PATCHES IN THE POLAR IONOSPHERE - UT AND SEASONAL DEPENDENCE

The seasonal and UT dependencies of patches in the polar ionosphere ar e simulated using the Utah State University time dependent ionospheric model (TDIM). Patch formation is achieved by changing the plasma conv ection pattern in response to temporal changes in the interplanetary m agnetic field (IMF) B(y), component during periods of southward IMF. T his mechanism redirects the plasma flow from the dayside high-density region, which is the source of the tongue of ionization (TOI) density feature, through the throat and leads to patches, rather than a contin uous TOI. The model predicts that the patches are absent at winter sol stice (northern hemisphere) between 0800 and 1200 UT and that they hav e their largest seasonal intensity at winter solstice between 2000 and 2400 UT. Between winter solstice and equinox, patches are strong and present all day. Patches are present in summer as well, although their intensity is only tens of percent above the background density. These winter-to-equinox findings are also shown to be consistent with obser vations. The model was also used to predict times at which patch obser vations could be performed to determine the contributions from other p atch mechanisms. This observational window is +/- 20 days about winter solstice between 0800 and 1200 UT in the northern hemisphere. In this observational window the TOI is either absent or reduced to a very lo w density. Hence the time dependent electric field mechanism considere d in this study does not produce patches, and if they are observed, th en they must be due to some other mechanism.