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ABSTRACT
This paper describes a new method of deriving a planetary index 
for F2 layer ionization. It is shown that planetary F2 layer ionization exhibits remarkable seasonal enhancement with biannual maxima which occur regularly and persistently during the months of April and October every year. A correlation coefficient of 0·95 has been found to exist between the observed values of and those calculated on the basis of this planetary index and solar activity for the period January 1953 to July 1962.