The chromospheric Lyα line of neutral hydrogen (1216 Å) is the most intense emission line in the solar spectrum, yet until recently observations of flare-related Lyα emission have been scarce. Here, we examine the relationship between nonthermal electrons accelerated during the impulsive phase of three M3 flares that were co-observed by RHESSI, GOES, and the Solar Dynamics Observatory, and the corresponding response of the chromosphere in Lyα. Despite having identical X-ray magnitudes, these flares showed significantly different Lyα responses. The peak Lyα enhancements above the quiescent background for these flares were 1.5%, 3.3%, and 6.4%. However, the predicted Lyα enhancements from FISM2 were consistently <2.5%. By comparing the properties of the nonthermal electrons derived from spectral analysis of hard X-ray observations, flares with a “harder” spectral index were found to produce a greater Lyα enhancement. The percentage of nonthermal energy radiated by the Lyα line during the impulsive phase was found to range from 2.0% to 7.9%. Comparatively, the radiative losses in He ii (304 Å) were found to range from 0.6% to 1.4% of the nonthermal energy while displaying enhancements above the background of 7.3% to 10.8%. FISM2 was also found to underestimate the level of He ii emission in two of the three flares. These results may have implications for space weather studies and for modeling the response of the terrestrial atmosphere to changes in the solar irradiance, and will guide the interpretation of flare-related Lyα observations that will become available during Solar Cycle 25.

Despite the energetic significance of Lyman-alpha (Ly $\alpha $α ; 1216 Å) emission from solar flares, regular observations of flare related Ly $\alpha $α have been relatively scarce until recently. Advances in instrumental capabilities and a shift in focus over previous solar cycles mean it is now routinely possible to take regular co-observations of Ly $\alpha $α emission in solar flares. Thus, it is valuable to examine how the instruments selected for flare observations may influence the conclusions drawn from the analysis of their unique measurements. Here, we examine three M-class flares each observed in Ly $\alpha $α by GOES-14/EUVS-E, GOES-15/EUVS-E, or GOES-16/EXIS-EUVS-B, and at least one other instrument from PROBA2/LYRA, MAVEN/EUVM, ASO-S/LST-SDI, and SDO/EVE-MEGS-P. For each flare, the relative and excess flux, contrast, total energy, and timings of the Ly $\alpha $α emission were compared between instruments. It was found that while the discrepancies in measurements of the relative flux between instruments may be considered minimal, the calculated contrasts, excess fluxes, and energetics may differ significantly – in some cases up to a factor of five. This may have a notable impact on multi-instrument investigations of the variable Ly $\alpha $α emission in solar flares and estimates of the contribution of Ly $\alpha $α to the radiated energy budget of the chromosphere. The findings presented in this study will act as a guide for the interpretation of observations of flare-related Ly $\alpha $α from upcoming instruments during future solar cycles and inform conclusions drawn from multi-instrument studies.

The Lyman-alpha (Ly $\alpha $ α ; 1216 Å) line is the brightest emission line in the quiescent solar spectrum and radiates a significant fraction of the available nonthermal energy during flares. Despite its importance, there is a lack of detailed studies of Ly $\alpha $ α spectral variability during flares. Recently, spectrally resolved Ly $\alpha $ α flare observations from the SORCE/SOLSTICE instrument have become available. This study examines Ly $\alpha $ α spectral variability and its relationship with HXR emission from nonthermal electrons, using observations of two M-class flares from SORCE/SOLSTICE and RHESSI. Imaging observations from STEREO/SECCHI EUVI and SDO/AIA provide further context. Enhancements across the Ly $\alpha $ α line profile were found to closely correlate with bursts of HXR emission, suggesting a primarily nonthermal origin. Red enhancement asymmetries at the peak of each flare were attributed to chromospheric evaporation, while blue wing enhancement and blue asymmetry were linked to a bright filament-eruption seen in SDO/AIA 1600 Å images. These findings contribute to the understanding of spectral Ly $\alpha $ α variability during flares and highlight the need for future studies using a higher quality and quantity of spectral Ly $\alpha $ α flare observations. Such studies will further characterise the physical mechanisms driving Ly $\alpha $ α flare variability.

