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

We statistically study the relationship between the Ly alpha and 1-8 A 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 Ly alpha 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 Ly alpha 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 Ly alpha 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 Ly alpha that appear in the impulsive phase follow the Neupert effect in general. This indicates that such Ly alpha emissions are related to the nonthermal electron beam heating. While the main-peak (for type III) and subpeak (for type I) emissions of Ly alpha that appear in the gradual phase are supposed to be primarily contributed by the thermal plasma that cools down.