Two decades ago, the U.S. Air Force Air Weather Service space forecasting group began generating what was termed an effective sunspot number (SSNe) by fitting a model of the critical frequency of the F2 layer (ƒoF2) to observedƒoF2 values. Initially a preprocessing step in a larger analysis package, this parameter has taken on a life of its own and is now used in various applications for both forecasts and specification of the globalƒoF2 field. This paper describes the various ways in which this parameter is calculated, investigates the behavior of this parameter over solar cycle 21 (1976 through 1986), and compares it with other solar-ionospheric indices, including R12, IF2, IG, and the Ionospheric Prediction Service (IPS) T index.
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