ON THE FORMATION OF CURRENT SHEETS IN THE SOLAR CORONA

Several theoretical studies have proposed that, in response to photospheric footpoint motions, current sheets can be generated in the solar corona without the presence of a null point in the initial potential magnetic field. In these analytic models, current sheets form wherever the coronal magnetic field dips down and is parallel to the photosphere. A fundamental assumption in these analyses - commonly referred to as the linetying assumption - is that all coronal field lines are anchored to a boundary surface representing the top of the dense, gas pressure-dominated photosphere. In the present work, however, we show that line-tying cannot be applied indiscriminately to "dipped" coronal fields and hence that the conclusions of the line-tied models are incorrect. We contend that current sheets will not form if the photosphere-corona interface is represented by a physically valid model. To support our theoretical arguments, we have investigated numerically the response of a "dipped" potential magnetic field in a hydrostatic-equilibrium atmosphere to shearing motions of the footpoints, using a new 2.5-dimensional magnetohydrodynamic (MHD) code. Our results show that, in the absence of artificial line-tying conditions, a current sheet indeed does not form at the location of the dip. Rather, the dipped magnetic field rises, causing upflows of photospheric and chromospheric plasma.