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

TL;DR

This study uses SOHO/SUMER data to measure electron densities and opacity in the solar transition region, revealing higher densities than previous radiance-based estimates and highlighting the need for improved models.

Contribution

It provides new measurements of electron density and opacity in the solar transition region using spectroscopic data, addressing discrepancies with classical estimates.

Findings

Electron opacity at S VI line center is about 0.05.

Average electron density is approximately 2.4×10^16 m^-3 at 2×10^5 K.

Derived pressure aligns with previous studies, but opacity estimates differ significantly.

Abstract

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.