STRUCTURE AND OCCURRENCE OF POLAR IONIZATION PATCHES

We use measurements from the retarding potential analyzer and ion drif t meter on the DE 2 spacecraft to examine the density and velocity str ucture of 225 northern hemisphere polar ionization patches. An ionizat ion patch is loosely defined as a large-scale (greater than or equal t o 100 km) region where the F region plasma density is significantly (g reater than or equal to 100%) above the background level. We examine t he occurrence frequency of ionization patches as a function of interpl anetary magnetic field (IMF), season, and UT. While for all values of B-z patches tend to occur mostly in the northern hemisphere winter, wh en B-z<0, the greatest frequency of patch occurrence is in the 1000-20 00 UT range, and when B-z>0, the UT distribution is much more uniform We also examine the plasma velocity structure inside and conjoint with patches as a function of IMF in order to determine their stability wi th time. A distinct IMF variation in the velocity structure of patches was found With a much lower level of velocity structure during southw ard IMF conditions. As patches convect across the polar cap, intermedi ate-scale (similar to 15 km) irregularities are found both on the edge s of and inside a patch. The irregularity level is found to be only sl ightly higher on the trailing edge of a patch than the leading edge. E xamination of the average density gradient on the leading and trailing edges of convecting patches shows that the leading edge tends to be s teeper. This information is related to current theoretical mechanisms on the generation and evolution of patches.