Axisymmetric Dynamos in Differentially Rotating Spacetimes
American Astronomical Society, AAS Meeting #221, #143.06
Published in Jan 2013
Accretion onto black holes is widely believed to drive jets and outflows when magnetic fields are present, as well as to be affected by magnetic fields through mechanisms such as the magnetorotational instability. Unfortunately the source of magnetic fields in accretion disks is still not well understood. Shear amplification of magnetic fields from tidally disrupted stars far from the black holes is probably insufficient to drive jets, but in or near the ergosphere amplification may become significant. Amplification of magnetic fields in plasmas is termed dynamo action. In a flat spacetime axisymmetric dynamo action cannot occur. This well known result is commonly referred to as Cowling's anti-dynamo theorem. However, in the Kerr spacetime around a rotating black hole violations of this theorem may be possible and the differential rotation of spacetime itself may be able to drive a dynamo. We examine the kinematic dynamo problem for a general relativistic magnetohydrodynamic (GRMHD) dynamo near a rotating black hole using a 3+1 foliation of spacetime. We seek physically acceptable growing modes and discuss effects deriving from the generalized GRMHD Ohm's law.
(c) 2013: American Astronomical Society