Transverse oscillation of a coronal loop induced by a flare-related jet^⋆

¹ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, CAS, Nanjing 210023, PR China
e-mail: zhangqm@pmo.ac.cn
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
³ State Key Laboratory of Lunar and Planetary Sciences, Macau University of Science and Technology, Macau, PR China

Received: 23 July 2020
Accepted: 11 December 2020

Abstract

Context. Kink oscillations in coronal loops are ubiquitous, and we apply the observed parameters of oscillations to estimate the magnetic field strength of the loops.

Aims. In this work, we report our multiwavelength observations of the transverse oscillation of a large-scale coronal loop with a length of ≥350 Mm. The oscillation was induced by a blowout coronal jet, which was related to a C4.2 circular-ribbon flare (CRF) in active region 12434 on 2015 October 16. We aim to determine the physical parameters in the coronal loop, including the Alfvén speed and the magnetic field strength.

Methods. The jet-induced kink oscillation was observed in extreme ultraviolet (EUV) wavelengths by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). Line-of-sight magnetograms were observed by the Helioseismic and Magnetic Imager (HMI) on board the SDO. We took several slices along the loop to assemble time-distance diagrams and used an exponentially decaying sine function to fit the decaying oscillation. The initial amplitude, period, and damping time of kink oscillations were obtained. Coronal seismology of the kink mode was applied to estimate the Alfvén speed and the magnetic field strength in the oscillating loop. In addition, we measured the magnetic field of the loop through nonlinear force-free field (NLFFF) modeling using the flux rope insertion method.

Results. The oscillation is most pronounced in AIA 171 and 131 Å. The oscillation is almost in phase along the loop with a peak initial amplitude of ∼13.6 Mm, meaning that the oscillation belongs to the fast standing kink mode. The oscillation lasts for ∼3.5cycles with an average period of ∼462 s and an average damping time of ∼976 s. The values of τ/P lie in the range of 1.5–2.5. Based on coronal seismology, the Alfvén speed in the oscillating loop is estimated to be ∼1210 km s⁻¹. Two independent methods are applied to calculate the magnetic field strength of the loop, resulting in 30–43 G using coronal seismology and 21–23 G using NLFFF modeling.

Conclusions. The magnetic field strength estimated using two different approaches are on the same order of magnitude, which confirms the reliability of coronal seismology by comparing with NLFFF modeling.