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 < )100 nT) throughout the solar activity cycle for the last three cycles and ®nd that it shows di€erent 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 ̄ares. We ®nd 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 pro®les. The temporal distribution of ̄ares varies from that of sunspots and is more in agreement with the distribution of intense geomagnetic storms, but ̄ares 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 ̄aring 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 ®nd that it also shows di€erent 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 ̄are in one of the few active regions adjacent to a coronal hole. Introduction 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 < )100 nT. They found that IGS distribution is not in agreement with the evolution of the sunspot number. Figure 1 shows the yearly number of IGSs (bars) from 1965 to 1994 (cycles 20, 21 and 22) and the corresponding yearly sunspot numbers (dots) obtained from the Solar Geophysical Data. We can see that in cycles 20 and 22 the maximum frequency of IGSs occurred at about sunspot maximum. In cycle 21, however, the frequency of IGSs has a peak during the ascending phase of the cycle, then a minimum at sunspot maximum, and then a higher peak in the declining phase. Although the occurrence of an IGS is, no doubt, associated with solar activity, there is no general agreement on the particular kind of solar events that can produce them. Fast and large coronal mass ejections (CMEs), ̄ares, and the eruption of prominences have been invoked by di€erent authors (see review by Joselyn, 1995). In previous works we have shown that most of the IGSs can be associated with eruptive events ( ̄ares or prominence eruptions) occurring near a coronal hole near the solar central meridian (Bravo and Rivera, 1994; Bravo, 1997). In this work we shall study the solar cycle evolution of active regions near coronal holes and compare them with the distribution of IGSs. A comparison with the solar cycle evolution of CMEs and ̄ares will be presented as well. The solar cycle evolution of coronal mass ejections Recently it has been claimed by some authors that the origin of the interplanetary disturbances leading to IGSs are large and fast CMEs (e.g. Gosling, 1993). If this is so, we would expect some similarity between the solar Correspondence to: S. Bravo [email protected] Ann. Geophysicae 16, 49±54 (1998) Ó EGS ± Springer-Verlag 1998