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| Titel |
Semiannual and annual variations in the height of the ionospheric F2-peak |
| VerfasserIn |
H. Rishbeth, K. J. F. Sedgemore-Schulthess, T. Ulich |
| 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 ; 18, no. 3 ; Nr. 18, no. 3, S.285-299 |
| Datensatznummer |
250013932
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| Publikation (Nr.) |
 copernicus.org/angeo-18-285-2000.pdf |
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| Zusammenfassung |
Ionosonde data from sixteen stations are used
to study the semiannual and annual variations in the height of the ionospheric
F2-peak, hmF2. The semiannual variation, which peaks shortly after
equinox, has an amplitude of about 8 km at an average level of solar activity
(10.7 cm flux = 140 units), both at noon and midnight. The annual variation has
an amplitude of about 11 km at northern midlatitudes, peaking in early summer;
and is larger at southern stations, where it peaks in late summer. Both annual
and semiannual amplitudes increase with increasing solar activity by day, but
not at night. The semiannual variation in hmF2 is unrelated to the
semiannual variation of the peak electron density NmF2, and is not
reproduced by the CTIP and TIME-GCM computational models of the quiet-day
thermosphere and ionosphere. The semiannual variation in hmF2 is
approximately "isobaric", in that its amplitude corresponds quite well
to the semiannual variation in the height of fixed pressure-levels in the
thermosphere, as represented by the MSIS empirical model. The annual variation
is not "isobaric". The annual mean of hmF2 increases with solar
10.7 cm flux, both by night and by day, on average by about 0.45 km/flux unit,
rather smaller than the corresponding increase of height of constant
pressure-levels in the MSIS model. The discrepancy may be due to solar-cycle
variations of thermospheric winds. Although geomagnetic activity, which affects
thermospheric density and temperature and therefore hmF2 also, is
greatest at the equinoxes, this seems to account for less than half the
semiannual variation of hmF2. The rest may be due to a semiannual
variation of tidal and wave energy transmitted to the thermosphere from lower
levels in the atmosphere.
Key words: Atmospheric composition and structure
(thermosphere - composition and chemistry) - Ionosphere (mid-latitude
ionosphere) |
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