Seasonal and universal time distribution of patches in the northern and southern polar caps

Ion density measurements from the Dynamics Explorer 2 and Defense Meteorolo gical Satellite Program F8 and F9 satellites are used to examine hemispheri cal differences in the occurrence patterns of polar ionization patches as a function of season, universal time (UT), and interplanetary magnetic field (IMF). When B-z < 0, the greatest frequency of patch occurrence in the nor thern hemisphere is in the winter in the 1000-2200 UT range. This time corr esponds to the interval when the northern magnetic pole (and hence the cusp ) lies the farthermost toward the dayside. This fact is often used to expla in the creation of patches in terms of the entrainment of dayside plasma in to the cusp by high-latitude convection. In the southern hemisphere we see that the occurrence frequencies peak at over twice the northern hemisphere values in the same general UT region (1000-2300 UT). However, the southern hemisphere cusp is most dayward at approximately 0300 UT, a time of minimum patch formation. Examination of the relationship of the terminator to the polar cap boundaries in each hemisphere leads to a simple explanation in te rms of the differing offset of the magnetic poles (and the ionospheric conv ection pattern) from the geographic poles.