A statistical survey of SCNAs over half solar cycle

A study of the characteristics of sudden cosmic noise absorption (SCNA) is made in order to investigate into their variations from IGY to IQSY. The necessary data for the investigation are taken from CRPL-Part B (now NOAA) series as reported by five observatories in the American Zone. The entire period of analysis (1958-65) is divided into three groups, viz. periods of high, medium and low solar activity, and the optical flares associated with the SCNA are divided into three classes. The analysis reveals that the SCNA size has an increasing tendency with decreasing solar activity. The growth-time associated with class 1 flares is observed to be decreasing with decreasing solar activity, while for the class 2 and class 3 flares the reverse is the case. The time of decay of the SCNA shows a decrease with decrease in the level of solar activity. No systematic latitudinal variation is observed in the size, growth-time, and decay-time of the SCNAs.