Solar flares are driven by the release of free magnetic energy and are often associated with the restructurization of the magnetic field topology. Yet, observations of the evolving magnetic field in the flaring volume are limited to very few cases, including the 2017 September 10 X8.2 limb flare; thus, a verification of whether a similar evolution takes place in other solar flares is needed. Here, we report one more: the 2022 October 2 X1.1-class solar flare, seen on the disk, whose microwave data permit mapping the magnetic field over the flaring source and tracking the magnetic field evolution over the course of the flare. We find that the coronal magnetic field shows a prominent decay with a rate up to 10 G s-1 in several (above the) looptop locations. The magnetic field is also confidently measured at the loop legs and the bottom part of the erupting filament. Prominent acceleration of electrons is detected where the magnetic field decays. We develop 3D models of the flare, whose magnetic field shows resemblance to and also deviation from the magnetic field inferred from the microwave data. This study confirms that the coronal magnetic field decays during the rise phase of the solar flare. The amount of released magnetic energy is sufficient to support other components of the flare energy.

Solar flares are a powerful engine capable of accelerating ambient plasma particles to high nonthermal energies. A key characteristic of these particles is their energy spectrum. Numerous studies of nonthermal hard X-ray emission established that a typical spectral evolution of the nonthermal electrons with energies of dozens keV follows a soft-hard-soft pattern in impulsive events and soft-hard-harder pattern in some long-duration events. Here we extend the study of the spectral evolution to the case of flare-accelerated mildly relativistic electrons primarily responsible for the flare microwave emission. We examine the spatially resolved microwave emission observed with the Expanded Owens Valley Solar Array in twelve solar flares and find that they also typically follow a soft-hard-soft spectral evolution. However, the revealed spectral evolution of the mildly relativistic electrons covers the spectral index range from 2-4 to 15 or more and, thus, is much more prominent than for the X-ray-producing electrons. The evolution of the spectral index is closely correlated with the emission flux and does not show any noticeable time delay. The spectral index displays a prominent correlation with the brightness temperature with a pattern similar to all events considered. We conclude that the revealed relationships and the evolutionary pattern is an inherent property of particle acceleration in the solar flares.

In order to understand solar atmospheric heating, it is important to test heating models against spatially resolved data from solar active regions. Here, we model a small active region, AR 12760, observed on 2020 April 28, with the GX Simulator package by fitting the extreme-ultraviolet (EUV) intensities in wave bands observed by the Solar Dynamics Observatory's Atmospheric Imaging Assembly. We assume the temporally and spatially averaged heating rate along a loop has a power-law dependence on loop length, L, and average magnetic field strength along the loop, Bavg. We find that the best-fit heating model for the 211 & Aring; band is < Q >approximate to 7 & times;10-3(Bavg/100G)1.5(L/109cm)-1 erg cm-3 s-1, but that there is a range of parameters that give qualitatively reasonable fits, which we conclude is due to a correlation between Bavg and L. In addition, we find that the models of the bands including cooler emission (131 and 171 & Aring;) greatly underestimate the extent of the emission in the legs of the longer loops at the peripheries of the active region that are the strongest contributors of the emission in those bands. We conclude that this is because the modeling assumes that all transition-region emission is confined to the loop footpoints, but in reality the upper transition region of longer loops extends significantly farther into the loop. It will be important to consider this aspect of the transition region in future efforts to model EUV emission.

Solar flares are driven by release of free magnetic energy and often associated with restructurization of the magnetic field topology. Yet, observations of evolving magnetic field in the flaring volume are limited to very few cases including the 2017-09-10 X8.2 limb flare; thus, a verification of whether a similar evolution takes place in other solar flares is needed. Here we report one more, 2022-10-02, X1.1 class solar flare but seen on disk, whose microwave data permit mapping the magnetic field over the flaring source and tracking magnetic field evolution over the course of the flare. We found that the coronal magnetic field shows a prominent decay with the rate up to 10 G s^-1 in several (above) loop-top locations. The magnetic field is also confidently measured at the loop legs and the bottom part of the erupting filament. Prominent acceleration of electrons is detected where the magnetic field decays. We developed 3D models of the flare, whose magnetic field shows resemblance and also deviation from the magnetic field inferred from the microwave data. This study confirms that the coronal magnetic field decays during the rise phase of the solar flare. The amount of released magnetic energy is sufficient to support other components of the flare energy.

