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

ABSTRACT We present the first kinematic study of an -dynamo in the General Relativistic Magneto-HydroDynamics (GRMHD) regime, applied to thick disks orbiting around Kerr black holesand using a fully covariant mean field dynamo closure for the Ohm law. We show that the -dynamo mechanism leads to a continuous exponential growth of the magnetic field within thedisk and to the formation of dynamo waves drifting away or toward the equatorial plane. Sincethe evolution of the magnetic field occurs qualitatively in the same fashion as in the Sun, wepresent also butterfly diagrams that characterize our models and show the establishment of anadditional timescale, which depends on the microscopic properties of the turbulent motions,possibly providing an alternative explanation to periodicities observed in many high-energyastrophysical sources where accretion onto a rotating black hole is believed to operate.Key words: magnetic fields – MHD – dynamo – accretion – relativistic processes – plasmas. 1 INTRODUCTIONOrdered, large-scale magnetic fields are believed to be a fundamen-tal ingredient of the accretion processes that power many of theastrophysical sources of high-energy emission, like jets from Ac-tive Galactic Nuclei (McKinney & Blandford 2009) or Gamma RayBursts (Bucciantini et al. 2009; Rezzolla et al. 2011). This is par-ticularly true for the Blandford-Znajek mechanism (Blandford ZSantos-Lima, de Gouveia Dal Pino & Lazarian 2012) have shownhow the magnetic braking due to a preexisting large-scale magneticfield can extract angular momentum from the cloud fast enough toprevent the formation of a disk. Turbulent reconnection can effi-ciently inhibit the magnetic braking by removing magnetic flux,and therefore a turbulent magnetic field allows the formation ofa disk. However, turbulent motions and small-scale instabilities inaccretion disks, such as the Magneto-Rotational Instability (MRI)(Balbus 2003), could enhance the level of turbulent magnetic fields,but the tangled final configuration would not be able to drive largescale outflows.