Hardly had I sent off my earlier message than I came across this
brand-new paper (6 days old, publ. 22 Feb 13) on arXiv that lucidly explains all the basic terms and ideas in galactic MHD. The lead author is Rainer Beck, a familiar name on enthusiast forums. For a startling visual analog to a late-stage SNr only on a grand scale, check out the polarization map of the MW on p.13.