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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]

In this study, a comparative analysis was carried out on the preflare and flare conditions for six flares accompanied by coronal mass ejections (CMEs) and five events not accompanied by CMEs. All of the events occurred in the period 2011–2014. The features of the spatial dynamics of microwave emission of active regions (data from the Nobeyama Radioheliograph) prior to the selected events and during flares were studied. For the same 11 events, the fine structure of active ultraviolet (UV) sources was examined based on SDO/AIA data. Certain differences were found in the behavior of microwave emission for events with CMEs. The features of the frequency radio spectra from the RATAN-600 data were found for some active regions. [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 time delays between sub-terahertz (sub-THz) and soft X-ray (SXR) emission from solar flares with a positive spectrum slope at frequencies of >100 GHz are considered. Based on a cross-correlation analysis of the time profiles of sub-THz and SXR emission from 11 flare events obtained from the KOSMA (230 GHz), SST (212 GHz), and RT-7.5 (93 GHz) telescopes, as well as the GOES satellite in channel 1–8 Å, we identified two main types of sub-THz events. Type I includes four X-class events with a well-defined Neupert effect, i.e., the sub-THz flux outpaces the SXR flux by 2–9 min. Type II includes seven M-class flares, in which the time profiles behave similarly and the delays do not exceed 30 s. The different mechanisms of radio emission in different layers of the solar atmosphere may be responsible for the positive slope of the sub-THz spectrum of flares. [ABSTRACT FROM AUTHOR]

The temporal evolution of thermal bremsstrahlung in the sub-terahertz range is calculated for F-CHROMA models of the flare chromosphere and transition region (https://www.fchroma.org). These models are calculated based on the numerical solution of the system of equations of radiation hydrodynamics with the RADYN code, which describes the response of chromospheric plasma to the flux of accelerated electrons in the form of a triangular pulse. It is found that the positive slope of the spectrum in the frequency range of 93–400 GHz is formed during the entire burst. The sub-THz radiation peak can be both ahead and behind the maximum of the function of the injection of energetic electrons depending on the number and index of the spectrum of precipitating particles. In some cases, the time profile of the sub-THz radiation is quite complex and pulsating due to gas-dynamic phenomena in the chromosphere and transition region. [ABSTRACT FROM AUTHOR]

The spatial dynamics of microwave emission (data from the Nobeyama Radioheliograph) of active regions with a complex magnetic field configuration before powerful flares, as well as the fine structure of active sources of ultraviolet (UV) emission based on AIA/SDO data, are studied. It is shown that there is unsteady behavior of microwave sources, along with the ignition of a system of crossed UV loops (X-structures). A class-M X-ray flare occurs after several hours at the site of ignition of the intersecting UV loops. The similar behavior observed in microwave and ultraviolet emission in the five active regions studied by us likely serves as a sign of a preflare situation in the active region on the Sun. [ABSTRACT FROM AUTHOR]

This paper is a study of the spatial dynamics of microwave emission (data from the Nobeyama Radioheliograph) from two active regions NOAA 12673 and NOAA 11261 for several hours prior to powerful class M x-ray flares. The spatial distribution of the radio brightness and emission intensity in certain parts of the active region is found to be highly nonstationary. Data in the ultraviolet (AIA/SDO) are also used for studying the fine structure of the active emission sources. It is shown that the spatial dynamics of the microwave sources correlates well with the position and appearance of bright ultraviolet (UV) loops in the 94 Å line, i.e., with magnetic loops where the plasma is heated to high temperatures ~107 K. A combined analysis of the UV and radio emission charts with magnetic field charts (HMI/SDO) for NOAA 12673 shows that the highly nonstationary behavior of the microwave sources develops simultaneously with the ignition of a system of intersecting UV loops (X-structures). It is also shown that the bright UV loops and microwave sources arise in the area where new magnetic fluxes emerge. [ABSTRACT FROM AUTHOR]

The study analyzes the millimeter emission of a solar flare on April 2, 2017, observed by the Bauman Moscow State Technical University telescope RT-7.5. Based on the flare atmosphere model proposed by Machado et al. (1980) we inferred the emission contribution function of the source and determined its main characteristics. The results show that the subterahertz emission is formed in a thin, chromospheric layer with a thickness of about 10 km with a plasma temperature of ~105 K. The proposed homogeneous model of the layer makes it possible to explain the observed spectral features. The obtained results suggest the chromospheric source heating with heat fluxes generated in the overlying flare areas. [ABSTRACT FROM AUTHOR]

Abstract: Millimeter (93 and 140 GHz) emission of the М6.4 solar flare detected on April 2, 2017 in the NOAA 12644 active region by the RT-7.5 telescope of the Bauman Moscow State Technical University is analyzed using the observational data provided by the Radio Solar Telescope Network (4.9, 8.8, and 15.4 GHz); the SDO/AIA satellites (Etreme Ultraviolet); and GOES, RHESSI, and Konus-Wind (X-rays). It is found that the spectral flux density of millimeter emission increases with frequency throughout the entire burst. The similarity between the profiles of millimeter and soft-X-ray radiation suggests that the burst is of a thermal nature. It follows from the results of calculations of the differential emission measure of coronal plasma based on the SDO/AIA data that its contribution to millimeter emission of the flare is negligible. The simulation of thermal emission of chromospheric flare plasma in the model of Machado (Machado et al., 1980) yields millimeter fluxes that are by several times lower than the observed ones. The physical implications of these results are discussed. [ABSTRACT FROM AUTHOR]

A three-dimensional simulation of the microwave emission of nonthermal electrons within a flare magnetic loop that takes into account the influence of the chromosphere has been carried out. The paper investigates the possibility of the generation of a microwave spectrum (observed for some solar flares) with a maximum in the centimeter wavelength range and a positive slope in the millimeter one under the some distribution of the magnetic field strength and the parameters of the anisotropic nonthermal electrons along the loop. By the example of the event on July 5, 2012, it is shown that nonthermal electrons can be responsible not only for the centimeter bell-shaped emission spectrum generated in the coronal part of the loop but also for the increasing millimeter spectrum generated in the chromosphere. [ABSTRACT FROM AUTHOR]

The ordinary mode of gyrosynchrotron radiation was identified to be predominant in some segments of flare loops in solar flares of July 19, 2012, and October 22, 2014. These events were studied by investigation of the quasi-transverse propagation effect on the observed polarization. The analysis involved reconstruction of the magnetic field topology at the linear force-free approximation based on the data of the SDO HMI space telescope and the subsequent simulation of radio emission of flare loops with the GX Simulator software package. The quasi-transverse propagation effect was established to be characteristic for both events, but its influence on the radio emission polarization at a frequency of 17 GHz was observed only in the October 22, 2014 flare. [ABSTRACT FROM AUTHOR]

An Erratum to this paper has been published: https://doi.org/10.1134/S0016793221070288 [ABSTRACT FROM AUTHOR]