Abstract:
Using ionogram data at Kodaikanal (geomagnetic lat. 06 deg N, Dip 3.5 deg N) over a seven year period, Jan. 1957-Dec. 1963, a study is made of the seasonal and sunspot cycle variations in the nocturnal occurrence of amplitude and frequency type of spread-F configurations. It is found that the occurrence of both amplitude and frequency type spread-F exhibits a positive dependence on the phase of the sunspot cycle. There is also a significant positive correlation between the occurrence patterns of amplitude and frequency type spread-F. The occurrence of amplitude type spread-F shows a prominent peak around 20-22 hrs LT, irrespective of season and phase of the sunspot cycle, except during local summer months of low sunspot years when it is shifted to the postmidnight period around 00-03 hrs LT. The peak occurrence of amplitude spread-F (for two hours around the peak) shows a semi-annual variation during years of moderate and low sunspot activity. A comparison of these results with those at other equatorial stations revealed a marked longitudinal dependence of the characteristics (nocturnal, seasonal and sunspot cycle changes) of amplitude and frequency type spread-F.
