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| Titel |
Time variations of geomagnetic activity indices Kp and Ap: an update |
| VerfasserIn |
G. K. Rangarajan, T. Iyemori |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 15, no. 10 ; Nr. 15, no. 10, S.1271-1290 |
| Datensatznummer |
250012974
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| Publikation (Nr.) |
 copernicus.org/angeo-15-1271-1997.pdf |
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| Zusammenfassung |
| Kp and Ap indices covering the period 1932 to
1995 are analysed in a fashion similar to that attempted by Bartels for the
1932–1961 epoch to examine the time variations in their characteristics. Modern
analysis techniques on the extended data base are used for further insight. The
relative frequencies of occurrence of Kp with different magnitudes and the
seasonal and solar cycle dependences are seen to be remarkably consistent
despite the addition of 35 years of observations. Many of the earlier features
seen in the indices and special intervals are shown to be replicated in the
present analysis. Time variations in the occurrence of prolonged periods of
geomagnetic calm or of enhanced activity are presented and their relation to
solar activity highlighted. It is shown that in the declining phase the
occurrence frequencies of Kp = 4–5 (consecutively over 4 intervals) can be used
as 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 not
only the Ap index but also the occurrence frequencies of Kp index with different
magnitudes. Lack of dependence of the amplitude of semi-annual variation on
sunspot number is emphasised. Singular spectrum analysis of the mean monthly Ap
index shows some distinct periodic components. The temporal evolution of ~44
month, ~21 month and ~16 month oscillations are examined and it is postulated
that while QBO and the 16 month oscillations could be attributed to solar wind
and IMF oscillations with analogous periodicity, the 44 month variation is
associated with a similar periodicity in recurrent high speed stream caused by
sector boundary passage. It is reconfirmed that there could have been only one
epoch around 1940 when solar wind speed could have exhibited a 1.3-year
periodicity comparable to that seen during the post-1986 period. |
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