Time variations of geomagnetic activity indices Kp and Ap: an update

摘要

Kp and Ap indices covering the period 1932 to1995 are analysed in a fashion similar to that attempted by Bartels for the1932–1961 epoch to examine the time variations in their characteristics. Modernanalysis techniques on the extended data base are used for further insight. Therelative frequencies of occurrence of Kp with different magnitudes and theseasonal and solar cycle dependences are seen to be remarkably consistentdespite the addition of 35 years of observations. Many of the earlier featuresseen in the indices and special intervals are shown to be replicated in thepresent analysis. Time variations in the occurrence of prolonged periods ofgeomagnetic calm or of enhanced activity are presented and their relation tosolar activity highlighted. It is shown that in the declining phase theoccurrence frequencies of Kp = 4–5 (consecutively over 4 intervals) can be usedas a precursor for the maximum sunspot number to be expected in the next cycle.The semi-annual variation in geomagnetic activity is re-examined utilising notonly the Ap index but also the occurrence frequencies of Kp index with differentmagnitudes. Lack of dependence of the amplitude of semi-annual variation onsunspot number is emphasised. Singular spectrum analysis of the mean monthly Apindex shows some distinct periodic components. The temporal evolution of ~44month, ~21 month and ~16 month oscillations are examined and it is postulatedthat while QBO and the 16 month oscillations could be attributed to solar windand IMF oscillations with analogous periodicity, the 44 month variation isassociated with a similar periodicity in recurrent high speed stream caused bysector boundary passage. It is reconfirmed that there could have been only oneepoch around 1940 when solar wind speed could have exhibited a 1.3-yearperiodicity comparable to that seen during the post-1986 period.