  This Letter analyzes 3-dimensional simulations of Kerr black hole
magnetospheres that obey the general relativistic equations of perfect
magnetohydrodynamics (MHD). Particular emphasis is on the event horizon
magnetosphere (EHM) which is defined as the the large scale poloidal magnetic
flux that threads the event horizon of a black hole (This is distinct from the
poloidal magnetic flux that threads the equatorial plane of the ergosphere,
which forms the ergospheric disk magnetosphere). Standard MHD theoretical
treatments of Poynting jets in the EHM are predicated on the assumption that
the plasma comprising the boundaries of the EHM plays no role in producing the
Poynting flux. The energy flux is electrodynamic in origin and it is
essentially conserved from the horizon to infinity, this is known as the
Blandford-Znajek (B-Z) mechanism. To the contrary, within the 3-D simulations,
the lateral boundaries are strong pistons for MHD waves and actually inject
prodigious quantities of Poynting flux into the EHM. At high black hole spin
rates, strong sources of Poynting flux adjacent to the EHM from the ergospheric
disk will actually diffuse to higher latitudes and swamp any putative B-Z
effects. This is in contrast to lower spin rates, which are characterized by
much lower output powers and modest amounts of Poynting flux are injected into
the EHM from the accretion disk corona.
