Flare heating of the chromosphere: Observations of flare continuum from GREGOR and IRIS

¹ Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 251 65 Ondřejov, Czech Republic
² Astronomical Institute, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 182 00 Prague, Czech Republic
³ Center of Scientific Excellence – Solar and Stellar Activity, University of Wrocław, Mikołaja Kopernika 11, 51-622 Wrocław, Poland
⁴ Christian Doppler Grammar School, Zborovská 621/45, 150 00 Prague, Czech Republic

Received: 22 June 2024
Accepted: 8 August 2024

Abstract

Context. On 2022 May 4, an M5.7 flare erupted in the active region NOAA 13004, which was the target of a coordinated campaign between GREGOR, IRIS, Hinode, and ground-based instruments at the Ondřejov observatory. A flare kernel located at the edge of a pore was co-observed by the IRIS slit and GREGOR HiFI+ imagers.

Aims. We investigated the flare continuum enhancement at different wavelength ranges in order to derive the temperature of the chromospheric layer heated during the flare.

Methods. All datasets were aligned to IRIS slit-jaw images. We selected a pixel along the IRIS slit where the flare kernel was captured and evaluated multi-wavelength light curves within it. We defined a narrow IRIS near-UV band that comprises only continuum emission. The method, which assumes that the flare continuum enhancement is due to optically thin emission from hydrogen recombination processes, was applied to obtain a lower limit on the temperature in the layer where the continuum enhancement was formed.

Results. We determined a lower limit for the temperature and its time evolution in the chromospheric layer heated during the flare in the range of (3–15) ×10³ K. The mean electron density in that layer was estimated to be ∼1 × 10¹³ cm⁻³.

Conlcusions. Multi-wavelength flare co-observations are a rich source of diagnostics. Due to the rapidly evolving nature of flares, the sit-and-stare mode is key to achieving a high temporal cadence that allows one to thoroughly analyse the same flare structure.