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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 13, issue 3
Ann. Geophys., 13, 262–276, 1995
© European Geosciences Union 1995
Ann. Geophys., 13, 262–276, 1995
© European Geosciences Union 1995

  31 Mar 1995

31 Mar 1995

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

H. Ranta, H. Yamagishi, and P. Stauning H. Ranta et al.

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.

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