Probing the thermodynamic conditions of the heliosheath plasma by shock wave propagation

Abstract

Context. The pressure equilibrium between the inner heliosheath and the outer heliosheath (referred to as the local interstellar medium) is an eminent theoretical and practical problem; theoretical, because the relevant pressure carriers have to be identified, and practical, because data must be gathered in order to confirm such a pressure equilibrium. The problem is closely connected with the stability of the heliopause, that is, of the tangential discontinuity between these two counterflowing media, and is of utmost importance for understanding the stability of the whole circumsolar plasma structure. Aims. In this paper we analyze the thermodynamic conditions of the multi-fluid plasma between the solar wind termination shock and the heliopause determining the total heliosheath pressure. We look into this problem from a theoretical standpoint and revisit theoretical descriptions of the solar wind plasma after its passage over the solar wind termination shock, thereafter forming the subsonic heliosheath region. Methods. Hereby we take into account the 3D magnetohydrodynamics shock conditions and the resulting 3D temperature structure of the downstream plasma flow. We use a kind of seismological procedure to probe the heliosheath plasma by inquiring into the propagation conditions of traveling shock wave perturbations in this predetermined 3D heliosheath plasma structure. We discuss the fact that the front geometry of such a traveling shock wave most probably does not remain spherical, if it was to begin with, due to asymmetric shock propagation conditions. In contrast, the wave front is likely to become strongly deformed into an upwind bulge. Results. Concerning the plasma pressure, in addition to solar wind and pick-up proton pressures, we have to take into account the solar wind electron pressure which as a surprise turns out to be of comparable magnitude. As a consequence, the characteristic propagation speed of the traveling shock wave in the weakly magnetized heliosheath plasma is given as a mixed speed expressed by the sound speeds of the protons and the electrons. We describe local low-energy proton density signatures that can be found in Voyager-2 proton data as a consequence of traveling shock wave passages and show that the total local plasma pressure can be directly derived from them.

Galactic Cosmic Rays Throughout the Heliosphere and in the Very Local Interstellar Medium

“… 2019a ), constituents of this total pressure are as follows: , , solar wind dynamic , ACRs and , pickup . The remaining is not presently accounted for – possibly due to electrons (Fahr and Heyl 2020b , a ), which are not directly measured by Voyager or IBEX. Recent studies of pressures in the heliosheath show general agreement (Dialynas et al.…”

The Role of Electrons and Helium Atoms in Global Modeling of the Heliosphere

“…The results mentioned above were obtained by fitting the Voyager/Low-Energy Charged Particle (LECP) electron observations at energies 20 keV using a single isotropic kappa distribution function. Further theoretical analysis of kappa distributions has been carried out recently by Fahr & Dutta-Roy (2019), Scherer et al (2019), andHeyl (2020). Since electrons below 20 keV were not measured, another possible scenario is that the bulk electrons in the IHS have properties similar to those of thermal protons, and that suprathermal electrons do not contribute significantly to the pressure.…”

Nearly a Decade of Cosmic Ray Observations in the Very Local Interstellar Medium

“…Constituents of this total pressure are as follows: 1.3% = thermal; 2.2% = magnetic; ~12% = dynamic pressure of the solar wind; ~21% = anomalous and galactic cosmic rays; ~45% = pickup ions. This leaves 18.5% that is presently unaccounted for (e.g., not directly measured by Voyager or IBEX), possibly due to electrons [70,71].…”