Thoughts on galactic magnetism ∗

Magnetic fields in spiral galaxies have strengths of order few 10−6G, and are coherent on scales of several kpc [1]. In several disk galaxies, like M51 and NGC 6946, they are also highly correlated (or anti-correlated) with the optical spiral arms. How do such ordered, large-scale fields arise? One possibility is dynamo amplification of a weak but nonzero seed field ∼ 10−19 − 10−23G, provided the galactic dynamo can operate efficiently to exponentiate the field by a factor ∼ 30− 40. We critically review here some of the issues relevant to the operation of the galactic dynamo, the problems which arise and possible solutions. We also touch upon the origin of the seed magnetic field, needed for dynamo amplification. The evolution of the magnetic field, in the MHD approximation, is described by the induction equation