Study of the Electron Distribution Function Downstream of the Solar Wind Termination Shock: A Touch with the Interstellar Medium

Abstract

Hans J Fahr

We theoretically describe the evolution of the solar wind electron distribution function downstream of the termination shock under the influence of electron impact ionizations of interstellar H- and He- atoms that enter the heliosheath from the upwind hemisphere and continue to move along the heliotail region. We start from a kinetic phasespace transport equation in the bulk frame of the heliosheath plasma flow that takes into account convective changes, cooling processes, whistler wave-induced energy diffusion, and electron injection into and removal from velocity-space cells due to electron impact ionization processes of interstellar neutral atoms. From this kinetic equation we can ascend to an associated pressure moment of the electron distribution function and there with arrive at a so-called pressure transport equation describing the evolution of the electron pressure in the bulk-velocity frame of the plasma flow. Assuming that the local electron distribution can be represented by a kappa function with a K- parameter that varies with the streamline coordinate s, we obtain an ordinary differential equation for K as function of s. With this result we first gain the heliosheath electron distribution function downstream of the termination shock, and at second, obtain a newly based estimate of the ionization probability of interstellar neutral atoms like H and He at the passage over the heliosheath. The latter information will enable us to quantitatively predict the interstellar inflow of neutral He- and H- atoms into the heliosheath. As we shall show the effect of H- and He impact ionizations especially gives its signature to the electron distribution along the extended down-tail region of the heliosheath. This is why we especially study this 100AU- extended down-tail region here in this article.

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