Sun-as-a-Star Observation of Flares in Lyman {\alpha} by the PROBA2/LYRA radiometer

TL;DR

This study reports the detection of solar flare signatures in the Lyman-alpha line at 121.5 nm using the PROBA2/LYRA radiometer, highlighting the flare's energy contribution and temporal profile in this spectral line.

Contribution

First detailed analysis of Lyman-alpha flare signatures in solar irradiance data, demonstrating the flare's energy and temporal characteristics in this spectral line.

Findings

Lyman-alpha flux increased by 0.6% during the flare.

Lyman-alpha emission was about twice the energy of the GOES soft X-ray emission.

Lyman-alpha flare profile peaks before other wavelengths.

Abstract

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 has 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.