2026-10-06T11:06:24Z https://api.figshare.com/v2/oai

oai:figshare.com:article/32641455 2026-10-01T16:23:19Z portal_1172 item_type_8 month_year_10_2026

Observational analysis of Lyman-alpha emission in solar flares Harry Joseph Greatorex (24169980) PUREID: 643655859 solar solar flare sun astrophysics observation emission lyman-alpha The work presented in this thesis details a multi-instrument analysis of Lyman-alpha (Lya; 1216 Å) emission from solar flares, focusing on its energetic significance, behaviour during flares, correlation with nonthermal electrons, and the observational consistency across past and present instrumentation. Although Lya is the most intense line in the solar spectrum, flare-related observations have been historically limited. Using photometric observations and hard X-ray spectroscopic analysis, we examine the relationship between flare-accelerated electrons and the Lya response in three M3-class flares co-observed by RHESSI, GOES, and SDO. Despite identical X-ray classifications, Lya enhancements varied from 1.5% to 6.4%, exceeding the <2.5% expected from FISM2. Flares with harder electron spectra produced larger Lya enhancements, with Lya radiative losses accounting for 2.0%–7.9% of total nonthermal energy. 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. 2026-10-01T16:23:19Z Text Thesis 10.17034/32641455.v1 https://figshare.com/articles/thesis/Observational_analysis_of_Lyman-alpha_emission_in_solar_flares/32641455 All Rights Reserved