The multi-fluid pressures downstream of the solar wind termination shock

Abstract

In this paper we consider a multi-fluid plasma that describes the upstream solar wind at its passage over the solar wind termination shock. In one respect, the plasma at the shock reacts like a joint fluid that is described by a single compression ratio. This ratio depends on all upstream and downstream pressures of the magnetohydrodynamic (MHD) plasma. In another respect, the distinguished plasma fluids in their downstream properties show fluid-specific reactions, thet we describe by using additional kinetic information on the plasma constituents, such as the Liouville theorem, the conservation of typical particle invariants, and the species-specific influence of the electric shock ramp. We thus obtain the resulting distribution functions of the seperate fluid particles and their associated velocity moments for the downstream region, especially their separate fluid pressures. We show that the different fluid pressures in different forms depend on the shock compression ratio and on the tilt angle between the upstream magnetic field and the shock surface normal. The dominant downstream pressures are connected with the pick-up protons and with the solar wind electrons, one dominating under some given shock conditions, the other dominating under some other shock conditions. Since the downstream distributions of solar wind protons and pick-up protons partly overlap in velocity space, we look for a joint distribution of the joint proton population in the form of a joint Kappa distribution and find that the associated Kappa index and the "Gaussian velocity width" are functions of the pick-up ion abundance, of the joint compression ratio, and of the tilt angle. Owing to the strongly heated electrons the energy-per-mass density ratio of the downstream plasma turns out to be fairly different from all that was expected up to now. This might also give a hint as to why the heliosheath plasma flow lines seen by Voyagers are different from all MHD simulations so far.

Electrons under the dominant action of shock-electric fields

“…With this result, we determine the size of the demagnetization region (phase one) as Δz ≈ 10 8 cm for typical solar-wind parameters. In earlier work (see Chalov & Fahr 2013;Fahr & Siewert 2013, we emphasized the fact that the creation of energetic electrons at the plasma passage over the shock is essential to fulfill the thermodynamic entropy requirements (Fahr & Siewert 2015) and to arrive at downstream plasma properties that nicely fit the Voyager-2 measurements of Richardson et al (2008). Zieger et al (2015) recently published a multifluid study of the solar-wind termination shock, which strongly supports our claim for the occurrence of energetic downstream electrons by showing that the Voyager-2 measurements allow for a reasonably good theoretical fit with the model results only if the appearance of energetic downstream electrons is taken into account.…”

“…They treat the transition from the upstream side to the downstream side of the shock as an instantaneous kinetic reaction in the velocity distribution function via the Liouville-Vlasov theorem, predicting all of the relevant downstream plasma quantities. In this model, a semikinetic representation of the multifluid termination shock with mass-and charge-specific reactions of protons and electrons to the electric shock ramp leads to excessive electron heating as described by Fahr et al (2012) or Fahr & Siewert (2013). Based on these studies, we assume that electrons enter the downstream side as a strongly heated, massless plasma fluid that dominates the downstream plasma pressure.…”

Three-dimensional Features of the Outer Heliosphere Due to Coupling between the Interstellar and Heliospheric Magnetic Field. V. The Bow Wave, Heliospheric Boundary Layer, Instabilities, and Magnetic Reconnection

“…Approaches which are not based on the conservation-law principles and involving straightforward calculations of the PUI pressure derivatives across the TS (e.g., Usmanov 2016) are mathematically flawed. Fahr & Siewert (2013) (see also references therein)…”

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

“…In this framework, the Liouville-Vlasov theorem describes all relevant downstream plasma quantities as an instantaneous kinetic reaction in the velocity distribution function during the transition from upstream to downstream. The excessive electron heating is then the result of the mass-and charge-specific reactions to the electric shock ramp, as shown in the semikinetic models of the multifluid termination shock by Fahr et al (2012) and Fahr & Siewert (2013). According to these studies, electrons enter the downstream side as a strongly heated plasma fluid with negligible mass density that dominates the downstream plasma pressure.…”