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 cor onal magnetic field structures is described by quasi-static equilibriu m sequences. The magnetic field is represented by Euler potentials whi ch allow for a realistic description of the photospheric boundary cond itions, because the pressure and the magnetic footpoint displacement c an be prescribed separately. We use an improved numerical method suita ble for computing equilibrium sequences, allowing for larger domains a nd higher resolution than used in the previous work. With this method, we are able to show that, in contradiction to a supposition made by Z wingmann (1987), the results of the computations do strongly depend on the size of the computing domain. This has consequences for a possibl e physical interpretation of the model. We furthermore show that with the boundary conditions used in this model a shearing motion of the ma gnetic footpoints inevitably leads to the formation of singular curren t layers at the separatrix between field lines cutting the upper bound ary (open field lines) and field lines which are only connected with t he photosphere (closed held lines).