Entropy generation at the multi-fluid MHD solar wind termination shock

TL;DR

This paper develops a novel approach to calculate entropy production at the solar wind termination shock, revealing that ion entropy generation depends on shock compression and magnetic tilt, differing from previous models.

Contribution

It introduces an abundance-independent method for calculating ion entropy production at the shock, incorporating electron heating to satisfy thermodynamic limits.

Findings

Ion entropy production depends on shock compression ratio and magnetic tilt.

Electron heating is essential for total entropy production to meet thermodynamic limits.

The new model differs from earlier pseudo-polytropic approaches by removing dependence on pick-up ion abundance.

Abstract

In a series of earlier papers, we have developed expressions for ion and electron velocity distribution functions and theirvelocity moments at the passage over the solar wind termination shock. As we have shown there, with introduction of appropriate particle invariants and the use of Liouville`s theorem one can get explicit solutions for the resulting total downstream pressure adding up from partial pressure contributions of solar wind protons, solar wind electrons and pick-up protons. These expressions deliver in a first step the main contributions to the total plasma pressure in the downstream plasma flow and consistently determine the shock compression ratio. Here now we start out from these individual fluid pressures downstream of the shock and thereafter evaluate for the first time the shock-induced entropy production of the different fluids, when they are passing over the shock to the downstream side. As is shown here, the resulting ion entropy production substantially deviates from earlier calculations using a pseudo-polytropic reaction of the ions to the shock compression, with polytropies selected to describe fluid-specific reactions at the shock passage similar to those seen by the VOYAGERs. From these latter models ion entropy jumps are derived that depend on the pick-up ion abundance, while our calculations, to the opposite, deliver an abundance-independent ion entropy production which only depends on the shock compression ratio and the tilt angle between the upstream magnetic field and the shock surface normal. We also do show here that only when including the strongly heated electrons into the entropy balance one then arrives at the total entropy production that just fulfills the thermodynamically permitted limit.