New models of the solar chromosphere and transition region from SUMER observations

The SUMER atlas of the disk-center spectrum of the average quiet Sun between 67 and 161 nm, together with HRTS spectra to 173 nm, provide valuable observational data that can be used to determine the structure of the chromosphere, transition region, and low corona. The Pandora atmospheric modeling program is being used to determine atmospheric models and calculated spectra consistent with these observations, resulting in a new model of the average quiet chromosphere and transition region in better agreement with observations than before. We find that temperature variations of 400 K cause the computed chromospheric intensities, both in lines and the continuum, to change by a factor of about 4. This is greater than the observed intensity variations at chromospheric wavelengths 1) with time in quiet regions, and 2) with position from cell centers to bright network. These results appear to conflict with dynamical models that predict time variations of 1000K or more in the chromosphere. Also, we demonstrate that the optically-thin or coronal approxiniation that is often used to analyze observed line intensities from high stages of ionization is reasonabl.y accurate only: 1) for C IV and higher carbon ions. and C III for T > 50, 000 K, and 2) for 0 V and higher oxygen ions, and 0 IV for T > 150, 000 K..