Non-damping oscillations at flaring loops^⋆

¹ Key Laboratory for Dark Matter and Space Science, Purple Mountain Observatory, CAS, Nanjing, 210034, PR China
e-mail: lidong@pmo.ac.cn, ningzongjun@pmo.ac.cn
² Sate Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing, 100190 PR China
³ CAS Key Laboratory of Solar Activity, National Astronomical Observatories, Beijing, 100012, PR China
⁴ Institute of Space Science and Applied Technology, Harbin Institute of Technology, Shenzhen Campus, Shenzhen, 518055, PR China
⁵ School of Astronomy and Space Science, University of Science and Technology of China, Hefei, Anhui, 230026, PR China
⁶ MOE Key Laboratory of Fundamental Physical Quantities Measurements, School of Physics, Huazhong University of Science and Technology, Wuhan, 430074, PR China

Received: 9 March 2018
Accepted: 9 June 2018

Abstract

Context. Quasi-periodic oscillations are usually detected as spatial displacements of coronal loops in imaging observations or as periodic shifts of line properties (i.e., Doppler velocity, line width and intensity) in spectroscopic observations. They are often applied for remote diagnostics of magnetic fields and plasma properties on the Sun.

Aims. We combine the imaging and spectroscopic measurements of available space missions, and investigate the properties of non-damping oscillations at flaring loops.

Methods. We used the Interface Region Imaging Spectrograph (IRIS) to measure the spectrum over a narrow slit. The double-component Gaussian fitting method was used to extract the line profile of Fe XXI 1354.08 Å at the “O I” spectral window. The quasi-periodicity of loop oscillations were identified in the Fourier and wavelet spectra.

Results. A periodicity at about 40 s is detected in the line properties of Fe XXI 1354.08 Å, hard X-ray emissions in GOES 1−8 Å derivative, and Fermi 26−50 keV. The Doppler velocity and line width oscillate in phase, while a phase shift of about π/2 is detected between the Doppler velocity and peak intensity. The amplitudes of Doppler velocity and line width oscillation are about 2.2 km s⁻¹ and 1.9 km s⁻¹, respectively, while peak intensity oscillates with amplitude at about 3.6% of the background emission. Meanwhile, a quasi-period of about 155 s is identified in the Doppler velocity and peak intensity of the Fe XXI 1354.08 Å line emission, and AIA 131 Å intensity.

Conclusions. The oscillations at about 40 s are not damped significantly during the observation; this might be linked to the global kink modes of flaring loops. The periodicity at about 155 s is most likely a signature of recurring downflows after chromospheric evaporation along flaring loops. The magnetic field strengths of the flaring loops are estimated to be about 120−170 G using the magnetohydrodynamic seismology diagnostics, which are consistent with the magnetic field modeling results using the flux rope insertion method.