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.