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Variations in the solar cycle of the solar EUV flux (represented by F10.7, the Covington index) are shown to control annual mean foF2 cycle variations to a great extent. Counterclockwise in cycle 19 and clockwise in cycle 21 EUV flux hysteresis loops result in corresponding foF2 solar cycle variations, the latter being changed by the F2-layer storm effects due to enhanced geomagnetic activity during...
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