A method for predicting the F₁ layer critical frequency based on the Zurich smoothed sunspot number

A series of expressions to calculate the monthly median F 1 layer penetration frequency ( ƒoF 1 ) as a function of the ionospheric index IF 2 was reported by DuCharme et al. , [1971]. Using similar techniques and the same data base, development of a series of expressions to calculate ƒoF 1 as a function of the Zurich smoothed sunspot number R 12 is described. The derived expressions are evaluated for the period of maximum solar activity 1967 through 1969 and compared with observed ƒoF 1 data for this period, which are outside the 1954 through 1966 data base used to develop the expressions.