[26] The analysis of a series of midlatitude stations revealed the following effects in the behavior of the F2 region critical frequency foF2. The values of the near noon (1400 LT) and near midnight (0200 LT) values of foF2 for the same day are not independent: there is a negative correlation between these values. This fact was for the first time mentioned by Vanina and Danilov [2003]. They have detected this effect for the data of several midlatitude Russian stations considered in the search of the relation between stratosphere and ionosphere parameters. In this paper, a few western stations were added and more sophisticated analysis has been performed.
[27] Vanina and Danilov [2003] considered the correlation coefficient r(foF2) between foF2 (day) and foF2 (day) night for quiet conditions ( Ap< 8 ) only. In this paper a set of boundary values of Ap from 6 to 30 was considered. The results (see Figures 5 and 9) show the presence of a strong dependence on Ap (boun). The highest negative values of r(foF2) for all considered stations is found for quiet conditions ( Ap < 6 ) and for some stations reaches 0.8-0.9, whereas is falls with the increase in Ap (boun) and is negligibly small in moderately disturbed conditions.
[28] There is a seasonal effect: the minimum in r(foF2) is observed in March-April. The effect is observed at all stations and for the particular station at different Ap (see Figures 3 and 4). The maximum (by the amplitude) values of r(foF2) in the spring extreme show a pronounced dependence on solar activity (see Figure 6). The effect is maximal at high solar activity and small or statistically insignificant at low values of the solar index W. There is an indication to the presence of a latitudinal effect (see Figure 10). The higher the geomagnetic latitude, the higher the negative correlation coefficient r(foF2). However, it is not true for two auroral stations considered (Sodankylä and Kiruna). The cause of this fact is related to the physical processes of formation of the electron concentration maximum in the daytime and at night. In the auroral oval these processes differ from the ones governing the electron concentration at middle latitudes.
[29] This paper is deliberately aimed at the description of the morphological features of the effect. We are going to consider in detail the physical explanation of these features and also the related problems of long-term trends in the ionosphere and thermosphere parameters [Danilov, 2005] in a separate paper.
[30] The mechanism of the discussed effect presumably is related to day-to-day thermosphere circulation changes. The winter type of the circulation corresponds to an increased poleward neutral wind Vnx, while the summer type corresponds to a decreased one. Daytime electron concentration NmF2 under normal conditions is mainly controlled by photochemical processes being roughly speaking proportional to atomic oxygen concentration [O] [Mikhailov et al., 1995; Rishbeth and Barron, 1960], while vertical drifts generated by Vnx play a secondary role. An enhanced poleward Vnx brings to middle latitudes the air with increased atomic oxygen concentration and this corresponds to higher daytime NmF2 values. The summer type of the thermospheric circulation (a decreased poleward Vnx ) results in an opposite effect with decreased [O] and lower daytime NmF2 values. On the other hand, the nighttime F2 layer is uplifted by strong equatorward Vnx (irrespective of the circulation pattern) from the heights where chemical processes dominate and under the absence of direct solar ionization NmF2 does not "feel" these changes in [O]. However, the nighttime NmF2 is strongly controlled by the initial conditions formed during the sunset hours, and diurnal Vnx variations may play some role in the effect considered. A summer type of the circulation with a decreased poleward Vnx corresponds to earlier transition to positive vertical plasma drifts. This under a direct solar ionization (although small) helps in creating higher NmF2 sunset values. In contrast, a winter type of the circulation with an increased poleward Vnx lags in time the transition to positive nighttime drifts. The F2 layer remains in the high-recombination area for some time and this under low direct solar ionization results in lower "start" NmF2 values. Therefore different types of the circulation lead to an additional differentiation in the nighttime NmF2 values thus increasing the effect of negative correlation between NmF2 (day) and NmF2 (night). It is worth noting that the strongest effect is detected during the equinoctial periods when the winter/summer reversal of the circulation takes place and so the direction of the meridional circulation may undergo day-to-day changes [Mikhailov and Schlegel, 2001].
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