Annual variation of geomagnetic activity

摘要: Abstract The annual variation of geomagnetic activity is studied through the indices aa, Dst and AE, according to different levels intensity for each indices. For thresholds that correspond from moderate fairly intense storms (i.e., peak aa⩾ 90), distribution follows well-known pattern a seasonal variation, with maxima around equinoxes minima near solstices. Deviations this behavior are observed when refers associated occurrence more storms. In particular, days index aa greater than about 90, shows in July. contribution very (aa⩾ 210) July peak, seems be evenly distributed along 12 solar cycles covered by index. Furthermore, although restricted much shorter interval time (they have been recorded only since 1957), seem as well show existence study done AE gives some indication another November, also However, due lack longer records these indices, real questionable. A statistical analysis events presented. From it seen number occurrences above given level those can approximated an exponential law. estimation during cycle function (or ∗ ) has done.