Physics
Scientific paper
Aug 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...467..105p&link_type=abstract
Astrophysical Journal v.467, p.105
Physics
10
Black Hole Physics, Magnetohydrodynamics: Mhd, Waves
Scientific paper
This article presents a calculation of the wave nature of outgoing fast modes near the event horizon of a Kerr black hole magnetosphere. It is found that such waves become more and more feeble propagators of energy and angular momentum electromagnetic fluxes the closer to the hole they are. This result invalidates recent conjectures that the Blandford-Znajek model of electrodynamic energy extraction from a black hole is causally sound as a result of fast waves transporting enormous magnetic stresses from near the horizon to asymptotic infinity.
Alternatively phrased, the perfect magnetohydrodynamic (MHD) relations at the horizon cannot be used even as an approximate causal boundary condition in a calculation that determines the power radiated from a black hole magnetosphere. This new insight accentuates the role of the ergospheric physics farther out from the event horizon in determining any putative unipolar inductive (current generator) properties of a black hole magnetosphere. It is shown that the "dragging of inertial frames" in the ergosphere can only provide the microphysics appropriate to driving cross-field poloidal currents (as in a laboratory unipolar inductor). Again, this is in contradiction to the Blandford-Znajek mechanism, which is a predominantly field-aligned poloidal current flow throughout the ergosphere and outgoing wind zone.
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