In this paper, the dynamics of loop-like structures and related phenomena during the solar flare on February 24, 2023 are investigated. A new character of the dynamics of the coronal loop system during the flare has been studied, consisting in compression (lowering) of the loops both during the growth and decay phases of the flare. It was found that a sharp decrease in height began with the appearance of intense nonstationary plasma fluxes (ejections) observed mainly in the vicinity of the eastern footpoints of the coronal loop system. It was concluded that the rapid (at a speed of up to 25 km/s) compression of the coronal loop system can be explained by a decrease in free magnetic energy (a decrease in the vortex phi-component of the magnetic field) caused by the observed non-stationary plasma eruptions from the vicinity of the loops, as well as possible Joule dissipation of electric currents in the loops.

The SOL2012-05-17 event is remarkable in that it caused one of two ground-level enhancements (GLE71) in Solar Cycle 24. Despite the efforts spent studying this solar event, some aspects of it remain unclear. This relates to the development of a coronal mass ejection (CME), the history of the shock wave, and the flare. Our measurements reveal the following chain of phenomena. Two successive eruptions occurred within a few minutes. The rate of change of the reconnected magnetic flux shows a series of increases corresponding to the acceleration or deceleration of the erupting structures. The temporal profile of the magnetic-flux change rate is similar to the hard X-ray burst. Each eruption excited a disturbance that, propagating outward, accelerated all structures above it. This led to complex kinematic characteristics of the erupting structures that eventually formed a self-similarly expanding CME. The two disturbances became piston shocks and merged into a single, stronger shock. There are indications of transformation of the piston shock into a bow shock, but this occurs at distances exceeding ten solar radii. Components of the described picture were observed in a number of events and can serve as a guide for studies of eruptive flares.

Multi-wavelength observations of a coronal hole (CH) with two-dimensional spatial resolution have been made for the first time in the frequency range from 2.8 to 12 GHz. At frequencies below 6 GHz, the average brightness of the hole is 1.5 times lower than the brightness level of the quiet Sun. The distribution of radio brightness over the hole is inhomogeneous: the ratio of maximum to minimum brightness temperatures falls from several times at low frequencies to tenths of fractions at the upper received frequencies. At frequencies above 6 GHz, the temperature contrast between the CH and regions of the quiet Sun is small. Within the CH, there are compact sources that are bright relative to the quiet Sun. In general, observations of CHs with SRH are promising both for the research into the nature of CHs and for the applied problems of forecasting solar wind characteristics.

Analysis of GOES data for the SOL2012-06-29T04:09 flare, class C4.6, shows a thermal character of the energy release for several minutes before the impulsive stage. Plasma heating to temperatures above 10 MK leads to the appearance of plasma jets along open field lines and in large loops. This work examines the relationship between the heated plasma and the flare structure and its dynamics, using observations in the X-ray, extreme ultraviolet (EUV), and radio-wave ranges. Particular attention is drawn to the detection of narrow-band fine temporal structures of radio emission before and after the impulsive stage of the flare in dynamic spectra. In the initial stage, broadband pulses in the decimeter range are observed which can be associated with the formation of thermal fronts in the jets. A series of super-bright drifting bursts in the centimeter range occurs after the end of the impulsive energy release in the flare kernel. Using data from the Siberian Solar Radio Telescope (5.7 GHz), we managed to localize the position of the source of the fine structure of drifting bursts at the remote footpoint of the large-scale flare loop.

Analysis of GOES data for the SOL201206-29T04:09 flare, class C4.6, shows a thermal character of the energy release for several minutes before the impulsive stage. Plasma heating to temperatures above 10 MK leads to the appearance of plasma jets along open field lines and in large loops. This work examines the relationship between the heated plasma and the flare structure and its dynamics, using observations in the Xray, extreme ultraviolet (EUV), and radio-wave ranges. Particular attention is drawn to the detection of narrow-band fine temporal structures of radio emission before and after the impulsive stage of the flare in dynamic spectra. In the initial stage, broadband pulses in the decimeter range are observed which can be associated with the formation of thermal fronts in the jets. A series of super-bright drifting bursts in the centimeter range occurs after the end of the impulsive energy release in the flare core. Using data from the Siberian Solar Radio Telescope (5.7 GHz), we managed to localize the position of the source of the fine structure of drifting bursts at the remote footpoint of the large-scale flare loop.

