Li LX
Astrophys J. 2000 Mar 10;531(2):L111-L114. doi: 10.1086/312538.
When magnetic field lines thread a rotating black hole's horizon and connect with remote astrophysical loads, the rotational energy of the black hole can be extracted through the Blandford-Znajek mechanism. Due to the rotation of the black hole, the magnetic field lines are twisted and toroidal components are generated. Thus, poloidal electric currents are induced and the black hole's rotational energy is transported to the astrophysical loads through Poynting flux. The Blandford-Znajek mechanism has been considered to be a possible process for powering extragalactic jets. In this Letter, we show that because of the screw instability of the magnetic field, the toroidal components of the magnetic field, and thus the poloidal currents, cannot exceed the limits given by the Kruskal-Shafranov criterion. This significantly lowers the power of the Blandford-Znajek mechanism when the loads are far from the black hole. So the Blandford-Znajek mechanism can only work efficiently within the neighborhood of the black hole. The implications of the results for the scenario of extragalactic jets powered by the Blandford-Znajek mechanism are discussed.
当磁力线穿过旋转黑洞的视界并与遥远的天体物理负载相连时，黑洞的旋转能量可通过布兰福德-兹纳耶克机制提取。由于黑洞的旋转，磁力线会发生扭曲并产生环形分量。因此，会感应出极向电流，黑洞的旋转能量通过坡印廷通量传输到天体物理负载。布兰福德-兹纳耶克机制被认为是为河外喷流提供能量的一种可能过程。在本快报中，我们表明，由于磁场的螺旋不稳定性，磁场的环形分量以及极向电流不能超过由克鲁斯卡-沙夫拉诺夫判据给出的极限。当负载远离黑洞时，这会显著降低布兰福德-兹纳耶克机制的功率。所以，布兰福德-兹纳耶克机制只能在黑洞附近有效工作。我们还讨论了这些结果对由布兰福德-兹纳耶克机制驱动的河外喷流情形的影响。
