Spatial distribution of upstream magnetospheric ≥50 keV ions

We present for the first time a statistical study of less than or equal to 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 K-p, with an average solar wind speed as high as 620 km s(-1) and values of the index K-p > 2. The statistical results are consistent with (1) preferential leakage of similar to 50 keV magnetospheric ions from the dusk magnetopause, (2) nearly scatter free motion of similar to 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 similar to 16%-similar to 34% in the upstream region.