FOF2 HYSTERESIS VARIATIONS AND THE SEMIANNUAL GEOMAGNETIC WAVE

Seasonal and solar cycle variations of the foF2 hysteresis magnitude a re investigated. Data for the noon foF2 monthly medians for Slough (51 .48 degrees N, 0.57 degrees W), the monthly means for the sunspot numb ers, and for the geomagnetic activity index aa(N) for the northern hem isphere for the period 1933-1986, covering solar cycle from 17 to 21, are used. It is found that: (1) the greatest negative amplitudes of th e foF2 hysteresis variation are near the equinoxes, and (2) the solar cycle average noon foF2 hysteresis magnitude is linearly correlated wi th the solar cycle average semi-annual geomagnetic amplitude of the aa -index. These results support the hypothesis that the foF2 hysteresis is due to the geomagnetic activity variation during the sunspot cycle.