Formation of current sheets in structurally stable and structurally unstable magnetic configurations with two null lines

The nonlinear dynamics of magnetoacoustic and Alfven Mt-ID perturbations in structurally unstable magnetic configurations with two null lines (X-lines ) is studied both analytically and numerically. It is shown that these pert urbations cause the electric current to evolve nonlinearly in such a manner that a structurally unstable configuration of the magnetic field transform s into a structurally stable configuration. Such a transformation is forbid den in ideal magnetohydrodynamics but can occur in the process of magnetic field line reconnection. The final magnetic configuration to which the syst em evolves is shown to contain no separatrices connecting the null lines. ( C) 2000 MAIK "Nauka/Interperiodica".