34th Solar Physics Division Meeting, June 2003
Session 17 Corona III
Poster, Wednesday, June 18, 2003, 3:30-5:00pm, Mezzanine

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[17.06] Properties of the Lower Transition Region Deduced from Widths of Optically Allowed Lines in SUMER Spectra

G. A. Doschek (Naval Research Laboratory)

The widths of spectral lines in the ultraviolet (UV) and extreme ultraviolet (EUV) spectral regions that are formed in the solar transition region and corona are usually greater than the optically thin widths due to thermal Doppler broadening calculated under the assumption of ionization equilibrium. Although opacity can explain the widths of some lines, there are a host of optically thin lines for which the excess widths are attributed to nonthermal motions of unknown origin. Interest in these motions has lead to the measurement of spectral line profiles and widths throughout the solar UV and EUV spectrum. I find that for the quiet Sun the widths of the optically allowed lower transition region O III lines of the multiplet near 834 Å\ deduced from SUMER/SOHO spectra are considerably wider than predicted from simply scaling the previously measured (from Skylab) width of the optically thin O III 1666.15 Å\ intersystem line. The excess widths are not due to nonthermal motions as these are already included in the width of the 1666.15 Å\ line, and opacity in the 834 Å\ lines does not appear to be adequate in itself to explain the result. I find a similar result for optically allowed lines of other ions observed in SUMER spectra. I discuss possible causes for the excess widths and show that they can be explained by several effects. In some cases opacity is significant and excess broadening due to opacity provides a direct measure of the path length through lower transition region structures at Sun center. This work was supported by the Naval Research Laboratory basic research program.

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