SOLAR-CYCLE VARIATION OF FOF2

For solar cycle 19 (1954-1964), the 12 monthly mean values of noon-time foF2 at Ahmedabad (23-degrees-N, 73-degrees-E) show a large hysteresis effect when plotted against sunspot number or against geomagnetic Ap. However, a multiple regression analysis for the dependence of foF2 on solar 2800 MHz flux and geomagnetic Ap, simultaneously, shows a better matching. Thus, long-term predictions need to take into account not just sunspot number but some solar index and geomagnetic index as two key parameters, simultaneously.