Mirror and Firehose Instabilities in the Heliosheath

Abstract

We investigate the nature of the heliosheath plasma behind the termination shock across which jump relations in anisotropic MHD are formulated. Along side analytical results for downstream parameters in the strictly parallel and perpendicular cases we numerically solve the Rankine-Hugoniot relations for arbitrary shock angle and strength. We then focus on two temperature anisotropy driven instabilities which have attracted attention in many other astrophysical situations, namely the mirror and firehose instabilities. It is revealed that the firehose instability is mainly controlled by the shock strength with little influence of the shock angle contrary to the mirror instability for which both parameters intervene. We confirm results showing that the heliosheath plasma observed by Voyager 1 immediately behind the termination shock is mirror unstable. Similar conditions are probable in the heliosheath recently encountered by Voyager 2. Finally, by comparison with studies in the Earth's magnetosheath context, we formulate predictions on the shapes of mirror-associated magnetic fluctuations in the heliosheath. Both hole and peak magnetic structures were indeed observed by Voyager 1 and these shapes correspond to different stages of the mirror instability.