A new method of obtaining an ionospheric index of solar activity from the F2 layer critical frequencies

A new method of obtaining an ionospheric index of solar activity, based on the monthly median foF2 at 4 hours of the day (0, 6, 12, and 18 local hours), is described. The index for each month of the year has been determined, together with regression coefficients of quadratic equations for every hour at several typical stations, by minimizing the standard percentage error of foF2 estimates. The index has favorable features: the standard deviation of index above and below its running average is smaller; the standard percentage deviation of residual of foF2 and the standard percentage deviation of total error in forecasts of foF2 are much smaller than those of the index obtained by the current method for calculating an ionospheric index of solar activity. Reasons for considering the new index a better approximation to an ideal solar index for the ionosphere are presented.