ADAPTIVE IDENTIFICATION AND CHARACTERIZATION OF POLAR IONIZATION PATCHES

Dynamics Explorer 2 (DE 2) spacecraft data are used to detect and char acterize polar cap ''ionization patches'', loosely defined as large-sc ale (>100 km) regions where the F region plasma density is significant ly enhanced (greater than or similar to 100%) above the background lev el. These patches are generally believed to develop in or equatorward of the dayside cusp region and then drift in an antisunward direction over the polar cap. We have developed a flexible algorithm for the ide ntification and characterization of these structures, as a function of scale-size and density enhancement, using data from the retarding pot ential analyzer, the ion drift meter, and the langmuir probe on board the DE 2 satellite. This algorithm was used to study the structure and evolution of ionization patches as they cross the polar cap. The resu lts indicate that in the altitude region from 240 to 950 km ion densit y enhancements greater than a factor of 3 above the background level a re relatively rare. Further, the ionization patches show a preferred h orizontal scale size of 300 - 400 km. There exists a clear seasonal an d universal time dependence to the occurrence frequency of patches wit h a northern hemisphere maximum centered on the winter solstice and th e 1200-2000 UT interval.