Magnetic field of solar prominences obtained from spectropolarimetric observations in He  <scp>i</scp> 10830 Å

Abstract The magnetic field of solar prominences is an important quantity that determines their structures and energy balance. Some studies have estimated the magnetic field by spectropolarimetric observations, but the field direction and strength of prominences are discrepant among the studies. In this study, we performed spectropolarimetric observations of nine prominences on the solar limb including both quiescent and active region prominences in He i $10830$Å. Using the HAZEL inversion code, magnetic fields of each prominence were derived. In general, the inversion allows both quasi-horizontal and quasi-vertical solutions due to the Van Bleck ambiguity. We introduce an RGB $\chi ^2$ diagnostic map, which visualizes the spatial distribution of the likelihood of the solution and helps to identify regions where inversion degeneracy occurs. Regardless of the ambiguity, it is found that the field strengths of the quiescent prominences are less than $40$G, which is consistent with previous studies, while the field strengths of the active region prominences are less than $120$G, which is inconsistent with some of the previous studies which estimated field strengths of on-disk active region filaments as 600–$800$G. Our results support the statement by Díaz Baso et al. (2016, ApJ, 822, 50) that such strong signal is not attributed to the filament itself. One of our quiescent prominence is identical with the filament subsequently observed by Yamasaki et al. (2023, PASJ, 75, 660), and from the consistency of two results, we could determine a unique solution that is a quasi-horizontal magnetic configuration.