Polar patches: Auroral zone precipitation effects



[1] We look at patches observed by digital ionosondes in the northern polar cap. These observations show that patches have the following properties. (1) Patches are relatively large, typically ∼500 km in the sunward-antisunward direction and ∼1000 km in the dawn-dusk direction. (2) When the IMF Bz has abnormally large positive-negative swings the patches show correlation with the IMF Bz swings. (3) For typical days the patches show no average correlation with IMF By and only weak correlation with negative IMF Bz. (4) Patches are slightly weaker in summer. The typical patch max-to-min Ne variation in winter is ∼2 × 1011 el/m3 and in summer it is ∼1.5 × 1011 el/m3. (5) There is restructuring of patches, particularly the small-scale structures, as the patches convect across the polar cap. (6) Patches are formed prior to entering the polar cap and their properties stay about the same thereafter. We explain the patch generation and most of these properties by low-energy precipitation into the returning flow from the nightside to the dayside around the dawn convection cell.