Cusp energetic ions as tracers for particle transport into the magnetosphere

Abstract

The magnetospheric cusps are focal points in studies of both magnetic reconnection at the magnetopause and plasma entry into the magnetosphere. Contrary to a well‐understood precipitating thermal magnetosheath ion population, the origins of energetic ions in the cusp regions are still a matter of controversy. It has been suggested that these cusp energetic particles (CEP) with significant fluxes from magnetosheath energies up to several hundred keV/e are accelerated locally in the cusp. A recent paper has suggested local plasma conditions conducive to CEP acceleration in the cusp diamagnetic cavity (CDC). An alternative source region for CEP events is the quasi‐parallel bow shock, which is a well‐known particle accelerator. Energetic ions accelerated at the bow shock can be transported downstream and enter the cusp along newly reconnected field lines. Composition and energy spectra of these CEP events resemble those of bow shock energetic diffuse ions. We use recently developed techniques to determine the location of the reconnection site at the magnetopause, draping interplanetary magnetic field (IMF) lines over the magnetopause and mapping those field lines back into the solar wind to show the magnetic connection between the cusp regions, the Earth bow shock, and the upstream region. Several cusp crossings by the Polar satellite during variable IMF conditions are analyzed for patterns between the cusp, their connection to the upstream region, and the appearance of energetic ions in the cusp. Local plasma conditions in the cusp are also documented. This analysis reveals that the occurrence of CEP events is not uniquely determined by local plasma conditions. The flux of CEP ions depends on the location of the quasi‐parallel bow shock and the magnetic topology in the magnetosheath. Our analysis allows us to use CEP ions as tracers for plasma transport into the cusp and to better understand the magnetic topology between the solar wind and the ionosphere.