THEORETICAL-STUDY OF ONSET CONDITIONS FOR SOLAR ERUPTIVE PROCESSES - INFLUENCE OF THE BOUNDARIES

We investigate the influence of the finite extent of the computational domain and of specific boundary conditions on a theoretical model for solar eruptive processes originally proposed by Zwingmann (1987). In this model, the slow pre-onset time evolution of arcade-like solar coronal magnetic field structures is described by quasi-static equilibrium sequences. The magnetic field is represented by Euler potentials which allow for a realistic description of the photospheric boundary conditions, because the pressure and the magnetic footpoint displacement can be prescribed separately. We use an improved numerical method suitable for computing equilibrium sequences, allowing for larger domains and higher resolution than used in the previous work. With this method, we are able to show that, in contradiction to a supposition made by Zwingmann (1987), the results of the computations do strongly depend on the size of the computing domain. This has consequences for a possible physical interpretation of the model. We furthermore show that with the boundary conditions used in this model a shearing motion of the magnetic footpoints inevitably leads to the formation of singular current layers at the separatrix between field lines cutting the upper boundary (open field lines) and field lines which are only connected with the photosphere (closed held lines).