Cosmic Ray-Dominated Shocks in the Hot Ionized Interstellar Medium*

Galactic cosmic rays are energetic particles that are primarily accelerated by supernova remnant shocks. The back-reaction of energetic particles can modify the structure of the

shock wave and the magnetic field. The incoming plasma is decelerated by cosmic ray pressure and collisionless viscous stress associated with the cosmic rays. An instability

associated with the streaming of cosmic rays upstream of the shock generates turbulence that acts to scatter the cosmic rays, creating a nearly isotropic distribution function. The

dissipation of turbulence heats the background gas. Both the cosmic rays and the background thermal gas are treated as fluids coupled to the turbulence. We numerically

solve the steady-state system of equations, which include the gas dynamic equations, the cosmic ray pressure equation, and the turbulence transport equations, using typical parameters for the hot ionized interstellar medium. We find that the shock has no discontinuity, such as a subshock, but instead exhibits a very narrow but continuous transition. The self-generated turbulent magnetic field is found to be much larger than both the large-scale field and the pre-existing turbulent magnetic field. The resulting diffusion coefficient is substantially suppressed and is more than three orders smaller near the shock than it is far upstream. The results are qualitatively consistent with some observations of supernova shocks thought to be responsible for accelerating cosmic rays. This work, despite the vast separation in scales, has implications for the acceleration of solar energetic particles at so-called gradual events in the solar wind.