The results of a study to improve the global maps of monthly median values of the F2 region critical frequency, f0 F2, using values determined from a theoretical model are presented. Values of the midlatitude F2 region critical frequency that could be used to improve the prediction of f0 F2 in regions of the earth inaccessible to ground‐based measurements were obtained. This was accomplished by including in the theoretical calculations realistic physical processes along with a realistic geomagnetic field model. Parameters were adjusted so that agreement was achieved between calculated and observed f0 F2 values as a function of local time at different stations, and then these same input parameters (i.e., neutral wind field, and neutral atmospheric model) were assumed valid at all regions of interest where the major difference is the geomagnetic field line configuration. Coefficients that yield global representations of f0 F2 were then determined using the theoretically derived f0 F2 values, and the predicted critical frequencies were compared with observed values to estimate the degree of improvement.

Observations of the F 2 region critical frequency, foF2, and values determined from the timedependent continuity equation for ions and electrons in the ionosphere have been used to develop a new set of numerical coefficients to represent the global variation of foF2 . Like those in earlier investigations, the new coefficients permit monthly median hourly values OffoF 2 to be obtained at any location around the globe for any month of the year and solar activity level. Comparisons between foF2 determined using older sets of numerical coefficients and foF 2 determined using the new set of coefficients are given along with a description of how well each set of coefficients specifies and predicts the observed variations in the F 2 region critical frequency.

This paper describes observations of the reliability of the ionospheric HF communication channel within and close to the northern auroral zone. Automatic equipment has been developed to measure reliability and virtual reflection height as a function of frequency in an hourly cycle. Measurements have been made over two propagation paths within Norway. The data show diurnal and seasonal variations as well as dependence of the measured parameters on geomagnetic conditions. The observations have been compared with predictions obtained from the Ionospheric Communication Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC) prediction code. This code includes a model of the high‐latitude and polar ionospheres and their dependence upon geomagnetic disturbances. The results of the comparison show that ICEPAC represents an improvement over previous codes but that transmission losses are not properly included.

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A method is given for producing world maps of plasma frequency that includes variation with height above the surface of the earth as well as variations with latitude, longitude, and local mean time. The maps, based on a numerical analysis of ionospheric data, are expressed in a functional form suitable for computer calculation of the value of plasma frequency and any of its partial derivatives at a given position and instant of time. The representation of vertical profiles of plasma frequency is based on a technique introduced by Lanczos for accelerating the convergence of trigonometric approximations. Diurnal and geographic variations are represented by methods previously developed for ionospheric mapping. An illustration is given using the monthly average value of plasma frequency for November 1966 from vertical profiles at 36 stations.