Magnetohydrodynamic turbulence in supernova remnants

We present estimates of the angular power spectra of the synchrotron radiation intensity fluctuations at 6 and 20 cm for the shell type supernova remnant Cas A and the filled-centre Crab supernova remnant. We find that the intensity fluctuations of both sources have a power law power spectrum with index -3.24 +/- 0.03. This power law power spectrum is consistent with the magnetohydrodynamic turbulence in the synchrotron emitting plasma. For Cas A, there is a break in the power spectrum and the power law index changes from -3.2 to -2.2 at large angular scale. This transition occurs at an angular scale that corresponds to the shell thickness of Cas A. We interpret this as a transition from three dimensional turbulence to two dimensional turbulence on scales that are respectively smaller and larger than the shell thickness.