Concerning pressure and entropy of heliosheath electrons

Concerning pressure and entropy of heliosheath electrons

by Hans J. Fahr

Argelander Institute for Astronomy, University of Bonn, Bonn, Germany

We study the behaviour of shocked solar wind electrons downstream of the termination shock and give a theoretical description of the evolution of their distribution function under the influence of the shock-induced injection of KeV-energetic electrons. Hereby we start from a kinetic phasespace transport equation in the bulk frame of the heliosheath plasma flow that takes into account shock-induced electron injection, convective changes, cooling processes and whistler wave-induced energy diffusion. From this kinetic equation we proceed to an associated pressure moment of the electron distribution function and arrive at a corresponding pressure transport equation. The latter describes the evolution of the electron pressure in the comoving rest frame of the plasma flow along the plasma flow lines. We assume that the local electron distribution function, in view of the prevailing non-LTE conditions, is reasonably well represented by a local kappa function with local kappa parameter values kappa(s) and Theta(s) that vary with the streamline coordinate s downstream of the solar wind termination shock. We obtain a solution of the pressure transport equation which gives the electron pressure as function of the streamline coordinate s. Furthermore we demonstrate that, connected with this pressure, one obtains an expression for the entropy of the electron fluid which can also be quantitatively derived as a streamline function. Finally and surprisingly we do demonstrate that the heliosheath electron fluid can essentially be characterized as an isobaric and isentropic flow. These results may allow to conclude that astrotail plasma flows in general are characterized as isobaric, incompressible and isentropic electron flows.