Long-term trends in the relation between daytime and nighttime values of Ifo/IF2

摘要

The data from the vertical ionospheric sounding for 12 stations over theworld were analyzed to find the relation between the values of foF2 for 02:00 LTand 14:00 LT of the same day. It is found that, in general, there exists anegative correlation between foF2(02) and foF2(14). The value of thecorrelation coefficient R(foF2) can be in some cases high enough and reachminus 0.7–0.8. The value of R(foF2) demonstrates a well pronounced seasonalvariations, the highest negative values being observed at the equinoxperiods of the year. It is also found that R(foF2) depends on geomagneticactivity: the magnitude of R(foF2) is the highest for the choice of onlymagnetically quiet days (A_p<6), decreasing with the increase of thelimiting value of A_p. For a fixed limitation on A_p, the value of R(foF2)depends also on solar activity. Apparently, the effects found are related tothermospheric winds. Analysis of long series of the vertical sounding datashows that there is a long-term trend in R(foF2) with a statisticallysignificant increase in the R(foF2) magnitude after about 1980. Similaranalysis is performed for the foF2(02)/foF2(14) ratio itself. The ratio alsodemonstrates a systematic trend after 1980. Both trends are interpreted interms of long-term changes in thermospheric circulation.