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

TL;DR

This paper reports the first observation of very long-periodic pulsations in Halpha and EUV wavelengths before a solar flare, suggesting their potential as flare precursors for improved prediction.

Contribution

It presents the first detection of preflare-VLPs in Halpha and EUV wavelengths, expanding understanding of flare precursors and their modulation mechanisms.

Findings

Preflare-VLPs observed in Halpha, SXR, and EUV wavelengths.

Quasi-periodicity of ~9.3 minutes identified in preflare phase.

Preflare-VLPs may serve as useful flare prediction indicators.

Abstract

Very long-periodic pulsations during preflare phases (preflare-VLPs) have been detected in the full-disk solar soft X-Ray (SXR) flux (see Tan et al. 2016). They may be regarded as precursors to solar flares and may help us better understand the trigger mechanism of solar flares. In this letter, we report a preflare-VLP event before the onset of an M1.1 circular-ribbon flare on 2015 October 16. It was simultaneously observed in Halpha, SXR, and extreme ultraviolet (EUV) wavelengths, which were recorded by the NVST, GOES, EVE, and AIA respectively. The preflare-VLP is identified as the repeat and quasi-periodic pulses in light curves during preflare phase, which might be modulated by LRC-circuit oscillation in the current-carrying plasma loop. The quasi-periodicity can be determined from the Fourier power spectrum with Markov chain Monte Carlo (MCMC)-based Bayesian (e.g., Liang et al. 2020), such as ~9.3 minutes. We present the first report of a preflare-VLP event in the local Halpha 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.