Donald H. Zacharisen; Rayner K. Rosich; G. R. Enoch

Abstract: The Fl layer is not explicitly included in current ITS high frequency radio propagation predictions. Numerical maps of Fl layer characteristics, similar to those for F2, E, and Es characteristics now in use, were rejected in favor of a more concise representation by a formula expressing the strong dependence of monthly median foFI on the sun's zenith angle. Three formulas for predicting monthly median foFI were tested against 8 months of monthly median foFl data observed in 1958 and 1964, years of maximum and minimum, respectively, of the solar cycle. The root mean square residuals between observed data and predictions from the three formulas were compared. Recommendations are made for deriving an improved formula.

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