Ion heating by broadband electromagnetic waves in the magnetosheath and across the magnetopause

Abstract

We present statistical observations demonstrating the action of broadband electromagnetic waves or structures in heating magnetosheath ions in the Earth's magnetosheath and across the magnetopause in the low‐latitude boundary layer. Through the use of a generalized multicomponent fitting routine to the full 3‐D distribution function, we distinguish magnetosheath ions from magnetospheric and diffuse energetic ion populations through these regions. From this approach, we find that the energy density of magnetosheath ions is well correlated with the spectral energy density of electromagnetic field fluctuations on gyro‐radius scales. Through a dispersion analysis, we demonstrate that these field structures have the properties of kinetic Alfvén waves. We show that the heating provided by these waves is nearly entirely perpendicular to the magnetic field and, subject to a threshold condition being exceeded, occurs at a rate that is lognormally distributed as 10 ‐0.3±1.1 eVs ‐1 or on average ~1 eVs ‐1 . This rapid perpendicular heating is shown in turn to be limited by the threshold condition for anisotropy instabilities which when exceeded leads to further wave emission and the transfer of perpendicular energy to parallel particle energy. We suggest that this cycle of wave emission and heating may be expected to slow magnetosheath flow while driving irreversible changes in magnetosheath ion distributions.