Spatial distribution of upstream magnetospheric ≥50 keV ions

We present for the first time a statistical study of ≥50 keV 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 shows: (1) a dawn-dusk asymmetry in ion distributions, with most events and lower intensities upstream from the quasi-parallel pre-dawn side (4 LT-6 LT) 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 magnetopause, (2) nearly scatter free motion of ∼50 keV ions within the magnetosheath, and (3) final escape of magnetospheric 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.