Origin of the submillimeter radio emission during the time-extended phase of a solar flare

TL;DR

This study analyzes the origin of long-lasting submillimeter radio emission during a solar flare, demonstrating it is primarily thermal bremsstrahlung from coronal and chromospheric materials, challenging existing models.

Contribution

It provides a detailed multi-frequency analysis of the flare's extended phase, revealing the thermal origin and temperature distribution of the emitting plasma, and questions standard flare models.

Findings

Extended radio emission is thermal bremsstrahlung from hot and cool coronal plasma.

At 345 GHz, chromospheric material also contributes to the emission.

Results challenge existing semi-empirical models of flare chromosphere and transition region.

Abstract

Solar flares observed in the 200-400 GHz radio domain may exhibit a slowly varying and time-extended component which follows a short (few minutes) impulsive phase and which lasts for a few tens of minutes to more than one hour. The few examples discussed in the literature indicate that such long-lasting submillimeter emission is most likely thermal bremsstrahlung. We present a detailed analysis of the time-extended phase of the 2003 October 27 (M6.7) flare, combining 1-345 GHz total-flux radio measurements with X-ray, EUV, and H{\alpha} observations. We find that the time-extended radio emission is, as expected, radiated by thermal bremsstrahlung. Up to 230 GHz, it is entirely produced in the corona by hot and cool materials at 7-16 MK and 1-3 MK, respectively. At 345 GHz, there is an additional contribution from chromospheric material at a few 10^4 K. These results, which may also apply to other millimeter-submillimeter radio events, are not consistent with the expectations from standard semi-empirical models of the chromosphere and transition region during flares, which predict observable radio emission from the chromosphere at all frequencies where the corona is transparent.