Balmer continuum enhancement detected in a mini flare observed with IRIS

Abstract

Context. Optical and near-UV continuum emissions in flares contribute substantially to the flare energy budget. Two mechanisms play an important role for continuum emission in flares: hydrogen recombination after sudden ionization at chromospheric layers, and transportation of the energy radiatively from the chromosphere to lower layers in the atmosphere, the so-called back-warming. Aims. The aim of the paper is to distinguish between these two mechanisms for the excess of the Balmer continuum observed in a flare. Methods. We combined the observations of the Balmer continuum obtained with the Interface Region Imaging Spectrograph (IRIS) (spectra and slit-jaw images (SJIs) 2832 Å) and hard X-ray (HXR) emission detected by the Fermi/Gamma Burst Monitor (GBM) during a mini flare. The calibrated Balmer continuum was compared to non-local thermodynamic equilibrium (LTE) radiative transfer flare models, and the radiated energy was estimated. Assuming thick target HXR emission, we calculated the energy of the nonthermal electrons detected by the Fermi/GBM and compared it to the radiated energy. Results. The favorable argument of a relation between the Balmer continuum excess and the HXR emission is that there is a good time coincidence between them. In addition, the shape of the maximum brightness in the 2832 SJIs, which is mainly due to this Balmer continuum excess, is similar to that of the Fermi/GBM light curve. The electron-beam flux estimated from Fermi/GBM between 109 and 1010 erg s−1 cm−2 is consistent with the beam flux required in non-LTE radiative transfer models to obtain the excess of Balmer continuum emission observed in this IRIS spectra. Conclusions. The low-energy input by nonthermal electrons above 20 keV is sufficient to produce the enhancement in the Balmer continuum emission. This could be explained by the topology of the reconnection site. The reconnection starts in a tiny bald-patch region, which is transformed dynamically into an X-point current sheet. The size of the interacting region would be below the spatial resolution of the instrument.