Exact spirally symmetric galactic dynamos

Abstract

This paper extends the results of an earlier paper on scale invariant galactic spiral magnetic modes to time dependent, scale invariant, spirals. The examples given are all exact in that they may be described in terms of hypergeometric functions. We restrict the discussion to an infinitely conducting medium in order to avoid earlier approximation, which limited the solutions to cones lying within about twenty degrees to the plane. The magnetic disc spirals, `X type" poloidal fields, and the recent discovery of rotation measure screens in edge-on galactic halos were all recovered in such solutions.