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The ionization rate of helium in the thermosphere is inferred from AE‐E ion and neutral particle composition measurements, made on several days during solar cycle 21, in the context of a steady state photochemical model for helium ions. The inferred ionization rate varies by a factor of 4 between July 1976 and January 1979, a value consistent with the variation in the flux measured during the same period by the EUV spectrometer on board the AE‐E satellite.
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Oppenheimer et al. (1981) studied this question.
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