Acceleration and Precipitation of Energetic Ionospheric Ion Beams in the Auroral Region: Cluster Observations and Large Scale Modeling

The Cluster spacecraft offer a unique opportunity to investigate the origin of dispersed ion beams observed at mid-latitudes (4-5 RE) in the nightside auroral region. In this paper we present a detailed study of one event observed during the recovery phase of a magnetospheric substorm. In contrast to the suggestion that energetic ions sporadically originate in the distant PSBL, we argue that energetic (5-15 keV), field-aligned H+ ions observed by Cluster are ejected at the top of a steady and localized auroral acceleration region. These ions bounce on closed field-lines and are dispersed in latitude by the E x B magnetospheric filter. Echoes of energetic ionospheric H+ ions are observed by each of the Cluster spacecraft as they pass through the high latitude poleward edge of the plasmasheet. Such multiple-spacecraft measurements are well-suited to test our ability to model very complex magnetospheric situations. Detailed time-dependent modeling of this event will be presented. In order to correctly interpret the observed spatial dispersion, global MHD simulations demonstrate that vortices or channels of tailward convection are locally generated in the neutral sheet at 14-18 RE distance in the post-midnight sector, i.e., on field lines connected with the mid-altitude region crossed by Cluster. By using the Large Scale Kinetic (LSK) approach, millions of ion trajectories are followed in the time-dependent MHD fields, starting from an ionospheric source. These particles undergo very complex orbits, some of them are on adiabatic bouncing orbits but a non-negligible fraction are non-adiabatically accelerated during neutral sheet crossings and supply part of the observed echoes.