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 inclinations with respect to the bulk plasma motion. As a function of the compression ratio at the shock, we aim at the prediction of ion plasma properties downstream of the shock, especially the resulting downstream temperatures, pressures and pressure anisotropies as function of the upstream magnetic tilt angle. Using dynamical invariants governing the ion motions, we derive the independent reactions of the ion velocity components parallel and perpendicular to the local magnetic 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 pressures 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 ⊥/T∥≪1 at small tilt angles (quasi-parallel shock) and mirror-unstable values of anisotropies T⊥/T ∥≫1 at tilt angles near π/2 (quasi-perpendicular shock). © 2010 Author(s).