Twilight anomaly,midday recovery and cutoff latitudes during the intense polar cap absorption event of March 1991 |
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| Authors: | H Ranta H Yamagishi P Stauning |
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| Institution: | (1) Geophysical Observatory, SF-99600 Sodankylä, Finland;(2) National Institute of Polar Research, 9-10 Kaga, 1-chome, Itabashiku, Tokyo 173, Japan;(3) Danish Meteorological Institute, Solar-Terrestrial Physics Division, Research and Development Department, Lyngbyvej 100, DK-2100 Copenhagen, Denmark |
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| 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 cutoff lati- tudes during the noontime hours and is attributed to the daily variation in the proton cutoff latitudes. The maximum absorption during the PCA event was observed at high latitudes or near the cutoff 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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