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| Titel |
Why is there more ionosphere in January than in July? The annual asymmetry in the F2-layer |
| VerfasserIn |
H. Rishbeth, I. C. F. Müller-Wodarg |
| 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 ; 24, no. 12 ; Nr. 24, no. 12 (2006-12-21), S.3293-3311 |
| Datensatznummer |
250015705
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| Publikation (Nr.) |
 copernicus.org/angeo-24-3293-2006.pdf |
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| Zusammenfassung |
Adding together the northern and southern hemisphere
values for pairs of stations, the combined peak electron density NmF2 is
greater in December-January than in June–July. The same applies to the total
height-integrated electron content. This "F2-layer annual asymmetry" between
northern and southern solstices is typically 30%, and thus greatly
exceeds the 7% asymmetry in ion production due to the annual variation of
Sun-Earth distance. Though it was noticed in ionospheric data almost seventy
years ago, the asymmetry is still unexplained.
Using ionosonde data and also values derived from the International
Reference Ionosphere, we show that the asymmetry exists at noon and at
midnight, at all latitudes from equatorial to sub-auroral, and tends to be
greater at solar minimum than solar maximum. We find a similar asymmetry in
neutral composition in the MSIS model of the thermosphere. Numerical
computations with the Coupled Thermosphere-Ionosphere-Plasmasphere (CTIP)
model give a much smaller annual asymmetry in electron density and neutral
composition than is observed. Including mesospheric tides in the model makes
little difference. After considering possible explanations, which do not
account for the asymmetry, we are left with the conclusion that dynamical
influences of the lower atmosphere (below about 30 km), not included in our
computations, are the most likely cause of the asymmetry. |
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