Spatial distribution of upstream magnetospheric 350 keV ions

We present for the ®rst time a statistical study of 350 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 ̄uxes upstream from the nose/dusk side of the bow shock under an almost radial interplanetary magnetic ®eld (IMF) con®guration, 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 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) ®nal 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.