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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., Comment: 7 pages, 4 figures, Geomagnetism and Aeronomy, accepted

We present systematic analysis of spatio-temporal evolution of sources of hard X-ray (HXR) pulsations in solar flares. We concentrate on disk flares whose impulsive phase are accompanied by a series of more than three peaks (pulsations) of HXR emission detected in the RHESSI 50-100 keV channel with 4-second cadence. 29 such flares observed from February 2002 to June 2015 with time differences between successive peaks of 8-270 s are studied. The main observational result is that sources of HXR pulsations in all flares are not stationary, they demonstrate apparent displacements from pulsation to pulsation. The flares can be subdivided into two groups depending on character of dynamics of HXR sources. The group-1 consists of 16 flares (55%) with systematic dynamics of HXR sources from pulsation to pulsation with respect to a magnetic polarity inversion line (MPIL), which has simple extended trace on the photosphere. The group-2 consists of 13 flares (45%) with more chaotic displacements of HXR sources with respect to an MPIL having more complicated structure. Based on the observations we conclude that the mechanism of flare HXR pulsations is related to successive triggering of energy release in different magnetic loops. Group-1 flare regions consist of loops stacked into magnetic arcades extended along MPILs. Group-2 flare regions have more complicated magnetic structures and loops are arranged more chaotically. We also found that at least 14 (88%) group-1 flares and 11 (85%) group-2 flares are accompanied by coronal mass ejections, i.e. the majority of flares studied are eruptive events. This gives an indication that eruptive processes play important role in generation of HXR pulsations. We suggest that an erupting flux rope can act as a trigger of energy release. Its successive interaction with different loops can lead to apparent motion of HXR sources and to a series of HXR pulsations., Comment: 48 pages, 18 figures, 4 tables, 2 appendices

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., Comment: 18 pages, 6 figures