Magnetic fields: Their origin and manifestation in accretion disks around supermassive black holes

Abstract

The magnetic field dynamo in the inner part of accretion disks around supermassive black holes in AGNs may be an important mechanism for the generation of magnetic fields in galaxies and in extragalactic space. We consider dynamo with the necessary helicity generation produced by star-disk collisions. Gas heated by a star passing through the disk is buoyant and form rising and expanding plume of plasma. Due to Coriolis forces the flow produced by plumes have coherent helicity. This helicity is the source of alpha effect in the alpha-O dynamo in differentially rotating accretion disk. We apply the mean field dynamo theory to the ensemble of plumes produced by star-disk collisions. We demonstrate the existence of the dynamo and evaluate the growth rate of magnetic field. The estimate of the nonlinear saturated state of the dynamo gives the magnetic field exceeding equipartition with the thermal energy in the accretion disk. Thus, star-disk collision dynamo can be important in generating dynamically significant magnetic fields, which could alter the disk structure and be the source of the energy flow in extragalactic jets. We present results of numerical simulations of the kinematic dynamo produced by star-disk collisions. It was found that for about one star-disk collision per period of rotation of the inner edge of an accretion disk, the typical value of the threshold magnetic Reynolds number is of the order of 100. The generated mean magnetic field has predominantly even parity. We also present theoretical consideration of magnetic dynamo in New Mexico dynamo experiment, which is currently under construction. The experiment utilizes Couette flow and driven jets of liquid sodium to simulate astrophysical alpha-O dynamos in the laboratory. We perform numerical simulations with ideally conducting boundary and evaluate the changes, which vacuum boundary conditions introduce in our numerical results. We also develop the theory of the MHD Ekman boundary layer in differentially rotating conducting fluid. The Ekman layer is formed at the end plates in the experiment. We show that the Ekman layer does not influence the generation of the large scale magnetic field in the experiment.