Initial Observation of Low-Energy Charged Particles by Satellite OHZORA (EXOS-C)

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The low-energy particle experiment (ESP) on the EXOS-C satellite was designed to measure the energy spectrum and the pitch-angle distribution of electrons over the energy range of 6eV to 16keV and positive ions from 200eV to 16keV. The experiment has been successful and it has revealed global characteristics of ionospheric photoelectrons and magnetospheric energetic particles over a wide range of latitude, longitude, altitude (350-850km) and local time. The precipitation of auroral particles is observed at latitudes higher than the trapping boundary of higher-energy particles. The electron precipitation pattern is diffuse in the dayside auroral region, whereas it is quite discrete on the nightside. The energy spectrum of ions precipitating into the auroral region is found to be generally harder than the corresponding electrons. Energetic electrons with energies of 10-100keV have also been detected at dusk hours near the equator. The nature of these electrons is not yet known. The energy spectrum of photoelectrons is found to have a peak in an energy range from 20 to 30eV and a sharp cutoff near 60eV. However, the spectral shape in the energy range of 20 to 30eV is found to vary significantly depending on latitude, longitude and local time. A large peak at 20-30eV is observed occasionally over the region of the South-Atlantic geomagnetic anomaly.