Ffde

[Yi Pressimone]

Recent simulations of force-free, degenerate electrodynamic (ffde) black hole magnetospheres indicate that the fast mode radiated from (or near) the event horizon can modify the global potential difference in the poloidal direction orthogonal to the magnetic field, V, in a black hole magnetosphere. By contrast, in MHD (meaning perfect magnetohydrodynamics hereafter), a combination of Alfvn and fast waves are required to alter V in a magnetosphere. This distinction is significant because it changes the causal boundary surface for evolving V from the event horizon (the inner fast critical surface in ffde) to the inner Alfvn critical surface. Second, there is a fundamental contradiction in a wave that alters V coming from near the horizon. The background fields in ffde satisfy the ``ingoing wave condition'' near the horizon (which arises from the requirement that all matter is ingoing at the event horizon), yet outgoing waves are radiated from this region in the simulation. The resolution of these two issues is important to our understanding of causality in black hole magnetospheres and ffde as a faithful representation of tenuous MHD near a black hole. Studying the properties of the waves in the simulations are useful tools to this end. It is shown that the regularity of the stress-energy tensor in a freely falling frame requires that the outgoing (as viewed globally) waves near the event horizon are redshifted away and are ineffectual at changing V. It is also concluded that waves in massless MHD (ffde) are extremely inaccurate depictions of waves in a tenuous MHD plasma, near the event horizon, as a consequence of black hole gravity. Any analysis based on ffde near the event horizon is seriously flawed.

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