Relation between daytime and nighttime values of the critical frequency foF2

The relation between the daytime in the nighttime values of the critical frequencies (foF2) of the ionospheric F2 layer is considered. The correlation coefficient of foF2 measured at 1400 and 0200 LT of the same day is considered in various seasons of years with different solar activity (during the complete cycle of solar activity in 1979-1989). Special accent is made on the dependencies of the above mentioned correlation on a choice of magnetically quiet days with various limitations on maximal values of geomagnetic index Ap. It has been obtained that a statistically significant negative correlation between the foF2(1400) and foF2(0200) is more pronounced in the periods of high solar activity. The effect increases with increasing limitation of the considered days on value of Ap : the largest values of the correlation coefficient are observed if only very quiet days are considered (Ap < 6). There are preliminary indications that the considered relation between daytime and nighttime foF2 values depends on latitude.