PERIODIC VARIATION IN THE GEOMAGNETIC-ACTIVITY - A STUDY BASED ON THEAP INDEX

The monthly and daily samples of the Ap geomagnetic index for 51 years , 1932-1982, were investigated by means of the power spectrum techniqu e, In general, the results confirm previous findings about possible pe riodicities in the geomagnetic activity. However, in our opinion the f ollowing aspects are either new or they are being interpreted somewhat differently than other authors have done. The period around 4 years i n the monthly Ap power spectrum is associated to the double peak struc ture observed in the geomagnetic activity variation [Gonzalez et al., 1990]. Several of the peaks shown by the daily Ap spectrum are interpr eted as harmonics of the 6-month period and other peaks as caused by t he solar rotation periodicity, in such a way that the two series of Fo urier sequences are consider to be juxtaposed. A strong solar cycle mo dulation is observed in these series, particularly iii that related to the solar rotation period, which almost disappears for the solar maxi mum phase. The study of the seasonal variation was complemented by a s uperposed epoch analysis. The profiles resulting from this analysis se em to show a multiple origin of the 6-month periodicity, so that it do es not seem realistic to search for a unique cause for this well-known seasonal variation. This conclusion is also supported by the histogra ms of the occurrence of storms above a given intensity level, taken ov er short duration intervals (i.e., 8 days). According to these histogr ams, for large data samples the dates with largest number of storms ar e spread out around those predicted by the different theoretical model s, while for short intervals the semiannual periodicity may sometimes not even be present. Therefore these known mechanisms would combine to give a resulting modulation of the geomagnetic response to the random ly generated source of storms. It was also found that an additional se asonal peak seems to exist in July, with an amplitude comparable to th ose of the equinoctial peaks, for the range of the most intense storms (Ap greater-than-or-equal-to 150 nT). A weak periodicity around 158 d ays, well correlated to that of about 155 days observed in the solar a ctivity, has also been detected for some years during solar cycle 21.