EFFECT OF HIGH-LATITUDE IONOSPHERIC CONVECTION ON SUN-ALIGNED POLAR CAPS

A coupled magnetospheric-ionospheric (M-I) MHD model has been used to simulate the formation of Sun-aligned polar cap arcs for a variety of interplanetary magnetic field (IMF) dependent polar cap convection fie lds. The formation process involves launching an Alfven shear wave fro m the magnetosphere to the ionosphere where the ionospheric conductanc e can react self-consistently to changes in the upward currents. We as sume that the initial Alfven shear wave is the result of solar wind-ma gnetosphere interactions. The simulations show how the E region densit y is affected by the changes in the electron precipitation that are as sociated with the upward currents. These changes in conductance lead t o both a modified Alfven wave reflection at the ionosphere and the gen eration of secondary Alfven waves in the ionosphere. The ensuing bounc ing of the Alfven waves between the ionosphere and magnetosphere is fo llowed until an asymptotic solution is obtained. At the magnetosphere the Alfven waves reflect at a fixed boundary. The coupled M-1 Sun-alig ned polar cap arc model of Zhu et al. (1993a) is used to carry out the simulations. This study focuses on the dependence of the polar cap ar c formation on the background (global) convection pattern. Since the p olar cap arcs occur for northward and strong B(y)IMF conditions, a var iety of background convection patterns can exist when the arcs are pre sent. The study shows that polar cap arcs can be formed for all these convection patterns; however, the arc features are dramatically differ ent for the different patterns. For weak sunward convection a relative ly confined single pair of current sheets is associated with the impos ed Alfven shear wave structure. However, when the electric field excee ds a threshold, the arc structure intensifies, and the conductance inc reases as does the local Joule heating rate. These increases are faste r than a linear dependence on the background electric field strength. Furthermore, above the threshold, the single current sheet pair splits into multiple current sheet pairs. For the fixed initial ionospheric and magnetospheric conditions used in this study, the separation dista nce between the current pairs was found to be almost independent of th e background electric field strength. For either three-cell or distort ed two-cell background convection patterns the arc formation favored t he positive B(y) case in the northern hemisphere.