Statistical approach to frequency rising sub-terahertz emission from solar flares

Galina Motorina, Astronomical Institute ASCR

Sub-terahertz (sub-THz) emission from solar flares showing an increase of the spectral flux density with frequency has been of great interest, since its origin to date is unclear. Further, sub-THz radio observations may allow us to diagnose the properties of flaring plasma in the transition region and chromosphere, which is difficult using other methods. In this work, we conduct for the first time, a statistical study of flares with frequency rising sub-THz emission in the 200-400 GHz range. Assuming that thermal free-free emission is responsible for the sub-THz rising component, 17 solar flares with a growing spectrum have been considered in detail. Taking into account the effect of signal saturation, based on data from TRACE and SDO/AIA (1600 A), we estimate the areas of flare ribbons. The results demonstrate that large fluxes of sub-THz emission correspond to large flare ribbon areas. We conclude that the optically thick thermal plasma of the transition region makes the decisive contribution to the rising sub-THz component. Finally, we show that radiation losses from the heated dense plasma can provide the observed sub-second pulsations of the sub-THz emission.