Long-term trends in the ratio of the daytime and nighttime values of foF2

Ionospheric F1 layer long-term trends and the geomagnetic control concept

“…At present, there are no direct observations of such Vnx long-term increase. It is possible to mention the results by Danilov (2008) who has analyzed the correlation coefficient r between daytime and nighttime foF2 values (which normally is negative) and found an increase of abs(r) after 1980, which he has related to an increase in the poleward Vnx. Such an increase of Vnx does not look unreasonable as under decreasing geomagnetic activity (corresponding to a decrease of the auroral heating) the solar driven circulation (poleward during daytime) should enhance (e.g.…”

Time and spatial variations in the ratio of nighttime and daytime critical frequencies of the F2 layer

“…In the same way as in the previous publications (Danilov, 2006b, c;Danilov and Vanina-Dart, 2007;VaninaDart and Danilov, 2006), the foF2 values measured at 0200 and 1400 LT were considered as the nighttime and daytime values, respectively. These two moments represent well the daytime and nighttime conditions in the F2 layer, and at the same time, refer to the same calendar day, this fact simplifying the data description with the help of the geomagnetic activity index Ap (the same daily value of Ap corresponds to both nighttime and daytime values of foF2).…”

“…Finally, on the basis of the analysis of the data of midlatitude ionospheric stations on the territory of the former USSR, a preliminary conclusion was drawn that the observed effect has a latitudinal dependence: the amplitude of the effect decreases with a decrease in geomagnetic latitude. Danilov and Vanina-Dart (2007) continued studies of the correlation coefficient R(foF2). They found that a specific behavior of R(foF2) with time (a sort of a long-term trend) is observed: after approximately 1980 (for different stations this ''boundary date'' can vary by 1-2 years in both directions): the absolute value of the negative correlation coefficient grows for all studied stations (12 ionospheric stations located at middle geomagnetic latitudes were considered).…”

Critical frequency foF2 as an indicator of trends in thermospheric dynamics

“…Vanina-Dart and Danilov (2006a) considered the correlation coefficient R(foF2) between nighttime and daytime values of foF2. Later, Danilov (2008a) derived long-term trends in the minimal values of R(foF2) on the basis of the data of a few ionospheric stations. Danilov (2007) proposed to analyze the ratio of nighttime and daytime values of foF2: foF2(night)/foF2(day).…”

“…More stations (17) than in Danilov (2008a) are considered, and not only are the minimal values of R(foF2) over the year considered, but the maximum values as well. The new results on the R(foF2) behavior together with the results concerning the foF2 (night)/foF2(day) ratio and the changes in r(h,fo) are considered in this paper from the point of view of the concept of long-term trends in the circulation of the upper atmosphere.…”