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 eithe r in the dayside cusp or equatorward of the cusp in the sunlit hemisph ere. 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 pla sma patch varies from about 100 to 1000 km, and its density relative t o the background density varies from a few tens of percent to a factor of 100. Because propagating plasma patches might have a significant e ffect on the neutral atmosphere, a global thermospheric circulation mo del was used to calculate the response of the polar thermosphere to a ''representative'' plasma patch. The model predicts that a localized t hermospheric disturbance is induced by and moves along with the propag ating plasma patch. The moving disturbance is characterized by a neutr al density depletion, an enhanced wind speed, an elevated neutral temp erature, neutral gas upwelling, and O/N-2 composition changes. The the rmospheric disturbance persists for a time that is much longer than th e patch lifetime, and it spreads out over a region that is much larger than the patch dimensions as it dissipates.