Irregularities in the semiannual variation of the geomagnetic activity Original Research Article

The annual distribution of geomagnetic activity is studied through the geomagnetic indices Aa, Dst and AE, according to different levels of intensity for each of the indices. It is shown that the well known pattern of seasonal variation, with maxima around the equinoxes and minima near the solstices, is typical for thresholds that correspond to moderate to fairly intense storms, but may be not too realistic for levels associated to the occurrence of very intense storms. In fact, the monthly number of days with Aa ≥ 100 shows the possible existence of a peak in July. This occurrence seems to be evenly distributed along the 12 solar cycles covered by this index. The statistical analysis of the indices Dst and AE reinforce the hypothesis of the existence of this peak. These two indices seem also to indicate the presence of another peak in November in the annual distribution of very disturbed days.