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

Context. The steep temperature and density gradients that are measured in the coronal transition region challenge the model interpretation of observations. Aims. We derive the average electron density < ne > in the region emitting the S vi lines. We use two different techniques, which allow us to derive linearly-weighted (opacity method) and quadratically-weighted (emission measure method) electron density along the line-of-sight, to estimate a filling factor or derive the layer thickness at the formation temperature of the lines. Methods. We analyze SoHO/SUMER spectroscopic observations of the S vi lines, using the center-to-limb variations in radiance, the center-to-limb ratios of radiance and line width, and the radiance ratio of the 93.3-94.4 nm doublet to derive the opacity. We also use the emission measure derived from radiance at disk center. Results. We derive an opacity tau(0) at S VI 93.3 nm line center of the order of 0.05. The resulting average electron density < n(e)>, under simple assumptions concerning the emitting layer, is 2.4 x 1016 m(-3) at T = 2 x 10(5) K. This value is higher than (and inconsistent with) the values obtained from radiance measurements (2 x 10(15) m(-3)). The last value corresponds to an electron pressure of 10(-2) Pa. 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. It implies a low opacity of between 5 x 10(-3) and 10(-2). It remains unexplained why a direct derivation leads to a much higher opacity, despite tentative modeling of observational biases. Further measurements in S vi and other lines emitted at a similar temperature should be completed, and more realistic models of the transition region need to be used.