Regular changes in the critical frequency of the F2 layer of the quiet midlatitude ionosphere

A method for constructing the empirical model of the F 2 layer critical frequency ( foF 2) under magnetically quiet conditions, aimed at analyzing disturbances of any nature, is proposed. This method has been analyzed, and typical features of regular changes in foF 2 of the quiet ionosphere and day-to-day foF 2 variability are analyzed using the data from Irkutsk and Slough stations as an example. In particular, it has been obtained that this model differs from the international IRI model, and this difference is mainly caused by the fact that the foF 2 values in the IRI model do not correspond to quiet conditions. Therefore, this model gives a larger amplitude of the annual and semiannual variations in foF 2 than the IRI model. In addition, this model more accurately reproduces the rate of foF 2 annual variations at a fixed local time, especially in equinoxes, when foF 2 variations can exceed 1 MHz within one month.