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High-energy electron experiments (HEP) aboard the ERG (Arase) satellite
© The Author(s) 2018
- Received: 4 October 2017
- Accepted: 25 April 2018
- Published: 8 May 2018
- Radiation belts
- High-energy electrons
The mechanisms by which electrons are accelerated in geospace are a key research area in solar-terrestrial plasma physics. Relativistic electrons are trapped in the outer Van Allen radiation belt. The electron flux is known to decrease rapidly during the main phase of the magnetic storms, and then increases during the storms’ recovery phase (Baker et al. 1986; Nagai 1988; Reeves et al. 2003). The processes causing flux variation must be investigated with comprehensive in situ observations. Particle acceleration occurs in various environments around astronomical objects such as supernova remnants and pulsar-wind nebulae (Makishima 1999). Moreover, the Earth’s radiation belts offer a unique opportunity to observe these particles and waves in situ. Armed with an understanding of the acceleration mechanism in geospace, some insight might be gained into the phenomena of electron acceleration in other astronomical objects.
The exploration of energization and radiation in geospace (ERG) project explores the acceleration, transportation, and loss of relativistic electrons in the radiation belts and the dynamics of storms in geospace (Miyoshi et al. 2012; Miyoshi et al. in review). To gain a detailed understanding of the acceleration and transport processes, the electrons must be observed over a wide range of energies and electromagnetic fields with a wide range of frequencies. The experiment observes a wide range of geospace particles, including the warm electrons, hot electrons of the plasma sheet, and sub-relativistic and relativistic electrons of the radiation belts. The satellite uses four instruments, namely LEP-e, MEP-e, HEP, and XEP, in order to measure electrons over this wide range of energies (Kazama et al. 2017; Kasahara et al. 2018a; Higashio et al. in review). The high-energy electron experiments (HEP) onboard the ERG satellite detects 70 keV–2 MeV electrons and generates a three-dimensional velocity distribution of electrons for every period of the satellite’s spin. This energy range covers relativistic electrons and their seed electrons. The full suite of instruments aboard the ERG observes a wide range of electrons. Three additional devices have been installed: XEP (Higashio et al. in review) detects electrons with energies of 0.4–20 MeV, MEP-e (Kasahara et al. 2018a) detects electrons with energies of 7–87 keV, and LEP-e (Kazama et al. 2017) detects electrons with energies of 17 eV–20 keV. In MEP-e, the electrons are energy-filtered by an electrostatic analyzer and the maximum detected energy is 87 keV. To allow for continuous energy coverage, the lower detection range of HEP overlaps with the maximum MEP-e energy.
The ERG satellite, which is also known as ‘Arase,’ was launched from the Uchinoura Space Center at 11:00 on December 20, 2016, by using the Epsilon launch vehicle. The spacecraft attained an orbit with an apogee and perigee altitude of approximately 32,000 and 400 km, respectively. This allowed the satellite to record observations over the entire extent of the radiation belts. After all the instruments and functions were successfully checked, the scientific observations commenced in late March 2017.
This paper describes the design of the HEP instruments, prelaunch testing, and initial results of the in-orbit observations.
HEP performance and specifications. Resolution is indicated by full-width at half maximum (FWHM)
70 keV–1.0 MeV
Energy resolution (ΔE/E)
11% at 300 keV, 18% at 750 keV
17% at 750 keV, 12% at 1.2 MeV
Energy binning of onboard histograma
70, 100, 124, 153, 188, 230, 280 340, 412, 499, 605, 730, 850, 990, 1400, and 1800 (keV)
100, 153, 230, 340, 412, 499, 605, 730, 850, 990, 1200, 1400, 1600, 1800, and 2000 (keV)
Field of view
10° (Azimuth) × 60° (Elevation) for a module
10° (Azimuth) × 180° (Elevation) for three modules
4° ± 1°
15° ± 3°
3.1 × 10−4 cm2 sr (one module)
9.3 × 10−4 cm2 sr (three modules)
3.1 × 10−3 cm2 sr (one module)
9.3 × 10−3 cm2 sr (three modules)
8 s per full 3-D distribution function (for normal spin period of 8 s)
(15 histograms are generated for a 1/16th of spin period)
Flux dynamic range
Sizes of silicon strip detectors used in HEP
Strip pitcha (µm)
Strip lengtha (mm)
Number of strips
12.5 ± 2.5
50 ± 10
600 ± 10
300 ± 30
50 ± 10
600 ± 10
The total silicon thicknesses for the SSDs in HEP-L and HEP-H are 1.85 and 4.25 mm, respectively. These silicon thicknesses correspond to the range of 850 keV and 1.7 MeV electrons, respectively, according to the ESTAR web database of electron stopping powers and ranges, which is maintained by the National Institutes of Standards and Technology (NIST). The incident direction of the detected particles is determined from the position of the interaction at the first layer and the geometrical position of the collimator. To determine the interaction position, HEP uses SSDs, whose electrodes are subdivided into closely spaced strips. As summarized in Table 2, HEP-L consists of one 50-μm-thick SSD and three 600-μm-thick SSDs, while HEP-H consists of one 50-μm-thick SSD and seven 600-μm-thick SSDs. All SSDs were manufactured by HAMAMATSU Photonics. K. K. The full depletion voltage of the silicon wafers, from which the detectors were made, was 18 and 80 V for the 50- and 600-μm-thick SSDs, respectively. To choose the operation bias voltages for the SSDs, we measured the spectra by using a radioisotope while changing the bias voltage. We chose 20 and 200 V as the operation bias voltage for the 50- and 600-μm-thick SSDs, respectively.
