Ergospheric Disk Dynamos

Abstract

Since there is no unipolar induction associated with the event horizon, in order to elucidate the details of the ergospheric dynamo one can look at the GHM interaction of flux tubes that thread the equatorial plane of the ergosphere and therefore not the horizon. In this chapter we look at a highly idealized model in which a disk of ergospheric plasma is the dynamo (see Fig. 8.1). This has two simplifying features. Firstly, the equatorial disk acts as an MHD piston and thus it is essentially a boundary surface for the paired MHD wind system. Secondly, there is a direct analogy between the ergospheric disk and the Faraday wheel. The model has the same causal structure that was outlined in Chap. 7. This example clearly illustrates how the rotational energy of the hole powers the outgoing wind. The solution emphasizes the crucial role of the Alfvén wave radiation from the dynamo and strong dissipative cross-field currents in the dynamo region.