Overview

Reimei (INDEX) is a small, highly functional scientific satellite developed by the Institute of Space and Astronautical Science (ISAS) as a piggyback small satellite that can be launched together with larger satellites at extremely low cost, utilizing the surplus capacity of larger rockets.
It was launched on August 24, 2005 from the Baikonur Cosmodrome by a Dnepr rocket with the Optical Inter-orbit Communications Engineering Test Satellite “Kirari” (OICETS).
The aim of the project is to carry out small scale, high frequency science and engineering missions, train young scholars and engineers, and verify next generation technology in orbit.
Its main science mission is detailed observations of the aurora and the Earth’s magnetosphere.

It weighs approximately 70 kg, measures approximately 0.62 x 0.62 x 0.72 m and features a compact design.
It completed a nearly circular orbit with a perigee altitude of 610 km, an apogee altitude of 654 km, and an orbital system elevation angle of 97.8 degrees, completing one revolution in approximately 97 minutes.

Observation Instruments

Multi-Spectral Auroral Imaging Camera (MAC)

MAC is an instrument that performs optical auroral imaging observations with high temporal and spatial resolution to investigate the fine structure of the aurora.

Combining three independent CCD cameras and interference filters, it acquires monochromatic auroral image data in the N₂⁺ first negative band (427.8 nm), OI (557.7 nm), and N₂ first positive band (670 nm) with a 7.6 degree field of view. The MAC operates in the following two modes in the nightside auroral region:

(1) Simultaneous observation with the particle sensor (ESA/ISA): In this mode, an imaging area of ​​about 80 × 80 km (at an altitude of 100 km) centered on the magnetic footprint is observed with a spatial resolution of about 1.2 km and a temporal resolution of about 120 msec.

(2) Auroral altitude distribution measurement. The satellite’s attitude is changed to orient the MAC’s field of view toward the edge of the Earth. In this mode, the MAC’s observation range is approximately 270 × 270 km (2000 km from the satellite), the spatial resolution is approximately 4 km, and the time resolution is approximately 1 sec.

Electron/Ion Spectrum Analyzer (ESA/ISA)

ESA/ISA is an top-hat type electrostatic energy analyzer consisting of an ESA (Electron Spectrum Analyzer) and an ISA (Ion Spectrum Analyzer) that aims to investigate the formation mechanism of auroral fine structure.
It simultaneously acquires optical images of the pitch angle distribution of auroral single particles and auroral arcs that emit light in the ionospheric footprint region.
Each time a particle hits the MCP (Microchannel Plate) detector, a pulse of charge reaches the anode and is split into two groups, each propagating to the edge of the anode pattern where a TDC (time-to-digital converter) is installed. The time interval between the arrival timing of the two pulses is measured by the TDC, and from this time interval the hit position of the incident particle can be inferred. The minimum angular resolution of the EISA readout system is 5 degrees, which is mainly determined by the time resolution characteristics of the TDC. EISA can handle up to 2 × 10⁶ counts/s.

Plasma Current Monitor (CRM)

The CRM measures plasma density and temperature.
It consists of two pairs of double probes and one single probe, each measuring plasma parameters in a different direction.
The spatial resolution is designed to be roughly the same as that of MAC and ESA/ISA, and Langmuir curves are obtained once every 320 msec.
CRM data is not only useful for measuring the density and temperature of auroral plasma, but also for deepening our understanding of the spacecraft environment. Comparing the data with simulation analyses will also help elucidate the mechanism of satellite charging by auroral particles.

Achievements

Reimei achieved scientific results in both satellite engineering and space science, demonstrating the effectiveness of small satellites.
It also provided detailed observational data on the aurora, contributing to a deeper understanding of the Earth’s magnetosphere. In particular, new knowledge was gained about the mechanism by which auroras occur and their temporal variations. Furthermore, ESA/ISA’s observational data on electrons and ions is useful for analyzing particle movements and energy distribution within the magnetosphere.