Analysis of γ-rays in solar flares has suggested a distinct continuum component dominating at MeV energies, which differs from the well-studied X-ray continuum produced by flare-accelerated electrons with spectra steeply falling with energy. The origin, precise spatial location, and extent of this mysterious MeV component have been unknown up to now. If it is produced by bremsstrahlung, such a γ-ray component requires an unusual population of electrons peaked at a few MeV. Here we report a joint study of this MeV-peaked electron population in the 2017-Sep-10 solar flare with Fermi MeV γ-ray data and EOVSA spatially resolved microwave imaging spectroscopy data. We demonstrate that the microwave spectrum from the MeV-peaked distribution has a distinctly different shape from that produced by the electrons with falling energy spectrum. We inspected microwave maps of the flare and identified an evolving area where the measured microwave spectra matched the theoretically expected one for the MeV-peaked population, thus pinpointing the site where this MeV component resides. The locations are in a coronal volume adjacent to the region where prominent release of magnetic energy and bulk electron acceleration were detected, which implies that transport effects play a key role in forming this population.

Aims. Diagnosing solar flare conditions is essential for understanding coronal energy release. Using combined microwave and X-ray data, we aim to reconstruct 3D maps of the magnetic fields and plasma parameters in the SOL2021-05-07 flare. Methods. We used imaging spectroscopy from the Expanded Owens Valley Solar Array (EOVSA) to derive spatial maps of the magnetic field strength, as well as the thermal and nonthermal electron densities, along with the power-law index of nonthermal electrons via gyrosynchrotron modeling. Simultaneous X-ray observations from Hinode/X-Ray Telescope (Hinode/XRT) and Solar Orbiter/Spectrometer Telescope for Imaging X-rays (SolO/STIX), taken from different vantage points, enable a stereoscopic reconstruction of the flaring loop. By correlating the positions of microwave and thermal X-ray sources, we associated the 3D coordinates with the microwave-derived plasma parameters. Results. We derived observational 3D maps of magnetic field strength, Alfv & eacute;n speed, and plasma beta in a flaring volume, revealing a magnetically dominated environment. These spatially resolved diagnostics provide valuable constraints for models of magnetic reconnection and flare dynamics, representing a step toward a realistic 3D characterization of energy release in solar eruptive events.

Analysis of γ-rays in solar flares has suggested a distinct continuum component dominating at megaelectronvolt energies, which differs from the well-studied X-ray continuum produced by flare-accelerated electrons, with spectra steeply falling with energy. The origin, precise spatial location and extent of this mysterious megaelectronvolt component have been unknown up to now. If it is produced by bremsstrahlung, such a γ-ray component requires an unusual population of electrons peaked at a few million electron volts. Here we report a joint study of this megaelectronvolt-peaked electron population in the 2017 September 10 solar flare with Fermi megaelectronvolt γ-ray data and spatially resolved microwave imaging spectroscopy data obtained by the Expanded Owens Valley Solar Array. We demonstrate that the microwave spectrum from the megaelectronvolt-peaked distribution has a distinctly different shape from that produced by the electrons with a falling energy spectrum. We inspected microwave maps of the flare and identified an evolving area where the measured microwave spectra matched the theoretically expected ones for the megaelectronvolt-peaked population, thus pinpointing the site where this megaelectronvolt component resides. The locations are in a coronal volume adjacent to the region where prominent release of magnetic energy and bulk electron acceleration were detected. The results imply that transport effects play a key role in forming this population of high-energy particles, which is crucial for building a complete picture of the multifaceted solar flare phenomena. A group of extremely energetic electrons peaking at a few million electron volts is revealed in a large solar flare observed in microwaves. This megaelectronvolt-peaked population appears to originate near a coronal source where bulk electron acceleration occurs.

