Transverse ion energization and low‐frequency plasma waves in the mid‐altitude auroral zone: A case study

Abstract

The transport of ions from the ionosphere to the magnetosphere requires that ions acquire significant energy in directions both transverse and parallel to the magnetic field. There is a considerable body of experimental evidence that shows that transverse energization occurs over a wide range of altitudes on auroral field lines. Many recent analytical and simulation studies have addressed the microphysics involved in transverse ion energization. There are, however, remarkably few published high‐resolution plasma and plasma wave observations obtained in the mid‐altitude auroral region available to compare with the analytical and simulation studies. Several hundred hours of high‐resolution plasma data obtained from the Dynamics Explorer 1 satellite have been surveyed. A wide variety of plasma environments that are difficult to simply characterize were found. We present here a comprehensive set of high‐sensitivity, high‐resolution plasma wave, ion, and magnetometer data obtained from an evening auroral zone crossing at r/RE∼3. The total density, thermal structure, and composition of the plasma in this representative interval varied rapidly, as did the character (mode) of low‐frequency plasma waves observed. We did not find an unambiguous particle and wave signature of local transverse ion energization, but we did frequently find intervals where local transverse ion heating was consistent with the observations. We also found a downward flowing ion distribution that occurred simultaneously with a region of intense plasma wave emissions primarily below the lower hybrid resonance frequency.