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Solar eruptions are sudden release of the magnetic free energy accumulated within a quasi-static evolutionary process of the corona. Interestingly, many solar eruptions are preceded by a short-term slow-rise phase, during which the pre-eruption structure rises at a speed significantly larger than that of the quasi-static evolution. Here we suggest an explanation for the slow-rise phase based on a recent high-accuracy magnetohydrodynamic simulation for initiation of solar eruption. The simulation shows that by continuously shearing a bipolar magnetic arcade, an internal current sheet forms gradually, and an eruption begins once magnetic reconnection is triggered at the current sheet. We find in the simulation that the overlying field presents a slow-rise phase before the reconnection sets in. In addition, the rising speed is significantly larger than that of the core field during this phase. This slow rise is a manifestation of the growing expansion of the arcade in the process of approaching a fully open field state, which is inherent to the formation of a current sheet before the eruption. We also show three flare events with slow-rise phases that are highly consistent with these key characteristics in the simulation: an expansion of the overlying coronal loops with speeds much larger than the quasi-static evolution speed, and for those events with filament eruption, the slow rise of filament is much smaller than that of the overlying loops. In this type of events, the eruption might be initiated through the mechanism as shown in the simulation, and the expansion of overlying coronal loops is a better indicator of the slow-rise phase. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is widely believed to be the fundamental process in the solar atmosphere that underlies magnetic energy release and particle acceleration. This process is responsible for the onset of solar flares, coronal mass ejections, and other explosive events (e.g., jets). Here, we report direct imaging of a prolonged plasma/current sheet along with quasiperiodic magnetic reconnection in the solar corona using ultra-high-resolution observations from the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory and the Solar Dynamics Observatory/Atmospheric Imaging Assembly. The current sheet appeared near a null point in the fan–spine topology and persisted over an extended period (≈20 hr). The length and apparent width of the current sheet were about 6″ and 2″, respectively, and the plasma temperature was ≈10–20 MK. We observed quasiperiodic plasma inflows and outflows (bidirectional jets with plasmoids) at the reconnection site/current sheet. Furthermore, quasiperiodic reconnection at the long-lasting current sheet produced recurrent eruptions (small flares and jets) and contributed significantly to the recurrent impulsive heating of the active region. Direct imaging of a plasma/current sheet and recurrent null-point reconnection for such an extended period has not been reported previously. These unprecedented observations provide compelling evidence that supports the universal model for solar eruptions (i.e., the breakout model) and have implications for impulsive heating of active regions by recurrent reconnection near null points. The prolonged and sustained reconnection for about 20 hr at the breakout current sheet provides new insights into the dynamics and energy release processes in the solar corona. [ABSTRACT FROM AUTHOR]

An analysis of solar flares with simple temporal structures can help us to understand the features and mechanisms of energy and particle propagation. A weak impulsive solar flare that occurred on 2021 June 3 provided such opportunity. For the purposes of the study, we used microwave observations with spatial resolution from the Siberian Radioheliograph (3–6 GHz) combined with various spectral radio and X-ray data. Flare topology analysis revealed a configuration consisting of a small loop or dome-like structure associated with a compact bright source, and a long high loop system associated with the diffuse source. This indicates a compact and relatively low-lying site of acceleration and initial energy release. The radio metre and the microwave emission demonstrated a peak-to-peak correlation in three of the four bursts. The delays obtained from comparing microwave and metre radio ranges are in good agreement with the delays from the metre dynamic spectrum analysis, but the different radio bursts had different delays. We found that the electron energies derived from metre dynamic spectrum analysis are lower than those shown by hard X-ray emissions. According to the results of theoretical simulations, this can be explained by the expansion of magnetic loops with altitude. The difference in drift velocities for various radio bursts can be the result of the different size of the loop where the electron beams are propagated. This can be a feature related to the configuration type of the studied flare. [ABSTRACT FROM AUTHOR]

The current sheet is an essential feature in solar flares and is the primary site for magnetic reconnection and energy release. Imaging observations feature a long linear structure above the candle-flame-shaped flare loops, which resembles the standard flare model with the current sheet viewed edge-on. We investigate the thermal properties of plasmas surrounding the linear sheet during flares, using EUV observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The differential emission measure analyses show evidence of high temperatures in the plasma sheets (PSs), containing hot emissions from only a narrow temperature range, suggestive of an isothermal feature. The sheet temperature remains constant at different heights above the flare arcade, peaking at around log T = 7.0–7.1; while the well-studied 2017 September 10 X8.2 flare serves as an exception in that the temperature decreases with increasing height and peaks higher ( log T = 7.25) during the gradual phase. Most PS cases also hold similar emission measures and thicknesses; while the PS emissions drop exponentially above the flare arcade, the sheet thicknesses show no significant height association as for all the measurements. The characteristics of isothermal and steady temperature suggest balanced heating and cooling processes along the current sheet; in particular, additional heating may exist to compensate for the conductive and radiative cooling away from the reconnection site. Our results suggest a steady and uniform sheet structure in the macroscopic scale that results from flare reconnection. [ABSTRACT FROM AUTHOR]

Coronal mass ejections from the Sun are not always initiated along a radial trajectory; such non-radial eruptions are well known to be caused by the asymmetry of the pre-eruption magnetic configuration, which is primarily determined by the uneven distribution of magnetic flux at the photosphere. Therefore, it is naturally expected that the non-radial eruptions should be rather common, at least as frequent as radial ones, given the typically asymmetrical nature of photospheric magnetic flux. However, statistical studies have shown that only a small fraction of eruptions display non-radial behaviour. Here we aim to shed light on this counterintuitive fact, based on a series of numerical simulations of eruption initiation in bipolar fields with different asymmetric flux distributions. As the asymmetry of the flux distribution increases, the eruption direction tends to deviate further away from the radial path, accompanied by a decrease in eruption intensity. In case of too strong asymmetry, no eruption is triggered, indicating that excessively inclined eruptions cannot occur. Therefore, our simulations suggest that asymmetry plays a negative role in producing eruption, potentially explaining the lesser frequency of non-radial solar eruptions compared to radial ones. With increasing asymmetry, the degree of non-potentiality the field can attain is reduced. Consequently, the intensity of the pre-eruption current sheet decreases, and reconnection becomes less efficient, resulting in weaker eruptions. [ABSTRACT FROM AUTHOR]

