山崎 大輝

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.

The solar active region NOAA 12887 produced a strong X1.0 flare on 2021 October 28, which exhibits X-shaped flare ribbons and a circle-shaped erupting filament. To understand the eruption process with these characteristics, we conducted a data-constrained magnetohydrodynamics simulation using a nonlinear force-free field of the active region about an hour before the flare as the initial condition. Our simulation reproduces the filament eruption observed in the Ha images of GONG and the 304 angstrom images of SDO/AIA and suggests that two mechanisms can possibly contribute to the magnetic eruption. One is the torus instability of the pre-existing magnetic flux rope (MFR), and the other is upward pushing by magnetic loops newly formed below the MFR via continuous magnetic reconnection between two sheared magnetic arcades. The presence of this reconnection is evidenced by the SDO/AIA observations of the 1600 angstrom brightening in the footpoints of the sheared arcades at the flare onset. To clarify which process is more essential for the eruption, we performed an experimental simulation in which the reconnection between the sheared field lines is suppressed. In this case too, the MFR could erupt, but at a much reduced rising speed. We interpret this result as indicating that the eruption is not only driven by the torus instability, but additionally accelerated by newly formed and rising magnetic loops under continuous reconnection.

The Tandem Etalon Magnetograph (TEM) is one of the instruments of the Solar Magnetic Activity Research Telescope of Hida Observatory. The TEM is a partial disk (320" x240") filter magnetograph which scans the wavelength around a Fe I line at 6303 angstrom and achieves polarimetric sensitivity of < 5x10^-4 for each wavelength. To obtain the polarimeter response matrix of the instrument, we have carried out end-to-end polarization calibrations of the instrument. We have also measured the polarization characteristics of the polarization beam splitter (PBS), which is a crucial component of the instrument. As a result of end-to-end calibration, we found significant spatial variation in the response matrix across the field of view. From a laboratory test, we found that 1% of the magnitude of a circular diattenuation of the PBS was due to the retardation caused by the stress in the cube and the linear diattenuation of the film. Although the spatial variation across the field of view is more than ten times larger, to achieve the polarimetric sensitivity of < 5x10^-4, this can be well explained by the polarization characteristic of the PBS and corrected by using the response matrix obtained in the end-to-end calibration. In addition, we also obtained the daily variation of the polarization property of the TEM. We found that the crosstalk from Stokes Q to V changes an amount comparable to the tolerance through a day. In the present configuration, we require a pixel-by-pixel calibration every 100 minutes to meet the accuracy requirement.

Active region (AR) 12673 produced many M-class and several X-class flares, one of which being an X9.3 flare, which is recorded as the largest solar flare in solar cycle 24. We studied the evolution of the three-dimensional flare-productive magnetic field within AR 12673, using a time series of nonlinear force-free field extrapolations of every 12 hours from September 4th 00:00 UT to 6th 00:00 UT. Our analysis found that three magnetic flux ropes (MFRs) are formed by September 4th, one of which produced the X9.3 flare on September 6th. One MFR has positive magnetic twist which is a different sign from other two MFRs. Since the temporal evolution of the magnetic flux of the MFR accumulating the positive twist is consistent with the profile of the GOES X-ray flux well observed from September 4th to 6th, we suggest that the formation of the MFR having the positive twist is closely related to the occurrence of the M-class flares including an M5.5 flare. We further found a magnetic null in the magnetic field surrounding the MFRs, in particular, above the MFR having positive twist before the M5.5 flare which is the largest M-flare observed during this period. By comparing with the AIA 1600 angstrom images, we found that the footpoints of the overlying field lines are anchored to the area where the brightening was initially observed. Therefore, we suggest that reconnection induced by the torus instability of the positively twisted MFR at the null possibly drived the M5.5 flare.