Post-midnight onset of spread-F at Kodaikanal during the June solstice of solar minimum

At dip equatorial stations in the Indian zone, spread-F conditions are know n to develop preferentially around midnight during the June solstice (north ern summer) months of low solar activity, in association with a distinct in crease 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-Tayl or instability mechanism, with the gravitational and cross-field instabilit y factors (and hence F layer height) playing important roles. We have studi ed the quarter-hourly ionograms of Kodaikanal (10.2 degrees N; 77.5 degrees E; dip 4 degrees N) for the northern summer months (May-August) of 1994 an d 1995 to ascertain the ambient ionospheric conditions against which the po st-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 p urpose. 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 in dividual 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 p lay a pivotal role in the midnight onset of spread-F during the June solsti ce of solar minimum. The implications of this finding are discussed.