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

Abstract

The hydrogen Ly alpha (H I Ly alpha) emission during solar flares has rarely been studied in spatially resolved images, and its physical origin has not been fully understood. In this paper, we present novel Ly alpha images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-wave-band and multiperspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Ly alpha emission has a good temporal correlation with the thermal emissions at 1-8A and 5-7 keV, indicating that the flaring Ly alpha is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Ly alpha at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Ly alpha emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Ly alpha emission shows fairly similar features to the He II lambda 304 emission in light curve and spatiotemporal variation, along with small differences. These observational results improve our understanding of the Ly alpha emission in solar flares and also provide some insights for investigating the Ly alpha emission in stellar flares.