The Proton‐Alpha Instability as a Source of Ion Heating at Earth's Bow Shock

Abstract

Electrostatic waves are frequently observed at collisionless shocks and are believed to play a key role in energy dissipation. Recent spacecraft observations have identified a proton‐alpha instability in the Earth's bow shock as a potential mechanism to dissipate the energy associated with a relative drift between protons and alpha particles, . At plasma parameters relevant to this setting, we investigate the dependence of the evolution of this instability on through particle‐in‐cell simulations. We find that the instability produces strong ion heating, increasing ion temperatures by factors of two to five. The electrons, in contrast, experience only minimal heating . The heating is highly anisotropic and is most efficient when the wave vector is at a 45 angle to the alpha flow velocity. To interpret these results, we introduce a geometric model based on Landau resonance interaction bands in velocity space. It explains both the angular dependence and the magnitude of heating through the change in velocity distribution functions. At the same time, the wave properties are consistent with linear theory. Our results demonstrate that the proton‐alpha streaming instability can be an important mechanism to convert the bulk kinetic energy of the alphas into ion heating at collisionless shocks.