Coronal activity as a consequence of dynamos in astrophysical rotators

Abstract

We show that the presence of a steady $\ao$ dynamo in astrophysical rotators likely leads to an outflow of relative magnetic helicity and thus magnetic energy available for particle acceleration in a corona. The connection between energy and magnetic helicity arises because the generation of large scale field is linked to a generation of large scale magnetic helicity. In a steady state, the relative magnetic helicity associated with the large scale field can escape, accompanied by an equal and opposite small scale contribution from the field, since the total magnetic helicity in large magnetic Reynolds number flows is conserved. From the helicity flow, a lower limit on the magnetic energy deposited in the corona can be estimated. Steady coronal activity and the dissipation of magnetic energy is therefore a signature of an internal dynamo. Our theoretical estimate of the power delivered by a mean field dynamo is consistent with that inferred from observations to be delivered to the solar corona, the Galactic corona, and Seyfert I AGN coronae.