The Energetic Particle Spectrometer Hep onboard the Geotail Spacecraft

The instruments for the HEP (High Energy Particle) experiments of the Geotail mission, launched in July 1992, consist of 5 spectrometers (Ld, Bd, Mi-1, Mi-2 and Hi). The LD (Low energy particle Detector) and the Bd (Burst Detector) are mainly used to measure electrons, protons, helium and oxygen ions which reflect plasma dynamics in the magnetotail region. On the other hand, Mi-1, Mi-2 (Medium energy Isotope telescope-1, -2) and Hi (High energy Isotope telescope) are used to measure the isotope abundance of solar flare particles and cosmic ray particles which reflect physical conditions of interplanetary space and origin of these particles. In this paper, the objectives of these experiments, the details of the instruments and preliminary results from the observation in the magnetotail are given. The preliminary results obtained by the LD and BD show that a highly collimated beam of energetic particles appear in the plasma sheet just after the substorm onset and that directional distribution of particles can be radically different for different energies. In the Mi and HI telescopes, details a remarkable enhancement of anomalous cosmic ray components (N, O) in the quiet time of solar activity is reported, as well as the element composition of galactic cosmic rays.