Annual variation of geomagnetic activity

The annual variation of geomagnetic activity is studied through the geomagn etic indices aa, Dst and AE, according to different levels of intensity for each of the indices. For thresholds that correspond from moderate to fairl y intense storms (i.e., peak aa greater than or equal to 90), the distribut ion follows the well-known pattern of a seasonal variation, with maxima aro und the equinoxes and minima near the solstices. Deviations from this behav ior are observed when the distribution refers to levels associated with the occurrence of more intense storms. In particular, the annual distribution of days with a geomagnetic index an greater than about 90, shows the occurr ence of a peak in July. The contribution of very intense storms (aa greater than or equal to 210) to the July peak, seems to be evenly distributed alo ng the 12 solar cycles covered by this index. Furthermore, the indices Dst and AE, although restricted to a much shorter interval of time (they have b een recorded only since 1957), seem as well to show the existence of a peak of occurrence for July. The study done for the indices Dst and AE gives so me indication for the existence of another peak in November, also for thres holds associated with intense storms. However, due to the lack of longer re cords for these indices, the real existence of this peak in the geomagnetic activity is questionable. A statistical analysis of the distribution of ev ents according to the levels of intensity of the aa and Dst is also present ed. From this analysis it is seen that the number of occurrences of storms above a given level of intensity of those geomagnetic indices, can be appro ximated by an exponential law. Furthermore, an estimation of the occurrence of storms during a solar cycle as a function of the peak aa (or aa*) has b een also done. (C) 2001 Elsevier Science Ltd. All rights reserved.