The solar corona is much hotter than the photosphere and chromosphere, but the physical mechanism responsible for heating the coronal plasma remains unidentified. The thermal microwave emission, which is produced in a strong magnetic field above sunspots, is a promising but barely exploited tool for studying the coronal magnetic field and plasma. We analyzed the microwave observations of eight solar active regions obtained with the Siberian Radioheliograph in the years 2022-2024 in the frequency range of 6-12 GHz. We produced synthetic microwave images based on various coronal heating models, and determined the model parameters that provided the best agreement with the observations. The observations and simulations strongly favor either a steady-state (continuous) plasma heating process or high-frequency heating by small energy release events with a short cadence. The average magnetic field strength in a coronal loop was found to decrease with the loop length, following a scaling law with the most probable index of about -0.55. In the majority of cases, the estimated volumetric heating rate was weakly dependent on the magnetic field strength and decreased with the coronal loop length following a scaling law with an index of about -2.5. Among the known theoretical heating mechanisms, the model based on wave transmission or reflection in coronal loops acting as resonance cavities was found to provide the best agreement with the observations. The obtained results did not demonstrate a significant dependence on the emission frequency in the considered range.

The solar corona is much hotter than lower layers of the solar atmosphere—the photosphere and chromosphere. The coronal temperature is up to 1 MK in quiet Sun areas, while up to several megakelvins in active regions, which implies a key role of the magnetic field in coronal heating. This means that understanding coronal heating requires reliable modeling of the underlying 3D magnetic structure of an active region validated by observations. Here, we employ synergy between 3D modeling, optically thick gyroresonant microwave emission, and optically thin EUV emission to (i) obtain and validate the best magnetothermal model of the active region and (ii) disentangle various components of the EUV emission known as diffuse component, bright loops, open-field regions, and “moss” component produced at the transition region. Surprisingly, the best thermal model corresponds to high-frequency energy release episodes, similar to a steady-state heating. Our analysis did not reveal significant deviations of the elemental abundances from the standard coronal values.

A subclass of early impulsive solar flares, cold flares, was proposed to represent a clean case, where the release of the free magnetic energy (almost) entirely goes to acceleration of the nonthermal electrons, while the observed thermal response is entirely driven by the nonthermal energy deposition to the ambient plasma. This paper studies one more example of a cold flare, which was observed by a unique combination of instruments. In particular, this is the first cold flare observed with the Expanded Owens Valley Solar Array and, thus, for which the dynamical measurement of the coronal magnetic field and other parameters at the flare site is possible. With these new data, we quantified the coronal magnetic field at the flare site, but did not find statistically significant variations of the magnetic field within the measurement uncertainties. We estimated that the uncertainty in the corresponding magnetic energy exceeds the thermal and nonthermal energies by an order of magnitude; thus, there should be sufficient free energy to drive the flare. We discovered a very prominent soft-hard-soft spectral evolution of the microwave-producing nonthermal electrons. We computed energy partitions and concluded that the nonthermal energy deposition is likely sufficient to drive the flare thermal response similarly to other cold flares.

A subclass of early impulsive solar flares, cold flares, was proposed to represent a clean case, where the release of the free magnetic energy (almost) entirely goes to the acceleration of the nonthermal electrons, while the observed thermal response is entirely driven by the nonthermal energy deposition to the ambient plasma. This paper studies one more example of a cold flare, which was observed by a unique combination of instruments. In particular, this is the first cold flare observed with the Expanded Owens Valley Solar Array and, thus, for which the dynamical measurement of the coronal magnetic field and other parameters at the flare site is possible. With these new data, we quantified the coronal magnetic field at the flare site but did not find statistically significant variations of the magnetic field within the measurement uncertainties. We estimated that the uncertainty in the corresponding magnetic energy exceeds the thermal and nonthermal energies by an order of magnitude; thus, there should be sufficient free energy to drive the flare. We discovered a very prominent soft-hard-soft spectral evolution of the microwave-producing nonthermal electrons. We computed energy partitions and concluded that the nonthermal energy deposition is likely sufficient to drive the flare thermal response similarly to other cold flares.

