Acoustic emission and corrugational instability of shocks modified by strong particle acceleration

The effect of particles that undergo strong diffusive-shock-acceleration on the stability of the accelerating shock is investigated. A two-fluid model is employed in which the accelerated particles are treated as a fluid whose effect is incorporated as an additional pressure in the momentum equation. The Dyakov and Kontorovich stability criteria are used in order to study the stability of those steady-state shocks that contain a gas sub-shock. The downstream conditions of the latter are parametrized by the ratio of the upstream acclerated-particles pressure to the total pressure. For some range of values of that parameter, three possible downstream states are possible for each upstream state. It is shown that in that range of parameters the shocks are either corrugationally unstable or lose their energy by spontaneous emission of acoustic as well as entropy-vortex waves.