A functional dependence of foF2 variability on latitude

Considering that day-to-day foF2 variability is expressed by the fraction of hourly daily values to their corresponding monthly median and selecting the 25% level (quartile) as an appropriate variability estimate, a simple foF2 variability model with geomagnetic latitude is developed. A second order Fourier series scheme is applied separately for upper and lower quartiles and adequately describes the continuous variation of variability with latitude. In order to avoid a rough and unjustified segmentation of the day in local time windows, variability is examined in four discrete sections defined solely on solar zenith angle. The present variability model is overall successful at midlatitudes, being though somewhat limited in the geographical regions of high variability boundaries. Furthermore, the consideration of the solar zenith angle seems quite functional except around local sunrise hours.