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| Titel |
Model results for the ionospheric E region: solar and seasonal changes |
| VerfasserIn |
J. E. Titheridge |
| 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 ; 15, no. 1 ; Nr. 15, no. 1, S.63-78 |
| Datensatznummer |
250012574
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| Publikation (Nr.) |
 copernicus.org/angeo-15-63-1997.pdf |
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| Zusammenfassung |
| A new, empirical model for NO densities is
developed, to include physically reasonable variations with local time, season,
latitude and solar cycle. Model calculations making full allowance for secondary
production, and ionising radiations at wavelengths down to 25 Å, then give
values for the peak density NmE that are only 6% below the
empirical IRI values for summer conditions at solar minimum. At solar maximum
the difference increases to 16%. Solar-cycle changes in the EUVAC radiation
model seem insufficient to explain the observed changes in NmE,
with any reasonable modifications to current atmospheric constants. Hinteregger
radiations give the correct change, with results that are just 2% below the IRI
values throughout the solar cycle, but give too little ionisation in the E-F
valley region. To match the observed solar increase in NmE,
the high-flux reference spectrum in the EUVAC model needs an overall increase of
about 20% (or 33% if the change is confined to the less well defined radiations
at λ < 150 Å). Observed values of NmE show a
seasonal anomaly, at mid-latitudes, with densities about 10% higher in winter
than in summer (for a constant solar zenith angle). Composition changes in the
MSIS86 atmospheric model produce a summer-to-winter change in NmE
of about –2% in the northern hemisphere, and +3% in the southern hemisphere.
Seasonal changes in NO produce an additional increase of about 5% in winter,
near solar minimum, to give an overall seasonal anomaly of 8% in the southern
hemisphere. Near solar maximum, reported NO densities suggest a much smaller
seasonal change that is insufficient to produce any winter increase in NmE.
Other mechanisms, such as the effects of winds or electric fields, seem
inadequate to explain the observed change in NmE. It therefore
seems possible that current satellite data may underestimate the mean seasonal
variation in NO near solar maximum. A not unreasonable change in the data, to
give the same 2:1 variation as at solar minimum, can produce a seasonal anomaly
in NmE that accounts for 35–70% of the observed effect at all
times. |
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