Temperature Anisotropy in a Shocked Plasma: Mirror-Mode Instabilities in the Heliosheath

We show that temperature anisotropies induced at a shock can account for interplanetary and planetary bow shock observations. Shocked plasma with enhanced plasma β is preferentially unstable to the mirror-mode instability downstream of a quasi-perpendicular shock and to the fire-hose instability downstream of a quasi-parallel shock, consistent with magnetic fluctuations observed downstream of a large variety of shocks. Our theoretical analysis of the solar wind termination shock suggests that the magnetic holes observed by Voyager 1 in the heliosheath are produced by the mirror-mode instability. The results are also of astrophysical interest, providing an energy source for plasma heating.