EFFECT OF POLAR-CAP PATCHES ON THE THERMOSPHERE FOR DIFFERENT SOLAR-ACTIVITY LEVELS

Polar cap patches are regions of enhanced ionization that appear when the interplanetary magnetic field is southward. They are created eithe r in or near the cusp and then convect in an antisunward direction acr oss the dark polar cap at speeds of from 100 m/s to about 2 km/s. The size of these plasma patches varies from about 100 to 1000 km, and the ir density Varies from 10 percent to over an order of magnitude relati ve to background ionospheric densities. To determine their effect on t he thermosphere, a time-dependent global circulation model was used to simulate the thermospheric response to a plasma patch for both solar maximum and minimum conditions. The simulations indicate that for both solar maximum and minimum a propagating plasma patch acts as a collis ional snowplow, creating a hole in the thermosphere in and behind the patch and a density enhancement at the front of the patch. The neutral temperature and wind are also increased. For plasma patches of simila r strength (same horizontal and vertical dimensions and peak-to-backgr ound density ratio), there is a stronger and more localized thermosphe ric disturbance at solar minimum than at solar maximum. In fact, the n eutral density enhancement at the front of a moderate patch is negligi bly small at solar maximum even though the density depletion is still evident. These features can be explained by the lower neutral pressure and lower collision frequency in the neutral gas at solar minimum com pared to solar maximum. (C) 1997 Elsevier Science Ltd.