Origin of the 30 THz emission during the 2012 March 13 solar flare at 1720 UT

Solar observations in the infrared domain can bring important clues on the lower layers of the solar atmosphere and on their response to primary energy released during flares. The 10 micron (30 THz) observations of the flare SOL2012-03-13 recently presented and discussed in Kaufmann et al. (2013), is one of the few examples in the literature. However, no firm conclusions are drawn on the origin of the mid-infrared radiation. In this work we present a detailed multi-frequency analysis of SOL2012-03-13, including observations in radio millimeter and sub millimeter wavelengths, Hard X-Ray (HXR) and Gamma-Ray (GR), H-alpha, and white-light. HXR/GR spectral analysis allows estimating the electron double power-law energy distribution at the origin of the non-thermal bremsstrahlung continuum emission. It is also shown that the high-energy (> ~0.8 MeV) part of this distribution is responsible for the > 20 GHz radio emission during the flare. By comparing flaring and quiescent active region solar atmosphere models, we find that most of the radiation excess at 30 THz observed during the flare is compatible with optically thin thermal free-free emission. The emitting source of assumed 10″ size, with temperature T ~ 8000 K is located at an altitude range of 960-1100 km above the photosphere where the mass column density varies from ~ 3.5 10-3 to 6.2 10-3 g cm-2. We finally show that the chromospheric heating, which results in the 30 THz excess radiation, can be due to energy deposited by non-thermal flare accelerated electrons, protons and alpha particles in the quiescent atmosphere layer, which spans the same range of column densities.