The position resolution is determined by the pitch of the strips in the 50-μm-thick SSD. To estimate the number of strips that will detect an incident charge, we simulate the interaction of electrons with the detector by using the Geant4 toolkit. In the simulation, a monoenergetic pencil beam of electrons is irradiated onto the detector at normal incidence from the collimator input. 100- and 500-keV beams are simulated to test HEP-L, and 750-keV and 1.5-MeV beams are simulated to test HEP-H. The standard deviations of the strip ID in HEP-L, which detects the maximum energy in the first layer, are 8.2 strips for the 100-keV electrons and 2.9 strips for the 500-keV electrons, while those in HEP-H are 7.5 strips for the 750-keV electrons and 7.1 strips for the 1.5-MeV electrons. When calculated by the distance between the collimator and the detector, which is indicated by R in Fig. 3, these values correspond to the incidence angles of 9.7°, 3.5°, 7.9°, and 7.4°, respectively.
In accordance with the ERG science observation plans, in the normal observation phase of ERG, HEP operates in the normal observation mode by default and enters the S-WPIA mode several times during a one-orbit revolution.
On February 2, 2017, HEP was turned on for the first time while in orbit, and the initial checkout was successful. The bias voltage of the silicon detectors was limited below 50 V, and the count rates were monitored for several days.
The HEP instrument has successfully begun the observation of electrons with energies of 70 keV–2 MeV in the Earth’s inner magnetosphere. The HEP consists of three HEP-L modules and three HEP-H modules. HEP-L detects 70 keV–1 MeV electrons and has a maximum G-factor of 9.3 × 10−4 cm2 sr, while HEP-H observes 0.7 MeV–2 MeV electrons and has a maximum G-factor of 9.3 × 10−3 cm2 sr at maximum. These modules are pin-hole cameras consisting of mechanical collimators and SSDs. The signals from a total of 2355 strips are processed by 78 readout ASICs. Before the satellite was launched, all channels were evaluated with reference signals from radioactive isotopes and the overall HEP performance was evaluated with electron beams. In orbit, the waveforms of the calibration pulses indicated that the HEP functioned properly after it was launched. From the initial results of the energy–time spectrograms, the HEP recorded high electron count rates in the outer radiation belt.
A simulation of the detector is under development in order to convert the count data to physical quantities with higher precision. We will model the HEP detector geometry and particle interaction with the detectors and surrounding materials by utilizing the Geant4 library. After the validation tests of the simulator and the comparisons between the simulation results and the experimental results, we will be able to calculate the detector response by using the simulator. As shown in Fig. 10, monoenergetic electrons can be detected in lower energy channels. A spectrum detected in orbit is the superposition of signals from electrons with different energy. By using the simulator, we can estimate how many of the detected counts in the lower energy channels are contributed by higher-energy electrons, and thereby determine the incident flux with higher precision. Thus, we will be able to deduce the distribution of incident electrons with higher precision from the direction and energy detections in orbit. A detailed report regarding the simulator and its validation will be published in the future.
TM led the design and development of HEP. TT supported and offered advice during all phases of design and development. SK provided information necessary to the development with regard to the instruments of the ERG mission. WM worked on the basic development of the VATA460 series. MH worked on the initial collimator design and stacked SSD module. All authors read and approved the final manuscript.
We would like to thank all members of the ERG project for their long-lasting efforts to realize this mission. The HEP instrument was developed by Mitsubishi Heavy Industries Co, Ltd. The high-voltage board in HEP was developed by Meisei Electric Co., Ltd. The silicon detectors were manufactured by Hamamatsu Photonics K. K., and VATA460.3 was manufactured by IDEAS, in Norway. The development of VATA460.3 has profited from the design heritage of BepiColombo MMO and Astro-H SGD/HXI. From the Astro-H team, we would like to thank S. Watanabe and M. Kokubun in particular, for their support and sharing of experience. The electron beam facility used in the calibration of HEP is maintained by the Research and Development Directorate in JAXA. Quick plots are distributed by the ERG science center, which also makes level-2 data available. We also thank F. Makino, T. Goka, H. Tajima, and T. Mukai, who reviewed the design and development of HEP during each phase of its development.
The authors declare that they have no competing interests.
Availability of data and materials
The data and materials used in this research are available from the corresponding author upon reasonable request.
Consent for publication
Ethics approval and consent to participate
The ERG (Arase) project is funded by ISAS/JAXA.
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