Midday recoveries of polar cap absorption

Riometer observations of midday recoveries of polar cap absorption (PCA) show that: (a) the recoveries peak in the local time range of 0800 to 1500, mostly between 1000 and 1200 LT; (b) the recoveries are most pronounced at the edge of the polar cap and not evident deep inside the polar cap; and (c) the midday recovery region remains at the edge of the polar cap as the polar cap expands during a magnetic storm. Some satellite data indicate that midday recoveries are the result of the development of an anisotropic pitch-angle distribution of the solar protons, rather than a simple increase of cutoff rigidity. The anisotropy is of the nature of a depleted loss cone; in addition, the 90° pitch-angle component is apparently reduced below its isotropic, non-midday recovery value. Model calculations of absorption using a sin³ θ pitch-angle distribution suggest that anisotropy, without appreciable increase of vertical cutoff energy, can cause midday recoveries of the observed magnitudes. Two outstanding features of midday recoveries yet to be explained are their presence during only about 20% of PCA events, and the fact that midday recoveries are usually pronounced on only one day of an event.