Influence of the interplanetary magnetic field orientation on polar cap ion trajectories: Energy gain and drift effects

Abstract

The influence of the interplanetary magnetic field (IMF) orientation on the transport of low‐energy ions injected from the ionosphere is investigated using three‐dimensional particle codes. It is shown that, unlike the auroral zone outflow, the ions originating from the polar cap region exhibit drastically different drift paths during southward and northward IMF. During southward IMF orientation, a “two‐cell” convection pattern prevails in the ionosphere, and three‐dimensional simulations of ion trajectories indicate a preferential trapping of the light ions (H+) in the central plasma sheet, due to the wide azimuthal dispersion of the heavy ions (O+). In contrast, for northward IMF orientation, the “four‐cell” potential distribution predicted in the ionosphere imposes a temporary ion drift toward higher L shells in the central polar cap. In this case, while the light ions can escape into the magnetotail, the heavy ions can remain trapped, featuring more intense acceleration (from a few electron volts up to the keV range) followed by precipitation at high invariant latitudes, as a consequence of their further travel into the tail.