About The Seasonal Distribution Of Geomagnetic Activity

The annual distribution of geomagnetic activity is studied through the geomagnetic indices aa, Dst and AE,<br>according to different levels of intensity for each of the indices. For thresholds that correspond to moderate to<br>fairly intense storms, the distribution follows the well-known pattern of a seasonal variation, with maxima<br>around the equinoxes and minima near the solstices. However, the observed pattern deviates from this behavior<br>as the distribution refers to levels associated to the occurrence of more intense storms. For the latter type of<br>storms, the geomagnetic index aa shows the occurrence of a peak in July (but not in January, as a seasonal<br>symmetry would suggest). The contribution of very intense storms to the July peak seems to be evenly<br>distributed along the 11 solar cycles covered by this index. Furthermore, although the records for the indices<br>Dst and AE are restricted to shorter time intervals as compared to aa, they also show the possible existence of<br>this peak for July.<br>The present analysis gives also some indications for the existence of a peak in November in the distribution of<br>very intense storms. This peak shows up particularly for the indices Dst and AE, whose records go back only to<br>1957. Therefore, its real existence is more questionable than that of the peak for July.