The geomagnetic 'solar flare effect' (SFE) results from excess ionization in the Earth's ionosphere, famously first detected at the time of the Carrington flare in 1859. This indirect detection of a flare constituted one of the first cases of 'multimessenger astronomy', whereby solar ionizing radiation stimulates ionospheric currents. Well-observed SFEs have few-minute time-scales and perturbations of >10 nT, with the greatest events reaching above 100 nT. In previously reported cases, the SFE time profiles tend to resemble those of solar soft X-ray emission, which ionizes the D-region; there is also a less-well-studied contribution from Lyman α. We report here a specific case, from flare SOL2024-03-10 (M7.4), in which an impulsive SFE deviated from this pattern. This flare contained an 'early impulsive' component of exceptionally hard radiation, extending up to γ-ray energies above 1 MeV, distinctly before the bulk of the flare soft X-ray emission. We can characterize the spectral distribution of this early-impulsive component in detail, thanks to the modern extensive wavelength coverage. A more typical gradual SFE occurred during the flare's main phase. We suggest that events of this type warrant exploration of the solar physics in the 'impulse response' limit of very short time-scales. [ABSTRACT FROM AUTHOR]

On 21 April 2023, a significant M1.7 solar flare erupted from Active Region 13283, accompanied by a filament eruption and a full-halo Coronal Mass Ejection, which reached Earth on 23 April, triggering a severe geomagnetic storm, with Kp reaching 8 (G4) and Dst plummeting to −212 nT together with a sharply distinguished long-lasting negative double-dip behavior of the z -component of the interplanetary magnetic field. This event led to remarkable auroral displays, even at mid-latitudes in Europe. The flare-induced filament eruption caused distinct intensity dimming in the solar corona, observed in specific EUV wavelengths. We observed the dimming region growing at its fastest rate before the flare reached its peak of intensity. Notably, the proximity of the flare to a large southern coronal hole influenced the expansion and propagation of the coronal mass ejection toward Earth, probably impacting the solar wind speed and density. Additionally, we observed a sudden expansion of the coronal hole during the flare, leading us to speculating that the adjacent flare may have further stimulated the flow of solar-wind particles along the open magnetic-field lines. In accordance with the severe Dst-index disturbance, we also report changes in the potential of the pipeline of an Italian energy infrastructure company with respect to the surrounding soil as well as double-dip variation in the H-component of the terrestial magnetic field observed locally (reminiscent to what reported in Dst-index and IMF Bz) temporal profiles, confirming the effects of the geomagnetic storm at Italy mid-latitudes. Several solar radio events have been observed too. Therefore this study provides insights into the dynamic solar phenomena and their potential geomagnetic implications. [ABSTRACT FROM AUTHOR]

Multiple pieces of evidence have revealed the important role of turbulence in physical processes in solar eruptions, from particle acceleration to the suppression of conductive cooling. Radio observations of density variation have established a Kolmogorov-like spectrum for solar wind density disturbance. Close to the Sun, measurements from extreme ultraviolet (EUV) bands have been used to examine turbulence in the solar atmosphere. The Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory (SDO/AIA) has been frequently used for diagnosing plasma properties due to its complex coverage of temperature response. We compute structure functions (SFs) using SDO/AIA emission measurements for two example of plasma sheets. With the relationship of v ~ b ~ δη and δΙ ~ δ(η0 + δη)2 ~ δη (v, b, δη, and δΙ are turbulent velocity, magnetic field, number density, and intensity, respectively, and η0 is the background density), SFs of δΙ can be regarded as a proxy for those of the turbulent vand b fields in the plasma sheet. We show that by properly accounting for the radial dependence of the emission line intensity, an SF method is capable of probing the presence of turbulence from SDO/AIA emission lines. Compared to ίη situ observations, performing SFs on EUV emissions is advantageous in studying turbulence behavior in the wave-vector space, and it opens a new window for investigating turbulence from massive SDO/AIA observations. [ABSTRACT FROM AUTHOR]

Context. In 2020, the Transiting Exoplanet Survey Satellite (TESS) observed a rapidly rotating M7 dwarf, TIC 277539431, producing a flare at 81° latitude, the highest latitude flare located to date. This is in stark contrast to solar flares that occur much closer to the equator, typically below 30°. The mechanisms that allow flares at high latitudes to occur are poorly understood. Aims. We studied five sectors of TESS monitoring, and obtained 36 ks of XMM-Newton observations to investigate the coronal and flaring activity of TIC 277539431. Methods. From the observations, we infer the optical flare frequency distribution; flare loop sizes and magnetic field strengths; the soft X-ray flux, luminosity, and coronal temperatures; as well as the energy, loop size, and field strength of a large flare in the XMM-Newton observations. Results. We find that the corona of TIC 277539431 does not differ significantly from other low-mass stars on the canonical saturated activity branch with respect to coronal temperatures and flaring activity, but shows lower luminosity in soft X-ray emission by about an order of magnitude, consistent with other late M dwarfs. Conclusions. The lack of X-ray flux, the high-latitude flare, the star's viewing geometry, and the otherwise typical stellar corona taken together can be explained by the migration of flux emergence to the poles in rapid rotators like TIC 277539431 that drain the star's equatorial regions of magnetic flux, but preserve its ability to produce powerful flares. [ABSTRACT FROM AUTHOR]

High-resolution observations with GRAVITY-Very Large Telescope Interferometer (VLTI) instrument have provided abundant information about the flares in Sgr A*, the supermassive black hole in our Galactic centre, including the time-dependent location of the centroid (a 'hotspot'), the light curve, and polarization. Yuan et al. (2009) proposed a 'coronal mass ejection' model to explain the flares and their association with the plasma ejection. The key idea is that magnetic reconnection in the accretion flow produces the flares and results in the formation and ejection of flux ropes. The dynamical process proposed in the model has been confirmed by three-dimensional general-relativistic magnetohydrodynamic simulations in a later work. Based on this scenario, in our previous works the radiation of the flux rope has been calculated analytically and compared to the observations. In the present paper, we develop the model by directly using numerical simulation data to interpret observations. We first identify flux ropes formed due to reconnection from the data. By assuming that electrons are accelerated in the reconnection current sheet and flow into the flux rope and emit their radiation there, we have calculated the time-dependent energy distribution of electrons after phenomenologically considering their injection due to reconnection acceleration, radiative and adiabatic cooling. The radiation of these electrons is calculated using the ray-tracing approach. The trajectory of the hotspot, the radiation light curve during the flare, and the polarization are calculated. These results are compared with the GRAVITY observations and good consistencies are found. [ABSTRACT FROM AUTHOR]

