HF Absorption near the Polar Cap Edge during PCA Events

Sometimes during PCA events the HF (330 MHz) absorption observed at a riometer station near the polar cap edge decreases, although it remains constant or even increases at a higher-latitude station [Hultqvist et al., 1959; Leinbach, 1961, 1962, 1967]. The absorption stays depressed for a few hours and then returns to or near the predecrease level. The effect often occurs in the local forenoon hours, is called the midday recovery (MDR), and has been attributed to an increase in geomagnetic cutoff energy of the solar protons [Leinbach, 1962; Reid and $auer, 1967] or to development of an anisotropic pitch angle distribution of solar protons [Leinbach, 1967]. We discuss here two recoveries in relation to satellite proton observations along meridians near those of the ground station.