Amplification of magnetic fields in accretion discs by GRMHD dynamo

The accretion of magnetized plasma from an accreting torus onto a supermassive black hole has recently received considerable attention thanks to the recent imaging of the shadow of the black hole in M87 by the Event Horizon Telescope (EHT) collaboration. In the regime of general relativistic magnetohydrodynamics (GRMHD) typically adopted, an initial magnetic field, small but certainly not negligible, is amplified by the magnetorotational instability (MRI) that develops in the disk, in turn triggering accretion. Here we show the first results about the amplification of an initial tiny magnetic field in the accretion disk by means of GRMHD mean-field dynamo, in the fully non-linear regime. This process is able to produce an exponential growth of the field up to the values required to explain the observations.