We present results of a new model of the time-dependent ionospheric response to a generalized steady state Sun-aligned (SA) arc structure. The thermal and plasma structure of a ''prototype'' arc is compared to the general features of observed SA arcs. We find that the general features of electron density, Ne, electron temperature, Te, and ion temperature, Ti, are determined by the distribution of the particle precipitation and E¿B convection associated with the SA arc. The model results are extended to predict the possible variation of Ne, Te, and Ti due to such arcs at Defense Meteorological Satellite Program altitude. An important parameter in determining the overall density enhancement produced by a SA arc is the speed of plasma flow across the arc. This is demonstrated by calculating the delta total electron content, ΔTEC, enhancement for various values of cross flow. Since cross-flow is a relatively hard measurement to make, observations of ΔTEC associated with SA arcs can help place limits on the magnitude of the cross-flow velocities for SA arcs. In conjunction with the National Science Foundation Coupling Energetic Dynamics of the Atmospheric Regions/High-Latitude Plasma Structure initiative, we compare the results of the model with coordinated observations of several SA arcs which 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.