Prof. YAN Xiaoli and his collaborators from Yunnan Observatories (YNAO) studied the magnetic structures of a solar filament based on observations from the one-meter New Vacuum Solar Telescope (NVST), the Optical and Near-Infrared Solar Eruption Trace Telescope (ONSET), and the Solar Dynamics Observatory (SDO). Their research has recently been published in The Astrophysical Journal.
Solar filaments exhibit thin and long strips when projected on the solar disk using chromosphere lines. When they appear at the Sun’s limb, they exhibit emission structures called prominences. Their density is 100 times higher, and their temperature is 100 times lower, than those of the surrounding coronal environment. Based on their locations, solar filaments are classified into quiescent filaments, active-region filaments, and intermediate filaments.
Typically forming along the polarity inversion line of the solar magnetic field, solar filaments are closely related to solar eruptive activities. The eruption of a solar filament is often accompanied by a solar flare and a coronal mass ejection (CME), making it a major driver of space weather. Since the magnetic field structure of solar filaments cannot be observed directly, it is studied through theoretical models and numerical simulations. Currently, there are two prevailing viewpoints: one suggests that the magnetic field structure of filaments is composed of sheared magnetic arches, while the other posits that it is a twisted magnetic flux rope.
To investigate the magnetic field structure of solar filaments, especially before their eruptions, researchers utilized multi-wavelength optical and extreme ultraviolet image data from NVST, ONSET, and SDO, along with magnetic field data from SDO’s Helioseismic and Magnetic Imager (HMI). The researchers conducted a detailed study of an intermediate filament that appeared on the solar disk on April 3, 2018.
This filament, with one end in active region NOAA 12703 and the other in a quiet region, is a typical intermediate filament. As a B-class solar flare occurred in this active region, flare material was injected into the filament channel, with the hot flare material moving along the filament’s magnetic structure from one end to the other.
By tracking the movement of the hot plasma using methods such as overlaying images from different times and fusing images, the researchers traced the magnetic field structure of the filament, which cannot be observed directly. The researchers found that the filament has a very complex twisted structure, with the central part consisting of tightly intertwined magnetic field lines and several groups of weaker magnetic field lines wrapping around the central part. Some of the magnetic field lines entwining the main body of the filament are rooted in areas outside the filament’s footpoints.
This study reveals the true magnetic field structures of solar filaments, providing good observational evidence for theoretical models and numerical simulations of filament formation and eruption. It also serves as a useful reference for studying the formation and eruption of filaments on Sun-like stars.
The research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China’s Distinguished Young Scholar Fund, and the Yunnan Key Laboratory for Solar Physics.
Figure 1, left panel: NVST H-alpha line center observations and reconstructed Doppler maps of the filament；right panel：Reconstructed magnetic structures of the filament by using the maximum fusion method. Image by YAN.
Contact:
YAN Xiaoli
Yunnan Observatories , CAS
Email: yanxl@ynao.ac.cn
