Signatures of Magnetic Reconnection at the Footpoints of Fan-shaped Jets on a Light Bridge Driven by Photospheric Convective Motions

Xianyong Bai,Yuanyong Deng,Kaifan Ji,Wenda Cao,Dong Li,Hector Socas-Navarro,Daniel Nóbrega-Siverio,Jiangtao Su

doi:10.3847/1538-4357/aaf1d1

Abstract

Abstract Dynamical jets are generally found on light bridges (LBs), which are key to studying sunspot decay. So far, their formation mechanism is not fully understood. In this paper, we used state-of-the-art observations from the Goode Solar Telescope, the Interface Region Imaging Spectrograph, the Spectro-polarimeter on board Hinode, and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory to analyze the fan-shaped jets on LBs in detail. A continuous upward motion of the jets in the ascending phase is found from the Hα velocity that lasts for 12 minutes and is associated with the Hα line wing enhancements. Two mini jets appear on the bright fronts of the fan-shaped jets visible in the AIA 171 and 193 Å channels, with a time interval as short as 1 minute. Two kinds of small-scale convective motions are identified in the photospheric images, along with the Hα line wing enhancements. One seems to be associated with the formation of a new convection cell, and the other manifests as the motion of a dark lane passing through the convection cell. The finding of three-lobe Stokes V profiles and their inversion with the NICOLE code indicate that there are magnetic field lines with opposite polarities in LBs. From the Hα −0.8 Å images, we found ribbon-like brightenings propagating along the LBs, possibly indicating slipping reconnection. Our observation supports the idea that the fan-shaped jets under study are caused by magnetic reconnection, and photospheric convective motions play an important role in triggering the magnetic reconnection.

Highlights

Light bridges (LBs), which commonly occur during the sunspot decay phase, are bright and elongated structures that separate umbra cores or are embedded in the umbra
The fan-shaped jets on the LB can be found in the region R1 in Figures 1(b) and (c), corresponding to the velocity derived from the Hα line using 11 points and the Interface Region Imaging Spectrograph (IRIS) slit-jaw images (SJI) in the 1400 Å passband, respectively
The jets’ fronts show brightenings (the region marked by the arrow in Figures 1(d)–(i)) in the abovementioned passbands, while the footpoint of the jets on the LB has some transient brightenings, especially in the IRIS 1400 Å image

Summary

Introduction

Light bridges (LBs), which commonly occur during the sunspot decay phase, are bright and elongated structures that separate umbra cores or are embedded in the umbra. The aims are to reveal the fine structures of the photospheric convection on LBs and to find the relationship among the small-scale photospheric flow, brightenings, and the jet-like activities on their upper layers, which helps us understand the coupling between mass and energy on LBs. To that end, we have used joint observations from the Goode Solar Telescope (GST, previously called the New Solar Telescope; Cao et al 2010), the Interface Region Imaging Spectrograph (IRIS; De Pontieu et al 2014), the Spectro-polarimeter (SP) instrument on board Hinode (Lites et al 2013), and the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO; Lemen et al 2012).

Observations and Data Reduction

Analysis and Results

The TR Spectrum at the Footpoint and Bright Front of the Fan-shaped Jets

Summary and Discussion