Measurements of the Daytime Abundance of Upper Atmospheric Sodium

Measurements made in 1961 by Blamont and Donahue on daytime sodium emission indicated that the sodium abundance was several times larger than during twilight. If this is true, then the sodium twilight emission should be profoundly affected, but no such effect has been observed. It is, therefore, of considerable importance that the daytime abundance measurement be confirmed by an independent method. This paper reports measurements of the daytime sodium abundance made at Saskatoon (52° 08′ N., 106° 40′ W.) by observing the variation of absorption of solar radiation by the terrestrial sodium as a function of the solar elevation. The apparatus consisted basically of a low-resolution Fabry–Perot spectrometer and a sodium-vapor cell. The results suggest that the daytime abundance is about 4.6 times higher than that during twilight, but for two reasons this result must be accepted with reservation. First, the seasonal variation of daytime abundance appears to be the same as that for twilight, whereas Blamont and Donahue found them to be opposite in phase. Second, the abundance values obtained from the D1 line differ, sometimes by as much as a factor of 2, from those obtained from the D2 line. Several reasons, none of them conclusive, are offered for this inconsistency.