Observational analysis of Lyman-alpha emission in solar flares

Radiative losses in He II (304 Å) were smaller but this line also displayed notable background enhancements. Additionally, a broader study of 14 flares observed by GOES-15/EUVS-E with Lya contrasts >10% revealed no clear correlation between the exceptional Lya excess and nonthermal parameters derived from RHESSI and Fermi spectroscopy or impulsivity, suggesting an additional driver influencing this behaviour. Finally, we examine the influence of instrumental variability on Lya flare measurements using three M-class events observed by the EUVS instrument on GOES-14, -15, -16 and at least one of PROBA2/LYRA, MAVEN/EUVM, SDO/EVE, or ASO-S/LST.

While relative flux trends were consistent, significant differences in Lya contrasts and energetics were observed—up to a factor of five, highlighting the importance of instrument-specific characteristics in flare diagnostics. These findings advance understanding of Lya flare emissions and their role in chromospheric energetics, while providing guidance for interpreting multi-instrument flare observations. The results will inform analysis during Solar Cycle 25, especially with forthcoming missions such as Solar-C, which will offer unprecedented Lya spectroscopic capabilities.