  As the Lyman-alpha (Lya) line of neutral hydrogen is the brightest emission
line in the solar spectrum, detecting increases in irradiance due to solar
flares at this wavelength can be challenging due to the very high background.
Previous studies that have focused on the largest flares have shown that even
these extreme cases generate enhancements in Lya of only a few percent above
the background. In this study, a superposed-epoch analysis was performed on
~8500 flares greater than B1 class to determine the contribution that flares
make to changes in the solar EUV irradiance. Using the peak of the 1-8A X-ray
emission as a fiducial time, the corresponding time series of 3123 B- and 4972
C-class flares observed in Lya emission by the EUV Sensor on GOES-15 were
averaged to reduce background fluctuations and improve the flare signal. The
summation of these weaker events showed that they produced a 0.1-0.3%
enhancement to the solar Lya irradiance. For comparison on average, the same
technique 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 their
heliographic angle to investigate any potential center-to-limb variation. For
each GOES class, the relative enhancement in Lya at the flare peak was found to
diminish for flares that occurred closer to the solar limb due to the opacity
of the line, and/or foreshortening of the footpoints. One modest event included
in 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 this
magnitude have hitherto only been associated with a small number of X-class
flares.
