Twilight anomaly, midday recovery and cutoff latitudes during the intense polar cap absorption event of March 1991

Abstract. A study was made of the polar cap absorption (PCA) event on 23-24 March 1991 produced by the largest solar proton event at E>10 MeV since August 1972. This PCA event was related to a solar flare in the eastern hemisphere lasting only 2 days and exhibiting a long time delay between the flare and the increase of ionospheric absorption. Midday recovery occurred regularly each PCA day near the cut-off latitudes during the noontime hours and is attributed to the daily variation in the proton cut-off latitudes. The maximum absorption during the PCA event was observed at high latitudes or near the cut-off latitudes where ionization may be due to both solar protons and trapped particles. The minimum in the absorption values during the night-time hours would appear to be caused by the chemistry of the D-region as well as access of the solar protons into the polar cap area.