors and high-energy particle detectors. This is due to the low-energy (< 30 keV) particles that are conventionally measured by electrostatic analyzers, while the high-energy (> 50 keV) particles are covered by solid-state detectors. Both techniques have difficulties regarding accuracy of the measurements at the ends of their energy ranges.
For the measurements of medium-energy electrons onboard the ERG, we designed an electron sensor consist of a cusp-type electrostatic analyzer and APDs. The ESA determines the energy of an incoming electron, while rejecting ions and photons. The APDs are used instead of the classical electron detectors, such as microchannel plates (MCPs) and channel electron multipliers (CEMs), because the quantum efficiencies of MCPs and CEMs fall off at energies above a few keV and it has been difficult to accurately predict the efficiency curve for the medium-energy range. Furthermore, the signal charge multiplication by the APDs, enhancing the signal-to-noise ratio, is a significant advantage over classical solid-state detectors. In addition, the ability of the APDs to measure particle (and photon) energies is especially useful for background reduction during observations in a harsh radiation environment, since spurious signals are discarded by a consistency check of the energy, determined by the ESA and APDs independently.
Cusp-type electrostatic analyzer
Note that the APD noise level is degraded by high temperature. In order to keep the minimum detectable energy below 10 keV, it is essential to achieve a low temperature. As shown in Fig. 2a, the sensor chassis is half covered by black kapton multilayered thermal insulators to avoid solar irradiation, while the other half side is painted white for radiative cooling against the dark sky (UPI WHITE LT48, made by UBE industries, Ltd.; it is conductive, and its solar absorptance and infrared emissivity are ~ 0.2 and 0.8–0.9, respectively). Thus, the ESA works as a cooler for APDs. Furthermore, the APD board (shown in Fig. 7) and the ESA are thermally isolated from the rest of the instrument, which is a significant heat source due to the power consumption of the analog and digital electronics boards, by a thermally insulating polyimide structure. As a result, APDs are kept below − 10 °C throughout the orbit. There is no significant temperature difference among the 16 detectors (less than a few degrees centigrade).
Energy coincidence method for background rejection
Using both the ESA and APDs, two independent values indicating the electron incident energy can be compared. Such a two-parameter analysis enables effective background rejection, since independently measured energies are rarely the same for noise pulses, while they should be consistent for true signals (Kasahara et al. 2009).
The PH tables in the CPU software are written based on the in-flight data. For this purpose, the list data, from which the APDs’ PH distribution can be produced, are acquired, as shown in “In-flight performance” section. Although the APD gain can also drift due to radiation damage (Kasahara et al. 2012), the long-term trends during the flight can be checked and the tables will be updated on the CPU accordingly.
Figure 9 also illustrates that the background is not completely eliminated by this energy coincidence method, since the energy coincidence can occur by chance for the background pulses. Subtraction of these backgrounds on the ground may be important especially when the relativistic-energy electron flux is intense.
Evaluation of sensor specification
The performances of the ESA and APDs are evaluated using a ground beam facility in Nagoya University, Japan. For the evaluation of the performance of the ESA, we utilized not only an electron but also a proton beam, since it is much easier to obtain straight and uniform proton beams, compared to electron beams, due to the geomagnetic field. Therefore, during a part of the calibration period, the ESA was connected to an external high-voltage–power supplier (HVPS) with an opposite polarity compared to the internal HVPS.
After these verifications, we assembled the flight HV board and further checked the response to the electron beam. As we mentioned above, it is generally difficult to compare the simulation with the laboratory results for electron beams due to the deflection by the geomagnetic field. Nonetheless, we confirmed the predicted relativistic effect (e.g., at 70 keV, the SV value corresponding to the peak count shifted 7% compared to the nonrelativistic case). Figure 10d shows the simulation result for 70 keV with the relativistic effect (in gray), fitting well to the laboratory result for a 70 keV electron beam (in cyan). Through these results, we confirmed that the analyzer was manufactured and assembled properly as per the design. In addition, we checked the EUV rejection, using a D2 lamp with a photon flux similar to the solar irradiance. We confirmed there was no increase in noise from the background level.
Operation mode and data product
Here we introduce the science/engineering data products of the MEP-e. When the MEP-e is in the normal mode, it produces count and list data. In the table-dump mode, the MEP-e sends the table data for a read-back check. The structures of each data set are described below. The CPU software receives data from the sensor FPGA and then transmits the data to the mission data processor after compression.
Normal data of the MEP-e
Time indicator, spin phase, etc
12-bit counter: SV(16) × Azm (16)
12-bit counter: SV(16) × Azm (16)
Raw (noise-remaining) data
List data (6 bytes per event, 160 events at maximum, first-come base)
One event includes SV step (4 bits), pulse height (12 bits), azimuth channel (4 bits), APD temperature (6 bit × 4), and SV table ID (1 bit). 10 events per SV step at maximum
The sensor FPGA prepares two 12-bit counters for each SV step. One is for the energy coincidence counts (including only signals for which the two energy determinations are consistent), and the other is for all counts (the result of the energy check result is not considered). For the purpose of the ground check, both types of count data are dumped. Thus, there are 2 × 12 bit × 16 SV steps × 16 azimuthal channels = 768 bytes for the count data per energy sweep.
