Daiki Yamasaki

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.

Abstract Unstable states of the solar coronal magnetic field structure result in various flare behaviors. In this study, we compared the confined and eruptive flares that occurred under similar magnetic circumstances in the active region 12673, on 2017 September 6, using the twist number, decay index, and height of magnetic field lines to identify observational behaviors of the flare eruption. We investigated the parameters from the magnetic field lines involved in an initial energy release, which were identified from the positions of the core of flare ribbons, i.e., flare kernels. The magnetic field lines were derived by nonlinear force-free field modeling calculated from the photospheric vector magnetic field obtained by the Solar Dynamics Observatory SDO/Helioseismic and Magnetic Imager, and flare kernels were identified from the 1600 Å data obtained by the SDO/Atmospheric Imaging Assembly. The twist number of all the magnetic field lines in the confined flare was below 0.6; however, the twist number in seven out of 24 magnetic field lines in the eruptive flare was greater than 0.6. These lines were tall. It is found that the decay index is not a clear discriminator of the confined and eruptive flares. Our study suggests that some magnetic field lines in the kink instability state may be important for eruptive flares, and that taller magnetic field lines may promote flare eruption.

Solar filaments are dense and cool plasma clouds in the solar corona. They are supposed to be supported in a dip of coronal magnetic field. However, the models are still under argument between two types of the field configuration; one is the normal polarity model proposed by Kippenhahn & Schlueter (1957), and the other is the reverse polarity model proposed by Kuperus & Raadu (1974). To understand the mechanism that the filaments become unstable before the eruption, it is critical to know the magnetic structure of solar filaments. In this study, we performed the spectro-polarimetric observation in the He I (10830 angstrom) line to investigate the magnetic field configuration of dark filaments. The observation was carried out with the Domeless Solar Telescope at Hida Observatory with a polarization sensitivity of 3.0x10^-4. We obtained 8 samples of filaments in quiet region. As a result of the analysis of full Stokes profiles of filaments, we found that the field strengths were estimated as 8 - 35 Gauss. By comparing the direction of the magnetic field in filaments and the global distribution of the photospheric magnetic field, we determined the magnetic field configuration of the filaments, and we concluded that 1 out of 8 samples have normal polarity configuration, and 7 out of 8 have reverse polarity configuration.