highest frequency at which a radio wave undergoes reflection in an ionospheric layer on which it is incident vertically
Note 1 to entry: There is usually one critical frequency for each magneto-ionic component
Note 2 to entry: The critical frequency for the ordinary wave is the plasma frequency corresponding to the maximum electron density of the layer.
Note 3 to entry: The critical frequencies for the ordinary wave and the extraordinary wave are respectively represented by the symbols fo and fx, followed by the designation of the layer, for example: foF2.
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