Magnetic Helicity and Dynamos in Galaxies

Magnetic helicity is a robustly conserved topological invariant, even in the presence of turbulence. It produces a back reaction which suppresses kinematic dynamos. In shearing and rotating objects, like galaxies, turbulence produces a magnetic helicity flux, aligned with the axis of the system. The resulting accumulation of magnetic helicity dominates over the kinetic helicity in one eddy turnover time and drives a dynamo which can grow much more rapidly than a conventional kinematic dynamo. We will discuss the implications for the early history of galactic magnetic fields and their saturation strengths.