The electron distribution function downstream of the solar-wind termination shock: Where are the hot electrons?

TL;DR

This paper explains why hot electrons downstream of the solar-wind termination shock are undetectable by spacecraft, due to spacecraft charging effects and altered plasma wave propagation caused by suprathermal electron populations.

Contribution

It reveals how hot electron populations influence spacecraft charging and plasma wave behavior, providing a new method to infer electron temperatures from wave observations.

Findings

Hot electrons cause negative spacecraft charging, reducing detectable flux.

Debye length increases by a factor of about 10^6 in hot-electron plasma.

Electrostatic waves at large wavelengths can reveal electron temperatures.

Abstract

In the majority of the literature on plasma shock waves, electrons play the role of "ghost particles," since their contribution to mass and momentum flows is negligible, and they have been treated as only taking care of the electric plasma neutrality. In some more recent papers, however, electrons play a new important role in the shock dynamics and thermodynamics, especially at the solar-wind termination shock. They react on the shock electric field in a very specific way, leading to suprathermal nonequilibrium distributions of the downstream electrons, which can be represented by a kappa distribution function. In this paper, we discuss why this anticipated hot electron population has not been seen by the plasma detectors of the Voyager spacecraft downstream of the solar-wind termination shock. We show that hot nonequilibrium electrons induce a strong negative electric charge-up of any spacecraft cruising through this downstream plasma environment. This charge reduces electron fluxes at the spacecraft detectors to nondetectable intensities. Furthermore, we show that the Debye length $\lambda _{\mathrm D}^{\kappa}$ grows to values of about $\lambda _{\mathrm D}^{\kappa}/\lambda _{\mathrm D}\simeq 10^{6}$ compared to the classical value $\lambda _{\mathrm D}$ in this hot-electron environment. This unusual condition allows for the propagation of a certain type of electrostatic plasma waves that, at very large wavelengths, allow us to determine the effective temperature of the suprathermal electrons directly by means of the phase velocity of these waves. At moderate wavelengths, the electron-acoustic dispersion relation leads to nonpropagating oscillations with the ion-plasma frequency $\omega _{\mathrm p}$ , instead of the traditional electron plasma frequency.