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Using the spread-F (SF) data observed by DPS-4 digisonde at Hainan ionospheric observatory station from March 2002 to February 2005, occurrence features of four types of SF, i.e., frequency SF (FSF), range SF (RSF), mixed SF (MSF), and strong range SF (SSF), are studied. The results show a variation in the occurrence of the four types of the SF as a function of season, Kp and SunSpot Number (SSN). During equinoxes and summer, the occurrence frequency of all types of spread-F shows a little increase when Kp varies from 0 to 1 and then a decrease when Kp varies from 1 to 8. During winter, the occurrence frequency in days of each type of spread-F is small, however, it also shows an increase with smaller values of Kp and then shows a decrease with higher values of Kp. The RSF and SSF occurrence maximum at Hainan station are most frequent during the equinoctial months in 2002 and 2004. However, the FSF and MSF occurrence maximum are most frequent during the summer months in 2003 and 2004. The RSF and SSF occurrence rates are increasing with SSN during equinoctial and summer months. The FSF and MSF occurrence rates are decreasing with SSN during summer months. It seems that all spread-F occurrence rates are nearly independent with SSN during winter months. In this paper, we also make a comparative study on these results with the published statistics for Brazilian (Cachoeira Paulista) and Indian (Ahmedabad) sectors. |
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