Letter to the Editor: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

J. H. Sastri (1)

1 IIA - Indian Institute of Astrophysics

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.

Domaines: Océan, Atmosphère; Sciences de la Terre

J. H. Sastri. Letter to the Editor: Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum. Annales Geophysicae, 1999, 17 (8), pp.1111-1115. ⟨hal-00319057⟩