In the MHD description of plasma phenomena the concept of magnetic fieldlines frozen into the plasma turns out to be very useful. We present here amethod of introducing Lagrangian coordinates into relativistic MHD equations ingeneral relativity, which enables a convenient mathematical formulation for thebehaviour of flux tubes. With the introduction of these Lagrangian, so--called``frozen--in'' coordinates, the relativistic MHD equations reduce to a set ofnonlinear 1D string equations, and the plasma may therefore be regarded as agas of nonlinear strings corresponding to flux tubes. Numerical simulationshows that if such a tube/string falls into a Kerr black hole, then the leadingportion loses angular momentum and energy as the string brakes, and tocompensate for this loss, momentum and energy is radiated to infinity toconserve energy and momentum for the tube. Inside the ergosphere the energy ofthe leading part turns out to be negative after some time, and the rest of thetube then gets energy from the hole. In our simulations most of the compensatedpositive energy is also localized inside the ergosphere because the inwardspeed of the plasma is approximately equal to the velocity of the MHD wavewhich transports energy outside. Therefore, an additional physical process hasto be included which can remove energy from the ergophere. Magneticreconnection seems fills this role releasing Maxwellian stresses and producinga relativistic jet.
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