Constraints on electron temperature anisotropies in sheath regions of interplanetary shocks

A statistical study of electron data observed within magnetosheath regions after interplanetary (IP) shocks driven by magnetic clouds has been performed to understand the constraints on temperature anisotropies of suprathermal electron component due to collective effects of electron wave-particle interactions with plasma micro-instabilities. Results show that although shock heating and acceleration, downstream selective reflection and leakage into upstream region, and wave-particle scattering all can contribute to suprathermal electron temperature anisotropy in downstream sheath regions of IP shocks, the consequent whistler anisotropy instabilities triggered by the sufficiently large temperature anisotropies can constrain temperature anisotropy increase to a more nearly isotropic distribution as plasma beta increases.

Statistical counts of the electron temperature anisotropies Tr=T⊥e/T∥e versus electron parallel plasma beta βPe for the suprathermal electrons in downstream sheath regions of IP shocks. The dashed lines represent theoretical thresholds with three different maximum growth rate γm/|Ωe| for the whistler anisotropy (Tr>1) and the resonant fire hose (Tr<1) instability in bi-Maxwellian distribution.Figure optionsDownload high-quality image (118 K)Download as PowerPoint slideHighlights
► We focus on the sheath regions after interplanetary shocks driven by magnetic clouds.
► Study constraints on suprathermal electron temperature anisotropies by instabilities.
► Whistler instabilities scatter electrons and constrain anisotropies.
► Electron distributions tend to be more nearly isotropic as plasma beta increases.