Magnetic fields in galaxies

Disk galaxies, of which the Milky Way is a typical example, possess large scale magnetic fields. The interstellar gas in which the fields reside is removed and replaced on a timescale of about 1/10 of the age of the Galaxy, and the new gas must acquire a magnetic field of the correct strength and direction on this timescale in order to maintain a steady state. The outstanding problem facing all theories for the origin and evolution of galactic magnetic fields is identifying the mechanisms for magnetic diffusion. Diffusion is necessary to prevent the buildup of magnetic energy at the resistive scale, to magnetically assimilate new material, and to remove the field from the galaxy. Some of these processes require topological changes in the field, and necessitate and resistive or collisionless effects. Others only require that the field diffuse with respect to the center of mass, and could occur through ion-neutral drift. In addition to diffusion, there must be a mechanism for maintaining the large scale field.