Upstream Energetic Ions: Leakage or Fermi Acceleration

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2114 Energetic Particles (7514), 2154 Planetary Bow Shocks, 7845 Particle Acceleration, 7851 Shock Waves (4455)

Scientific paper

Energetic (> 20 keV) ion data collected by the Interball-1 spacecraft upstream from the Earth's bow shock over a period of five years (1995-2000) are analyzed. Most of the enhanced energetic ion flux events occurred when Interball-1 was magnetically connected to the bow shock. In these connected events which occupy ~ 43% of total upstream events, characteristics of bow shock accelerated ions and magnetospheric energetic ions emerge for different ranges of ion energy. Upstream energetic ion flux is anti-correlated with θBn. This correlation weakens with increasing ion energy but greatly improves with more isotropic ion distribution. For diffusive events, ion flux at energies up to ~100 keV becomes strongly anti-correlated with θBn. Other parameters, such as local time, distance from Interball-1 to the shock, Kp and Dst indices, have little influence on the θBn dependence for lower-energy ions. For ions with energy above 150 keV, ion flux is better correlated with Dst than θBn. These results suggest that Fermi acceleration accounts for most of the observed lower-energy ions and contribution from the magnetospheric leakage becomes important for the higher-energy ions. Both solar wind and magnetosphere contribute to the seed particles for the Fermi process at the quasi-parallel bow shock. For bow shock accelerated ions with energy below ~50 keV, seed particles are mainly drawn from the solar wind but above 50 keV, contribution from the magnetosphere is significant and may exceed solar wind contribution.

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