Using non-linear force free field (NLFFF) extrapolation, 3D magnetic fields were modeled from the 12-min cadence Solar Dynamics Observatory Helioseismic and Magnetic Imager (HMI) photospheric vector magnetograms, spanning a time period of 1 hour before through 1 hour after the start of 18 X-class and 12 M-class solar flares. Several magnetic field parameters were calculated from the modeled fields directly, as well as from the power spectrum of surface maps generated by summing the fields along the vertical axis, for two different regions: areas with photospheric |6z| > 300 G (active region -- AR) and areas above the photosphere with the magnitude of the non-potential field (BNP) greater than three standard deviations above |BNP| of the AR field and either the unsigned twist number |Tw| > 1 turn or the shear angle ¥ > 80° (non-potential region -- NPR). Superposed epoch (SPE) plots of the magnetic field parameters were analyzed to investigate the evolution of the 3D solar field during the solar flare events and discern consistent trends across all solar flare events in the dataset, as well as across subsets of flare events categorized by their magnetic and sunspot classifications. The relationship between different flare properties and the magnetic field parameters was quantitatively described by the Spearman ranking correlation coefficient, rs. The parameters that showed the most consistent and discernable trends among the flare events, particularly for the hour leading up to the eruption, were the total unsigned flux 0), free magnetic energy (EFree), total unsigned magnetic twist (rTot), and total unsigned free magnetic twist (pTot). Strong (|rs| e [0.6, 0.8)) to very strong (|rs| e [0.8, 1.0]) correlations were found between the magnetic field parameters and the following flare properties: peak X-ray flux, duration, rise time, decay time, impulsiveness, and integrated flux; the strongest correlation coefficient calculated for each flare property was 0.62, 0.85, 0.73, 0.82, -0.81, and 0.82, respectively. [ABSTRACT FROM AUTHOR]

The acceleration and transport of solar energetic particles (SEPs) cause their abundance, measured at a constant velocity, to be enhanced or suppressed as a function of the magnetic rigidity of each ion, and hence, of its atomic mass-to-charge ratio of A/Q. Ion charges, in turn, depend upon the source electron temperature. In small "impulsive" SEP events, arising from solar jets, acceleration during magnetic reconnection causes steep power-law abundance enhancements. These impulsive SEP events can have 1,000-fold enhancements of heavy elements from sources at ~2.5 MK and similar enhancements of ³He/4He and of streaming electrons that drive type-III radio bursts. Gamma-ray lines show that solar flares also accelerate ³He-rich ions, but their electrons and ions remain trapped in magnetic loops, so they dissipate their energy as X-rays, γ-rays, heat, and light. "Gradual" SEPs accelerated at shock waves, driven by fast coronal mass ejections (CMEs), can show power-law abundance enhancements or depressions, even with seed ions from the ambient solar corona. In addition, shocks can reaccelerate seed particles from residual impulsive SEPs with their pre-existing signature heavy-ion enhancements. Different patterns of abundance often show that heavy elements are dominated by a source different from that of H and He. Nevertheless, the SEP abundance, averaged over many large events, defines the abundance of the corona itself, which differs from the solar photosphere as a function of the first ionization potential (FIP) since ions, with FIP <10 eV, are driven upward by forces of electromagnetic waves, which neutral atoms, with FIP >10 eV, cannot feel. Thus, SEPs provide a measurement of element abundance in the solar corona, distinct from solar wind, and may even better define the photosphere for some elements. [ABSTRACT FROM AUTHOR]

Using numerical simulations, we study the formation and dynamics of solar flares in a local region of the solar atmosphere. The magnetohydrodynamics (MHD) equations describe the dynamic evolution of flares, including space-dependent and anomalous magnetic resistivity and highly anisotropic thermal conduction on a 2.5 D slice. We adopt an initial solar atmospheric model in magnetohydrostatic equilibrium, with a magnetic configuration consisting of a vertical current sheet, which helps trigger the magnetic reconnection process. Specifically, we study three scenarios, two with only resistivity and the third with resistivity plus thermal conduction. The main results of the numerical simulations show differences in the global morphology of the flares, including the post-flare loops and the current sheet in three cases. In particular, localized resistivity produces more substructure around the post-flare loops that could be related to the Ritchmyer–Meshkov Instability (RMI). Furthermore, in the scenario of anomalous resistivity, we identify the formation of a plasmoid and a jet at coronal heights. On the other hand, in the scenario with resistivity plus thermal conduction, the post-flare loops are smooth, and no apparent substructures develop. Besides, in the z -component of the current density for the Res + TC case, we observe the development of multiple magnetic islands generated due to the Tearing instability in the non-linear regime. [ABSTRACT FROM AUTHOR]

A significant impediment to solving the coronal heating problem is that we currently only observe active region loops in their cooling phase. Previous studies showed that the evolution of cooling loop densities and apex temperatures is insensitive to the magnitude, duration, and location of energy deposition. Still, potential clues to how energy is released are encoded in the properties of the cooling phase. The appearance of coronal rain, one of the most spectacular phenomena of the cooling phase, occurs when plasma has cooled below 1 MK, which sets constraints on the heating frequency, for example. Most observations of coronal rain have been made by imaging instruments. Here we report rare Hinode/EUV Imaging Spectrometer (EIS) observations of a loop arcade where coronal rain forms following an X2.1 limb flare. A bifurcation in plasma composition measurements between photospheric at 1.5 MK and coronal at 3.5 MK suggests that we are observing postflare-driven coronal rain. Increases in nonthermal velocities and densities with decreasing temperature (2.7–0.6 MK) suggest that we are observing the formation and subsequent evolution of the condensations. Doppler velocity measurements imply that a 10% correction of apparent flows in imaging data is reasonable. Emission measure analysis at 0.7 MK shows narrow temperature distributions, indicating coherent behavior reminiscent of that observed in coronal loops. The limitations on spatio-temporal resolution of EIS suggest that we are observing the largest features or rain showers. These observations provide insights into the heating rate, source, turbulence, and collective behavior of coronal rain from observations of the loop cooling phase. [ABSTRACT FROM AUTHOR]

Self-organization in continuous systems is associated with dissipative processes. In particular, for magnetized plasmas, it is known as magnetic relaxation, where the magnetic energy is converted into heat and kinetic energy of flow through the process of magnetic reconnection. An example of such a system is the solar corona, where reconnection manifests as solar transients like flares and jets. Consequently, toward investigation of plasma relaxation in solar transients, we utilize a novel approach of data-constrained MHD simulation for an observed solar flare. The selected active region NOAA 12253 hosts a GOES M1.3 class flare. The investigation of extrapolated coronal magnetic field in conjunction with the spatiotemporal evolution of the flare reveals a hyperbolic flux tube (HFT), overlying the observed brightenings. MHD simulation is carried out with the EULAG-MHD numerical model to explore the corresponding reconnection dynamics. The overall simulation shows signatures of relaxation. For a detailed analysis, we consider three distinct subvolumes. We analyze the magnetic field line dynamics along with time evolution of physically relevant quantities like magnetic energy, current density, twist, and gradients in magnetic field. In the terminal state, none of the subvolumes is seen to reach a force-free state, thus remaining in nonequilibrium, suggesting the possibility of further relaxation. We conclude that the extent of relaxation depends on the efficacy and duration of reconnection, and hence on the energetics and time span of the flare. [ABSTRACT FROM AUTHOR]

