Abstract:
A study is made of the seasonal and sunspot cycle trends in the effect of geomagnetic activity on the nocturnal occurrence of spread-F configurations at Kodaikanal (Geo. Mag. Lat. 0.60 N, dip 3.50 N), using ionogram data for the period 1957-1963. It is found that, in overall behaviour, the peak occurrence of range spread shows a systematic decrease with increase of Kp (upto a value of 6) during D and E months. Such a trend in the peak occurrence of frequency spread is noticed only during E months. The occurrence of range spread in the presunrise period is nearly independent of Kp during D and E months, while in J months it is relatively more for frequent quiet to moderately disturbed conditions (Kp m the range 1 to 6) compared to very quiet and highly disturbed periods (Kp 0 and above 6). A comparison of these and other features observed at Kodaikanal with those at Huancayo, reported recently, indicates marked differences in the seasonal and sunspot cycle trends of the effect of geomagnetic activity on spread-F configurations in the Indian and American equatorial regions.
