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.
Analysis of data for Washington and Watheroo indicates differences in the relationship between ƒ°F2 and sunspot number for the current and preceding sunspot cycles. The sunspot number is therefore not entirely satisfactory as an index for ionospheric variations. Consequently, ionospheric data for the current cycle only should be used in preparing ionospheric radio propagation predictions whenever possible.
Abstruct--A study has been undertaken to assess the effects of the ionosphere on the performance of a direct broadcasting satellite service operating in the high frequency (HF) band. Results relating to two issues of performance are presented: 1) determining under wjhat conditions a frequency allocated to the broadcasting service can be expected to reach the surface of the earth from a satellite and 2) the loss of signal strength due to passage through the ionosphere.It was found that if high frequency broadcast services are to be provided on a worldwide basis using satellite platforms, the satellites need to be optimally configured for the intended service area. Also, a 3 dB loss of signal strength is expected due to ionospheric absorption.
Analysis of data for Washington and Watheroo indicates differ-ences in the relationship between føF2 and sunspot number for the current and preceding sunspot cycles. The sunspot number is therefore not entirely satisfactory as an index for ionospheric variations. Consequently, ionospheric data for the current cycle only should be used in preparing ionospheric radio propagation predictions whenever possible.
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