This paper investigates the incidence of coherent emission in solar radio bursts, using a revised catalog of 3800 solar radio bursts observed by the Nobeyama Radio Polarimeters from 1988 to 2023. We focus on the 1.0 and 2.0 GHz data, where radio fluxes of order 1010 Jy have been observed. Previous work has suggested that these bursts are due to electron cyclotron maser (ECM) emission. In at least one well-studied case, the bright emission at 1 GHz consists of narrowband spikes of millisecond duration. Coherent emission at 1 GHz can be distinguished from traditional incoherent gyrosynchrotron flare emission based on the radio spectrum: Gyrosynchrotron emission at 1 GHz usually has a spectrum rising with frequency, so bursts in which 1 GHz is stronger than higher-frequency measurements are unlikely to be incoherent gyrosynchrotron. Based on this criterion, it is found that for bursts exceeding 100 sfu, three-quarters of all bursts at 1 GHz and half of all 2 GHz bursts have a dominant coherent emission component, assumed to be ECM. The majority of the very bright bursts at 1 GHz are highly circularly polarized, consistent with a coherent emission mechanism, but not always 100% polarized. The frequency range from 1 to 2 GHz is heavily utilized for terrestrial applications, and these results are relevant for understanding the extreme flux levels that may impact such applications. Further, they provide a reference for comparison with the study of ECM emission from other stars and potentially exoplanets.

We report for the first time the detection of thermal free-free emission from post-flare loops at 34GHz in images from the Nobeyama Radioheliograph (NoRH). We studied 8 loops, 7 of which were from regions with extremely strong coronal magnetic field reported by Fedenev et al. (2023). Loop emission was observed in a wide range of wavelength bands, up to soft X-rays, confirming their multi-temperature structure and was associated with noise storm emission in metric wavelengths. The comparison of the 17GHz emission with that at 34GHz, after a calibration correction of the latter, showed that the emission was optically thin at both frequencies. We describe the structure and evolution of the loops and we computed their density, obtaining values for the top of the loops between 1 and 6 x 10^10 cm^-3, noticeably varying from one loop to another and in the course of the evolution of the same loop system; these values have only a weak dependence on the assumed temperature, 2 x 10^6 K in our case, as we are in the optically thin regime. Our density values are above those reported from EUV observations, which go up to about 10^10 cm^-3. This difference could be due to the fact that different emitting regions are sampled in the two domains and/or due to the more accurate diagnostics in the radio range, which do not suffer from inherent uncertainties arising from abundances and non-LTE excitation/ionization equilibria. We also estimated the magnetic field in the loop tops to be in the range of 10 to 30G.

Observations at (sub-)millimeter wavelengths offer a complementary perspective on our Sun and other stars, offering significant insights into both the thermal and magnetic composition of their chromospheres. Despite the fundamental progress in (sub-)millimeter observations of the Sun, some important aspects require diagnostic capabilities that are not offered by existing observatories. In particular, simultaneous observations of the radiation continuum across an extended frequency range would facilitate the mapping of different layers and thus ultimately the 3D structure of the solar atmosphere. Mapping large regions on the Sun or even the whole solar disk at a very high temporal cadence would be crucial for systematically detecting and following the temporal evolution of flares, while synoptic observations, i.e., daily maps, over periods of years would provide an unprecedented view of the solar activity cycle in this wavelength regime. As our Sun is a fundamental reference for studying the atmospheres of active main sequence stars, observing the Sun and other stars with the same instrument would unlock the enormous diagnostic potential for understanding stellar activity and its impact on exoplanets. The Atacama Large Aperture Submillimeter Telescope (AtLAST), a single-dish telescope with 50 m aperture proposed to be built in the Atacama desert in Chile, would be able to provide these observational capabilities. Equipped with a large number of detector elements for probing the radiation continuum across a wide frequency range, AtLAST would address a wide range of scientific topics including the thermal structure and heating of the solar chromosphere, flares and prominences, and the solar activity cycle. In this white paper, the key science cases and their technical requirements for AtLAST are discussed.

