Ion passage over the solar wind termination shock under conservation of particle invariants in view of Voyager-2 observations

Abstract

We study the plasma passage over astrophysical MHD shocks with frozen-in magnetic fields in arbitrary incli- nations with respect to the bulk plasma motion. As a function of the compression ratio at the shock, we aim at the predic- tion of ion plasma properties downstream of the shock, espe- cially the resulting downstream temperatures, pressures and pressure anisotropies as function of the upstream magnetic tilt angle. Using dynamical invariants governing the ion mo- tions, we derive the independent reactions of the ion veloc- ity components parallel and perpendicular to the local mag- netic field at the shock passage. This allows us to determine not only the associated downstream ion velocities, but the ion distribution function and its velocity moments like pres- sures and temperatures. We find pronounced increases of the downstream ion temperatures with respect to corresponding upstream values. The down-to-up temperature ratios thereby strongly depend on the upstream magnetic tilt angle attaining maximum values in case of a quasi-perpendicular shock. We also obtain fire-hose-unstable temperature anisotropies with values T?/Tk 1 at small tilt angles (quasi-parallel shock) and mirror-unstable values of anisotropies T?/Tk 1 at tilt angles near / 2 (quasi-perpendicular shock).