The Lyα Emission in Solar Flares. I. A Statistical Study on Its Relationship with the 1–8 Å Soft X-Ray Emission

Abstract

We statistically study the relationship between the Lyα and 1–8 Å soft X-ray (SXR) emissions from 658 M- and X-class solar flares observed by the Geostationary Operational Environmental Satellite during 2006–2016. Based on the peak times of the two wave band emissions, we divide the flares into three types. Type I (III) has an earlier (a later) peak time in the Lyα emission than that in the SXR emission, while type II has nearly the same peak time (within the time resolution of 10 s) between the Lyα and SXR emissions. In these 658 flares, we find that there are 505 (76.8%) type I flares, 10 (1.5%) type II flares, and 143 (21.7%) type III flares, and that the three types appear to have no dependence on the flare duration, flare location, or solar cycle. Besides the main peak, the Lyα emission of the three type flares also shows sub-peaks which can appear in the impulsive or gradual phase of the flare. It is found that the main-peak (for type I) and subpeak (for type III) emissions of Lyα that appear in the impulsive phase follow the Neupert effect in general. This indicates that such Lyα emissions are related to the nonthermal electron beam heating. While the main-peak (for type III) and subpeak (for type I) emissions of Lyα that appear in the gradual phase are supposed to be primarily contributed by the thermal plasma that cools down.

The Lyα Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

“…These could be accompanied by some plasma motions such as chromospheric evaporation and coronal condensation, which have different directions along the loops. Compared with previous studies on flaring Lyα (e.g., Nusinov et al 2006;Rubio da Costa et al 2009;Jing et al 2020), our study provides a direct imaging evidence for the Lyα emission originating from flare ribbons as well as flare loops for the first time. In addition, the thermal origin of Lyα is supposed to be more important than the nonthermal origin in this small flare, which may be different with the case in large or major flares.…”

“…This Neupert effect (Neupert 1968) was also detected in some other flares for Lyα (e.g., Rubio da Costa et al 2009;Milligan & Chamberlin 2016;Milligan et al 2017;Chamberlin et al 2018;Dominique et al 2018;Wauters et al 2022). Interestingly, Jing et al (2020) studied 658 M-and X-class flares and found that in ∼20% of the events the Lyα peak appears later than the SXR peak. This delayed Lyα emission can mostly be contributed by thermal plasmas that cool down, suggesting a thermal origin.…”

Detection of flare multi-periodic pulsations in mid-ultraviolet Balmer continuum, Ly-alpha, hard X-ray, and radio emissions simultaneously

“…The hydrogen Balmer continuum enhancement at MUV wavelengths around 2000 Å was found to be highly synchronous with the enhancement of Lyα emission during a powerful solar flare (Dominique et al 2018), while the flare radiation in the Lyα and HXR ranges were demonstrated to have a close relationship (Nusinov et al 2006;Jing et al 2020;Lu et al 2021b). However, flarerelated QPPs were rarely observed simultaneously in these channels.…”

Observational Analysis of Lyα Emission in Equivalent-magnitude Solar Flares

“…A value of E FP /E IP 2 implies at least half of the total energy radiated during the flare is radiated after the impulsive phase, as was the case for SOL2011-09-24 and SOL2014-02-01. Possible contributions to the elevated flux may be attributed to evaporated material in the corona cooling to chromospheric temperatures (Jing et al 2020), failed filament eruptions (Wauters et al 2022), or evaporated material driven by thermal conduction (Zarro & Lemen 1988), for example. While we cannot necessarily determine the source of the emission using disk-integrated measurements alone, nonchromospheric contributions should not be neglected from the interpretation of the results presented here.…”