MACROSCALE MODELING AND MESOSCALE OBSERVATIONS OF PLASMA-DENSITY STRUCTURES IN THE POLAR-CAP

The seasonal and UT variation of mesoscale structures (10 km - 100 m) in the central polar cap has been obtained from an analysis of 250-MHz intensity scintillation observations made at Thule, Greenland. It has been established earlier [Su. Basu et al., 1990] that mesoscale struc tures causing scintillations of satellite signals may develop at the e dges of macroscale structures (several hundred km) such as discrete po lar cap plasma density enhancements or patches through the gradient dr ift instability process. As such, we examined the seasonal and UT vari ation of polar cap patches simulated by using the USU Time Dependent I onospheric Model (TDIM) under conditions of southward BZ. A fairly rem arkable similarity is found between the scintillation observations and the model predictions of patch occurrence. For instance, both the pat ch and scintillation occurrences are minimized during the winter solst ice (northern hemisphere) between 0800-1200 UT while also having their largest seasonal intensity between 2000-2400 UT. Little UT dependence of patches and scintillations is seen at equinox with high intensity being observed throughout the day, while during local summer the inten sity of macroscale patches and mesoscale irregularities are found to b e a minimum at all UT. These results indicate that macroscale features in the polar cap are routinely associated with plasma instabilities g iving rise to smaller scale structures and that the specific patch for mation mechanism assumed in the simulation is consistent with the obse rvations. This ability to bridge between macroscale modeling and mesos cale observations provides a natural framework for the modeling of mes oscale structures themselves. This mesoscale modeling, in rum can be u tilized in a variety of radar and communication systems applications i n the polar region.