Overview

“Arase” (ERG) is an Earth-orbiting satellite developed by JAXA’s Institute of Space and Astronautical Science (ISAS). It was launched on December 20, 2016, from the Uchinoura Space Center (USC) in Kagoshima Prefecture by an Epsilon rocket. The primary objective of this mission is to elucidate the acceleration and loss mechanisms of relativistic electrons in the Earth’s radiation belts (Van Allen belts) during geomagnetic disturbances like space storms. Particularly, it is crucial to conduct in-situ measurements of plasma, particles, and electromagnetic fields in the inner magnetosphere. “Arase” measures a continuous electron energy spectrum from 19 eV to 20 MeV, as well as a continuous electromagnetic waveform from DC to 20 kHz.

The satellite weighs approximately 350 kg, orbiting in an elliptical orbit with a periapsis of 460 km, an apoapsis of about 32,000 km, and an inclination of 31 degrees, with an orbital period of about 570 minutes. It employs a spin-stabilized attitude control method, with the spin axis directed 15 degrees (within the range of 4-20 degrees) towards the sun, and a spin period of approximately 8 seconds.

Observation Instruments

Low-Energy Particle Experiments - Electron Analyzer (LEP-e)

LEP-e is an instrument for observing the three-dimensional distribution function of low-energy electrons. The apparatus consists of an electrostatic analyzer, a multichannel plate (MCP), a high-voltage power supply, and a power unit, measuring electrons from ~19 eV to ~19 keV in 32 energy steps through electrostatic analysis. It enables three-dimensional observation by measuring in 16 spin phase directions using the satellite’s spin motion with a fan-shaped field of view. Electrons in this energy range dominate the inner magnetosphere, and measuring such electrons is essential for understanding magnetosphere dynamics and wave-particle interactions.

Low-Energy Particle Experiments - Ion Mass Analyzer (LEP-i)

LEP-i is an ion energy mass spectrometer covering the low energy range from <0.01 to 25 keV/q. The types of incoming ions are identified by a combination of electrostatic energy analysis per charge and time-of-flight techniques. This sensor has a planar field of view and covers 4π steradians using the satellite’s rotation motion.

Medium-Energy Particle Experiments - Electron Analyzer (MEP-e)

MEP-e is an electrostatic analyzer measuring the energy and arrival direction of incoming electrons in the medium energy range of 7-87 keV. The sensor covers a disk-shaped field of view of 2π radians with 16 detectors, achieving full solid angle coverage through the satellite’s spin motion. Electron energy is independently measured by an electrostatic analyzer and avalanche photodiode (APD), allowing significant background reduction by comparing detection pulse heights.

Medium-Energy Particle Experiments - Ion Mass Analyzer (MEP-i)

MEP-i is an ion mass analyzer in the medium energy range of 10-180 keV/q. Incoming ions are analyzed for energy-to-charge ratio (E/q) and arrival direction by an electrostatic analyzer (ESA). The mass-to-charge ratio (M/q) is identified by the ion’s time of flight (TOF), and the ion’s energy (E) is measured by a silicon solid-state detector (SSD) to determine the charge (q).

High-Energy Electron Experiments (HEP)

HEP is an instrument for measuring high-energy electrons in the range of 7o keV-2 MeV. It uses a pinhole camera with a solid-state detector, employing a single-sided silicon strip semiconductor detector for one-dimensional position resolution. It resolves within a 60-degree field of view determined by the positions of the entrance and the detector, generating data every 12 degrees. To measure up to MeV electrons, thin silicon semiconductors are stacked, and energy is measured by summing the measurements of each layer.

Extremely High-Energy Electron Experiments (XEP)

XEP observes extremely high-energy electrons in the range of 0.4-20 MeV. It consists of five solid-state silicon detectors (SSD) and one scintillator (GSO+PMT), with a plastic scintillator surrounding the GSO scintillator to eliminate contamination from the sides. The low-energy range of the energy measurement is detected by the SSD, while the high-energy range is detected using the GSO.

Plasma Wave Experiment (PWE)

PWE is an instrument for measuring plasma waves in the range from DC to 20 kHz. It comprises orthogonal electric field sensors (WPT: wire probe antennas), a three-axis magnetic sensor (MSC: magnetic search coils), an electric field detector (EFD), a waveform capture and onboard frequency analyzer (WFC/OFA), and a high-frequency analyzer (HFA). These sensors and receivers are designed to measure DC electric fields and plasma waves in the inner magnetosphere. Using these sensors and receivers, PWE covers a wide frequency range from DC to 10 MHz for electric fields and from a few Hz to 100 kHz for magnetic fields. It generates continuous wave spectra in the ELF/VLF/HF ranges and ELF range waveforms 24 hours a day. Additionally, it creates spectral matrices as continuous data for wave direction finding and two types of burst data—“chorus burst” and “EMIC burst”—intermittently. The live waveform data are also input into the Software-Type Wave-Particle Interaction Analyzer (S-WPIA) to derive the direct correlation between waves and particles.

Software-Type Wave-Particle Interaction Analyzer (S-WPIA)

S-WPIA is a system that analyzes wave-particle interactions in real-time based on software. This allows for detailed data acquisition regarding the energy exchange between waves and particles.

Magnetic Field Experiment (MGF)

MGF is designed to verify the particle acceleration processes in the inner magnetosphere. Accurate measurements of the strength and direction of magnetic fields are essential for studying particle motion, the characteristics of waves interacting with particles, and magnetic field variations caused by currents. Observing temporal magnetic field variations provides deeper insights into magnetohydrodynamic and electromagnetic ion cyclotron waves in the ultra-low frequency range that cause the generation and loss of relativistic electrons and ring current particles. The hardware and software design of the magnetic field experiment (MGF) has been optimized to meet the requirements for studying these phenomena.
MGF measures at a sampling rate of 256 vectors per second, with data averaged onboard to fit the telemetry budget. The magnetometer switches its dynamic range between ±8000 and ±60,000 nT depending on the local magnetic field strength. The experiment is calibrated through pre-flight tests and on-orbit data analysis.
MGF observes geomagnetic variations and provides data on the environmental variations in the radiation belt.

Achievements

“Arase” provides crucial data for elucidating the acceleration and loss mechanisms of relativistic electrons in the Earth’s radiation belts. Based on this data, research on geomagnetic environmental variations and radiation belt dynamics during space storms is progressing. Further collection of detailed observation data is expected in the future.