Frequency rising sub-THz emission from solar flare ribbons

Abstract

Observations of solar flares at sub-THz frequencies (mm and sub-mm wavelengths) over the last two decades often show a spectral component rising with frequency. Unlike a typical gyrosynchrotron spectrum decreasing with frequency, or a weak thermal component from hot coronal plasma, the observations can demonstrate a high flux level (up to ∼ 10 4 s.f.u. at 0.4 THz) and fast variability on sub-second time scales. Although, many models has been put forward to explain the puzzling observations, none of them have clear observational support. Here we propose a scenario to explain the intriguing sub-THz observations. We show that the model, based on free-free emission from the plasma of flare ribbons at temperatures 10 4 − 10 6 K, is consistent with all existing observations of frequency-rising sub-THz flare emission. The model provides a temperature diagnostic of the flaring chromosphere and suggests fast heating and cooling of the dense transition region plasma.

Incoherent Solar Radio Emission

“…Thermal free-free emission has also been detected in weak transient brightenings observed at microwaves (White et al, 1995) and millimeter wavelengths (Nindos et al, 2020). For the sake of completeness we note that (i) non-thermal emissions have occasionally also been reported in the decay phase of flares (e.g., Kundu et al, 2001aKundu et al, , 2004a, and (ii) sub-THz emission may originate from optically thick and relatively hot free-free sources located at the chromospheric footpoints of flaring loops (Kontar et al, 2018;Morgachev et al, 2018Morgachev et al, , 2020.…”

Incoherent Solar Radio Emission

“…For the sake of completeness we note that (i) nonthermal emissions have occassionally also been reported in the decay phase of flares (e.g. Kundu et al, 2001aKundu et al, , 2004a, and (ii) sub-THz emission may originate from optically thick and relatively hot free-free sources located at the chromospheric footpoints of flaring loops (Morgachev et al, 2018(Morgachev et al, , 2020Kontar et al, 2018).…”

What aspects of solar flares can be clarified with mm/submm observations?

“…Fleishman and Kontar (2010) discussed some of them and proposed a new one -Cherenkov emission at some spectral windows where the dielectric permeability is above one due to contributions of atomic or molecular transitions. Kontar et al (2018) noted that the sub-mm radio flux is correlated with the area of the flare ribbons and proposed that the raising submm component could be produced by thermal emission of these ribbons. This would imply that the sources must be rather large, larger than the nonthermal ones.…”