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E. H. Avrett (Harvard-Smithsonian Center for Astrophysics)
Vernazza, Avrett, and Loeser (1981) used Skylab observations of the inhomogeneous structure of the quiet Sun in the EUV wavelength range 40-140 nm to determine separate chromospheric models for six observed brightness components, ranging from dark cell centers to very bright network elements. The intensity at the head of the C I continuum at 110 nm varies by a factor of 4 between cell center and bright network, but the corresponding temperature in the middle chromosphere responsible for this emission varies from about 6000 K to 7000 K. These observations and recent ones from the SUMER instrument on SOHO show variations with time, but the time variations appear to be smaller than the spatial variations. If so, time-averaged models for each separate brightness component seem to be justified as a first approximation. Using separate models for each component seems a reasonable approximation since the cell and network features are large compared with the thickness of the chromosphere. Large excursions of the chromospheric temperature on short time scales appear to be ruled out given the moderate temporal variations of EUV continuum and line intensities observed to date. The EUV observations seem to indicate a chromospheric temperature rise at all locations and at all times.