Electrons under the dominant action of shock-electric fields

¹ Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
e-mail: hfahr@astro.uni-bonn.de
² Space Science Center, University of New Hampshire, 8 College Road, Durham, NH 03824, USA

Received: 4 December 2015
Accepted: 14 January 2016

Abstract

We consider a fast magnetosonic multifluid shock as a representation of the solar-wind termination shock. We assume the action of the transition happens in a three-step process: In the first step, the upstream supersonic solar-wind plasma is subject to a strong electric field that flashes up on a small distance scale Δz ≃ U₁/ Ω_e (first part of the transition layer), where Ω_e is the electron gyro-frequency and U₁ is the upstream speed. This electric field both decelerates the supersonic ion flow and accelerates the electrons up to high velocities. In this part of the transition region, the electric forces connected with the deceleration of the ion flow strongly dominate over the Lorentz forces. We, therefore, call this part the demagnetization region. In the second phase, Lorentz forces due to convected magnetic fields compete with the electric field, and the highly anisotropic and energetic electron distribution function is converted into a shell distribution with energetic shell electrons storing about 3/4 of the upstream ion kinetic energy. In the third phase, the plasma particles thermalize due to the relaxation of free energy by plasma instabilities. The first part of the transition region opens up a new thermodynamic degree of freedom never before taken into account for the electrons, since the electrons are usually considered to be enslaved to follow the behavior of the protons in all velocity moments like density, bulk velocity, and temperature. We show that electrons may be the downstream plasma fluid that dominates the downstream plasma pressure.