A Critical Review of Galactic Dynamos

Abstract

▪ Abstract I present the general picture of how galactic magnetic fields grow in disks according to the alpha-Omega dynamo theory. Emphasis is placed on following the lines of force during the dynamo process. The dynamo equation is presented together with a simple growing solution for the galactic disk. Then I take up the various critical questions that have been raised concerning the galactic dynamo theory. These are (1) the importance of the escape of flux from the disk in order for the magnetic field to grow; (2) the physics of turbulent diffusion and its mixing of field lines together so that the rms field is possibly greater than the mean field; (3) whether magnetic reconnection plays a role in the galactic dynamo; (4) whether small-scale fields can grow large enough to swamp the dynamo. Then I discuss the possible seed fields from which the dynamo starts and their relation to the primordial hypothesis. Finally I take up the question of the final evolution of the galactic field after the alpha effect saturates. My conclusion is that all these problems warrant attention but none of them seem to be serious enough to cast any real doubt on the dynamo as the most likely generator of galactic fields.