First Detection of Solar Flare Emission in Mid-ultraviolet Balmer Continuum

We present the first detection of solar flare emission at mid-ultraviolet wavelengths around 2000 Å by the channel 2 of the Large-Yield RAdiometer (LYRA) on board the PRoject for OnBoard Autonomy 2 mission. The flare (SOL20170906) was also observed in the channel 1 of LYRA centered at the H i Lyα line at 1216 Å, showing a clear non-thermal profile in both channels. The flare radiation in channel 2 is consistent with the hydrogen Balmer continuum emission produced by an optically thin chromospheric slab heated up to 10,000 K. Simultaneous observations in channels 1 and 2 allow the separation of the line emission (primarily from the Lyα line) from the Balmer continuum emission. Together with the recent detection of the Balmer continuum emission in the near-ultraviolet by the Interface Region Imaging Spectrometer, the LYRA observations strengthen the interpretation of broadband flare emission as the hydrogen recombination continua originating in the chromosphere.