The 'meissner effect' and the Blandford-Znajek mechanism in conductive black hole magnetospheres

Abstract

The expulsion of axisymmetric magnetic fields from the event horizons of rapidly rotating black holes has been seen as an astrophysically important effect that may significantly reduce or even nullify the efficiency of the Blandford-Znajek mechanism of powering the relativistic jets in active galactic nuclei and gamma-ray bursts. However, this Meissner-like effect is seen in the vacuum solutions of black hole electrodynamics, whereas the Blandford-Znajek mechanism is concerned with plasma-filled magnetospheres. In this Letter we argue that conductivity dramatically changes the properties of axisymmetric electromagnetic solutions-even for a maximally rotating Kerr black hole, the magnetic field is pulled inside the event horizon. Moreover, the conditions resulting in an outgoing Poynting flux in the Blandford-Znajek mechanism exist not on the event horizon but everywhere within the black hole ergosphere. Thus, the 'Meissner effect' is unlikely to be of interest in the astrophysics of black holes, at least not in the way this has been suggested so far. These conclusions are supported by the results of time-dependent numerical simulations that used three different computer codes. The test problems involve black holes with the rotation parameter ranging from a = 0.999 to a = 1. The pure electrodynamic simulations deal with the structure of conductive magnetospheres of black holes placed in a uniform-at-inflnity magnetic field (Wald's problem) and the magne-tohydrodynamic simulations are used to study the magnetospheres arising in the problem of disc accretion. © 2007 The Authors. Journal compilation © 2007 RAS.