Abstract
Abstract. We study the annual frequency of occurrence of intense geomagnetic storms (Dst < –100 nT) throughout the solar activity cycle for the last three cycles and find that it shows different 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 cycle distribution of storms with the corresponding evolution of coronal mass ejections and flares. We find that, as the frequency of occurrence of coronal mass ejections seems to follow very closely the evolution of the sunspot number, it does not reproduce the storm profiles. The temporal distribution of flares varies from that of sunspots and is more in agreement with the distribution of intense geomagnetic storms, but flares show a maximum at every sunspot maximum and cannot then explain the small number of intense storms in 1980. In a previous study we demonstrated that, in most cases, the occurrence of intense geomagnetic storms is associated with a flaring event in an active region located near a coronal hole. In this work we study the spatial relationship between active regions and coronal holes for solar cycles 21 and 22 and find that it also shows different temporal evolution in each cycle in accordance with the occurrence of strong geomagnetic storms; although there were many active regions during 1980, most of the time they were far from coronal holes. We analyse in detail the situation for the intense geomagnetic storms in 1980 and show that, in every case, they were associated with a flare in one of the few active regions adjacent to a coronal hole.
Highlights
The solar cycle distribution of the frequency of occurrence of intense geomagnetic storms (IGSs) was studied by Gonzalez et al, (1990), who de®ned intense storms as those when Dstmax < A100 nT
From an analysis of the 10 IGSs that occurred between 1978 and 1979, combining interplanetary scintillation (IPS) tracking of interplanetary disturbances and solar eruptive event observations, we found that in most of the cases IGSs were associated with the occurrence of aare or the eruption of a prominence near a coronal hole, near the solar central meridian (Bravo and Rivera, 1994)
The analysis presented here lends further support to the ®ndings that most of intense geomagnetic storms are associated with eruptive solar events near coronal holes
Summary
The solar cycle distribution of the frequency of occurrence of intense geomagnetic storms (IGSs) was studied by Gonzalez et al, (1990), who de®ned intense storms as those when Dstmax < A100 nT. To see if the combined evolution of coronal holes and active regions aects, in general, the occurrence of IGSs, we study the spatial relation between active regions and coronal holes from 1977 to 1990, covering the maxima of sunspot cycles 21 and 22, to compare with the frequency of occurrence of IGSs. The coronal hole maps were obtained from McIntosh (private communication), Solar Geophysical Data, and the catalogue of Stewart et al (1985).
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