The transition region is a very thin but most peculiar layer in the solar atmosphere located between the solar chromosphere and the corona. It is a key region for understanding coronal heating, solar eruption triggers, and the origin of solar winds. Here, almost all physical parameters (density, temperature, and magnetic fields) have the maximum gradient. Therefore, this region should be highly dynamic, including fast energy releasing and transporting, plasma heating, and particle accelerating. The physical processes can be categorized into two classes: thermal and non-thermal processes. Thermal processes can be observed at ultraviolet (UV) and extreme ultraviolet (EUV) wavelengths via multi-wavelength images. Non-thermal processes accelerate non-thermal electrons and produce radio emissions via the gyrosynchrotron mechanism resulting from the interaction between the non-thermal electrons and magnetic fields. The frequency range spans from several GHz to beyond 100 GHz, in great number of bursts with narrowband, millisecond lifetime, rapid frequency drifting rates, and being referred to as transition region small-scale microwave bursts (TR-SMBs). This work proposes a new type of Solar Ultra-wide Broadband Millimeter-wave Spectrometer (SUBMS) that can be used to observe TR-SMBs. From SUBMS observations, we can derive rich dynamic information about the transition region, such as information about non-thermal energy release and propagation, the flows of plasma and energetic particles, the magnetic fields and their variations, the generation and transportation of various waves, and the formation and evolution of the source regions of solar eruptions. Such an instrument can actually detect the non-thermal signals in the transition region during no flare as well as the eruptive high-energy processes during solar flares. [ABSTRACT FROM AUTHOR]

Fast electron scattering on plasma ions due to stimulated Bremsstrahlung is investigated and modeled. Comparison with Coulomb scattering suggests that stimulated Bremsstrahlung scattering can be dominant in low-density, radiation-driven plasmas, provided that the radiation spectrum has a sufficiently high brightness temperature in the neighborhood of the plasma frequency. While stimulated Bremsstrahlung scattering cannot be easily observed in laboratory plasmas due to their small size, it should operate in large-scale astrophysical plasmas, such as those met in the flaring solar corona. The effect of the solar microwave radiation on fast-electron scattering is evaluated through a parameterized flaring corona model. We find that stimulated Bremsstrahlung greatly enhances the fast-electron scattering frequency in the flare magnetic loop, leading the transport of deka-keV electrons to occur in the diffusion regime, characterized by significant precipitation rates. This prediction is consistent with the interpretation of the above-loop-top hard x-ray and microwave emissions from the X3.1 flare of August 24, 2002. Our analysis indicates that stimulated Bremsstrahlung may play an essential role in the dynamics of fast electrons trapped in solar flare loops. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a crucially important process for energy conversion in plasma physics, with the substorm cycle of Earth's magnetosphere and solar flares being prime examples. While 2D models have been widely applied to study reconnection, investigating reconnection in 3D is still, in many aspects, an open problem. Finding sites of magnetic reconnection in a 3D setting is not a trivial task, with several approaches, from topological skeletons to Lorentz transformations, having been proposed to tackle the issue. This work presents a complementary method for quasi-2D structures in 3D settings by noting that the magnetic field structures near reconnection lines exhibit 2D features that can be identified in a suitably chosen local coordinate system. We present applications of this method to a hybrid-Vlasov Vlasiator simulation of Earth's magnetosphere, showing the complex magnetic topologies created by reconnection for simulations dominated by quasi-2D reconnection. We also quantify the dimensionalities of magnetic field structures in the simulation to justify the use of such coordinate systems. [ABSTRACT FROM AUTHOR]

Force-free magnetic fields can be used to compute the magnetic structure above active regions where the field strength is large. Using vector magnetograms for two active regions that gave rise to solar flares, the force-free fields are obtained for times before and after the flare. The current density in the upper atmosphere obtained from nonlinear force-free fields is analyzed using Singular Value Decomposition (SVD) methods to determine how it is distributed around the times of flare occurrence. Regions of localized and relatively intense currents are sought at the different scales since these are usually linked to magnetic reconnection. It is found that the probability distribution functions (PDF) for small scales exhibit long tails and they are compared for times before and after the flare. Previous to the flare they seem to be slightly wider but not significant enough to draw a definite conclusion. This is likely due to break down of the applicability of the Force-Free fields after the flare. The effect of having long-tailed PDFs is equivalent to the one found in temperature fluctuations about the flare occurrence (1) which could point to a correlation between electrical currents and temperature. It is also obtained that the energy content in the middle scales of the currents is larger before than after the flare which would point to a magnetic dissipation at those scales during the flare. [ABSTRACT FROM AUTHOR]

The purpose of this work was to study maps of the temperature distribution in the corona above the NOAA active region (AR) 12 192 outside solar flares and during individual flare events of high X-ray classes as well as to determine the role of electric currents in heating the coronal matter. Data on the distribution of magnetic field vector components in the photosphere provided by the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO) are used to detect the large-scale electric current and calculate its magnitude. Photogeliograms of the solar corona in ultraviolet (UV) channels 131, 171, 193, and 211 Å provided by the Atmospheric Imaging Assembly (AIA/SDO) instrument are used to estimate the temperature in the corona above active regions (ARs). The following results have been obtained: (1) Outside the time of solar flares, a coronal structure with a temperature of 10 MK or more, which was observed for the entire time interval of AR monitoring and marked the location of a large-scale electric current channel at coronal heights, is detected in the central part above the studied AR. (2) The existence of the high-temperature coronal structure over a long time interval indicates a stationary mode of coronal matter heating due to the ohmic dissipation of large-scale electric currents. (3) It is shown that effective stationary heating of coronal matter by electric currents requires anomalous values of plasma conductivity (σ = 1010 s–1) and filamentation of the current channel into elements with a cross section of the order of 108 cm or less. (4) Heating of a coronal loop (loop system) with a cross section of 108 cm and a length of 1010 cm to a temperature of 10 MK on time scales of a few hours can be implemented under the condition of anomalous plasma conductivity by an electric current in the corona of the order of 109 A. (5) During solar flare events, a decrease (in percentage terms) in the role of electric currents in the processes of coronal matter heating and the activation of additional heating mechanisms are observed. [ABSTRACT FROM AUTHOR]

