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An empirical formula giving attenuation as a function of F2 peak ion density and hydromagnetic-wave frequency has been applied to a nine-month data sample of hydromagnetic emissions (quasi-sinusoidal oscillations of the earth's magnetic field in the frequency interval around 1 cps). Under the hypothesis that these signals are attenuated as they propagate downward through the ionosphere as hydromagnetic waves, the amplitude and frequency spectrum above the ionosphere at middle latitudes has been deduced.
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R. C. Wentworth (1964) studied this question.
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