Energy extraction from a rotating black hole via magnetic reconnection: the plunging bulk plasma and orientation angle

Magnetic reconection provides a new avenue to extract energy from a rotating black hole in astrophysical scenarios. There have been many works studying the feasibility of extracting energy via magnetic reconnection in the scheme of magnetohydrodynamics. However, most of them focused on circularly flowing bulk plasma only, and the influence of orientation angle, the angle between flowing direction of bulk plasma and ejection direction of plasmoids, was never carefully considered. In this work, we would like to study the energy extraction via magnetic reconnection, via the so-called Comisso-Asenjo process specifically, in the plunging plasma. More importantly, we analyze the influence of orientation angle in depth. Actually, we consider the magnetic reconnection occurring in bulk plasma with two kinds of streamlines, one being the circular flow and the other being the plunging flow from the ISCO. We find that the plunging plasma has higher energy-extraction efficiency. Moreover, we notice that it is favorable for energy extraction when the orientation angle is suitably increased if the bulk plasma plunges. We further define the covering factor. Under the assumption that the magnetic reconnection occurs equally probable along any direction on any position within ergosphere, this value help us to primitively judge the probability of energy extraction via magnetic reconnection, through which we conclude that the plunging bulk plasma is much more capable in energy extraction than the circularly flowing one.