EVOLVING MAGNETIC-FIELDS AND THE CONSERVATION OF MAGNETIC-MOMENT

It is shown that the magnetic moment mu is a conserved quantity not on ly in MHD, but also in general electrodynamics under certain not very restrictive conditions. The propagation of magnetic moment from a regi on D with an evolving current system (e.g., due to dynamo action) is d iscussed for the two cases of vacuum and a conducting medium, respecti vely, surrounding D. In the case of vacuum, the MHD approximation no l onger herds and the weak electromagnetic wave emitted from D is import ant, as was pointed out by Sokoloff (1997). In the case of an unbounde d conducting medium, the classical definition of mu is generalised and mu is shown to propagate diffusively, undisturbed by the newly genera ted magnetic field.