According to modern concepts, Type I noise storms in the meter wavelength range (30–400 MHz), consisting of a large number of narrow-band short bursts, are excited in the rarefied solar corona. These radiations are characterized by a high brightness temperature and circular polarization. Despite many advances in theory and confidence that the mechanism of radiation formation is coherent, there is currently no generally accepted understanding of the nature of Type I noise storms. In this paper, we show that many important properties of radiation can be explained as a result of the BPA (beam-pulse-amplifier) mechanism in a rarefied magnetized plasma with energetic electrons. Short noise electromagnetic pulses with a suitable carrier frequency, with circular polarization and wave normal angle can be amplified in the plasma layer at an extremely high rate, which is characteristic of quasi-hydrodynamic type instability. When this mechanism occurs, high energy and the rate of change in the spectral forms of radiation can take place even in the absence of a noticeable anisotropy of the distribution function of energetic electrons. [ABSTRACT FROM AUTHOR]

Several strong eruptive events that occurred near the solar limb were reported generating off-limb features detectable in the SDO/HMI's continuum intensity. These observations offer new insights into the emission mechanisms of off-limb flaring loops, magnetic strength in the loops, and electron density distributions, among others. However, only a limited number of such events were reported, and it is unclear whether off-limb white-light features are popular or only associated with specific eruptions. In this study, we surveyed all the flaring events that occurred between May 2010 and August 2023 with a magnitude stronger than M2.0 and a heliographic longitude larger than 65 ∘ . We found that among the 189 flares that met our selection criteria, 78 (41.3%) had off-limb features associated with them. Further statistical analysis showed, unsurprisingly, that the stronger the flare, the more likely it has an off-limb white-light feature, and the closer the flare is to the limb, the more likely an off-limb feature is detectable. We then categorized these off-limb white-light events into four types, closed-loop eruptions, open-loop eruptions, fast ejection, and flare arcades, and identified the events with visible flare ribbons. Coupling two examples of the white-light observations with simultaneous UV/EUV observations, we demonstrate the usefulness of the former in studying the flare dynamics and emission mechanisms. Our catalogue provides a rather complete list of the off-limb white-light events, which will benefit the community interested in studying such events. [ABSTRACT FROM AUTHOR]

Using non-linear force free field (NLFFF) extrapolation, 3D magnetic fields were modeled from the 12-min cadence Solar Dynamics Observatory Helioseismic and Magnetic Imager (HMI) photospheric vector magnetograms, spanning a time period of 1 hour before through 1 hour after the start of 18 X-class and 12 M-class solar flares. Several magnetic field parameters were calculated from the modeled fields directly, as well as from the power spectrum of surface maps generated by summing the fields along the vertical axis, for two different regions: areas with photospheric |Bz|≥ 300 G (active region—AR) and areas above the photosphere with the magnitude of the non-potential field (BNP) greater than three standard deviations above |BNP|̄ of the AR field and either the unsigned twist number |Tw| ≥ 1 turn or the shear angle Ψ ≥ 80° (non-potential region—NPR). Superposed epoch (SPE) plots of the magnetic field parameters were analyzed to investigate the evolution of the 3D solar field during the solar flare events and discern consistent trends across all solar flare events in the dataset, as well as across subsets of flare events categorized by their magnetic and sunspot classifications. The relationship between different flare properties and the magnetic field parameters was quantitatively described by the Spearman ranking correlation coefficient, rs. The parameters that showed the most consistent and discernable trends among the flare events, particularly for the hour leading up to the eruption, were the total unsigned flux ϕ), free magnetic energy (EFree), total unsigned magnetic twist (τTot), and total unsigned free magnetic twist (ρTot). Strong (|rs| ∈ [0.6, 0.8)) to very strong (|rs| ∈ [0.8, 1.0]) correlations were found between the magnetic field parameters and the following flare properties: peak X-ray flux, duration, rise time, decay time, impulsiveness, and integrated flux; the strongest correlation coefficient calculated for each flare property was 0.62, 0.85, 0.73, 0.82, −0.81, and 0.82, respectively.

Magnetic reconnection is a crucially important process for energy conversion in plasma physics, the substorm cycle of Earth's magnetosphere and solar flares being prime examples. While 2D models have been widely applied to study reconnection, investigating reconnection in 3D is still in many aspects an open problem. Finding sites of magnetic reconnection in a 3D setting is not a trivial task, with several approaches from topological skeletons to Lorentz transformations proposed to tackle the issue. This work presents a complementary method by noting that the magnetic field structures near reconnection lines exhibit two-dimensional features that can be identified in a suitably chosen local coordinate system. We present applications of this method to a hybrid-Vlasov Vlasiator simulation of the Earth's magnetosphere, showing the complex magnetic topologies created by reconnection. We also overview the dimensionalities of magnetic field structures in the simulation to support the use of such coordinate systems. [ABSTRACT FROM AUTHOR]

Three generations of the Miniature X-ray Solar Spectrometer (MinXSS) have flown on small satellites with the goal "to explore the energy distribution of soft X-ray (SXR) emissions from the quiescent Sun, active regions, and during solar flares, and to model the impact on Earth's ionosphere and thermosphere." The primary science instrument is the Amptek X123 X-ray spectrometer that has improved with each generation of the MinXSS experiment. This third-generation MinXSS-3 has a higher energy resolution and larger effective area than its predecessors and is also known as the Dual-zone Aperture X-ray Solar Spectrometer (DAXSS). It was launched on the INSPIRESat-1 satellite on 2022 February 14, and INSPIRESat-1 has successfully completed its 6 month prime mission. The INSPIRESat-1 is in a dawn–dusk, Sun-synchronous orbit and therefore has had 24 hr coverage of the Sun during most of its mission so far. The rise of Solar Cycle 25 has been observed by DAXSS. This paper introduces the INSPIRESat-1 DAXSS solar SXR observations, and we focus the science results here on a solar occultation experiment and multiple flares on 2022 April 24. One key flare result is that the reduction of elemental abundances appears greatest during the flare impulsive phase, thus highlighting the important role of chromospheric evaporation during flares to inject warmer plasma into the coronal loops. Furthermore, these results are suggestive that the amount of chromospheric evaporation is related to flare temperature and intensity. [ABSTRACT FROM AUTHOR]

