Abstract
Abstract. 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 (Ap<6), decreasing with the increase of the limiting value of Ap. For a fixed limitation on Ap, 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.
Highlights
The problem of long-term changes in the ionosphere is an object of a close attention of specialists in various scientific groups
The main goal of this paper is to try to take a look at the trends in the relation between the nighttime and daytime values of foF2. The reason for such an attempt is that foF2 in the daytime and at night is governed by different processes: photochemistry and composition changes dominate in the daytime, whereas dynamical processes govern the nighttime values of foF2
For the sake of comparison we considered taking only the spring (March–April) values and found that principally the results are the same, but the statistics is certainly better in the former case
Summary
The problem of long-term changes (trends) in the ionosphere is an object of a close attention of specialists in various scientific groups (see the summarizing paper by Lastovicka et al, 2006). The main goal of this paper is to try to take a look at the trends in the relation between the nighttime and daytime values of foF2. Vanina-Dart and Danilov (2006) were the first to draw attention to the fact that there is a significant negative correlation between the nighttime and daytime values of foF2 for the same day. The correlation coefficient R(foF2) was calculated over a three-month running interval with a step of one month. Danilov: Long-term trends in the relation between daytime and nighttime values of foF2
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