A case study of the variation of ionospheric parameter during typhoons at Xiamen

The variation of the F2-layer critical frequency(foF2),the frequencies of occurences of the sporadic E-layer(Es) and spread-F during 3 typhoons in 2007 observed by Xiamen Ionosonde Station are investigated in this paper.The results indicate that the foF2 is disturbed,and the Es and spread-F occurring rates increase distinctly during the three typhoons approaching the station.One day before Sepat landing,the foF2 enhancement reached 40%(4 MHz) comparing with the month median value,and the foF2 quickly fell to the month median value after Sepat landing;the foF2 was a little higher than the month median value before Wipha landing,and quickly dropped below the median value after it landing with the decreasing amplitude of about 30%and lasted 4 days.On the other hand,the foF2 was below the month median value 2 days before Krosa landing; the foF2 restored the month median value during its landing,and then foF2 decreased in the following 1 day and increased continuously in the successive 3 days.Based on the preliminary analyses we consider that the interaction between the atmosphere, ocean and land may influence the altitude of the ionosphere,and bring about the foF2,Es,and spread-F variations.Although the foF2 decrease is able to be well explained by the mechanism proposed in Wang(2005),further investigation is necessary to interpret the foF2 increase when the typhoon landing as revealed.