Topology and Large Scale Magnetic Fields

Abstract: Fluids with small resistivities (e.g. stars, galaxies, ionized accretion disks) robustly preserve magnetic helicity even in the presence of turbulence. This has been used to set limits on kinematics dynamos in astrophysics. I will discuss how differential rotation drives a magnetic helicity flux and how this leads to large scale dynamos. This mechanism always dominates over kinematic dynamos. The latter are not relevant in astrophysical settings. I will discuss the implications for dynamos in stars and show how this explains the scaling of magnetic field strengths with rotation. If time allows I will also discuss the origin of galactic magnetic fields and the implications of this work for computational simulations of accretion disks.