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| Titel |
Origins of the semiannual variation of geomagnetic activity in 1954 and 1996 |
| VerfasserIn |
E. W. Cliver, L. Svalgaard, A. G. Ling |
| 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 ; 22, no. 1 ; Nr. 22, no. 1, S.93-100 |
| Datensatznummer |
250014752
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| Publikation (Nr.) |
 copernicus.org/angeo-22-93-2004.pdf |
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| Zusammenfassung |
We investigate the cause of the unusually strong semiannual
variation of geomagnetic activity observed in the solar minimum years of 1954
and 1996. For 1996 we separate the contributions of the three classical
modulation mechanisms (axial, equinoctial, and Russell-McPherron) to the six-month
wave in the aam index and find that all three contribute about equally.
This is in contrast to the longer run of geomagnetic activity (1868-1998) over
which the equinoctial effect accounts for ∼70% of the semiannual variation. For
both 1954 and 1996, we show that the Russell-McPherron effect was enhanced
by the Rosenberg-Coleman effect (an axial polarity effect) which increased the
amount of the negative (toward Sun) [positive (away from Sun)] polarity field
observed during the first [second] half of the year; such fields yield a southward
component in GSM coordinates. Because this favourable condition occurs only
for alternate solar cycles, the marked semiannual variation in 1954 and 1996 is a
manifestation of the 22-year cycle of geomagnetic activity. The 11-year evolution of
the heliospheric current sheet (HCS) also contributes to the strong six-month
wave during these years. At solar minimum, the streamer belt at the base of the
HCS is located near the solar equator, permitting easier access to high speed
streams from polar coronal holes when the Earth is at its highest heliographic
latitudes in March and September. Such an axial variation in solar wind speed
was observed for 1996 and is inferred for 1954.
Key words. Magnetosphere (solar wind – magnetosphere
interactions; storms and substorms) |
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