The protonopause — An ion composition boundary in the magnetosheath of comets, Venus and Mars

Abstract

Abstract The interaction of the solar wind with heavy ions continuously produced by ionization from a background neutral gas cloud is studied in the framework of a collisionless 2D bi-ion fluid model in which the motion of both ion species is coupled exclusively by electromagnetic forces. It is shown that a magnetosheath plasma boundary is formed which has signatures of a bi-ion double layer separating the plasma mixture of protons and heavies from the purely heavy ion plasma. At the self generated potential jump the solar wind protons are stopped and deflected whereas the heavy ions have enough energy to penetrate it. This discontinuity, called here protonopause, is formed in a region where the heavy ion density becomes comparable to the solar wind proton density. We suggest that the protonopause reflects main features of the hitherto unexplained plasma boundaries at comets (cometopause, pile-up boundary), Venus (rarefaction wave, intermediate transition) and Mars (planetopause, mass-loading boundary).