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**Session 7 - Gas and Dust in the ISM.**

*Display session, Monday, June 10*

*Great Hall, *

## [7.15] Instability of the Density Spike in the Evolution of Cosmic-Ray modified Shocks

*T. W. Jones, B. Jun (U. Minnesota)*

We study the formation mechanism and evolution of the density enhancement
(density spike) that appears downstream during the evolution of cosmic-ray modified
shocks. The density spike results from double compression of the
flow by the cosmic-ray precursor and the gas subshock as the
cosmic-ray pressure modifies the gas shock. Density in this
overcompressed region increases as Mach number increases.
Theoretically, the spike's maximum density can be as high
as 16 for \gamma_g = \gamma_c = 5/3, and 28 for \gamma_g = 5/3 and
\gamma_c = 4/3, where \gamma_g and \gamma_c
are the adiabatic index for gas and cosmic-rays, respectively. As the
shock structure reaches a steady state, the density spike lags behind
and is further compressed. Consequently, the density can be
higher than 16 for \gamma_c = 5/3. We found that this density spike
is unstable under the modified Rayleigh-Taylor instability criterion.
Our linear analysis shows that the flow is unstable when the
gradients of total pressure (gas pressure + cosmic-ray pressure) and
gas density have opposite signs. The growth of the instability is
followed by two-dimensional numerical simulations by solving the
two-fluid equations. It grows
impulsively at early stage and decays afterward. This region can become
turbulent. The appearance of
the density spike is common whenever the shock becomes cosmic-ray modified,
and whenever a cosmic-ray modified shock undergoes a period of enhanced
particle acceleration. That can happen, for example, if the shock
encounters a dense interstellar cloud.
Therefore, observational discovery of this
unstable density spike, possibly through strong radio emission (due to
amplified magnetic field by the instability), will provide
evidence for the diffusive shock acceleration of cosmic-ray particles
and the existence of cosmic-ray modified shock structures. This work
is supported by the NSF, by NASA and by the Univesity of Minnesota
Supercomputer Institute.

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