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The filling-in or Ring effect for the three Fraunhofer lines 4227 Å CaI, 4326 Å FeI, and 4340 Å HI has been observed in the day sky under clear and cloudy conditions. A pronounced diurnal variation for all three lines shows a midday minimum with a three-fold increase as the solar zenith angle increases from 30° to 90°.
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A. W. Harrison (1976) studied this question.
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