EFFECT OF SUN-ALIGNED ARCS ON THE POLAR THERMOSPHERE

Sun-aligned polar cap arcs cause regions of enhanced ionization that o ften appear when the interplanetary magnetic field is near zero or nor thward. They are typically associated with a shear in the convection e lectric field, which leads to electron precipitation and plasma densit y enhancements that can be up to 10 times background plasma densities. The Sun-aligned arcs are 100-300 km in width and 1000 km in length. B ecause the arcs offer resistance to the basically antisunward thermosp heric flow, they could cause an appreciable thermospheric perturbation . To determine their effect, a time-dependent, three-dimensional, high -resolution model of the global thermosphere was used to calculate the thermospheric response to ''representative'' Sun-aligned arcs. The mo del predicts that the largest perturbations occur when the arcs are as sociated with a sheared electric field and when multiple arcs are pres ent. In this case, there are both density and temperature enhancements on the upstream side of the arcs and depressions on the downstream si de. There is about a 20% neutral density variation and about a 100 deg rees K neutral temperature variation across the arcs.