Electron density in the quiet solar coronal transition region from SoHO/SUMER measurements of S VI line radiance and opacity

Context: The sharp temperature and density gradients in the coronal transition region are a challenge for models and observations. Aims: We set out to get linearly- and quadratically-weighted average electron densities in the region emitting the S VI lines, using the observed opacity and the emission measure of these lines. Methods: We analyze SoHO/SUMER spectroscopic observations of the S VI lines, using the center-to-limb variations and radiance ratios to derive the opacity. We also use the Emission Measure derived from radiance at disk center. Results: We get an opacity at S VI line center of the order of 0.05. The resulting average electron density is 2.4 10^16 m^-3 at T = 2 10^5 K. This value is higher than the values obtained from radiance measurements. Conversely, taking a classical value for the density leads to a too high value of the thickness of the emitting layer. Conclusions: The pressure derived from the Emission Measure method compares well with previous determinations and implies a low opacity of 5 10^-3 to 10^-2. The fact that a direct derivation leads to a much higher opacity remains unexplained, despite tentative modeling of observational biases. Further measurements need to be done, and more realistic models of the transition region need to be used.