Spatial distributions of ion pitch angle anisotropy in the near‐Earth magnetosphere and tail plasma sheet

Abstract

We have quantified anisotropy of ion pitch angle distributions observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and determined statistically how anisotropy varies with particle energy, as well as spatial distributions and dependences on geomagnetic activity. In the tail plasma sheet, ions from a few keV to a few tens of keV are mostly isotropic. The locations and energy ranges for these isotropic ions and their changes with Dst are consistent with ions being isotropized by current sheet scattering predicted using empirical magnetic field models. Ions of a few hundreds of keV in the tail have cigar‐shaped or unidirectional pitch angle distribution (PAD) and are likely a result of Speiser motion. The majority of ions in the near‐Earth magnetosphere are expected to conserve their first and second adiabatic invariants as they move with pitch angle dependent drift. This gives drift shell splitting, which plays an important role in generating pancake‐shaped PAD observed from ~1 keV up to hundreds of keV. The magnetic local time of the pancake PAD rotates with increasing energy. Loss of near 90° ions due to magnetopause shadowing can further explain the butterfly‐shaped PAD observed at the postmidnight sector at energies above 30 keV. For ions below a few hundreds of eV in the tail plasma sheet and the near‐Earth magnetosphere, their PAD is dominantly bidirectional, which is likely due to ionosphere outflow. High‐energy ions on the dayside become less anisotropic during higher AE, when pitch angle scattering by electromagnetic ion cyclotron waves may play an important role.