The corona-dynamo connection in accretion disks

The theory of mean-field dynamos in accretion disks has been discussed at length in numerous papers, dating back almost to the first theoretical papers on accretion disks. In the last decade the theory of mean-field dynamos in almost perfectly conducting fluids has been subjected to strong criticisms, based on general theoretical considerations and as well as numerical tests. Paradoxically, the numerical evidence for mean-field dynamo activity in accretion disks, driven by the magnetorotational instability (MRI), has become extremely strong. I will discuss a reformulation of mean-field dynamo theory, based on magnetic helicity conservation, which is consistent with previous theoretical and numerical analyses. Applying this to accretions disks, we find that the MRI drives a magnetic helicity current aligned with the spin axis of the disk. As this current emerges into the disk corona it is carried by magnetic structures of steadily increasing size. Consequently the energy flux associated with the magnetic helicity current drops, leading to the dissipation of energy in the corona. This represents a minimal level of coronal heating A similar dynamo operating in the collisionless plasma of the corona can produce additional coronal heating, in which case the magnetic helicity current driven by the corona will increase the efficiency of the disk dynamo.