Inference of the chromospheric magnetic field configuration of solar plage using the Ca II 8542 {\AA} line

TL;DR

This study uses high-quality spectro-polarimetric data and advanced inversion techniques to directly measure the magnetic field strength and inclination in solar plage chromosphere, providing new constraints for solar models.

Contribution

It presents the first direct, physics-based estimates of chromospheric magnetic field strength and inclination in a plage region using NLTE inversions of Ca II 8542 Å data.

Findings

Magnetic field strength in plage: 440 ± 90 G.

Inclination in plage: 10° ± 16°.

Fibril magnetic field: 300 ± 50 G with 50° ± 13° inclination.

Abstract

It has so far proven impossible to reproduce all aspects of the solar plage chromosphere in quasi-realistic numerical models. The magnetic field configuration in the lower atmosphere is one of the few free parameters in such simulations. The literature only offers proxy-based estimates of the field strength, as it is difficult to obtain observational constraints in this region. Sufficiently sensitive spectro-polarimetric measurements require a high signal-to-noise ratio, spectral resolution, and cadence, which are at the limit of current capabilities. We use critically sampled spectro-polarimetric observations of the \cair line obtained with the CRISP instrument of the Swedish 1-m Solar Telescope to study the strength and inclination of the chromospheric magnetic field of a plage region. This will provide direct physics-based estimates of these values, which could aid modelers to put constraints on plage models. We increased the signal-to-noise ratio of the data by applying several methods including deep learning and PCA. We estimated the noise level to be $1\cdot10^{-3} I_c$. We then used STiC, a non-local thermodynamic equilibrium (NLTE) inversion code to infer the atmospheric structure and magnetic field pixel by pixel. We are able to infer the magnetic field strength and inclination for a plage region and for fibrils in the surrounding canopy. In the plage we report an absolute field strength of $|B| =440 \pm 90$ G, with an inclination of $10^\circ \pm 16^\circ$ with respect to the local vertical. This value for $|B|$ is roughly double of what was reported previously, while the inclination matches previous studies done in the photosphere. In the fibrillar region we found $|B| = 300 \pm 50$ G, with an inclination of $50^\circ \pm 13^\circ$.