Limit shocks of relativistic magnetohydrodynamics, Punsly's waveguide   and the Blandford-Znajek solution
  In this paper we examine various issues closely related to the ongoing discussion on the nature of the Blandford-Znajek mechanism of extraction of rotational energy of black holes. In particular, we show that switch-on and switch-off shocks are allowed by the shock equations of relativistic MHD and have similar properties to their Newtonian counterparts. Just like in Newtonian MHD they are limits of fast and slow shock solutions and as such they may be classified as weakly evolutionary shocks. The analysis of Punsly's MHD waveguide problem shows that its solution cannot have the form of a traveling step wave and that both fast and Alfv\'en waves are essential for generating the flow in the guide. Causality considerations are used to argue that the Blandford-Znajek perturbative solution is in conflict with the membrane paradigm. An alternative interpretation is presented according to which the role of an effective unipolar inductor in the Blandford-Znajek mechanism is played by the ergospheric region of a rotating black hole. Various implications of this are discussed. 
S.S. Komissarov