Recently, we established a fundamental mechanism of solar eruption initiation in which an eruption can be initiated from a bipolar field through magnetic reconnection in the current sheet (CS) that is formed slowly in the core field as driven by photospheric shearing motion. Here, using a series of fully 3D MHD simulations with a range of different photospheric magnetic flux distributions, we extended this fundamental mechanism to the quadrupolar magnetic field containing a null point above the core field, which is the basic configuration of the classical breakout model. As is commonly believed, in such a multipolar configuration, the reconnection triggered in the CS originated at the null point (namely, the breakout reconnection) plays the key role in eruption initiation by establishing a positive feedback loop between the breakout reconnection and the expansion of the core field. However, our simulation showed that the key to eruption initiation in such a multipolar configuration remains the slow formation of the CS in the sheared core, rather than the onset of fast breakout reconnection. The breakout reconnection only helps the formation of the core CS by letting the core field expand faster, but the eruption cannot occur when the bottom surface driving is stopped well before the core CS is formed, even though the fast reconnection has already been triggered in the breakout CS. This study clarified the role of breakout reconnection and confirmed formation of the core CS as the key to the eruption initiation in a multipolar magnetic field. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is a fundamental process in astrophysical and space plasmas which is responsible for particle acceleration, in particular, in solar flares. Separator reconnection is one type of steady-state three-dimensional magnetic reconnection. In this paper, by considering static electric and magnetic fields associated with separator reconnection, we investigate first the acceleration features of a test-particle and then discuss acceleration of a randomly injected large population of protons and electrons in the vicinity of reconnection site with input parameters for the solar corona. It is shown that protons and electrons can be accelerated up to tens of MeV and even higher kinetic energies. The variation of energy spectra for different strengths of electric field amplitude is also discussed. [ABSTRACT FROM AUTHOR]

Solar cosmic rays (SCRs) are generated during the primordial energy release in solar flares. This explosive process takes place in the solar corona above the active region. It represents the fast release of the magnetic field energy of the current sheet, which is formed near a singular magnetic field line. Solar cosmic rays appear as a result of the acceleration of charged particles, mainly protons, by an inductive electric field in the current sheet equal to the field E = V × B/c (with V the speed of plasma and B the magnetic field near the current sheet, and c the speed of light). To study the mechanism of solar flares and obtain conditions for studying SCR acceleration, it is necessary to carry out magnetohydrodynamic (MHD) simulations of flare situations in the solar corona above a real active region. Methods of stabilization were developed which made it possible to partially solve the problem of numerical instabilities. MHD simulations shows complicated configurations near the singular line. Comparison of the results of the MHD simulations with observations showed the general agreement of the positions of the current sheets with regions of intense flare radiation. However, there are some problems with the details of such coincidences. The results obtained in this paper show the possibility of improving the methods of MHD simulation in order to solve the problems that arise during solving of MHD equations. [ABSTRACT FROM AUTHOR]

Type II solar bursts are radio signatures of shock waves in the solar corona driven by solar flares or coronal mass ejections (CMEs). Therefore, these bursts present complex spectral morphologies in solar dynamic spectra. Here, we report meter–decameter radio observations of a type II burst on 2014 July 25 made with the Ukrainian radio telescopes UTR-2 (8.25–33 MHz) and GURT (8.25–78 MHz). The burst demonstrates fundamental and harmonic components, band splitting, a herringbone structure, and a spectral break. These specific spectral features, observed jointly in a single type II burst, are rarely detected. To contribute to our understanding of such puzzling type II events, we carried out a detailed analysis of the recorded type II dynamic spectrum. In particular, the herringbone pattern has been exploited to study electron density turbulence in the solar corona. We calculated the power spectral densities of the flux variations in selected herringbones. The spectral index is in the range of α = −1.69 to −2.00 with an average value of −1.897, which is slightly higher than the Kolmogorov spectral index of −5/3 for fully developed turbulence. We also recognized that the second type II burst consists of three drifting lanes. The lane onset times coincide with the spectral break in the first type II burst. We regard that the CME/shock passage through a streamer caused the spectral break and triggered the multilane type II radio emission. Thus, we support one of the proposed scenarios for type II burst occurrence as being the result of CME/shock–streamer interaction. [ABSTRACT FROM AUTHOR]

This paper poses the problem of studying the role of large-scale electric currents propagating in the upper layers of the solar atmosphere in processes of coronal heating of the sun. For detecting and calculating the magnitude of the large-scale electric current, data on the distribution of the components of the magnetic field vector in the photosphere provided by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) were used. Photoheliograms of the sun's corona in the ultraviolet radiation channels at 131, 171, 193, and 211 Å provided by the Atmospheric Imaging Assembly (AIA/SDO) were used to estimate the temperature in the corona above active regions (ARs). The dynamics of the large-scale current and the average temperature in 9 regions with different levels of flare activity of the corona above the ARs have been studied and charts of the spatial distribution of the temperature in the corona above the ARs have been constructed. The following results have been obtained: 1. Heating of the coronal matter owing to ohmic dissipation of large-scale electric currents proceeds in a stationary regime. 2. The increase in the average temperature in the corona above an AR during solar flares to < log T ¯ > = 6.3 - 6.5 (2.0-3.2 MK) is caused, not only by heating of coronal structures by large-scale electric currents, but also by other processes at coronal elevations. 3. For the NOAA 11899 and 12494 regions a reduction in the average temperature of the corona to < log T ¯ > = 5.7 (0.5-0.6 MK) was observed with a simultaneous drop in the values of the large-scale electric current to zero. These observations indicate that the mechanism for heating of the corona by ohmic dissipation of electric currents is shut off at zero values (within the computational errors) of the large-scale electric current. 4. In the NOAA regions 12192 and 12371, when constructing charts of the temperature distribution in the corona outside flare events, hot structures with temperatures ≥ 10 MK were observed outside the flare events which appear to mark the location of the channel of a large-scale electric current at coronal elevations. For the NOAA region 12192 this assumption is confirmed by a numerical simulation carried out in 2016. [ABSTRACT FROM AUTHOR]

Solar ares are sudden bursts in the solar atmosphere, which emit from radio wavelengths to gamma rays, and according to their energy, are classified into different classes (A, B, C, M, and X, respectively). Predicting the time of the are occurrence and determining its class type can help reduce its destructive effects. One of the observable structures that can be seen before a are occurs are oscillations with very long period pulsations (VLPs) of the order of 8-30 minutes, which occur about one to two hours before the are onset. In this paper, using GOES data, we observed typical VLPs before are-onset for six ares in class C and M. Periodicities that we calculated for the VLPs of these ares are between 14.6 and 28.2 minutes, which is in agreement with the results of Tan et al. (2016). The number of pulses observed in each pre-are is between 4 and 6. For the other two remaining ares of our selection, no typical pre-are VLP was observed. [ABSTRACT FROM AUTHOR]

We present X-ray imaging spectroscopy of one of the weakest active region (AR) microflares ever studied. The microflare occurred at ~11:04 UT on 2018 September 9 and we studied it using the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic Observatory's Atmospheric Imaging Assembly (SDO/AIA). The microflare is observed clearly in 2.5-7 keV with NuSTAR and in Fe XVIII emission derived from the hotter component of the 94 Å SDO/AIA channel. We estimate the event to be three orders of magnitude lower than a GOES A class microflare with an energy of 1.1 × 1026 erg. It reaches temperatures of 6.7 MK with an emission measure of 8.0 × 1043 cm-3. Non-thermal emission is not detected but we instead determine upper limits to such emission. We present the lowest thermal energy estimate for an AR microflare in literature, which is at the lower limits of what is still considered an X-ray microflare.

