Observations of the development of electron temperature anisotropies in Earth's magnetosheath

Abstract

Electron velocity distributions in Earth's magnetosheath exhibit a number of nonequilibrium characteristics, including a flat-topped shape at thermal energies and anisotropies relative to the magnetic field, with T_⊥e > T_∥e. These features are related to processes at the bow shock and within the magnetosheath. In the present investigation we focus our attention on the development of the electron temperature anisotropy in the magnetosheath, employing the simultaneous multispacecraft data obtained by the Plasma Electron and Current Experiment (PEACE) on board Cluster. Such studies remove the temporal ambiguities introduced by single-spacecraft analyses. We find that the electron velocity space distributions just behind the bow shock are nearly isotropic with a slight T_∥e > T_⊥e anisotropy, whereas deeper into the magnetosheath the electrons exhibit a significant T_⊥e > T_∥e anisotropy. We find observational evidence for two processes that contribute to the sheath anisotropy. There is a clear decrease of suprathermal electrons at 0° and 180° pitch angles, suggesting that this population suffers losses (e.g., into the upstream region) related to the global configuration. Additionally, an ongoing local mechanism inflates the 90° pitch angle suprathermal electron population.