Properties of the lower transition region: The widths of optically allowed and intersystem spectral lines

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. Interest in these motions for coronal heating theories has led to the measurement and comparison of spectral line profiles/widths throughout the solar UV and EUV spectrum. We find that for the quiet Sun the widths of some optically allowed lower transition region lines, deduced from spectra obtained by the Solar Ultraviolet Measurements of Ultraviolet Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO) spacecraft, are considerably larger than predicted from simply scaling previously measured wavelengths of other lines from the same ion. For example, the O III lines of the multiplet near 834 Angstrom are considerably wider than predicted from the previously measured (from Skylab) width of the optically thin O III 1666.15 Angstrom intersystem line. The excess widths are not due to nonthermal motions, as these are already included in the width of the 1666.15 Angstrom line. In this paper, we analyze the widths of some prominent optically allowed lines and discuss possible causes for discrepancies with previous measurements of intersystem lines.