The abundances of low first ionization potential (FIP) elements are three to four times higher in the closed loop active corona than in the photosphere, known as the FIP effect. Observations suggest that the abundances vary in different coronal structures. Here, we use the soft X-ray spectroscopic measurements from the Solar X-ray Monitor (XSM) onboard the Chandrayaan-2 orbiter to study the FIP effect in multiple A-class flares observed during the minimum of Solar Cycle 24. Using time-integrated spectral analysis, we derive the average temperature, emission measure, and the abundances of four elements – Mg, Al, Si, and S. We find that the temperature and emission measure scales with the sub-class of flares while the measured abundances show an intermediate FIP bias for the lower A-flares (e.g. A1), while for the higher A-flares, the FIP bias is near unity. To investigate it further, we perform a time-resolved spectral analysis for a sample of the A-class flares and examine the evolution of temperature, emission measure, and abundances. We find that the abundances drop from the coronal values towards their photospheric values in the impulsive phase of the flares and, after the impulsive phase, they quickly return to the usual coronal values. The transition of the abundances from the coronal to photospheric values in the impulsive phase of the flares indicates the injection of fresh unfractionated material from the lower solar atmosphere to the corona due to chromospheric evaporation. However, explaining the quick recovery of the abundances from the photospheric to coronal values in the decay phase of the flare is challenging. [ABSTRACT FROM AUTHOR]

It has been widely accepted that solar magnetic field manipulates all solar activities, especially violent solar bursts in solar corona. Thus, it is extremely important to reconstruct three-dimentional (3D) magnetic field of solar corona from really observed photospheric magnetogram. In this paper, a large-scale dataset of 3D solar magnetic fields of active regions is built by using the nonlinear force-free magnetic field (NLFFF) extrapolation from vector magnetograms of Helioseismic and Magnetic Imager (HMI) on Solar Dynamics Observatory (SDO). In this dataset, all space-weather HMI active region patches (SHARPs) with the corresponding serial numbers of national oceanic and atmospheric administration (NOAA) are included. They are downloaded from the SHARP 720 s series of JSOC every 96 minutes. In addition, each sample is labelled with a finer grained label for solar flare forecast. This paper is with the purpose of open availability of data resource and source code to the peers for refraining from repeated labor of data preparation. Meanwhile, with such a large-scale, high spatio-temporal resolution and high quality scientific data, we anticipate a wide attention and interest from artificial intelligence (AI) and computer vision communities, for exploring AI for astronomy over such a large-scale dataset. [ABSTRACT FROM AUTHOR]

The R -value is a measure of the strength of photospheric magnetic polarity inversion lines in active regions (ARs). This work investigates the possibility of a relation between R -value variations and the occurrence of X-class flares in ARs, not in the solar photosphere, as usual, but above it in regions closer to where flares occur. The modus operandi is to extrapolate the Solar Dynamic Observatory’s Helioseismic and Magnetic Imager magnetogram data up to a height of 3.24 Mm above the photosphere and then compute the R -value based on the extrapolated magnetic field. Recent studies have shown that certain flare-predictive parameters such as the horizontal gradient of the vertical magnetic field and magnetic helicity may improve flare prediction lead times significantly if studied at a specific height range above the photosphere, called the optimal height range (OHR). Here, we define the OHR as a collection of heights where a sudden but sustained increase in R -value is found. For the eight case studies discussed in this paper, our results indicate that it is possible for OHRs to exist in the low solar atmosphere (between 0.36 and 3.24 Mm), where R -value spikes occur 48–68 hr before the first X-class flare of an emerging AR. The temporal evolution of R -value before the first X-class flare for an emerging AR is also found to be distinct from that of nonflaring ARs. For X-class flares associated with nonemerging ARs, an OHR could not be found.

Solar energetic events are caused by the release of magnetic energy accumulated in the solar atmosphere. To understand their initiating physical mechanisms, the dynamics of the coronal magnetic fields must be studied. Unfortunately, the dominant mechanisms are still unclear due to a lack of direct measurements. Numerical simulations based on magnetohydrodynamics (MHD) can reproduce the dynamical evolution of solar coronal magnetic field, providing a useful tool to explore flare initiation. Data-driven MHD simulations, in which the time-series observational data of the photospheric magnetic field is used as the simulation boundary condition, can explore different mechanisms. To investigate the accumulation of free magnetic energy through a solar eruption, we simulated the first of several large flares in NOAA active region 11283. We used a data-driven model that was governed by zero-beta MHD, focusing on the free magnetic energy accumulation prior to the M5.3 flare (2011 September 6 at 01:59 UT). We reproduced the flare-associated eruption following the formation of twisted magnetic fields, or a magnetic flux rope (MFR), formed by photospheric motions at its footpoints. We found that the eruption was first triggered by the growth of the torus instability. The erupting MFR caused magnetic reconnections with neighboring magnetic field lines located along the direction of the eruption. Using the simulation results and an axial-radial decay index centered on the MFR, we find a natural explanation for the inclination of the eruption and a possible approach to predict the direction of solar eruptive events.

The current sheet is an essential feature in solar flares and is the primary site for magnetic reconnection and energy release. Imaging observations feature a long linear structure above the candle-flame-shaped flare loops, which resembles the standard flare model with the current sheet viewed edge-on. We investigate the thermal properties of plasmas surrounding the linear sheet during flares, using EUV observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The differential emission measure analyses show evidence of high temperatures in the plasma sheets (PSs), containing hot emissions from only a narrow temperature range, suggestive of an isothermal feature. The sheet temperature remains constant at different heights above the flare arcade, peaking at around $\mathrm{log}T$ = 7.0–7.1; while the well-studied 2017 September 10 X8.2 flare serves as an exception in that the temperature decreases with increasing height and peaks higher ( $\mathrm{log}T$ = 7.25) during the gradual phase. Most PS cases also hold similar emission measures and thicknesses; while the PS emissions drop exponentially above the flare arcade, the sheet thicknesses show no significant height association as for all the measurements. The characteristics of isothermal and steady temperature suggest balanced heating and cooling processes along the current sheet; in particular, additional heating may exist to compensate for the conductive and radiative cooling away from the reconnection site. Our results suggest a steady and uniform sheet structure in the macroscopic scale that results from flare reconnection.