Observations at (sub-)millimeter wavelengths offer a complementary perspective on our Sun and other stars, offering significant insights into both the thermal and magnetic composition of their chromospheres. Despite the fundamental progress in (sub-)millimeter observations of the Sun, some important aspects require diagnostic capabilities that are not offered by existing observatories. In particular, simultaneous observations of the radiation continuum across an extended frequency range would facilitate the mapping of different layers and thus ultimately the 3D structure of the solar atmosphere. Mapping large regions on the Sun or even the whole solar disk at a very high temporal cadence would be crucial for systematically detecting and following the temporal evolution of flares, while synoptic observations, i.e., daily maps, over periods of years would provide an unprecedented view of the solar activity cycle in this wavelength regime. As our Sun is a fundamental reference for studying the atmospheres of active main sequence stars, observing the Sun and other stars with the same instrument would unlock the enormous diagnostic potential for understanding stellar activity and its impact on exoplanets. The Atacama Large Aperture Submillimeter Telescope (AtLAST), a single-dish telescope with 50m aperture proposed to be built in the Atacama desert in Chile, would be able to provide these observational capabilities. Equipped with a large number of detector elements for probing the radiation continuum across a wide frequency range, AtLAST would address a wide range of scientific topics including the thermal structure and heating of the solar chromosphere, flares and prominences, and the solar activity cycle. In this white paper, the key science cases and their technical requirements for AtLAST are discussed.

Observations at (sub-)millimeter wavelengths offer a complementary perspective on our Sun and other stars, offering significant insights into both the thermal and magnetic composition of their chromospheres. Despite the fundamental progress in (sub-)millimeter observations of the Sun, some important aspects require diagnostic capabilities that are not offered by existing observatories. In particular, simultaneously observations of the radiation continuum across an extended frequency range would facilitate the mapping of different layers and thus ultimately the 3D structure of the solar atmosphere. Mapping large regions on the Sun or even the whole solar disk at a very high temporal cadence would be crucial for systematically detecting and following the temporal evolution of flares, while synoptic observations, i.e., daily maps, over periods of years would provide an unprecedented view of the solar activity cycle in this wavelength regime. As our Sun is a fundamental reference for studying the atmospheres of active main sequence stars, observing the Sun and other stars with the same instrument would unlock the enormous diagnostic potential for understanding stellar activity and its impact on exoplanets. The Atacama Large Aperture Submillimeter Telescope (AtLAST), a single-dish telescope with 50m aperture proposed to be built in the Atacama desert in Chile, would be able to provide these observational capabilities. Equipped with a large number of detector elements for probing the radiation continuum across a wide frequency range, AtLAST would address a wide range of scientific topics including the thermal structure and heating of the solar chromosphere, flares and prominences, and the solar activity cycle. In this white paper, the key science cases and their technical requirements for AtLAST are discussed.

Context. Solar coronal jets seen in extreme ultraviolet (EUV) are ubiquitous on the Sun, and they have been found in and at the edges of active regions, at the boundaries of coronal holes, and in the quiet Sun. Jets have various shapes, sizes, brightness, velocities, and durations in time, which complicates their detection by automated algorithms. So far, solar jets reported in the Heliophysics Event Knowledgebase (HEK) have been mostly reported by humans looking for them in the data, with different levels of precision regarding their timing and positions. Aims. We created a catalog of solar jets observed in EUV at 304 Å containing precise and consistent information on the jet timing, position, and extent. Methods. We designed a citizen science project, Solar Jet Hunter, on the Zooniverse platform, to analyze EUV observations at 304 Å from the Solar Dynamic Observatory/Atmospheric Imaging Assembly (SDO/AIA). We created movie strips for regions of the Sun in which jets have been reported in HEK and ask the volunteers to 1) confirm the presence of at least one jet in the data and 2) report the timing, position, and extent of the jet. Results. We report here the design of the project and the results obtained after the analysis of data from 2011 to 2016. We note that 365 “coronal jet” events from HEK served as input for the citizen science project, equivalent to more than 120 000 images distributed into 9689 “movie strips”. Classification by the citizen scientists resulted in 883 individual jets being identified. Conclusions. We demonstrate how citizen science can enhance the analysis of solar data with the example of Solar Jet Hunter. The catalog of jets thusly created is publicly available and will enable statistical studies of jets and related phenomena. This catalog will also be used as a training set for machines to learn to recognize jets in further datasets.

