Examination of the solar cycle variation of foF2 for cycles 22 and 23

The variations of monthly median noon values of foF2 at Slough and Rome are examined by using different solar activity indices (solar flare index, relative sunspot number, solar flux at 2800 MHz, and Mg II index) for solar cycles 22 and 23 (1986-2006). We compared the dependence of foF2 on solar activity indices by using a single regression analysis, and showed that qualitative similarity of the ionospheric foF2 with the solar indices depends on the solar cycle. We found that hysteresis effect shows generally lower foF2 for the rising branches compared with the falling branches of the two solar cycles, and the strength of the hysteresis effect varies from index to index as well as from cycle to cycle.