Effect of polar cap patches on the polar thermosphere

Polar cap patches are regions of enhanced ionization that appear when the interplanetary magnetic field is southward. They are created either in the dayside cusp or equatorward of the cusp in the sunlit hemisphere. Once formed, they convect in an antisunward direction across the dark polar cap at speeds of 100 m/s to about 2 km/s. The size of a plasma patch varies from about 100 to 1000 km, and its density relative to the background density varies from a few tens of percent to a factor of 100. Because propagating plasma patches might have a significant effect on the neutral atmosphere, a global thermospheric circulation model was used to calculate the response of the polar thermosphere to a “representative” plasma patch. The model predicts that a localized thermospheric disturbance is induced by and moves along with the propagating plasma patch. The moving disturbance is characterized by a neutral density depletion, an enhanced wind speed, an elevated neutral temperature, neutral gas up welling, and O/N2 composition changes. The thermospheric disturbance persists for a time that is much longer than the patch lifetime, and it spreads out over a region that is much larger than the patch dimensions as it dissipates.