We identify a set of 100 "cold" solar flares and perform a statistical analysis of them in the microwave range. Cold flares are characterized by a weak thermal response relative to nonthermal emission. This work is a follow up of a previous statistical study of cold flares, which focused on hard X-ray emission to quantify the flare nonthermal component. Here we focus on the microwave emission. The thermal response is represented by the soft X-ray emission measured by the GOES X-ray sensors. We obtain spectral parameters of the flare gyrosynchrotron emission and investigate patterns of the temporal evolution. The main results of the previous statistical study are confirmed: as compared to a "mean" flare, the cold flares have shorter durations, higher spectral peak frequencies, and harder spectral indices above the spectral peak. Nonetheless, there are some cold flares with moderate and low peak frequencies. In a majority of cold flares, we find evidence suggesting the presence of the Razin effect in the microwave spectra, indicative of rather dense flaring loops. We discuss the results in the context of electron acceleration efficiency.

A distinctive feature of the September 6, 2012 event was that sources of narrow-band (2–4 GHz) sub-second pulses (SSP) were observed in small areas of flare loops with so-called bright ultraviolet knots with high plasma density up to 1011 10¹¹ cm⁻³. Time profiles of hard X-rays of the flare, although similar to microwave light curves, do not have structures corresponding to SSP. Analysis of microwave, X-ray, and ultraviolet data has shown that the observable pulses of microwave radiation with a narrow spectral band are coherent in nature and are generated by electrons with energies of several tens of kiloelectronvolt in bright knots at a double plasma frequency. The results of the observations suggest that the appearance of bright knots is associated with local processes of energy release due to interaction of flare loops.

Radio observations of weak events are one of the promising methods for studying energy release and non-thermal processes in the solar corona. The development of instrumental capabilities allows for radio observations of weak transient coronal events, such as quasi-stationary brightenings and weak flares of X-ray class B and below, which were previously inaccessible for analysis. We have measured the spectral parameters of microwave radiation for thirty weak solar flares with X-ray classes ranging from A to C1.5, using observations from the Badary Broadband Microwave Spectropolarimeter (BBMS). The spectra indicate that plasma heating is caused by the appearance of non-thermal electron fluxes, which can be detected by bursts of microwave radiation, predominantly with an amplitude ~5–6 solar flux units (SFU) at 4–5 GHz frequencies. One solar flux unit (SFU) of radio emission is equal to 10–22 W/(m•Hz). The range of low-frequency spectrum growth indices fα varies widely from α=0.3 to 15. The distribution of high-frequency decay indices is similar to the distributions of regular flares. One of the explanations for the appearance of large fα values is the Razin effect, which can influence the shape of the gyrosynchrotron spectrum during the generation of bursts in dense plasma under relatively weak magnetic fields. We have detected two events in which the appearance of non-thermal electrons led to the generation of narrowband bursts at frequencies near the double plasma frequency. SRH test trials have shown the potential for measuring the structure of flare sources with fluxes of the order of 1 SFU, indicating the high diagnostic potential of the radioheliograph for detecting acceleration processes in weak flare events and their localization in active regions.

The current state of research on microwave subsecond pulses (SSPs) based on observations of radio-frequency radiation from solar flares with high temporal, spatial, and spectral resolutions is reviewed. It is shown that knowledge about the localization of SSP sources in the flare region, available with the help of large radio interferometers, radically affects the understanding of the nature of SSPs. Based on the unique observational material accumulated in radio observations over the past 30 years, the characteristics of SSPs have been studied, they have been classified according to their spectral characteristics, and the mechanisms of their generation have been determined.

