Dawn-Dusk Asymmetries in the Near-Earth Plasma Sheet: Ion Observations

Dawn-dusk asymmetries in the distributions of plasma sheet ions give us clues about the sources, transport, acceleration, and loss processes in the magnetosphere. In this chapter, we combine the latest studies on spatial ion distributions at different energy ranges focusing on Cluster observations. We consider different ion species and different distances from the Earth, and we discuss the dependence on solar parameters and geomagnetic conditions. The solar wind causes a dawnward asymmetry of the proton spatial distribution. Ionospheric ions are transported toward the duskward side. The combination of both leads to a rather symmetric ion density in the near-Earth magnetotail. During active geomagnetic periods, enhanced solar wind dynamic pressure (SW P dyn) and southward Interplanetary Magnetic Field (IMF) intervals, energetic ions > 100 keV exhibit a higher intensity in the duskward sector, indicating that the ions are accelerated inductively and then drift duskward because of a strong magnetic field gradient curvature. At the dayside, such an asymmetry in the energetic ion distribution is explained by ion losses through the magnetopause. However, it is still unclear why the near-Earth (8 RE) ionospheric ion density shows a dawnward asymmetry during active geomagnetic times