Thermal and Supra-thermal Electron Properties at Quasi-perpendicular Shocks

Abstract:

We present a statistical study of how the solar wind electron core and halo sub-populations are individually changed when crossing the Earth's quasi-perpendicular bow shock. Thermodynamical quantities are defined and obtained by fitting model functions to the electron distributions sampled by the THEMIS spacecraft. We find that while the core is compressed, the halo is often deflated behind the shock. Both are isotropically heated on a timescale of less than three seconds, and the ratio of the downstream to upstream temperature is a factor five to six for both populations.

The partial pressure ratio between the solar wind core and halo populations is of the order of 10 both upstream and downstream. The $\kappa$ of the halo Kappa distribution goes from an upstream value of $\kappa_{up}=2.5$ to a downstream value $\kappa_{down}=4.5$, taking the halo from a critically balanced non-equilibrium thermodynamical state in the solar wind to a state in thermal near-equilibrium behind the shock.
We find that there is an inverse dependence of the core and halo temperature, density, $\kappa$ and energy density upon the Mach number. Slower shocks are more effective at compressing and heating the core and the halo.
The difference in electron energy density normalized to incident proton kinetic energy density is inversely dependent on Mach number. At Earth's bow shock, the core electron heating accounts for around 10\% of the available incident ram energy, while the halo only 1\%. This quantitiy depends on Mach number but does not seem to depend on the shock $\theta_{Bn}$ angle.