The Lyman-$\alpha$ Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter

TL;DR

This study presents spatially resolved Ly$ ext{α}$ images of a small solar flare, revealing its thermal origin, minor nonthermal electron contribution, and similarities with He II 304 Å emission, enhancing understanding of Ly$ ext{α}$ in flares.

Contribution

First spatially resolved Ly$ ext{α}$ images of a solar flare, showing its thermal origin and detailed emission mechanisms, with implications for stellar flare studies.

Findings

Ly$ ext{α}$ emission correlates with thermal X-ray emissions.

Nonthermal electrons have a minor role in Ly$ ext{α}$ production.

Ly$ ext{α}$ emission features are similar to He II 304 Å emission.

Abstract

The hydrogen Lyman-$\alpha$ (H {\sc 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-waveband and multi-perspective 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--8 \AA\ 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 with the He {\sc ii} 304 \AA\ emission in light curve and spatio-temporal 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.