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The spatial relationship between active regions and coronal holes and the occurrence of intense geomagnetic storms throughout the solar activity cycle
We study the annual frequency of occurrence of intense geomagnetic storms (Dst < A100 nT) throughout the solar activity cycle for the last three cycles and ®nd that it shows dierent structures. In cycles 20 and 22 it peaks during the ascending phase, near sunspot maximum. During cycle 21, however, there is one peak in the ascending phase and a second, higher, peak in the descending phase separated by a minimum of storm occurrence during 1980, the sunspot maximum. We compare the solar cycledoi:10.1007/s005850050578 fatcat:gy7vy5uoqrcfllrg6f5cpsnsrq