Ion gyroradius effects on particle trapping in kinetic Aflven waves along auroral field lines

Authors: P.A. Damiano, J.R. Johnson

Abstract: In this study,  a 2D self-consistent  hybrid gyrofluid-kinetic  electron  model is used to investigate Alfven wave propagation along dipolar ma netic field lines for a range of ion to electron temperature ratios.  The focus of the investigation  is on understanding the role of these effects on electron trapping in kinetic Alfven waves sourced in the plasma sheet and  the role of this trapping in contributing to the overall electron energization at the ionosphere. This work also builds on our previous effort [Damiano  et al., 2015] by considering a similar system in the limit of fixed initial  parallel current, rather than fixed initial perpendicular electric field. It is found that  the effects of particle trapping  are strongest  in the cold ion limit and the kinetic Alfven wave is able to carry trapped  electrons a large distance  along the field line yielding a relatively large net energization of the trapped  electron  population as the phase speed of the wave is increased.  However, as the ion temperature is increased,  the ability of the kinetic Alfven wave to carry and energize trapped electrons is reduced by more significant wave energy dispersion perpendicular  to the ambient  magnetic field which reduces the amplitude of the wave. This reduction of wave amplitude in-turn  reduces both  the parallel current and the extent of the high energy tails evident in the energized electron populations  at the ionospheric boundary (which may serve to explain the limited extent  of the broadband electron energization seen in observations).  Even in the cold ion limit, trapping  effects in kinetic Alfven waves lead to only modest electron energization for the parameters considered (on the order of tens of eV) and the primary energization of electrolns to keV levels coincides with the  arrival of the wave at the ionospheric boundary.

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