Satellite Observations of the Extreme Ultraviolet and Far Ultraviolet Radiation Fields.

Observations of the astronomical extreme ultraviolet (EUV) and far ultraviolet (FUV) radiation fields in the 600-1200 (ANGSTROM) range are presented. Such observations are of interest both because the principal sources expected (stellar photospheres and interstellar coronal gas) have seldom been studied at these wavelengths and because the effects of EUV/FUV radiation upon the interstellar medium are substantial. The data were obtained with a spectrometer launched on board the P78-1 satellite in 1979. The instrument, a near normal incidence Rowland spectrometer with imaging microchannel plate detectors, is described in detail, along with the laboratory calibration procedures employed. Coverage of 12% of the sky is obtained from periods when the spectral coverage of the spectrometer extended longward of 912 (ANGSTROM). Airglow emissions are identified, flat fielding of the detector is performed and corrections are made for grating scattering and internal background. The fully reduced spectra are compared with previous observations and theoretical expectations. The upper limits derived at high galactic latitudes are significantly more stringent than those of previous observations, but are still above theoretical predictions for those directions. At low latitudes, the signal detected longward of 912 (ANGSTROM) is in excellent agreement with model atmosphere predictions, suggesting that the FUV radiation field is dominated by a small number of bright early type stars. Limits are also derived on the properties of hot coronal gas in the interstellar medium. The data presented here rule out high pressure, low filling factor sources for the EUV background observed at higher energies. It is argued that the intensity of the sub-912 (ANGSTROM) radiation field is lower than implied by previous observations and predictions. Known sources are shown to be too faint or too absorbed to provide the predicted flux. Further, it is argued that the observed neutrality of the very local interstellar medium precludes the presence of a significant local EUV radiation field. Finally, limits to the radiative lifetime of massive neutrinos are derived. The resulting lower limits are longer than the age of the universe.