Frequency rising sub-THz emission from solar flare ribbons

¹ School of Physics and Astronomy, University of Glasgow, Kelvin Building, Glasgow, G12 8QQ UK
e-mail: motorinagalina@gmail.com
² Central Astronomical Observatory at Pulkovo of Russian Academy of Sciences, St. Petersburg, 196140 Russia
³ Ioffe Institute, Polytekhnicheskaya, 26, St. Petersburg, 194021 Russia
⁴ Crimean Astrophysical Observatory, Nauchny, Russia
⁵ New Jersey Institute of Technology, University Heights, Newark, NJ, 07102-1982 USA

Received: 23 August 2018
Accepted: 7 October 2018

Abstract

Observations of solar flares at sub-THz frequencies (millimetre and sub-millimetre 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 ∼10⁴ solar flux units at 0.4 THz) and fast variability on sub-second timescales. Although, many models have been put forward to explain the puzzling observations, none of them has 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 10⁴ − 10⁶ 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.