One of the most important tasks in solar physics is the study of particles and energy transfer from the lower corona to the outer layers of the solar atmosphere. The most sensitive methods for detecting fluxes of non-thermal electrons in the solar atmosphere is observing their radio emission using modern large radioheliographs. We analyzed joint observations from the 13 April 2019 event observed by LOw-Frequency ARray (LOFAR) at meter wavelengths, and the Siberian Radio Heliograph (SRH) and the Badary Broadband Microwave Spectropolarimeter (BBMS) spectropolarimeter in microwaves performed at the time of the second PSP perihelion. During a period without signatures of non-thermal energy release in X-ray emission, numerous type III and/or type J bursts were observed. During the same two hours we observed soft X-ray brightenings and the appearance of weak microwave emission in an abnormally narrow band around 6 GHz. At these frequencies the increasing flux is well above the noise level, reaching 9 sfu. In the LOFAR dynamic spectrum of 53−80 MHz a region is found that lasts about an hour whose emission is highly correlated with 6 GHz temporal profile. The flux peaks in the meter waves are well correlated with extreme UV (EUV) emission variations caused by repeated surges from the bright X-point. We argue that there is a common source of non-thermal electrons located in the tail of the active region, where two loop systems of very different sizes interacted. The frequencies of type III and/or type J bursts are in accordance with large loop heights around 400 Mm, obtained by the magnetic field reconstruction. The microwave coherent emission was generated in the low loops identified as bright X-ray points seen in soft X-ray and EUV images, produced by electrons with energies several tens of keV at about twice the plasma frequency.

In this paper, we study the dynamics of a single loop structure in a C1.3 flare on January 15, 2022. A new type of behavior of flare loop was discovered. Unlike the previously known cases of visible contraction and expansion of flare loops in the phases of rise and decay of the flare radiation flux, respectively, the flare on January 15, 2022 exhibited a directly opposite process: the loop increases in its height during the flare intensity rise phase and contracts during the decay phase. Characteristic changes in the height, duration, and rate of the expansion and contraction of the loop structure were established. An increase of the electric current in the loop during the rise phase and its decrease during the contraction phase are proposed as an explanation of this behavior.

We discuss results of test observations of the 3–6 GHz range array of the Siberian Radio Heliograph (SRH). A method for calibrating brightness temperatures of images was verified using measurements of the brightness temperature of the quiet Sun at a minimum between solar activity cycles 20 and 21 known in the literature. The obtained time dependences of the integral solar flux at 2.8 GHz are similar to those measured at the Dominion Radio Astrophysical Observatory (DRAO), but the absolute values of SRH fluxes are lower relative to the DRAO fluxes by 10–15 %. The spectral density of the solar microwave flux at a frequency of 2.8 GHz, the so-called F10.7 index, is one of the main solar activity indices used as input parameters in models of Earth’s ionosphere. The paper considers the relationship between total radio fluxes and changes in the structure of sources on the solar disk during an interval of 50 days. During the period of daily observations from September 1 to October 20, 2021, the number of active regions on the disk changed several times, and the integral flux density at 2.8 GHz changed up to 1.5 times. We determine the relative contributions to the integral flux of bremsstrahlung of near-limb brightenings and plage regions, as well as bremsstrahlung in magnetic fields of active regions. The measured brightness temperatures of SRH radio maps are compared to the model temperatures calculated from observations of extreme ultraviolet emission (EUV) with the AIA/SDO telescope. The results of the analysis can be used to organize regular measurements of the corrected solar activity proxy index F10.7 at SRH, in which the contribution of gyroresonance emission is excluded.

A response has been found in a narrow band 5–7 GHz of microwave emission to the appearance of a coronal X-ray point. The emission source is a short X-ray loop located in the tail part of an active region and occurring when magnetic fields are reconnected near the footpoints of high and low loops rooted in nearby magnetic pores of the opposite polarity. The power of energy release is low and no response of the hot plasma component was observed in hard X-rays. Analysis of images in soft X-ray and extreme UV radiation shows that microwave emission has a coherent nature and is generated at a frequency of about twice the plasma frequency by electrons with energies above several tens of keV. The result indicates a high diagnostic potential of microwave observations to detect acceleration processes in weak transitory events and can be useful for observation planning with new generation radioheliographs currently under development.

