On the origin of 140 GHz emission from the 4 July 2012 solar flare

TL;DR

This study analyzes a 2012 solar flare observed at sub-THz frequencies, concluding that the millimeter emission likely originates from thermal bremsstrahlung in plasma around 0.1 MK, not from hotter coronal plasma.

Contribution

It provides new insights into the origin of sub-THz emission in solar flares, suggesting thermal bremsstrahlung from cooler plasma as the source.

Findings

Spectral flux increases with frequency between 93 and 140 GHz.

Thermal coronal plasma above 0.5 MK cannot explain the observed emission.

Millimeter emission likely produced by thermal bremsstrahlung from plasma at about 0.1 MK.

Abstract

The sub-THz event observed on the 4 July 2012 with the Bauman Moscow State Technical University Radio Telescope RT-7.5 at 93 and 140~GHz as well as Kislovodsk and Mets\"ahovi radio telescopes, Radio Solar Telescope Network (RSTN), GOES, RHESSI, and SDO orbital stations is analyzed. The spectral flux between 93 and 140 GHz has been observed increasing with frequency. On the basis of the SDO/AIA data the differential emission measure has been calculated. It is shown that the thermal coronal plasma with the temperature above 0.5~MK cannot be responsible for the observed sub-THz flare emission. The non-thermal gyrosynchrotron mechanism can be responsible for the microwave emission near $10$~GHz but the observed millimeter spectral characteristics are likely to be produced by the thermal bremsstrahlung emission from plasma with a temperature of about 0.1~MK.