The anisotropy of suprathermal electrons in the Martian ionosphere

Suprathermal electrons are an important population of the Martian ionosphere, either produced by photoionization of atmospheric neutrals or supplied from the Solar Wind (SW). This study is dedicated to an in-depth investigation of the pitch angle distribution of suprathermal electrons at two representative energies, 19−55 eV and 124−356 eV, using the extensive measurements made by the Solar Wind Electron Analyzer on board the Mars Atmosphere and Volatile Evolution. Throughout the study, we focus on the overall degree of anisotropy, defined as the standard deviation of suprathermal electron intensity among different directions which is normalized by the mean omni-directional intensity. The available data reveal the following characteristics: (1) In general, low energy electrons are more isotropic than high energy electrons, and dayside electrons are more isotropic than nightside electrons;(2) On the dayside, the anisotropy increases with increasing altitude at low energies but remains roughly constant at high energies, whereas on the nightside, the anisotropy decreases with increasing altitude at all energies;(3) Electrons tend to be more isotropic in strongly magnetized regions than in weakly magnetized regions, especially on the nightside. These observations indicate that the anisotropy is a useful diagnostic of suprathermal electron transport, for which the conversion between the parallel and perpendicular momenta as required by the conservation of the first adiabatic invariant, along with the atmospheric absorption at low altitudes, are two crucial factors modulating the observed variation of the anisotropy. Our analysis also highlights the different roles on the observed anisotropy exerted by suprathermal electrons of different origins.

Statistical study on the suprathermal electrons properties around dipolarization fronts in Earth's magnetotail

In order to investigate the suprathermal electron flux(>30 ke V) around dipolarization fronts(DFs), we statistically studied the suprathermal electron flux variations and pitch angle distributions of hundreds of earthward propagating DFs observed by THEMIS spacecraft during its tail seasons in years 2008–2009. We focused on the electron flux variations across DFs and electron anisotropies behind DFs. We divided DF into three sectors in the equatorial plane: Dusk, central and dawn sectors. The sectors are defined according to the DF normals with respect to DF's meridian in the equatorial plane(the symmetric line of DF). We found that events with electron flux increases and decreases behind the fronts had no particular dependence on the observation locations. In addition, there was no obvious dependence of electron anisotropy behind DF on the different sectors of DF.