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

¹ Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: hfahr@astro.uni-bonn.de
² Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
e-mail: jdr@space.mit.edu
³ Space Science Center, University of New Hampshire, 8 College Road, Durham, NH 03824, USA
e-mail: daniel.verscharen@unh.edu

Received: 21 January 2015
Accepted: 9 May 2015

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 λ_D^κ grows to values of about λ_D^κ/λ_D ≃ 10⁶ compared to the classical value λ_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 ω_p, instead of the traditional electron plasma frequency.