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Annales Geophysicae An open-access journal of the European Geosciences Union
Ann. Geophys., 17, 1111-1115, 1999
© European Geosciences Union 1999
31 Aug 1999
Letter to the Editor: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum
J. H. Sastri Indian Institute of Astrophysics, Bangalore 560 034, India, e-mail:
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.

Key words. Ionosphere (equatorial ionosphere; ionospheric irregularities)

Citation: Sastri, J. H.: Letter to the Editor: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum, Ann. Geophys., 17, 1111-1115, doi:10.1007/s00585-999-1111-4, 1999.
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