<1> The effect detected earlier by L. B. Vanina and A. D. Danilov (2003) is discussed in detail. The effect is that a high enough and statistically significant negative correlation coefficient is observed between the near-noon and near-midnight values of the critical frequency foF2. In this paper, the peculiarities of the effect (dependence on season, solar and geomagnetic activity, and geomagnetic latitude) are considered in detail on the basis of the data of a few ionospheric stations located in various geographic zones.