Polar cap patches are regions of enhanced ionization
that appear when the interplanetary magnetic field is southward. They are created either in or near the cusp and then convect in an antisunward direction across 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 their density varies from 10 percent to over an order of magnitude relative to background ionospheric densities. To determine their effect on the 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 collisional 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 similar strength (same horizontal and vertical dimensions and peak-to-background density ratio), there is a stronger and more localized thermospheric disturbance at solar minimum than at solar maximum. In fact, the neutral density enhancement at the front of a moderate patch is negligibly 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 compared to solar maximum.