FIRST SPECTRAL OBSERVATIONS OF THE DIFFUSE BACKGROUND WITH THE EXTREME-ULTRAVIOLET EXPLORER

We present the first results from the analysis of the spectroscopic ob servations of diffuse extreme ultraviolet (EUV) emission taken with th e Extreme Ultraviolet Explorer (EUVE) spectrometers in the wavelength range 160-740 Angstrom. Although not designed or optimized for diffuse observation, the EUVE spectrometers are the most sensitive diffuse EU V spectrometer in orbit. The spectral resolution for diffuse emission of the medium and long-wavelength spectrometers are 17 and 34 Angstrom FWHM, respectively. During the period from 1992 July 25 to 1992 Augus t 19, the spectrometers surveyed a 2.0 degrees x 20 degrees field scan ned from (l, b) = (24 degrees, -28 degrees) to (44 degrees, -74 degree s) with a total effective exposure time of 575,232 s. The only emissio n lines detected were those of He I and He II (584, 537, and 304 Angst rom) with intensities consistent with local geocoronal and/or interpla netary scattering of solar radiation (584 Angstrom = 1.30 rayleighs; 5 37 Angstrom = 0.040 R; and 304 Angstrom = 0.029 R). Models of the soft X-ray background, which results from a 10(6) K plasma (Local Bubble) surrounding the neutral gas near the Sun (Local Cloud), predict that m ost of the flux from the hot plasma appears as emission lines in the E UV. We have compared these spectral predictions with our observations to place limits on the emission measure versus temperature of the prop osed hot plasma. Using the same plasma model, we derived emission meas ures for our data and the C and B soft X-ray bands of the Wisconsin ro cket survey. We find that our limits for the plasma emission measure a re a factor of 5-10 below the C- and B-band emission measures over the temperature range from 10(5.7) to 10(6.4) K. We explore possible scen arios that could reconcile our results with the X-ray surveys and conc lude that depletion or a nonequilibrium plasma state rather than absor ption are the more likely explanations of the discrepancy. We also sho w that our spectrum is inconsistent with the spectrum from the similar to 10(5) K gas at the conductive interface between the hot Local Bubb le and the cooler Local Cloud given by Slavin (1989). In addition, we place new limits on the helium ionization parameter in the Sun's vicin ity caused by the 10(6) K plasma in the Local Bubble.