Models for Frequency Rising sub-THz Emission from Solar Flares: Progress and Challenges

Alexander Stepanov, Pulkovo Observatory

Progress and challenges in existing models for the mysterious positive slope in the frequency spectra of sub-THz emission from solar flares distinct from the microwave component are discussed. Examples for the sub-THz frequency rising emission observed with KOSMA (230 and 345 GHz), SST (212 and 405 GHz), and RT-7.5 of Bauman Technical University (93 and 140 GHz) are presented. Both thermal and non-thermal models of solar sub-THz emission are considered. It is shown that the most of sub-THz events with frequency rise can be interpreted in terms of free-free emission of hot flare plasma of the chromosphere and/or the transition region. The non-thermal models suggest Vavilov-Cherenkov emission, gyrosynchrotron radiation of accelerated electrons, and plasma radiation. The possible mechanisms for electron acceleration in non-thermal models are presented. The origin of second and sub-second temporal structures in sub-THz emission from solar flares is also discussed. The further progress in modeling of peculiarities of the spectrum and origin of sub-THz and THz emission is related to the idea of Pierre Kaufmann about observations at 3, 7 and 30 THz.