As measured by the Solar Mesosphere Explorer satellite, the density of nitric oxide at low latitudes (30°S to 30°N) and at 110 km (E‐region) decreased from a mean value of 3 × 107 molecules/cm³ in January 1982 to a mean value of 4 × 106 molecules/cm³ in April 1985. In addition, the nitric oxide density varied ±50% with a 27‐day period during times of high solar activity. The variation of nitric oxide correlates with variations in the solar Lyman‐alpha irradiance which is also measured by the Solar Mesosphere Explorer satellite. The Lyman alpha irradiance is interpreted as an index of the variations in the solar EUV and soft X‐ray flux. The hypothesis is proposed that the solar X‐ray flux between 20 and 100 Å has a larger variation than the solar EUV flux between 100 and 1050 Å and that the solar X‐rays produce photoelectrons which are the source of the nitric oxide.

A rocket measurement of thermospheric nitric oxide (NO) is used to evaluate the production of odd nitrogen by solar soft X rays (18-50 /•). The rocket observation was performed over White Sands Missile Range on November 9, 1981, at 1500 LT for solar maximum conditions (F10.7 = 233). The peak observed NO density was 6.3 x 107 cm -3 at 102 km. A photochemical model which included soft X rays was used for comparison with the data. The soft X rays create photoelectrons which lead to enhanced ionization of N2 and thus increased odd nitrogen production. A good fit to the data was achieved using a soft X ray flux of 0.75 erg cm-2 s-1. INTRODUCTION Nitric oxide (NO) is an important trace constituent in theEarth's therrnosphere. It is both chemically and radiatively active and thus plays an important role in determining the composition and structure of the atmosphere above 100 km [Roble and Emery, 1983; Roble et al., 1987]. It is also thought [Solomon et al., 1982] that NO may be transported down from the thermosphere to the stratosphere where it can react with ozone. The emission of NO in the (1,0) gamma band near 2150/!, is one of the brightest features of the Earth's ultraviolet spectrum and thus allows the NO density to be inferred from rocket or satellite observations. This technique was pioneered by Barth [1964] and has since been used to observe NO from the O130 4, Atmosphere Explorer (AE) C and D, and Solar Mesosphere Explorer (SME) satellites [Rusch and Barth, 1975; Cravens and Stewart, 1978; Stewart and Cravens, 1978; Cravens et al., 1985; Barth et al., 1988; Siskind et al., 1989a, b] as well as from a number of rockets [e.g., Thomas, 1978; McCoy, 1983; Ogawa et al., 1984; Cleary, 1986]. These measurements have shown that the peak of the NO layer is generally in the range 100-120 km and that the value of the NO density at the peak is quite variable. Barth et al. [1988] showed from 5 years of SME observations that equatorial NO at 110 km varied by a factor of 8 from 3 x 107 cm-3 at solar cycle maximum in 1982 to 4 x 106 cm -3 at solar cycle m'mimum in 1986. In addition, the NO varied with the 27-day period of solar rotation. E region temperatures (<400 K) although it is strongly temperature dependent. The primary loss of NO is through excited ultraviolet dayglow, Eos Trans. AGU, 68, 368, 1987. Conway, R. R., Photoabsorption and photoionization cross sections of O, 0 2, and N 2 for photoelectron production calculations: A compilation of recent laboratory measurements, Memo. Rep. 6155, Naval Res. Lab., Washington, D.C., 1988. Cravens, T. E., Nitric oxide gamma band emission rate factor, Planet. Space Sci., 25, 369,1977. Cravens, T. E., and A. I. Stewart, Global morphology of nitric oxide in the lower E region, The global distribution of nitric oxide in the thermosphere as determined by the Atmosphere Explorer D satellite, J. Geophys. Res., 90, 9862, 1985. Cummings, W. P., and L. G. Piper, The rate coefficient for quenching N(2D) by O('3P), Eos Trans. AGU, 70, 414, 1989. Earls, L. T., Intensities in the 2-2 tran...

Observations of the Earth's thermospheric dayglow were obtained on August 10, 1982, from Poker Flat, Alaska, using a rocket‐borne spectrometer covering the wavelength range 1950 to 2170 Å. This part of the spectrum is dominated by emissions from the gamma, delta and epsilon bands of nitric oxide. Column densities of nitric oxide were determined by fitting these bands with synthetic spectra. At low altitudes the (1, 0) and (2, 0) gamma bands exhibit large self‐absorption. Three bands of the weak NO epsilon system, (0, 1), (0, 2), and (0, 3) were positively identified and used to determine NO densities between 105 and 125 km. The fluorescence efficiency of the NO delta bands was found to be ∼0.25 based on the fit to synthetic spectra. A one‐dimensional diffusive photochemical model of the earth's thermosphere was used for comparison with the rocket observations. The data was reproduced by the model using a 10 keV characteristic energy for the primary aurora electrons with a constant flux of 0.5 ergs cm−2 s−1. The production yield of N(²D) from the dissociation of N2 by electron impact used in this calculation was 0.6. An analysis was performed to show the sensitivity of the conclusions to the various model parameters.

Spectroscopic measurements of the thermospheric dayglow in the wavelength range 1900 to 3400 Å are presented. These measurements were made during two rocket experiments conducted on March 30, 1990, and March 19, 1992, from White Sands Missile Range, New Mexico. The data are presented to provide reference spectra in the lower, middle, and upper thermosphere. The 1990 observations, which were made during high geomagnetic activity, showed considerably enhanced nitric oxide (NO) intensities. Self‐absorption theory is applied to the υ″ = 0 bands of the NO γ system. It is found that a recently published self‐absorption algorithm correctly accounts for the attenuation of the γ(1,0) bands. There is a small discrepancy between the theory and observation for the (2,0) band and the (0,0) band intensity. The fact that there is reasonable agreement for all three bands suggests that both the NO slant column density and oscillator strengths for these bands are correct.

Abstract. Previous models for dayslow and auroral emissions of the N2(A 3•Eu+--->XVegard