Letter to the Editor

Preflare very long-periodic pulsations observed in Hα emission before the onset of a solar flare^⋆

¹ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, CAS, Nanjing 210033, PR China
e-mail: lidong@pmo.ac.cn
² State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing 100190, PR China
³ CAS Key Laboratory of Solar Activity, National Astronomical Observatories, Beijing 100101, PR China
⁴ Yunnan Key Laboratory of Computer Technology Application, Faculty of Information Engineering and Automation, Kunming University of Science and Technology, Kunming 650500, PR China
⁵ MOE Key Laboratory of Fundamental Physical Quantities Measurements, School of Physics, Huazhong University of Science and Technology, Wuhan 430074, PR China

Received: 12 May 2020
Accepted: 23 June 2020

Abstract

Context. Very long-periodic pulsations during preflare phases (preflare-VLPs) have been detected in the full-disk solar soft X-ray (SXR) flux. They may be regarded as precursors to solar flares and may help us better understand the trigger mechanism of solar flares.

Aims. In this Letter, we report a preflare-VLP event prior to the onset of an M1.1 circular-ribbon flare on 2015 October 16. It was simultaneously observed in Hα, SXR, and extreme ultraviolet (EUV) wavelengths.

Methods. The SXR fluxes in 1−8 Å and 1−70 Å were recorded by the Geostationary Operational Environmental Satellite (GOES) and Extreme Ultraviolet Variability Experiment, respectively; the light curves in Hα and EUV 211 Å were integrated over a small local region, which were measured by the 1 m New Vacuum Solar Telescope and the Atmospheric Imaging Assembly (AIA), respectively. The preflare-VLP is identified as the repeat and quasi-periodic pulses in light curves during preflare phase. The quasi-periodicity can be determined from the Fourier power spectrum with Markov chain Monte Carlo-based Bayesian.

Results. Seven well-developed pulses are found before the onset of an M1.1 circular-ribbon flare. They are firstly seen in the local light curve in Hα emission and then discovered in full-disk SXR fluxes in GOES 1−8 Å and ESP 1−70 Å, as well as the local light curve in AIA 211 Å. These well-developed pulses can be regarded as the preflare-VLP, which might be modulated by LRC-circuit oscillation in the current-carrying plasma loop. The quasi-period is estimated to be ∼9.3 min.

Conclusions. We present the first report of a preflare-VLP event in the local Hα line and EUV wavelength, which could be considered a precursor of a solar flare. This finding should therefore prove useful for the prediction of solar flares, especially for powerful flares.