Sun-as-a-Star Observation of Flares in Lyman α by the PROBA2/LYRA Radiometer

There are very few reports of flare signatures in the solar irradiance at H i Lyman alpha at 121.5 nm, i.e. the strongest line of the solar spectrum. The LYRA radiometer onboard PROBA2 has observed several flares for which unambiguous signatures have been found in its Lyman-alpha channel. Here we present a brief overview of these observations followed by a detailed study of one of them: the M2 flare that occurred on 8 February 2010. For this flare, the flux in the LYRA Lyman-alpha channel increased by 0.6 %, which represents about twice the energy radiated in the GOES soft X-ray channel and is comparable with the energy radiated in the He ii line at 30.4 nm. The Lyman-alpha emission represents only a minor part of the total radiated energy of this flare, for which a white-light continuum was detected. Additionally, we found that the Lyman-alpha flare profile follows the gradual phase but peaks before other wavelengths. This M2 flare was very localized and had a very brief impulsive phase, but more statistics are needed to determine if these factors influence the presence of a Lyman-alpha flare signal strong enough to appear in the solar irradiance.