Dynamo action in thick discs around Kerr black holes: High-order resistive GRMHD simulations

Abstract

We present the first kinematic study of an αω-dynamo in the general relativistic magnetohydrodynamics regime, applied to thick discs orbiting around Kerr black holes and using a fully covariant mean-field dynamo closure for the Ohm law. We show that the αω-dynamo mechanism leads to a continuous exponential growth of the magnetic field within the disc and to the formation of dynamo waves drifting away or towards the equatorial plane. Since the evolution of the magnetic field occurs qualitatively in the same fashion as in the Sun, we present also butterfly diagrams that characterize our models and show the establishment of an additional time-scale, which depends on the microscopic properties of the turbulent motions, possibly providing an alternative explanation to periodicities observed in many high-energy astrophysical sources where accretion on to a rotating black hole is believed to operate. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.