Effect of Sun‐aligned arcs on the polar thermosphere

Sun‐aligned polar cap arcs cause regions of enhanced ionization that often appear when the interplanetary magnetic field is near zero or northward. They are typically associated with a shear in the convection electric field, which leads to electron precipitation and plasma density 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. Because the arcs offer resistance to the basically antisunward thermospheric 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 model predicts that the largest perturbations occur when the arcs are associated with a sheared electric field and when multiple arcs are present. In this case, there are both density and temperature enhancements on the upstream side of the arcs and depressions on the downstream side. There is about a 20% neutral density variation and about a 100°K neutral temperature variation across the arcs.