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Solar observations in the infrared domain can bring important clues on theresponse of the low solar atmosphere to primary energy released during flares.At present the infrared continuum has been detected at 30 THz (10 $\mu$m) inonly a few flares. In this work we present a detailed multi-frequency analysisof SOL2012-03-13, including observations at radio millimeter and sub-millimeterwavelengths, in hard X-rays (HXR), gamma-rays (GR), H-alpha, and white-light.HXR/GR spectral analysis shows that the event is a GR line flare and allowsestimating the numbers of and energy contents in electrons, protons and alphaparticles produced during the flare. The energy spectrum of the electronsproducing the HXR/GR continuum is consistent with a broken power-law with anenergy break at ~800 keV. It is shown that the high-energy part (above ~800keV) of this distribution is responsible for the high-frequency radio emission(> 20 GHz) detected during the flare. By comparing the 30 THz emission expectedfrom semi-empirical and time-independent models of the quiet and flareatmospheres, we find that most (~80%) of the observed 30 THz radiation can beattributed to thermal free-free emission of an optically-thin source. Using theF2 flare atmospheric model this thin source is found to be at temperaturesT~8000 K and is located well above the minimum temperature region. We arguethat the chromospheric heating, which results in 80% of the 30 THz excessradiation, can be due to energy deposition by non-thermal flare acceleratedelectrons, protons and alpha particles. The remaining 20% of the 30 THz excessemission is found to be radiated from an optically-thick atmospheric layer atT~5000 K, below the temperature minimum region, where direct heating bynon-thermal particles is insufficient to account for the observed infraredradiation.

Origin of the 30 THz emission detected during the 2012 March 13 solar flare at 17:20 UT