Some studies propose the transfer of flare-accelerated protons in an erupting flux rope until its reconnection with an open structure releases the trapped protons. Coulomb collisions in the dense flux-rope body deplete the low-energy part of the proton spectrum. On the other hand, shock acceleration progressively replenishes this part of the spectrum. These processes form a double power-law proton spectrum that is usually observed at the Earth’s orbit. We analyze the correlations between the slopes of near-Earth proton spectra below and above the break energy, on the one hand, and photon indices of the corresponding hard X-ray (HXR) bursts and speeds of associated coronal mass ejections (CMEs), on the other hand. We use catalogs of proton events in 1991 – 2006, HXR spectra obtained by Yohkoh and the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), and CME catalogs. Significant correlations have been found between the proton spectral slopes i) above the break energy and HXR spectral indices (0.86), and ii) below the break energy and CME speeds (−0.75). The results indicate a statistical predominance of flare acceleration at higher proton energies and shock acceleration at their lower energies. The highest-energy proton spectra reconstructed in ground-level events exhibit the second break with the steepest slope above it. Neither this slope nor the second-break energy correlates with any other parameter. This peculiarity requires understanding.

We study quasi-periodic pulsations (QPPs) during the impulsive phase of the C8.3 flare SOL2002-08-06T01:43. The shape of an extended 5.7 GHz source is similar to a tadpole with the head located above the region of a negative magnetic polarity, surrounded by positive polarity patches and with a remote tail source. The flare configuration includes bright extreme ultraviolet (EUV) ropes with footpoints near the boundary of the negative magnetic field region and it can be identified as a circular ribbon flare. We use simultaneous observations carried out by the Siberian Solar Radio Telescope at 5.7 GHz, the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz, the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI)/HXR, and the Transition Region and Coronal Explorer (TRACE) imaging in the extreme ultraviolet. The flare HXR emission is produced by a compact source located at the south periphery of the Negative Magnetic field Region (NMR). The QPPs are observed during a one-minute interval after the start of the impulsive phase, when this HXR source appeared. The remote source is detected on the variation maps of the of the brightness temperature at 17 GHz and is located at the end of tadpole tail about 60 arcsec eastward. More than a dozen cotemporal HXR and microwave pulses with timescales from 1.5 s up to about 8 s were observed in the flare kernel. At 5.7 GHz, the pulses are more prominent near the remote source where they are highly polarized and generated by the electron beams propagating from the flare kernel. The main tone of the QPP periodicity corresponds to the oscillations with a period of 8 s and is accompanied by the variations in the hardness of nonthermal electrons, that is, in the efficiency of the acceleration mechanism. The second intensity harmonic (about a 3-s period) appears due to a double peak structure of the QPP event. Such pulse shapes suggest oscillations of the current sheet during the loop coalescence as a modulation mechanism of the flare energy release.

An Erratum to this paper has been published: https://doi.org/10.3103/S1062873822220014

Primordial release of solar flare energy high in corona (at altitudes 1/40 - 1/20 of the solar radius) is explained by release of the magnetic energy of the current sheet. The observed manifestations of the flare are explained by the electrodynamical model of a solar flare proposed by I. M. Podgorny. To study the flare mechanism is necessary to perform MHD simulations above a real active region (AR). MHD simulation in the solar corona in the real scale of time can only be carried out thanks to parallel calculations using CUDA technology. Methods have been developed for stabilizing numerical instabilities that arise near the boundary of the computational domain. Methods are applicable for low viscosities in the main part of the domain, for which the flare energy is effectively accumulated near the singularities of the magnetic field. Singular lines of the magnetic field, near which the field can have a rather complex configuration, coincide or are located near the observed positions of the flare.

We studied 87 events of solar cosmic rays, i.e., solar energetic particle (SEP) emissions associated with solar flares of at least M5 class (according to the GOES classification) that occurred during solar cycle 23. A relationship between the spectral parameters of the SEP events and the microwave (MW) emission properties of the related solar flares was analysed. The peak frequency of the MW spectrum was used as an indicator of the acceleration processes during solar flares, which may characterize both the strength of magnetic field in the emission source and the intensity of the accelerated particle flux. The magneto-morphological classification (MMC) was applied to take into account the features of the magnetic topology of the active regions (ARs) that generated flares. Our analysis showed that most of ARs, associated with SEP events with large proton flux, violated at least one of the empirical laws established for sunspot groups (Hale’s law, Joy’s law, or another). The relationships between the spectral properties of electrons and protons and the properties of MW radiation were analyzed and discussed, taking into account the MMC of ARs generated flares associated with SEP events.