Spatial distribution of upstream magnetospheric ³50 keV ions
Anagnostopoulos, Georgios C.
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We present for the first time a statistical study of >=50keV ion events of a magnetospheric origin upstream from Earth's bow shock. The statistical analysis of the 50-220 keV ion events observed by the IMP-8 spacecraft showes: (1) a dawn-dusk asymmetry in ion distributions, with most events and lower intensities upstream from the quasi-parallel pre-dawn side (4LT-6LT) of the bow shock, (2) highest ion fluxes upstream from the nose/dusk side of the bow shock under an almost radial interplanetary magnetic field (IMF) configuration, and (3) a positive correlation of the ion intensities with the solar wind speed and the index of geomagnetic index Kp, with an average solar wind speed as high as 620 km s^-1 and values of the index Kp >2. The statistical results are consistent with (1) preferential leakage of ~50 keV magnetospheric ions from the dusk magentopause, (2) nearly scatter free motion of ~5 keV ions within the magnetosheath, and (3) final escape of magnetoshperic ions from the quasi-parallel dawn side of the bow shock. An additional statistical analysis of higher energy (290-500 keV) upstream ion events also shows a dawn-dusk asymmetry in the occurrence frequency of these events with the occurrence frequency ranging between ~16%-~34% in the upstream region.