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| Titel |
On the long term change in the geomagnetic activity during the 20th century |
| VerfasserIn |
F. Ouattara, C. Amory-Mazaudier, M. Menvielle, P. Simon, J.-P. Legrand |
| 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 ; 27, no. 5 ; Nr. 27, no. 5 (2009-05-05), S.2045-2051 |
| Datensatznummer |
250016523
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| Publikation (Nr.) |
 copernicus.org/angeo-27-2045-2009.pdf |
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| Zusammenfassung |
The analysis of the aa index series presented in this paper clearly
shows that during the last century (1900 to 2000) the number of
quiet days (Aa<20 nT) drastically diminished from a mean annual
value greater than 270 days per year at the end of the nineteenth
century to a mean value of 160 quiet days per year one hundred years
later. This decrease is mainly due to the decrease of the number of
very quiet days (Aa<13 nT). We show that the so-evidenced
decrease in the number of quiet days cannot be accounted for by
drift in the aa baseline resulting in a systematic underestimation
of aa during the first quarter of the century: a 2–3 nT
overestimation in the aa increase during the 20th century would lead
to a 20–40% overestimation in the decrease of the number of quiet
days during the same period.
The quiet days and very quiet days correspond to periods during
which the Earth encounters slow solar wind streams flowing in the
heliosheet during the period where the solar magnetic field has a
dipolar geometry. Therefore, the observed change in the number of
quiet days is the signature of a long term evolution of the solar
coronal field topology. It may be interpreted in terms of an
increase in the magnitude of the solar dipole, the associated
decrease of the heliosheet thickness accounting for the observed
decrease in the number of quiet days. |
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