Spectropolarimetric synthesis of forbidden lines in magnetohydrodynamic models of coronal bright points

¹ Instituto de Astrofísica de Canarias, E-38205 La Laguna, Tenerife, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain
³ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern 0315 Oslo, Norway
⁴ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern 0315 Oslo, Norway

^⋆ Corresponding author: ernest.alsina@iac.es

Received: 14 May 2025
Accepted: 30 June 2025

Abstract

Context. The inference of the magnetic field vector from spectropolarimetric observations is crucial for understanding the physical processes governing the solar corona.

Aims. We investigated what information on the magnetic fields of coronal bright points (CBPs) can be gained from the intensity and polarization of the Fe XIII 10747 Å, Fe XIV 5303 Å, Si X 14301 Å, and Si IX 39343 Å forbidden lines.

Methods. We applied the P-CORONA synthesis code to a CBP model in the very low corona, obtained with the Bifrost code, and to a larger global model to study the impact of the outer coronal material along the line of sight (LoS).

Results. The enhanced density within the CBP produces an intensity brightening, but suppresses the linear polarization. The circular polarization from such regions often approaches 0.1% of the intensity. The relative contribution from the coronal material along the LoS depends strongly on its temperature, and is weaker for lines with a peak response at higher temperatures (Fe XIII 10747 Å at 1.7 MK and Fe XIV 5303 Å at 2 MK). The weak field approximation (WFA) provides information on the longitudinal magnetic fields in the strongest-emitting spatial intervals along the LoS, and we find it to be more reliable in the regions of the CBP where the field does not change sign. This tends to coincide with the regions where there is a strong correlation between the circular polarization and the wavelength derivative of the intensity. Considering roughly 30 minutes of time evolution, the CBP signals are somewhat attenuated, but are still clearly identifiable, and the area where the WFA can be suitably applied remains substantial.

Conclusions. The circular polarization of the Fe XIV 5303 Å and especially Fe XIII 10747 Å lines are valuable diagnostics for the magnetic fields in the higher-temperature regions of the CBP, which could be exploited with future coronagraphs with similar capabilities to Cryo-NIRSP/DKIST, but designed to observe below 1.05 R_⊙.