X-ray and EUV solar flare emission cause increases in the Earth’s dayside ionospheric electron density. While the response of the lower ionosphere to X-rays is well studied, the delay between EUV flare emission and the response of the ionospheric F-region has not been investigated. Here, we calculate the delays between incident He II 304 Å emission, and the TEC response for 10 powerful solar flares, all of which exhibit delays under 1 min. We assess these delays in relation to multiple solar and geophysical factors, and find a strong negative correlation (∼−0.85) between delay and He II flux change and a moderate negative correlation (∼−0.55) with rate of increase in He II flux. Additionally, flare magnitude and the X-ray-to-He II flux ratio at peak He II emission show strong negative correlations with delay (∼−0.80 and ∼−0.75, respectively). We also identify longer delays for flares occurring closer to the summer solstice. These results may have applications in upper-ionospheric recombination rate calculations, atmospheric modelling, and other solar–terrestrial studies. We highlight the importance of incident EUV and X-ray flux parameters on the response time of the ionospheric electron content, and these findings may also have implications for mitigating disruptions in communication and navigation systems.

Since its launch in 2010, the Solar Dynamics Observatory (SDO) has provided continuous, high-cadence, multi-wavelength observations of the Sun, capturing thousands of solar flares and offering new insights into coronal dynamics. Among the discoveries enabled by SDO is the EUV late-phase (ELP), characterised by a secondary enhancement in warm coronal emission occurring tens of minutes after the main flare. While recent work has demonstrated the relevance of the ELP for space weather, the statistical behaviours and physical origins are not fully understood. Here, we present the most comprehensive statistical analysis of the ELP to date, based on 15-years of Fe xvi (335 Å) observations from the Atmospheric Imaging Assembly onboard SDO (SDO/AIA). From a sample of 5335 isolated flares between 2010 and 2025, we identify and validate 467 ELP events. The overall ELP occurrence rate was found to be 9%, with no significant dependence on the solar cycle and only a modest enhancement in the low-mid M-class range. The ELP typically exhibited an onset delay of 19 minutes, a peak-to-peak delay of 88 minutes, and a duration of 93 minutes. Strong correlations were found between ELP rise and decay rates ( $\rho =0.76$ ρ = 0.76 ), and between flare and ELP impulsivity ( $\rho =0.61$ ρ = 0.61 ). However, a comprehensive pairwise analysis revealed no significant correlation between the flare and ELP phases ( $|\rho |<0.3$ | ρ | < 0.3 ). A Principal Component Analysis of flare and ELP properties identified several semi-independent axes of variability, broadly associated with late-phase temporal scale, impulsive heating characteristics, and the relative prominence of flare and late-phase intensity measures. These results highlight the continuing importance of SDO’s long-term, high-resolution observations for uncovering new aspects of solar flare evolution and improving understanding of the Sun-Earth connection.

The chromospheric Lyman-alpha line of neutral hydrogen (Ly$α$; 1216 Å) is the most intense emission line in the solar spectrum, yet until recently observations of flare-related Ly$α$ emission have been scarce. Here, we examine the relationship between nonthermal electrons accelerated during the impulsive phase of three M3 flares that were co-observed by RHESSI, GOES, and SDO, and the corresponding response of the chromosphere in Ly$α$. Despite having identical X-ray magnitudes, these flares show significantly different Ly$α$ responses. The peak Ly$α$ enhancements above quiescent background for these flares were 1.5%, 3.3%, and 6.4%. However, the predicted Ly$α$ enhancements from FISM2 were consistently <2.5%. By comparing the properties of the nonthermal electrons derived from spectral analysis of hard X-ray observations, flares with a harder spectral index were found to produce a greater Ly$α$ enhancement. The percentage of nonthermal energy radiated by the Ly$α$ line during the impulsive phase was found to range from 2.0-7.9%. Comparatively, the radiative losses in He II (304 Å) were found to range from 0.6-1.4% of the nonthermal energy while displaying enhancements above the background of 7.3-10.8%. FISM2 was also found to underestimate the level of He II emission in two out of the three flares. These results may have implications for space weather studies and modelling the response of the terrestrial atmosphere to changes in the solar irradiance, and will guide the interpretation of flare-related Ly$α$ observations that will become available during Solar Cycle 25.