Pushing solar spectropolarimetry to the limit

by Jaime de la Cruz Rodriguez (Stockholm University)

Friday 28 Nov 2025, 10:30 → 11:30 Europe/Stockholm

FC61 (AlbaNova Main Building)

The heating of the solar chromosphere and corona remains one of the foremost open questions in solar and stellar physics: how are the outer layers of the Sun heated from a few thousand to several million degrees as we move outwards from its surface? Chromospheric heating is greatly modulated by radiation, gas pressure gradients, magnetic fields, and waves. High-resolution observations and numerical simulations both indicate that the combination of processes that supply energy to the chromopshere are likely to change within very small spatial scales. However, the role of the magnetic field is particularly challenging to assess because of the inherent difficulties of reconstructing the magnetic field vector in the chromosphere.

Moreover, inferring the physical state of the chromospheric plasma remains an outstanding challenge for solar observers. Because of the relatively low density of the chromosphere, the radiative rates can match or surpass the collisional ones. Therefore, the assumption of local thermodynamic equilibrium (LTE) is not valid in the chromosphere and there is no direct (analytical) translation between the observed intensities and the thermodynamic state of the plasma. Modern non-LTE inversion methods have posed one of the most successful approaches to infer high-angular resolution depth-stratified model atmospheres including a photosphere and a chromosphere. The catch is that the depth resolution and height sensitivity are directly affected by the type and number of spectral lines that are simultaneously included in the inversion.

Very few diagnostics have enough opacity to sample the chromosphere and they are spread over a very wide spectral range. As a result, we have been forced to combine data originating from different solar facilities, usually acquired at remarkably different spatial resolutions. Unfortunately, some of the assumptions adopted in traditional inversion codes hold when the input data are acquired at nearly constant spatial resolution but they break when there is a large spatial resolution discrepancy.

In this seminar, I will present how modern data inversion techniques are allowing us to reconstruct high-fidelity 3D models of the outer layers of the Sun from observational data. These models are providing very valuable constraints on the chromospheric energy budget and thermodynamic properties of the chromosphere. The combination of results from inversion methods and numerical simulations are allowing us to take steps towards solving the chromospheric heating problem in a (hopefully) not-too-distant future.