D-layer ionization during auroras was studied in the north of Sweden during the IGY. It was found that this type of D layer is closely associated with sudden auroral increase (SAI). The lowest measured heights of this D layer are ∼65 km. The latitude variations of this D layer are discussed. It seems likely that both primary electrons and secondary X rays may contribute to the formation of the auroral D ionization down to 65 km.
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Stoffregen et al. (1960) studied this question.
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