Time delays in the correlation between solar activity and the F2 region plasma frequency

► Studied the day-to-day correlation between solar activity and foF2 during 1967-2003. ► The data from mid-latitude ionospheric stations at 0° and 100° longitude used. ► The maximum correlation appears with some days delay. ► The sense of Δ depends on the position of the interval studied in the solar cycle. ► The existence of an 18-year periodicity seems possible.