Analysis of Asymmetric Ion Pitch Angle Distributions Observed by MMS HPCA

Abstract:

The Pitch Ange Distributions (PADs) of ions in the magnetosphere have several shapes (butterfly, pancake, beam and isotropic) and the spatial distributions of PADs in the outer magnetosphere (L > 6) are not well known. In this study, we report the statistical analysis of 3years of PADs near the magnetic equatorial plane observed by the Magnetospheric Multiscale (MMS) Hot Plasma Composition Analyzer (HPCA). We find an average asymmetry in warm-ion PADs favoring ions emerging from the northern hemisphere (NH) over those from the southern hemisphere (SH). Specifically, we find that phase space density (PSD) in the anti-parallel direction (pitch angle > 135°) is higher than in the parallel direction (< 45°) when their corrected energy shifted upwards to account for the spacecraft potential is less than 100 eV. To understand the cause of this NH-SH asymmetry, we sort the distributions by the direction of IMF, and the result shows that PSD of the anti-parallel direction becomes stronger when By < 0. We compute solar insolation angles by mapping MMS to the ionosphere to find the relationship with the ion outflow. The dependence of the solar insolation angles indicates that the outflow from the NH is higher when the NH footpoint of the field line is sunlit. In contrast, when the SH footpoint is the sunlit, outflow from the SH is not significantly higher and the PAD is only mildly asymmetric, or it is symmetric. Based on these results, we suggest there may generally be interhemispheric asymmetry of ion outflow that favors the NH.