The evolution of adiabatic supernova remnants in a turbulent, magnetized medium

We present the results of three-dimensional calculations for the MHD evolution of an adiabatic supernova remnant (SNR) in both a uniform and turbulent interstellar medium using the RIEMANN framework of Balsara. In the uniform case, which contains an initially uniform magnetic field, the density structure of the shell remains largely spherical, while the magnetic pressure and synchrotron emissivity are enhanced along the plane perpendicular to the field direction. This produces a bilateral or barrel-type morphology in synchrotron emission for certain viewing angles. We then consider a case with a turbulent external medium as in Balsara & Pouquet, characterized by v(A)(rms)/c(s) = 2. Several important changes are found. First, despite the presence of a uniform Deld, the overall synchrotron emissivity becomes approximately spherically symmetric, on the whole, but is extremely patchy and time variable, with flickering on the order of a few computational time steps. This is reminiscent of the Cas A observations of Anderson & Rudnick, although that remnant has a much more complicated pre-supernova evolution than what is considered here. We suggest that the time and spatial variability of emission in early phase SNR evolution provides information on the turbulent medium surrounding the remnant. The interaction of the outwardly propagating SNR shock with interstellar turbulence is shown to amplify the turbulence in the postshock region, thereby having important consequences for relativistic particle acceleration. The shock-turbulence interaction is also shown to be a strong source of helicity generation and, therefore, has important consequences for magnetic field generation. We compare our calculations to the Sedov-phase evolution and discuss how the emission characteristics of SNRs may provide a diagnostic on the nature of turbulence in the pre-supernova environment.