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As the Lyman-alpha (Lya) line of neutral hydrogen is the brightest emissionline in the solar spectrum, detecting increases in irradiance due to solarflares at this wavelength can be challenging due to the very high background.Previous studies that have focused on the largest flares have shown that eventhese extreme cases generate enhancements in Lya of only a few percent abovethe background. In this study, a superposed-epoch analysis was performed on~8500 flares greater than B1 class to determine the contribution that flaresmake to changes in the solar EUV irradiance. Using the peak of the 1-8A X-rayemission as a fiducial time, the corresponding time series of 3123 B- and 4972C-class flares observed in Lya emission by the EUV Sensor on GOES-15 wereaveraged to reduce background fluctuations and improve the flare signal. Thesummation of these weaker events showed that they produced a 0.1-0.3%enhancement to the solar Lya irradiance. For comparison on average, the sametechnique was applied to 453 M- and 31 X-class flares, which resulted in a 1-4%increase in Lya emission. Flares were also averaged with respect to theirheliographic angle to investigate any potential center-to-limb variation. Foreach GOES class, the relative enhancement in Lya at the flare peak was found todiminish for flares that occurred closer to the solar limb due to the opacityof the line, and/or foreshortening of the footpoints. One modest event includedin the study, a C6.6 flare, exhibited an unusually high increase in Lya of 7%that may have been attributed to a failed filament eruption. Increases of thismagnitude have hitherto only been associated with a small number of X-classflares.

Solar Irradiance Variability Due To Solar Flares Observed in Lyman-alpha Emission