Back to the Abstracts Index Page
P. Jelinsky, J. V. Vallerga, and J. Edelstein
Center for EUV Astrophysics, 2150 Kittredge Street, University of California, Berkeley, CA 94720
We present the first results from the analysis of the spectroscopic observations of diffuse extreme ultraviolet (EUV) emission taken with the Extreme Ultraviolet Explorer (EUVE) spectrometers in the wavelength range of 160-740Å. Though not designed or optimized for diffuse observation, the \euve\ spectrometers are the most sensitive diffuse EUV spectrometer in orbit. The spectral resolution for diffuse emission of the Medium and Long Wavelength spectrometers are 17 and 34Å FWHM, respectively. During the period from 1992 July 25 to 1992 August 19, the spectrometers surveyed a 2.0 deg x 20 deg field scanned from (l^II, b^II) = (24 deg -28 deg) to (44 deg, -47 deg) with a total effective exposure time of 575,232 sec. The only emission lines detected were those of HeI and HeII (584, 537, and 304Å), with intensities consistent with local geocoronal and/or interplanetary scattering of solar radiation (584Å = 1.30 rayleighs; 537Å = 0.040 R; and 304Å = 0.029 R).
Models of the soft X-ray background, which results from a 10E+06 K plasma (Local Bubble) surrounding the neutral gas near the Sun (Local Cloud), predict that most of the flux from the hot plasma appears as emission lines in the EUV. We have compared these spectral predictions with our observations to place limits on the emission measure vs. temperature of the proposed hot plasma. Using the same plasma model, we derived emission measures for our data and the C and B soft X-ray bands of the Wisconsin rocket survey. We find that our limits for the plasma emission measure are a factor of 5 to 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 scenarios that could reconcile our results with the X-ray surveys and conclude that depletion or a non-equilibrium plasma state rather than absorption are the more likely explanations of the discrepancy. We also show that our spectrum is inconsistent with the spectrum from the ~10E+05 K gas at the conductive interface between the hot Local Bubble and the cooler Local Cloud given by Slavin (1989). In addition, we place new limits on the helium ionization parameter in the Sun's vicinity caused by the 10E+06 K plasma in the Local Bubble.
keywords: ISM: abundances --- ISM: structure --- ultraviolet: interstellar --- ISM: clouds
Back to the Abstracts Index Page
