Spread-F at Kodaikanal

Abstract

Quarter-hourly ionograms of Kodaikanal for the period January 1968 to December 1969 have been examined for occurrence of Spread-F. Spread-F occurrence as a feature on ionograms is more or less a regular nocturnal event with little seasonal variation. There is a significant positive correlation between Spread-F occurrence and mean value of the H-component of the magnetic field. The effect of SC type magnetic storms is in general to inhibit the Spread-F incidence either on the same night or the succeeding night for both the pre- and post-midnight Spread-F. Several instances have been noticed when there is disappearance and subsequent appearance of Spread-F during a night within a span of a couple of hours or even less. Preliminary indication is that this feature is not dependent on the height of the layer crossing the threshold value for the production of irregularities and hence Spread-F.