SEARCH

SEARCH BY CITATION

We use measurements from the retarding potential analyzer and ion drift meter on the DE 2 spacecraft to examine the density and velocity structure of 225 northern hemisphere polar ionization patches. An ionization patch is loosely defined as a large-scale (≧ 100 km) region where the F region plasma density is significantly (≧ 100%) above the background level. We examine the occurrence frequency of ionization patches as a function of interplanetary magnetic field (IMF), season, and UT. While for all values of Bz patches tend to occur mostly in the northern hemisphere winter, when Bz <0, the greatest frequency of patch occurrence is in the 1000–2000 UT range, and when Bz >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 with time. A distinct IMF variation in the velocity structure of patches was found with a much lower level of velocity structure during southward IMF conditions. As patches convect across the polar cap, intermediate-scale (∼15 km) irregularities are found both on the edges of and inside a patch. The irregularity level is found to be only slightly higher on the trailing edge of a patch than the leading edge. Examination of the average density gradient on the leading and trailing edges of convecting patches shows that the leading edge tends to be steeper. This information is related to current theoretical mechanisms on the generation and evolution of patches.