The relative line strength and intensity of the N II 2143 doublet

The doublet emission from N II at 2139.7 A and 2143.6 A was observed by a 1.4-m scanning spectrometer with 3.1 A resolution in the daytime, high-altitude thermosphere during moderate levels of solar activity. The spectrometer viewed the earth's limb 5 deg below the local horizontal to give a nominal tangent height of 152 km. Both sub band heads of the nitric oxide gamma band system were resolved in the data at the resolution used. The emission features from N II are clearly evident on the short wavelength shoulder of the (1, 0) band. Synthetic profiles of the (1, 0) gamma band and the (0, 3) delta band of nitric oxide were fitted to the data using a chi-square analysis. These contributions were removed from the data leaving a residual emission, considered to be the N II doublet. A chi-square minimization of the data relative to a synthetic intensity profile was done. The minimum was for a line strength ratio between the 2139 A and 2143 A lines of 0.58 + or - 0.08. The mean solar EUV flux deduced from the intensity of the N II emission in this experiment is lower than other reported observations, consistent with a lower solar activity level.