Giant post-flare loops in active regions with extremely strong coronal magnetic fields

TL;DR

This study reports the first detection of thermal free-free emission from post-flare loops at 34GHz, revealing their multi-temperature structure, density, and magnetic field, especially in regions with extremely strong coronal magnetic fields.

Contribution

It provides the first observational evidence of thermal radio emission from post-flare loops at 34GHz and offers detailed diagnostics of their physical properties.

Findings

Detected thermal free-free emission at 34GHz from post-flare loops.

Loop densities range from 1 to 6 x 10^10 cm^-3, higher than EUV estimates.

Magnetic fields at loop tops are estimated between 10 and 30 G.

Abstract

We report for the first time the detection of thermal free-free emission from post-flare loops at 34GHz in images from the Nobeyama Radioheliograph (NoRH). We studied 8 loops, 7 of which were from regions with extremely strong coronal magnetic field reported by Fedenev et al. (2023). Loop emission was observed in a wide range of wavelength bands, up to soft X-rays, confirming their multi-temperature structure and was associated with noise storm emission in metric wavelengths. The comparison of the 17GHz emission with that at 34GHz, after a calibration correction of the latter, showed that the emission was optically thin at both frequencies. We describe the structure and evolution of the loops and we computed their density, obtaining values for the top of the loops between 1 and 6 x 10^10 cm^-3, noticeably varying from one loop to another and in the course of the evolution of the same loop system; these values have only a weak dependence on the assumed temperature, 2 x 10^6 K in our case, as we are in the optically thin regime. Our density values are above those reported from EUV observations, which go up to about 10^10 cm^-3. This difference could be due to the fact that different emitting regions are sampled in the two domains and/or due to the more accurate diagnostics in the radio range, which do not suffer from inherent uncertainties arising from abundances and non-LTE excitation/ionization equilibria. We also estimated the magnetic field in the loop tops to be in the range of 10 to 30G.