Magnetic field diagnostics of a solar active region filament

Abstract We performed spectropolarimetric observations of an active region filament in He i 10830 Å and Si i 10827 Å lines to investigate its magnetic field structure. We carried out full-Stokes inversions with the HAZEL code, which takes into account the Zeeman and Hanle effects. As a result, we yielded a mean field strength of 101 $\pm$ 33 G and a horizontal field nearly parallel to the filament axis, such that the distinction between the two classical normal- and reverse-polarity models becomes physically insignificant. In addition, we found Zeeman-like signatures in the linear polarization, characterized by double-peaked symmetric profiles, in some pixels of our observations. Since these profiles could not be well reproduced by modeling that included both the Zeeman and Hanle effects, we performed inversions assuming only the Zeeman effect. The inversion yielded a strong magnetic field of approximately 500 G. However, simultaneous observations of Si i 10827 Å indicate a photospheric magnetic field weaker than 100 G. Therefore, the scenario proposed by Díaz Baso, Martínez González, and Asensio Ramos (2016, ApJ, 822, 50), in which the strong field inferred from He i 10830 Å originates from contamination by the underlying photosphere, does not apply to our filament. The Zeeman-like profiles are preferentially found in optically thick regions ($\tau \sim 1.4$–2.5), where the simplifying assumptions adopted in HAZEL are expected to become less reliable. Our results suggest that these profiles reveal limitations of the current inversion framework in optically thick regions and motivate future radiative-transfer modeling incorporating self-consistent radiation fields, differential illumination of the multiplet components, and possibly partial frequency redistribution.