THE IONOSPHERIC FOF2 DATA OVER ISTANBUL AND THEIR RESPONSE TO SOLAR-ACTIVITY FOR THE YEARS 1964-1969 AND 1993

A Polish-made vertical ionosonde (VI) has been operated at the Kandill i Observatory in Istanbul, for almost one year (May 1993 - April 1994) as part of the COST 238, PRIME Project. The critical frequencies were obtained for every half-hour interval. The data obtained during this campaign, on the descending branch of solar cycle 22, and the data mea sured earlier in Istanbul for cycle 20 were analysed and the character istic behaviour of the F2 region ionosphere over Istanbul has been det ermined. This is a unique data set for this area. Several markers of t he solar cycle activities in terms of the daily relative sunspot numbe rs, F10.7 cm solar radio flux and solar flare index, and the magnetic daily index of Ap were then used to seek the possible influence of the solar and ionospheric activities on the critical frequencies observed in Istanbul. It was found that the solar flare index, as a solar acti vity index, was more reliable in determining quiet ionospheric days. i t is shown that the minimum and maximum time values of the solar activ ity are more convenient for ionospheric prediction and modelling.