RATIO AND ABSOLUTE BRIGHTNESS MEASUREMENTS OF THE SODIUM D LINES IN THE TWILIGHT GLOW

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Abstract
Simultaneous measurements of the ratio and absolute brightness of the sodium D lines in the twilight glow have been made with a Fabry–Perot spectrometer. The atomic sodium abundance can be obtained from either of these quantities alone, using the resonance scattering calculations made by others. It was expected that simultaneous measurements of these quantities would provide a good test of the theory and its assumptions. Unfortunately, one experimental parameter is not precisely known; the ratio of the amounts of exciting radiation at the bottom of the solar D1 and D2 lines. With a suitable choice for this parameter, satisfactory agreement between the calculations and the measurements was obtained. In particular, the results reported here do not support the recent suggestion that the sodium abundance on the day side of the earth is usually much larger than in the twilight region.

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