Magnetohydrodynamic models of coronal transients in the meridional plane. I - The effect of the magnetic field

The dynamic interaction between the magnetic field and the material ejected during a solar event is examined in the meridional plane with the use of numerical solutions of the time-dependent magnetohydrodynamic (MHD) equations of motion. The solar event is simulated by perturbations in the thermodynamic variables (pressure, density, and temperature) at the base of an initially hydrostatic atmosphere. Two magnetic field configurations are considered: one configuration allows the ejected material to flow essentially along the field lines to the outer corona (open configuration). and the other retards the outward motion by the buildup of Lorentz forces (closed configuration). The effect of the magnitude of the magnetic field is also considered. We use magnetic fields with magnitudes at the coronal base such that B (the ratio of thermal pressure to magnetic pressure) has the values of 1 and 0.1 at the base in the ''pre-event'' state. For all cases investigated, a disturbance propagates upward and laterally through the corona, with the leading edge of the disturbance having the shape of an expanding loop. The lateral motion is appreciably less for the low B, open configuration than for the other cases, with the major portion of the distrubance being confined to amore » region directly over the solar even. The leading portion of the disturbance contains only coronal material whose properties have been altered by the preceding waves (which may strengthen into shocks) created by the explosive nature of the solar event. For both values of ..beta.. the closed configuration does not allow any of the material ejected in the event to flow to the outer corona; the ejecta merely rises above the surface for a short time and then falls downward. In the open configuration the distrubance is preceded by a fast-mode MHD shock for ..beta..=1 and a fast-mode MHD wave for ..beta..=0.1. The coronal medium is set in motion by the MHD wave.« less