First Detection of Solar Flare Emission in Middle-Ultraviolet Balmer Continuum

TL;DR

This study reports the first detection of solar flare emission in the middle-ultraviolet range around 2000 A, using LYRA onboard PROBA2, revealing hydrogen Balmer continuum emission from the chromosphere.

Contribution

It provides the first direct observation of the Balmer continuum in the middle-ultraviolet, confirming hydrogen recombination as a key component of solar flare emission.

Findings

Detection of flare emission at 2000 A by LYRA.

Separation of line and continuum emission in UV channels.

Supports hydrogen recombination as a source of broadband flare emission.

Abstract

We present the first detection of solar flare emission at middle-ultraviolet wavelengths around 2000 A by the channel 2 of the Large-Yield RAdiometer (LYRA) onboard the PROBA2 mission. The flare (SOL20170906) was also observed in the channel 1 of LYRA centered at the H I Lyman-{\alpha} line at 1216 A, 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 10000 K. Simultaneous observations in channels 1 and 2 allow the separation of the line emission (primarily from the Lyman-{\alpha} line) from the Balmer continuum emission. Together with the recent detection of the Balmer continuum emission in the near-ultraviolet by IRIS, the LYRA observations strengthen the interpretation of broadband flare emission as the hydrogen recombination continua originating in the chromosphere.