MODELING SUN-ALIGNED POLAR-CAP ARCS

We present results of a new model of the time-dependent ionospheric re sponse to a generalized steady state Sun-aligned (SA) arc structure. T he thermal and plasma structure of a ''prototype'' arc is compared to the general features of observed SA arcs. We find that the general fea tures of electron density, N(e), electron temperature, T(e), and ion t emperature, T(i), are determined by the distribution of the particle p recipitation and E x B convection associated with the SA arc. The mode l results are extended to predict the possible variation of N(e), T(e) , and T(i) due to such arcs at Defense Meteorological Satellite Progra m altitude. An important parameter in determining the overall density enhancement produced by a SA arc is the speed of plasma flow across th e arc. This is demonstrated by calculating the delta total electron co ntent, DELTATEC, enhancement for various values of cross flow. Since c ross-flow is a relatively hard measurement to make, observations of DE LTATEC associated with SA arcs can help place limits on the magnitude of the cross-flow velocities for SA arcs. In conjunction with the Nati onal Science Foundation Coupling Energetic Dynamics of the Atmospheric Regions/High-Latitude Plasma Structure initiative, we compare the res ults of the model with coordinated observations of several SA arcs whi ch were observed at Qanaq, Greenland, on December 11, 1990. Comparison of our results and observations indicates a relatively low value for the cross-flow speed (less than 25 m s-1) for these SA arcs.