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Properties of the solar radio spectrum, as well as the temporal profiles of flare emission, indicate the thermal nature of the sub-terahertz (sub-THz) component observed as the growth of radio emission in the frequency range of 100–1000 GHz. The sub-THz flare onset can be ahead of the impulsive phase for several minutes. However, the origin of the pre-impulsive and impulsive sub-THz emission remains unclear. The present work is devoted to a detailed analysis of the M4.0 X-class solar flare observed on March 28, 2022 with the Bauman Moscow State Technical University Radio Telescope RT-7.5 at 93 GHz. We supply these data with multiwavelength solar observations in the X-ray (GOES, GBM/Fermi), extreme ultraviolet (AIA/SDO), and microwave ranges. The differential emission measure (DEM) responsible for EUV emission is determined by solving the inverse problem based on the AIA/SDO data. Using the DEM and assuming a thermal free-free emission mechanism in pre-impulsive and impulsive phases, we calculated the millimeter emission flux of coronal plasma of the flare source, which turned out to be much smaller than the observed values. We concluded that electrons accelerated in the corona and heat fluxes from the coronal loop top cannot be responsible for heating the sub-THz emission source located in the transition region and upper chromosphere. A possible origin of chromospheric heating in the pre-impulsive phase of the solar flare is discussed. [ABSTRACT FROM AUTHOR]

Based on observations at the RT-7.5 radio telescope of the Bauman Moscow State Technical University at a wavelength of 3.2 mm (93 GHz), along with other ground-based and space instruments (Siberian Radioheliograph, Solar Dynamics Observatory (SDO), Metsähovi Radio Observatory), the origin of millimeter radiation from X-ray class M 5.7 SOL2022-05-04T08:45 solar flare was investigated. An analysis of the time profiles of radiation in the X-ray and radio ranges showed that the millimeter emission source is not associated with hot (5 × 105–107 K) coronal plasma. This is also evidenced by the estimate of the sub-THz flux from radiating hot plasma according to the AIA/SDO data, which turned out to be much less than the observed values. Indications of the development of thermal instability in flare ultraviolet loops were obtained. The relationship between the millimeter emission of the flare and the heat source in the solar chromosphere has been substantiated. [ABSTRACT FROM AUTHOR]

The temporal evolution of thermal bremsstrahlung sub-THz emission from a flare chromosphere and transition region is analyzed based on the F-CHROMA solar flare model database (https://www.fchroma.org) and solar flare observations with the RT-7.5 radio telescope at 93 and 140 GHz on July 4, 2012. The model is based on the RADYN numerical code, which describes the response of chromospheric background plasma to the action of a beam of accelerated electrons in the form of a triangular pulse. The positive slope of the spectrum in the sub-THz frequency range has been formed over the entire burst. A thin layer of low temperature and high plasma density (the so-called chromospheric condensation) appears. It moves to the region of higher altitudes and effectively absorbs the sub-THz emission. It is shown based on the RНESSI hard X-ray observations that the chromospheric thermal plasma cannot provide the observed sub-THz emission fluxes during the solar flare on July 4, 2012. The cause of the obtained discrepancy is discussed. [ABSTRACT FROM AUTHOR]

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 Metsähovi 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. [ABSTRACT FROM AUTHOR]