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| Titel |
Letter to the Editor: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum |
| VerfasserIn |
J. H. Sastri |
| 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 ; 17, no. 8 ; Nr. 17, no. 8, S.1111-1115 |
| Datensatznummer |
250013803
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| Publikation (Nr.) |
 copernicus.org/angeo-17-1111-1999.pdf |
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| Zusammenfassung |
At dip equatorial stations in the Indian
zone, spread-F conditions are known to develop preferentially around midnight
during the June solstice (northern summer) months of low solar activity, in
association with a distinct increase in F layer height. It is currently held
that this onset of spread-F far away from the sunset terminator is due to the
generalised Rayleigh-Taylor instability mechanism, with the gravitational and
cross-field instability factors (and hence F layer height) playing important
roles. We have studied the quarter-hourly ionograms of Kodaikanal (10.2°N;
77.5°E; dip 4°N) for the northern summer months
(May-August) of 1994 and 1995 to ascertain the ambient ionospheric conditions
against which the post-midnight onset of spread-F takes place. A data sample of
38 nights with midnight onset of spread-F and 34 nights without spread-F is used
for the purpose. It is found that a conspicious increase in F layer height
beginning around 2100 LT occurs on nights with spread-F as well as without
spread-F. This feature is seen in the nocturnal pattern of F layer height on
many individual nights as well as of average F layer height for the two
categories of nights. The result strongly suggests that the F layer height does
not play a pivotal role in the midnight onset of spread-F during the June
solstice of solar minimum. The implications of this finding are discussed.
Key words. Ionosphere (equatorial ionosphere;
ionospheric irregularities) |
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