The data from the vertical ionospheric sounding for 12 stations over the
world were analyzed to find the relation between the values of foF2 for 02:00 LT
and 14:00 LT of the same day. It is found that, in general, there exists a
negative correlation between foF2(02) and foF2(14). The value of the
correlation coefficient R(foF2) can be in some cases high enough and reach
minus 0.7–0.8. The value of R(foF2) demonstrates a well pronounced seasonal
variations, the highest negative values being observed at the equinox
periods of the year. It is also found that R(foF2) depends on geomagnetic
activity: the magnitude of R(foF2) is the highest for the choice of only
magnetically quiet days (A_{p}<6), decreasing with the increase of the
limiting 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 to
thermospheric winds. Analysis of long series of the vertical sounding data
shows that there is a long-term trend in R(foF2) with a statistically
significant increase in the R(foF2) magnitude after about 1980. Similar
analysis is performed for the foF2(02)/foF2(14) ratio itself. The ratio also
demonstrates a systematic trend after 1980. Both trends are interpreted in
terms of long-term changes in thermospheric circulation.