The solar reference spectrum named R74113 announced at the 1975 International Association of Geomagnetism and Aeronomy symposium in Grenoble refers to the solar conditions of Aporil 23, 1974, and covers the wavelength range 14-2000/•. The fluxes for R74113 between 250 and 1940 A were based on a preliminary evaluation of data obtained from a rocket experiment flown on April 23, 1974. The fluxes outside of the wavelength range covered by the rocket experiment were estimated values for the solar condition of April 23, 1974. This reference spectrum has been revised because of refined evaluation of the rocket data and detailed experience gained from many scans of the solar spectrum for the entire wavelength range 140-1850 J• obtained from the extreme ultraviolet spectrometer experiment on the AE-E satellite. Although numerous changes have been made in the revised spectrum named F74113, summarized here, these changes have only a small effect on aeronomical calculations that have been based on the original reference spectrum. A solar EUV reference spectrum for the wavelength region from 14 to 2000 • was announced and summarized by Hinteregger [1976] at the 1975 International Association of Geomagnetism and Aeronomy (IAGA) symposium in Grenoble. Named R74113, this reference spectrum has been used in various aeronomical studies and also in the evaluation of data obtained from the extreme ultraviolet spectrometer (EUVS) experiments [Hinteregger et al., 1973; Hinteregger, 1976] on the Atmosphere Explorer (AE) satellites. The absolute fluxes fo? the part of R74113 in the wavelength range 250-1940/• were obtained from a rocket measurement of full-disk solar fluxes conducted on April 23, 1974 [Heroux and Higgins, 1977]. On the same day the EUVS experiment on AE-C also recorded solar fluxes over much of this wavelength region so that the rocket values of absolute flux could be used to establish absolute calibration factors for the EUVS experiment [Hinteregger, 1976; Hinteregger et al., 1977]. The fluxes

Solar radiation in the wavelength region 1220‐52 Å emitted from the whole solar disk has been measured by a rocket spectrometer flown from White Sands Missile Range on August 23, 1972. The intensities have been corrected for atmospheric absorption. The estimated errors in the intensities are ±30% between 1220 and 150 Å and ±50% between 150 and 52 Å. These data have been used to calculate electron densities in the altitude region 110–300 km. The calculated electron densities are in good agreement with densities measured by an ionosonde during the rocket flight.

The existence of marked structure in the 2‐ to 5‐eV energy spectra of near‐earth ionospheric photoelectrons has been predicted by theory and observed by experiment. This structure, arising from resonant vibrational excitation of N2, is seen as a valleylike dip in the spectra near 2.5 eV and is predicted to be most prominent at low altitudes (110 km), then to diminish with increasing altitude until it disappears in the 200‐ to 250‐km region. All measurements to date indicate that the overall magnitude of this valley or dip is significantly less than that predicted by theory, but confirm a trend toward diminishing prominence with increasing altitude above about 150 km. Below 140–150 km, however, the valley is observed in the present data to also diminish with decreasing altitude, a reversal of the predicted trend. The present measurements include the only observations, to our knowledge, of this spectral structure below 140 km, and these indicate that its maximum prominence occurs in the 140‐ to 150‐km altitude region, below which it is consistently observed to decline rapidly in magnitude. Recent refinement of theory may lead, in part, to an explanation of these discrepancies. The experimental results are summarized and the difficulties peculiar to space vehicle measurements of very low energy ambient electrons are examined.

A summary of full-disk solar fluxes for the wavelength region 250-1940 • compiled from data obtained from six Air Force Geophysics Laboratory rocket spectrometers flown during the period 1969-1976 is presented. The intense spectral lines in the wavelength region from 300 to 1220 A• are emitted predominantly from the solar chromosphere and the chromosphere-corona transition region. The data indicate that the flux in this wavelength region increases generally by less than 10% with increasing values of the solar Fxo. 7cm emission from 70 to 177 in units of 10 -• W m -• Hz -•. ultraviolet spectrum between 50 and 300 •, Astrophys. J., 181, 1009-1030, 1973. M anson, J. E., The solar spectral irradiance between 10 and 300 •, in The Solar Output and Its Variation, edited by O. R. White, University of Colorado Press, Boulder, 1976a. Manson, J. E., The solar extreme ultraviolet between 30 and 205 • on November 9, 1971, compared with previous measurements in this spectral region, J. Geophys. Res., 81, 1629-1635, 1976b. Munroe, R. H., A. K. Dupre•e, and G. L. Withbroe, Electron densities derived from line intensity ratios: Beryllium isoelectronic sequence, Solar Phys., 19, 347-355, 1971. Roble, R. G., Solar EUV flux variations during a solar cycle as derived from ionospheric modeling considerations, J. Geophys. Res., 81, 265-269, 1976. Schmidtke, G., EUV indices for solar-terrestrial relations, Geophys. Res. Lett., 3, 573-576, 1976. Timothy, A. F., and J. G. Timothy, Long-term intensity variations in the solar helium II Lyman alpha line, J. Geophys. Res., 75, 6950-6958, 1970.

A rocket borne spectrometer was flown to rneasure absolute intensities of extreme ultraviolet spectral lines from the three ions O\% Ncv[n, and Mgx present in the Sun. From these measurements, intensity ratios of lines from Ow, ratios of lines from Nevm, and ratios of lines from Mgx were formed. These experimental ratios were compared with ratios calculated by using specific theoretical values of the ionization equilibrium in which dielectronic recombination was included in lhe processes establishing ionization balance. The effects of the electron density and temperature gradient on the temperature distribution of the flux of the spectral lines in the solar atmosphere have been taken into account in the calculations of the ratios. The agreement between the experimental and calculated ratios is good Ibr the ions Nevm and Mgx and satisfactory for the ion Ow for which the calculated ratio is subject to large uncertainties. A reliable measurement of the electron temperature in the lower corona was obtained from the experimental ratios for Mgx. This experimental temperature is in good ag]'eemcnt with the emission temperature of the spectral lines of Mgx predicted from the theoretical values of the ionization equilibrium. The design and photometric calibration of a new rocket spectrometer developed to measure the intensity ratios over the broad spcctral region 50 to 1250 A are also described.