Two daytime observations of atomic nitrogen emissions in the thermosphere.

Abstract

The NII 2143 A doublet and the NI 8680 A multiplet have been observed in the daytime thermosphere as part of two rocket experiments conducted during moderate-to-high levels of solar activity. An ultraviolet grating spectrometer, a single channel photometer and a near-infrared grating spectrometer with a CCD detector were used in the experiments. Spectral data were obtained during limb viewing at tangent altitudes between 120 and 230 km. Molecular band shapes are calculated, and the atomic emissions are isolated by spectral fitting using a chi-square analysis. The ratio between the strengths of the doublet lines at 2139 A and 2143 A is found to be 0.58 $\pm$ 0.08. The measured intensity of the NII 2143 A doublet is consistent with production by photodissociative ionization, based on solar flux and photoelectron models and laboratory cross sections for photon and electron impact excitation. The experiment used to detect the NI 8680 A multiplet reveals spectral intensities higher than predicted for photoelectron impact excitation, and photon impact on N$\sb2$ is shown to play the dominant role in the production of the NI 8680 A emission. The magnitude of a cross section for this process is obtained from the data by assuming that the wavelength dependence is described by the excitation function measured for the NI 1200 A line. The peak value of the NI 8680 A photoexcitation cross section is found to be $1.5\times10\sp{-19}$ cm$\sp2$, accurate to within a factor of 5.