In addition to the count data, list data are also produced. This data type contains the information of the APD’s pulse height as well as the ESA energy step for each incoming particle and is mainly used for the in-flight calibration. The size of a data packet is 6 bytes per event (see Table 2 for the contents of the packet). Considering the expected maximum count rate is 5000 counts/s for one azimuthal channel, it is not reasonable to produce list data for all events (in that case, the data product rate is 7.5 kB per SV step, far beyond the capacity of the system data recorder and the down link rate). For this reason, the production of the list data packets is restricted to 10 events per SV step on a first-come basis (then the size of the list data is 960 bytes per spin phase).
In this mode, the sensor tables for the SV, threshold, and energy coincidence are dumped instead of the count data. No scientific data are acquired in this mode. This mode is mainly used for the check of the written table data.
Reduction mode list
No reduction is implemented
n-spin accumulated. n = 2 i (i = 1, 2, 3, 4), i can be selected by command
Non-reduced full spin data are obtained once per n spins. (n − 1)/n data are discarded. n = 2 i (i = 1, 2, 3, 4), i can be selected by command
Depending on the degree i, data in below spin phases are obtained, while others are discarded.
(i = 1) 0/2/4/6/8/10/12/14/16/18/20/22/24/26/28/30,
(i = 2) 0/4/8/12/16/20/24/28,
(i = 3) 0/4/8/12,
(i = 4) 0/8,
The degree i can be selected by command
One of the key objectives of the MEP-e (and ERG) is to quantify the energy transfer between particles and electromagnetic waves (Katoh et al. 2013; Hikishima et al. 2014). Of special interest is the Landau/gyro-resonance between electrons and whistler chorus waves. Although previous observations have addressed this issue, their focus has been limited to the correlation between the electron flux intensity and/or pitch-angle distribution and wave intensification. The critical problem of this approach is that not possible to distinguish cause and effect, or, in other words, the direction of the energy transfer. In order to unambiguously verify the wave growth or particle energization, it is essential to determine the particle velocity vector with the time resolution that is short enough compared to the period of whistler chorus waves. This requires a time resolution of tens of microseconds or better. However, if the count data packets are regularly produced in this time resolution, the data size and rate easily exceed the capacity of the system data recorder and the down link rate. In addition, precise synchronization is required between the particle and wave instruments with a time resolution of the order of microseconds.
In order to challenge this issue, a software-type wave–particle interaction analysis (S-WPIA) was implemented onboard the ERG. In this framework, three electron sensors, MEP-e, HEP, and XEP (Higashio et al. 2017), are directly connected to the wave instrument PWE (see Fig. 4). In order to synchronize the particle data with the wave data, an “S-WPIA clock” of 524.288 kHz is distributed from the PWE to the electron sensors. These electron sensors send an “S-WPIA event packet” for each event (i.e., particle detection) to the mission data recorder (MDR), when the “S-WPIA generation flag” is ON. This flag is distributed via a shared data packet, circulating in the mission system network. The CPU application software checks the flag once per second. The PWE also sends the wave data to the MDR, and the S-WPIA application calculates the physical values related to wave growth and particle energization. In addition to the calculated values, the raw burst data from the MEP-e, HEP, XEP, and PWE of short durations are also dumped.
S-WPIA data of MEP-e
TI data for clock synchronization
Event counter (used for data checking)
S-WPIA clock (24 bits), spin phase (5 bits), SV step (4 bits), energy coincidence flag (1 bit), azimuth channel (4 bits), SV table ID (1 bit)
Dummy data for tests
After more than 1 month of hibernation of the particle instruments during the initial spacecraft critical phases, the MEP-e was first turned-on on January 30, 2017. The HVPS initial turn-on was conducted after the other particle instruments. The nominal voltages were successfully applied without any sign of discharges. These initial checkouts were by real-time commands. The routine operations by timeline commands started on March 23, 2017.
Energy steps of the ESA measurements
Central energy (keV)
Applied voltage (V)
The MEP-e was developed for providing medium-energy electron measurements by ERG, and observations have now begun. It detects electrons with energies of 7–87 keV and obtains velocity distribution functions, which are the key to understanding the formation and decay of the radiation belt. Observations in combination with other instruments onboard the ERG, other magnetospheric explorers (Angelopoulos et al. 2008; Burch et al. 2016; Escoubet et al. 1997; Mauk et al. 2013; Nishida et al. 1994), as well as ground-based observatories (e.g., Shiokawa et al. 2017) and modeling output (Seki et al. 2018) are expected to shed new light on radiation belt physics.
SK designed, assembled, and tested the MEP-e hardware, as well as coded the application software. SY contributed in all aspects of the engineering/flight model development, including the assembly, environment tests, and ground calibration. TM, KA, TT, and MH supported the development of the MEP-e since the breadboard model phase. MH also developed the calibration facility. YS supported the thermal design of the MEP-e. All authors read and approved the final manuscript.
Development of the MEP-e was accomplished by the remarkable efforts of engineers in Mitsubishi Heavy Industries, Ltd., Meisei Electric Co., Ltd., and YS Design Co., Ltd. K. Mori and Minodenshi Co., Ltd., are also recognized for the design and fabrication of the amplifier board, which is the core of this instrument. We thank the reviewers of the instrument design reviews, T. Mukai, Y. Saito, S. Watanabe, I. Yoshikawa, and T. Kii, for their useful and critical comments. We are also grateful to L. Kistler for her assistance in improving this paper during her stay in the University of Tokyo. This work was partly supported by JSPS Kakenhi Grant No. 07J01222. The outstandingly dedicated works of the ERG Project members (and support from their families) were indispensable for the MEP-e and ERG spacecraft.
The authors declare that they have no competing interests.
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