Fan-spine magnetic topology is favorable for the occurrence of solar flares through null-point reconnection. Using the high-resolution imaging and spectral observations from the New Vacuum Solar Telescope (NVST), the Solar Dynamics Observatory (SDO), the Interface Region Imaging Spectrograph(IRIS), and the Geostationary Operational Environmental Satellite, and the high-quality vector magnetograms from the Hinode, a research group led by Prof. Shuhong Yang at National Astronomical Observatories of Chinese Academy of Sciences (NAOC) made the first simultaneous imaging and spectral study on the null point of a fan-spine magnetic topology during a solar flare in active region (AR) 12736.

Yang et al. extrapolated the coronal structures based on the observed photospheric magnetogram, and confirmed the existence of the fan-spine topology. The breakout-type magnetic reconnection first occurred at the null point, and consequently the flux rope within the fan began to rise and erupt (Figure 1). The cooler material within the flux rope moved along the longer field lines to the remote end of the spine, resulting in the remote brightening. At the flare maximum time, the total velocity of the material outflows from the reconnection site was found to be 144 km/s.

Superposed on the Si IV line profile, there were several deep absorption lines which have the same blueshift of -0.1 angstrom (Figure 2). The reason was inferred to be that the bright reconnection site observed in the ultraviolet channel was located under the cooler material appearing as dark features in the Hαline. The blueshifted absorption lines indicated that the cooler material moved toward the observer with the velocity of 22 km/s. The depth of the absorption lines depended on the amount of cooler material.

These results imply that this kind of spectral profiles can be used as a tool to diagnose the properties of cooler material above reconnection site. Moreover, the shift of the absorption lines can be used to convert the digital number in the Hα Doppler gram to the Hα Doppler velocity.

The paper has been published in The Astrophysical Journal on July 28, 2020.

Figure 1：Sketch of the magnetic structures illustrating the breakout-type reconnection at the null point of the fan-spine. (Credit: NAOC)

Figure 2：Simultaneous imaging and spectral observations from the NVST and IRIS. (Credit: NAOC)

The paper can be accessed at https://iopscience.iop.org/article/10.3847/1538-4357/ab9ac7