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| Titel |
Statistics of the largest geomagnetic storms per solar cycle (1844-1993) |
| VerfasserIn |
D. M. Willis, P. R. Stevens, S. R. Crothers |
| 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. 6 ; Nr. 15, no. 6, S.719-728 |
| Datensatznummer |
250012831
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| Publikation (Nr.) |
 copernicus.org/angeo-15-719-1997.pdf |
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| Zusammenfassung |
| A previous application of extreme-value
statistics to the first, second and third largest geomagnetic storms per solar
cycle for nine solar cycles is extended to fourteen solar cycles (1844–1993).
The intensity of a geomagnetic storm is measured by the magnitude of the daily aa
index, rather than the half-daily aa index used previously. Values of the
conventional aa index (1868–1993), supplemented by the Helsinki Ak
index (1844–1880), provide an almost continuous, and largely homogeneous, daily
measure of geomagnetic activity over an interval of 150 years. As in the earlier
investigation, analytic expressions giving the probabilities of the three
greatest storms (extreme values) per solar cycle, as continuous functions of
storm magnitude (aa), are obtained by least-squares fitting of the
observations to the appropriate theoretical extreme-value probability functions.
These expressions are used to obtain the statistical characteristics of the
extreme values; namely, the mode, median, mean, standard deviation and relative
dispersion. Since the Ak index may not provide an entirely homogeneous
extension of the aa index, the statistical analysis is performed
separately for twelve solar cycles (1868–1993), as well as nine solar cycles
(1868–1967). The results are utilized to determine the expected ranges of the
extreme values as a function of the number of solar cycles. For fourteen solar
cycles, the expected ranges of the daily aa index for the first, second
and third largest geomagnetic storms per solar cycle decrease monotonically in
magnitude, contrary to the situation for the half-daily aa index over
nine solar cycles. The observed range of the first extreme daily aa index
for fourteen solar cycles is 159–352 nT and for twelve solar cycles is 215–352
nT. In a group of 100 solar cycles the expected ranges are expanded to 137–539
and 177–511 nT, which represent increases of 108% and 144% in the respective
ranges. Thus there is at least a 99% probability that the daily aa index
will satisfy the condition aa < 550 for the largest geomagnetic storm in
the next 100 solar cycles. The statistical analysis is used to infer that
remarkable conjugate auroral observations on the night of 16 September 1770,
which were recorded during the first voyage of Captain Cook to Australia,
occurred during an intense geomagnetic storm. |
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