The Density Spike in Cosmic‐Ray–Modified Shocks: Formation, Evolution, and Instability

We examine the formation and evolution of the density enhancement (densityspike) that appears downstream of strong, cosmic-ray-modified shocks. Thisfeature results from temporary overcompression of the flow by the combinedcosmic-ray shock precursor/gas subshock. Formation of the density spike isexpected whenever shock modification by cosmic-ray pressure increases strongly.That occurence may be anticipated for newly generated strong shocks or forcosmic-ray-modified shocks encountering a region of higher external density,for example. The predicted mass density within the spike increases with theshock Mach number and with shocks more dominated by cosmic-ray pressure. Wefind this spike to be linearly unstable under a modified Rayleigh-Taylorinstability criterion at the early stage of its formation. We confirm thisinstability numerically using two independent codes based on the two-fluidmodel for cosmic-ray transport. These two-dimensional simulations show that theinstability grows impulsively at early stages and then slows down as thegradients of total pressure and gas density decrease. Observational discoveryof this unstable density spike behind shocks, possibly through radio emissionenhanced by the amplified magnetic fields would provide evidence for theexistence of strongly cosmic-ray modified shock structures.Comment: 26 pages in Latex and 6 figures. Accepted to Ap