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| Titel |
Annual and semiannual variations in the ionospheric F2-layer: II. Physical discussion |
| VerfasserIn |
H. Rishbeth, I. C. F. Müller-Wodarg, L. Zou, T. J. Fuller-Rowell, G. H. Millward, R. J. Moffett, D. W. Idenden, A. D. Aylward |
| 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. 8 ; Nr. 18, no. 8, S.945-956 |
| Datensatznummer |
250014044
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| Publikation (Nr.) |
 copernicus.org/angeo-18-945-2000.pdf |
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| Zusammenfassung |
| The companion paper by Zou et al.
shows that the annual and semiannual variations in the peak F2-layer electron
density (NmF2) at midlatitudes can be reproduced by a coupled
thermosphere-ionosphere computational model (CTIP), without recourse to external
influences such as the solar wind, or waves and tides originating in the lower
atmosphere. The present work discusses the physics in greater detail. It shows
that noon NmF2 is closely related to the ambient atomic/molecular
concentration ratio, and suggests that the variations of NmF2 with
geographic and magnetic longitude are largely due to the geometry of the auroral
ovals. It also concludes that electric fields play no important part in the
dynamics of the midlatitude thermosphere. Our modelling leads to the following
picture of the global three-dimensional thermospheric circulation which, as
envisaged by Duncan, is the key to explaining the F2-layer variations. At
solstice, the almost continuous solar input at high summer latitudes drives a
prevailing summer-to-winter wind, with upwelling at low latitudes and throughout
most of the summer hemisphere, and a zone of downwelling in the winter
hemisphere, just equatorward of the auroral oval. These motions affect
thermospheric composition more than do the alternating day/night (up-and-down)
motions at equinox. As a result, the thermosphere as a whole is more molecular
at solstice than at equinox. Taken in conjunction with the well-known relation
of F2-layer electron density to the atomic/molecular ratio in the neutral air,
this explains the F2-layer semiannual effect in NmF2 that prevails at low
and middle latitudes. At higher midlatitudes, the seasonal behaviour depends on
the geographic latitude of the winter downwelling zone, though the effect of the
composition changes is modified by the large solar zenith angle at midwinter.
The zenith angle effect is especially important in longitudes far from the
magnetic poles. Here, the downwelling occurs at high geographic latitudes, where
the zenith angle effect becomes overwhelming and causes a midwinter depression
of electron density, despite the enhanced atomic/molecular ratio. This leads to
a semiannual variation of NmF2. A different situation exists in winter at
longitudes near the magnetic poles, where the downwelling occurs at relatively
low geographic latitudes so that solar radiation is strong enough to produce
large values of NmF2. This circulation-driven mechanism provides a
reasonably complete explanation of the observed pattern of F2 layer annual and
semiannual quiet-day variations.
Key words: Atmospheric composition and structure
(thermosphere-composition and chemistry) - Ionosphere (mid-latitude ionosphere;
modelling and forecasting) |
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