IMF Dependence of Energetic Oxygen and Hydrogen Ion Distributions in the Near-Earth Magnetosphere

<p> Energetic ion distributions in the near-Earth plasma sheet can provide important information for understanding the entry of ions into the magnetosphere and their transportation, acceleration, and losses in the near-Earth region. In this study, 11 years of energetic proton and oxygen observations (&gt; ~274 keV) from Cluster/Research with Adaptive Particle Imaging Detectors were used to statistically study the energetic ion distributions in the near-Earth region. The dawn-dusk asymmetries of the distributions in three different regions (dayside magnetosphere, near-Earth nightside plasma sheet, and tail plasma sheet) are examined in Northern and Southern Hemispheres. The results show that the energetic ion distributions are in&fllig;uenced by the dawn-dusk interplanetary magnetic &filig;eld (IMF) direction. The enhancement of ion intensity largely correlates with the location of the magnetic reconnection at the magnetopause. The results imply that substorm-related acceleration processes in the magnetotail are not the only source of energetic ions in the dayside and the near-Earth magnetosphere. Energetic ions delivered through reconnection at the magnetopause signi&filig;cantly affect the energetic ion population in the magnetosphere. We also believe that the in&fllig;uence of the dawn-dusk IMF direction should not be neglected in models of the particle population in the magnetosphere.</p>