Imaging and Spectral Study on the Null Point of a Fan-spine Structure During a Solar Flare

Shuhong Yang,Jun Zhang,Zhi Xu,Ze Zhong,Qingmin Zhang,Yang Guo

doi:10.3847/1538-4357/ab9ac7

Abstract

Abstract Using the multi-instrument observations, we make the first simultaneous imaging and spectral study on the null point of a fan-spine magnetic topology during a solar flare. When magnetic reconnection occurs at the null point, the fan-spine configuration brightens in the (extreme-)ultraviolet channels. In the Hα images, the fan-spine structure is partly filled and outlined by the bidirectional material flows ejected from the reconnection site. The extrapolated coronal magnetic field confirms the existence of the fan-spine topology. Before and after the flare peak, the total velocity of the outflows is estimated to be about 60 km s−1. During the flare, the Si iv line profile at the reconnection region is enhanced both in the blue-wing and red-wing. At the flare peak time, the total velocity of the outflows is found to be 144 km s−1. Superposed on the Si iv profile, there are several deep absorption lines with the blueshift of several tens of km s−1. The reason is inferred to be that the bright reconnection region observed in Si iv channel is located under the cooler material appearing as dark features in the Hα line. The blueshifted absorption lines indicate the movement of the cooler material toward the observer. The depth of the absorption lines also depends on the amount of cooler material. These results imply that these kinds of spectral profiles can be used as a tool to diagnose the properties of cooler material above the reconnection site.

Highlights

Magnetic reconnection is a fundamental physical process during which magnetic energy is converted to the kinetic and thermal energy of plasma (Zweibel & Yamada 2009; Pontin 2011; Yang et al 2011, 2015a; Yang & Xiang 2016)
As revealed by many studies, the rising filament embedded within the fan dome plays an important role in triggering the magnetic reconnection at the null point, and the filament material can be ejected into the large-scale open or distantly closed field lines (e.g., Joshi et al 2015; Liu et al 2015; Xu et al 2017)
We study a fan-spine structure in active region (AR) 12736 on 2019 March 22, and focus on the null point where magnetic reconnection occurred during a B6.7 flare, combing the high-quality vector magnetograms from the Hinode (Kosugi et al 2007) with the high-resolution imaging and spectral observations from the New Vacuum Solar Telescope (NVST; Liu et al 2014), the Solar Dynamics Observatory (SDO; Pesnell et al 2012), and the Interface Region Imaging Spectrograph (IRIS ; De Pontieu et al 2014)

Summary

Introduction

Magnetic reconnection is a fundamental physical process during which magnetic energy is converted to the kinetic and thermal energy of plasma (Zweibel & Yamada 2009; Pontin 2011; Yang et al 2011, 2015a; Yang & Xiang 2016). As revealed by many studies, the rising filament (or sigmoid) embedded within the fan dome plays an important role in triggering the magnetic reconnection at the null point, and the filament material can be ejected into the large-scale open or distantly closed field lines (e.g., Joshi et al 2015; Liu et al 2015; Xu et al 2017). We study a fan-spine structure in active region (AR) 12736 on 2019 March 22, and focus on the null point where magnetic reconnection occurred during a B6.7 flare, combing the high-quality vector magnetograms from the Hinode (Kosugi et al 2007) with the high-resolution imaging and spectral observations from the New Vacuum Solar Telescope (NVST; Liu et al 2014), the Solar Dynamics Observatory (SDO; Pesnell et al 2012), and the Interface Region Imaging Spectrograph (IRIS ; De Pontieu et al 2014)

Observations and Data Analysis

Results

Conclusions and Discussion