ALFVENICALLY DRIVEN SLOW SHOCKS IN THE SOLAR CHROMOSPHERE AND CORONA

We consider, via a numerical simulation, the nonlinear evolution of a single torsional Alfvenic pulse launched from the solar photosphere on a thin vertical magnetic flux tube which extends into an open coronal region. The pulse steepens into a fast shock, and a slow shock is formed in the chromosphere behind the torsional pulse. Some of the Alfvenic energy is reflected downward by the transition region (TR) and by the steep rise of the Alfven speed in the upper chromosphere. However, the slow shock reflects part of this energy upward; the slow shock is thus able to enhance the flux of Alfvenic energy into the corona and to enhance the dynamical effects of the Alfven wave on the TR and upper chromosphere. The energy in the torsional pulse can lead to repeated upward ejections of the TR and underlying chromosphere. After three upward ejections there is a density plateau comparable to the densities found on solar spicules; moreover, shock heating can exceed adiabatic cooling and lead to a modest temperature increase in the plateau. However, the plateau is of insufficient height (and possibly too cool) to be identified with spicules. We find also that the nonlinear dynamics leads to very impulsive behavior, as manifested by the impulsive flux of energy into the corona and by the short-lived but large-amplitude transverse velocities in the corona. It is thus possible that at least some of the events that have been called microflares or explosive events are the consequence of nonlinear wave dynamics.