The Lyman-α Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter
Abstract
The hydrogen Lyman-α (H i Lyα) 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 Lyα images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-waveband and multi-perspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Lyα emission has a good temporal correlation with the thermal emissions at 1--8 \ and 5--7 keV, indicating that the flaring Lyα is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Lyα 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 Lyα 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 Lyα emission shows fairly similar features with the He ii 304 \ emission in light curve and spatio-temporal variation along with small differences. These observational results improve our understanding of the Lyα emission in solar flares and also provide some insights for investigating the Lyα emission in stellar flares.
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