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Title: | Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum |
Authors: | Sastri, J. H |
Keywords: | Ionosphere (equatorial ionosphere; ionospheric irregularities) |
Issue Date: | Aug-1999 |
Publisher: | European Geosciences Union |
Citation: | Annales Geophysicae, Vol. 17, No. 8, pp. 1111 - 1115 |
Abstract: | 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. |
URI: | http://hdl.handle.net/2248/3420 |
ISSN: | 0992-7689 |
Appears in Collections: | IIAP Publications |
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