Context. The 2003 October 28 (X17.2) eruptive flare was a unique event. The coronal electric field and the pi-decay gamma-ray emission flux displayed the highest values ever inferred for solar flares. Aims. Our aim is to reveal physical links between the magnetic reconnection process, energy release, and acceleration of electrons and ions to high energies in the chain of the magnetic energy transformations in the impulsive phase of the solar flare. Methods. The global reconnection rate, phi(t) phi and the local reconnection rate (coronal electric field strength), E-c(r, t), were calculated from flare ribbon separation in H alpha filtergrams and photospheric magnetic field maps. Then, HXRs measured by CORONAS-F/SPR-N and the derivative of the GOES SXR flux, & Idot;(SXR)(t) were used as proxies of the flare energy release evolution. The flare early rise phase, main raise phase, and main energy release phase were defined based on temporal profiles of the above proxies. The available results of INTEGRAL and CORONAS-F/SONG observations were combined with Konus-Wind data to quantify the time behavior of electron and proton acceleration. Prompt gamma-ray lines and delayed 2.2 MeV line temporal profiles observed with Konus-Wind and INTEGRAL/SPI were used to detect and quantify the nuclei with energies of 10-70 MeV. Results. The magnetic-reconnection rates, phi(t) and E-c(r, t), follow a common evolutionary pattern with the proxies of the flare energy released into high-energy electrons. The global and local reconnection rates reach their peaks at the end of the main rise phase of the flare. The spectral analysis of the high-energy gamma-ray emission revealed a close association between the acceleration process efficiency and the reconnection rates. High-energy bremsstrahlung continuum and narrow gamma-ray lines were observed in the main rise phase when E-c(r, t) of the positive (negative) polarity reached values of similar to 120 V cm(-1) (similar to 80 V cm(-1)). In the main energy release phase, the upper energy of the bremsstrahlung spectrum was significantly reduced and the pion-decay gamma-ray emission appeared abruptly. We discuss the reasons why the change of the acceleration regime occurred along with the large-scale magnetic field restructuration of this flare. Conclusions. The similarities between the proxies of the flare energy release with phi(t) and E-c(r, t) in the flare's main rise phase are in accordance with the reconnection models. We argue that the main energy release and proton acceleration up to subrelativistic energies began just when the reconnection rate was going through the maximum, that is, following a major change of the flare topology.

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.

Coronal mass ejections (CMEs) are the most important drivers of space weather. Central to most CMEs is thought to be the eruption of a bundle of highly twisted magnetic field lines known as magnetic flux ropes. A comprehensive understanding of CMEs and their impacts hence requires detailed observations of physical parameters that lead to the formation, destabilization, and eventual eruption of the magnetic flux ropes. Recent advances in remote-sensing observations of coronal cavities, filament channels, sigmoids, EUV "hot channels," white light CMEs, and in situ observations of magnetic clouds points to the possibility of significant progress in understanding CMEs. In this white paper, we provide a brief overview of the potential of radio diagnostics for CMEs and CME progenitors, with a particular focus on the unique means for constraining their magnetic field and energetic electron population. Using synthetic observations based on realistic 3D MHD models, we also demonstrate the transformative potential of advancing such diagnostics by using broadband radio imaging spectropolarimetry with a high image dynamic range and high image fidelity. To achieve this goal, a solar-dedicated radio facility with such capabilities is recommended for implementation in the coming decade.