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We have carried out two dimensional numerical simulations of the evolution of dense clumps of material moving supersonically through a background plasma in an effort to understand particle acceleration associated with them as well as the development of bright radio knots in such objects as supernova remnants and radio jets. The simulations use a two fluid model for the diffusive acceleration of high energy protons at shocks, including the backreaction of the particles on the gas dynamics. In addition we follow the evolution of the high energy electrons through a simple test particle theory for shock acceleration and evolve embedded magnetic fields in the weak field limit.
We find that the bow shocks of the clumps are moderately efficient at accelerating both protons and electrons, but that the synchrotron emission expected from such clouds may have little association with that development. Rather, significant magnetic field growth in sheared flows around the cloud as it breaks apart from Kelvin-Helmholtz instabilities is the dominant effect in generating synchrotron emission in the simulations. The clumps are radio bright primarily just before they break apart, at which times the emission comes mostly from remnants of the cloud boundary layer rather than the bow shock.
This work was supported in part by the NSF and by the University of Minnesota Supercomputer Institute.
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