8.13. The dynamo effect in magnetohydrodynamic accretion onto a rotating black hole

Extract

A rotaing black hole has interesting features which can't be seen in nonrotating cases. The most characteristic one is to induce differential rotaions at angular velocity ω in inertial frames. Astrophysically it might have various impacts on its surrounding accretion plasmas. In a stationary and axisymmetric magnetohydrodynamic picture, it enables to directly extract the rotation energy of the black hole in the form of the outgoing Poynting flux. This process needs a condition Ω_F(Ω_H – Ω_F) > 0 on the horizon r = r_H, where Ω_H ≡ ω|r_H, and Ω_F is the angular velocity of magnetosphere. However, it is not so clear how the extracted energy activates the magnetosphere. Recently, Kahnna and Camenzind (KC 1994,1995) proposed a possibility of a self-excitation mechanism of the electromagnetic fields, supported by a coupling between ω and the angular velocity of plasma Ω, through the magnetic diffusivity e of accretion plasmas. They called it ωΩ dynamo and tried to confirm this effect by numerical simulations. However no such growing cases were found in the initial conditions employed in the simulations (Brandenbrug 1996, KC 1996).