DARTS for Astrophysics

Akebono

Akebono (EXOS-D)

Mission Descriptions

Overview

The "Akebono (EXOS-D)" mission is an artificial satellite developed by the Institute of Space and Astronautical Science (ISAS) to observe the physical phenomena of the Earth's magnetosphere and auroras. As the first observation satellite in an collaborative science research project named International Solar-Terrestrial Energy Program (STEP) with NASA, the Soviet Union's Intercosmos, and the European Space Agency, it was launched on February 22, 1989, from the Kagoshima Space Center (now Uchinoura Space Center) and continued its operations until April 23, 2015. Over a long period of 26 years and 2 months, it continuously observed auroral phenomena in the polar regions and long-period variations in the Van Allen belts.

The satellite weighs approximately 295 kg and has an octagonal prism shape with a height of 1 m and a side dimension of 1.26 m. It is equipped with four solar panels, a 30 m long antenna, and extendable masts of 5 m and 3 m. It orbited in a highly elliptical orbit with a periapsis of 275 km, an apoapsis of 10,500 km, and an inclination of 75 degrees, completing one orbit in 211 minutes.

Observation Instruments

"Akebono" is equipped with nine scientific observation instruments to observe plasma, magnetic fields, electric fields, waves, and an auroral imaging camera.

Three-axis fluxgate magnetometer (MGF)

MGF is an instrument designed to measure the Earth's magnetic field with high precision in three axes. It consists of a three-axis fluxgate magnetometer and a search coil magnetometer, with extendable masts of 5 m and 3 m, respectively. The fluxgate magnetometer can automatically switch between four ranges (±1024 to ±65536 nT) with full scale, and the resolution for each range (0.031 to 2 nT) corresponds to a 16-bit A/D converter. The sampling rate is 32 vectors per second. The three-axis search coil magnetometer has a frequency characteristic up to 800 Hz, with signals above 100 Hz used for VLF and those below 100 Hz used for magnetic field experiments.

Electric field detector (EFD)

EFD is an instrument for measuring electric fields. It measures vector electric fields using both a standard double probe method (EFD-P) and a newly developed ion beam method (EFD-B). It detects the intensity and direction of electric fields to capture electric field variations in the plasma.

VLF wave detector (VLF)

VLF is an instrument for observing plasma waves in the very low frequency (VLF) range. It consists of a wide band analyzer (WBA), multi-channel analyzers (MCA), Poynting flux analyzers (PFX), ELF frequency analyzers, and a vector impedance probe (VIP), covering both electric and magnetic field components from a few Hz to 17.8 kHz. The main objectives of wave observations are to investigate wave phenomena closely related to high-energy particle precipitation in the auroral zone and the physics of wave-wave and wave-particle interactions.

HF wave detector and topside sounder (PWS)

PWS is an instrument for observing plasma waves in the high-frequency (HF) range. It consists of two subsystems for i) natural plasma wave observation (NPW) and ii) stimulated plasma wave experiments (SPW), as well as an instrument for measuring electron number density (NEI). It can accurately measure variations in plasma density and temperature. PWS provides diverse data on both natural plasma wave observations and active sounder experiments that observe the response of artificially transmitted plasma waves. The artificial excitation of plasma waves aims to sound plasma structures not only in auroral particle acceleration regions but also in the high regions of the ionosphere.

Low energy ion detector (LEP)

LEP is an instrument for observing low-energy ions. It measures the energy distribution and flow of ions to analyze the plasma environment within the Earth's magnetosphere. It performs three different types of charged particle observations:

Energy and pitch angle distribution of auroral electrons and ions.
Mass analysis per charge of positive ions.
Onboard detection of particle flux modulation in the HF and VLF bands.

Suprathermal ion spectrometer (SMS)

SMS was developed to study the distribution of thermal ions (0-25 eV) and suprathermal ions (25 eV to several keV) in the low-altitude magnetosphere. Its dynamic range covers 1-70 amu/e in mass and 0.001-100000 /cc in plasma density, periodically measuring the distribution functions of major and minor ions at the apoapsis (=104km) and periapsis (=300km). It features a high-frequency type mass spectrometer with a programmable mass resolution (Δm/m = 0.06-0.20) independent of the selected energy or mass. This instrument accurately measures ion energy spectra, analyzing the energy transport mechanisms within the magnetosphere.

Thermal electron detector (TED)

TED is an instrument for measuring the velocity distribution of thermal electrons. It measures the temperature and density of thermal electrons in the energy range of 0 to several eV, contributing to a detailed analysis of the plasma environment.

Radiation detection monitor (RDM)

RDM is an instrument for detecting radiation. It operates in three modes: electron temperature mode (TE), probe characteristics mode (SH-DC), and velocity distribution mode (SH-AC). This instrument monitors the spatial distribution of charged particle intensity within the Earth's magnetosphere, aiding in space weather forecasting and radiation protection research.

Visible and UV auroral imager (ATV)

ATV is a camera designed for studying auroral dynamics and global monitoring of magnetospheric activity with simultaneous satellite experiments. This camera has two channels for visible and ultraviolet regions, capable of capturing snapshots every 8 seconds.

Achievements

Some representative scientific achievements of "Akebono" include:

Demonstration of particle acceleration by electric fields parallel to magnetic field lines.
Quantitative research on ion outflow from the polar ionosphere.
Detailed study on the intensification of UHR waves at the equator.
Thermal structure of the low-altitude plasmasphere.
Discovery of partial density depletion in the plasmasphere during geomagnetic storms.
Long-term observation of particle variations in the Van Allen belts (radiation belts composed of electrons and protons trapped by the Earth's magnetic field, surrounding the Earth in a doughnut shape).

"Akebono" achieved observations over a complete solar activity cycle (11 years), providing significant data on the Earth's magnetosphere and auroral phenomena. It has deepened our understanding of the mechanisms of auroral formation and the structure of the magnetosphere. Additionally, this data has contributed to improving models for predicting variations in the Earth's space environment.