Physics
Scientific paper
Mar 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001jastp..63..367c&link_type=abstract
Journal of Atmospheric and Solar-Terrestrial Physics, Volume 63, Issue 4, p. 367-374.
Physics
9
Scientific paper
The annual variation of geomagnetic activity is studied through the geomagnetic indices /aa, /Dst and /AE, according to different levels of intensity for each of the indices. For thresholds that correspond from moderate to fairly intense storms (i.e., peak /aa>= 90), the distribution follows the well-known pattern of a seasonal variation, with maxima around the equinoxes and minima near the solstices. Deviations from this behavior 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 /aa greater than about 90, shows the occurrence of a peak in July. The contribution of very intense storms (/aa>= 210) to the July peak, seems to be evenly distributed along 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 been 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 some indication for the existence of another peak in November, also for thresholds associated with intense storms. However, due to the lack of longer records for these indices, the real existence of this peak in the geomagnetic activity is questionable. A statistical analysis of the distribution of events according to the levels of intensity of the /aa and /Dst is also presented. 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 approximated 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 been also done.
Clua de Gonzalez Alicia L.
Gonzalez Walter D.
Silbergleit Virginia M.
Tsurutani Bruce T.
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