Inversion of the F2 layer critical frequency with the solar zenith angle

Normalization of the F 2 layer critical frequency is put forward with a model related to the solar zenith angle at the time of observation and its local noon value. The method implies a physically based replacement of variable representing one of the key controlling parameters of the modern empirical ionospheric models. The inversion of critical frequency is analyzed with the hourly values of foF 2 of seven ionospheric stations for 2000–2006 (half the solar activity cycle). The normalized critical frequency fnF 2 reveals an improved correlation between the data of different stations and improved inter-seasonal correlation of the data of a particular station as compared with the initial foF 2 values. A correlation radius in summer is two times greater for normalized critical frequency than for observations. The analytical model of expansion of the diurnal variation of critical frequency with four hourly values at 00, 06, 12 and 18 h of local time exhibits improved statistical features of normalized critical frequency as compared with observed critical frequency.