  Mean field dynamos may explain the origin of large scale magnetic fields of
galaxies, but controversy arises over the extent of dynamo quenching by the
growing field. Here we explain how apparently conflicting results may be
mutually consistent, by showing the role of magnetic helicity conservation and
boundary terms usually neglected. We estimate the associated magnetic energy
flowing out of the Galaxy but emphasize that the mechanism of field escape
needs to be addressed.

Astrophysics