The origin of multiple peaks in light curves of various wavelengths remains illusive during flares. Here we discuss the flare of SOL2023-05-09T03:54M6.5 with six flux peaks as recorded by a tandem of new microwave and hard X-ray (HXR) instruments. According to its microwave spectra, the flare represents a high-turnover-frequency (>15 GHz) event. The rather-complete microwave and HXR spectral coverage provides a rare opportunity to uncover the origin of such an event together with simultaneous EUV images. We concluded that (1) the microwave sources originates around the top section of the flaring loops with a trend of source spatial dispersion with frequency; (2) the visible movement of the microwave source from peak to peak originates from the process of new flaring loops appearing sequentially along the magnetic neutral line; (3) the optically thin microwave spectra are hard with the indices ( α _tn ) varying from ∼−1.2 to −0.4, and the turnover frequency always exceeds 15 GHz; (4) higher turnover/peak frequency corresponds to stronger peak intensity and harder optically thin spectra. Using the Fokker–Planck and GX Simulator codes we obtained a good fit to the observed microwave spectra and spatial distribution of the sources at all peaks, if assuming the radiating energetic electrons have the same spatial distribution and single-power-law spectra but with the number density varying in a range of ∼30%. We conclude that the particle acceleration in this flare happens in a compact region nearing the loop-top. These results provide new constraints on the acceleration of energetic electrons and the underlying flare intermittent reconnection process.

Solar magnetic flux rope (MFR) plays a central role in the physics of coronal mass ejections (CMEs). It mainly includes a cold filament at typical chromospheric temperatures (∼10,000 K) and a hot channel at high coronal temperatures (∼10 MK). The warm MFR at quiescent coronal temperatures of a million Kelvin is, however, rarely reported. In this study, using multiwavelength images from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory and Extreme Ultraviolet Imager (EUVI) on board the Solar Terrestrial Relations Observatory-A, we present an eruption of a warm channel that represents an MFR with quiescent coronal temperatures (∼0.6–2.5 MK). On 2022 May 8, we observed the failed eruption of a hot channel, with the average temperature and emission measure (EM) of 10 MK and 1.1 × 10 ^28 cm ^−5 , using AIA high-temperature images in the active region (AR) 13007. This failed eruption was associated with a C8.2 flare, with no CME. Subsequently, we observed a warm channel that appeared in AIA and EUVI low-temperature images rather than in AIA high-temperature images. It then erupted and transformed into a semicircular shape. An associated C2.1 flare, along with the signatures of magnetic reconnection in AIA high-temperature images, were identified. Additionally, we observed a CME associated with this event. Compared with the hot channel, the warm channel is cooler and rarer with the average temperature and EM of 1.7 (1.6) MK and 2.0 × 10 ^26 (2.3 × 10 ^26 ) cm ^−5 . All the results suggest an unambiguous observation of the million-Kelvin warm MFR that erupted as a CME and fill a gap in the temperature domain of coronal MFRs.

A significant impediment to solving the coronal heating problem is that we currently only observe active region loops in their cooling phase. Previous studies showed that the evolution of cooling loop densities and apex temperatures is insensitive to the magnitude, duration, and location of energy deposition. Still, potential clues to how energy is released are encoded in the properties of the cooling phase. The appearance of coronal rain, one of the most spectacular phenomena of the cooling phase, occurs when plasma has cooled below 1 MK, which sets constraints on the heating frequency, for example. Most observations of coronal rain have been made by imaging instruments. Here we report rare Hinode/EUV Imaging Spectrometer (EIS) observations of a loop arcade where coronal rain forms following an X2.1 limb flare. A bifurcation in plasma composition measurements between photospheric at 1.5 MK and coronal at 3.5 MK suggests that we are observing postflare-driven coronal rain. Increases in nonthermal velocities and densities with decreasing temperature (2.7–0.6 MK) suggest that we are observing the formation and subsequent evolution of the condensations. Doppler velocity measurements imply that a 10% correction of apparent flows in imaging data is reasonable. Emission measure analysis at 0.7 MK shows narrow temperature distributions, indicating coherent behavior reminiscent of that observed in coronal loops. The limitations on spatio-temporal resolution of EIS suggest that we are observing the largest features or rain showers. These observations provide insights into the heating rate, source, turbulence, and collective behavior of coronal rain from observations of the loop cooling phase.

The coronal magnetic field over NOAA Active Region 11,429 during a X5.4 solar flare on 7 March 2012 is modeled using optimization based Non-Linear Force-Free Field extrapolation. Specifically, 3D magnetic fields were modeled for 11 timesteps using the 12-min cadence Solar Dynamics Observatory (SDO) Helioseismic and Magnetic Imager photospheric vector magnetic field data, spanning a time period of 1 hour before through 1 hour after the start of the flare. Using the modeled coronal magnetic field data, seven different magnetic field parameters were calculated for 3 separate regions: areas with surface |Bz|≥ 300 G, areas of flare brightening seen in SDO Atmospheric Imaging Assembly imagery, and areas with surface |B| ≥ 1000 G and high twist. Time series of the magnetic field parameters were analyzed to investigate the evolution of the coronal field during the solar flare event and discern pre-eruptive signatures. The data shows that areas with |B| ≥ 1000 G and |Tw|≥ 1.5 align well with areas of initial flare brightening during the pre-flare phase and at the beginning of the eruptive phase of the flare, suggesting that measurements of the photospheric magnetic field strength and twist can be used to predict the flare location within an active region if triggered. Additionally, the evolution of seven investigated magnetic field parameters indicated a destabilizing magnetic field structure that could likely erupt.

The M1.5-class flare and associated coronal mass ejection (CME) of 16 February 2011 was observed with the Extreme ultraviolet Imaging Spectrometer on board the Hinode spacecraft. Spray plasma associated with the CME is found to exhibit a Doppler blue-shift of 850 km s−1 – one of the largest values reported from spectroscopy of the solar disk and inner corona. The observation is unusual in that the emission line (Fe xii 193.51 Å) is not observed directly, but the Doppler shift is so large that the blue-shifted component appears in a wavelength window at 192.82 Å, intended to observe lines of O v , Fe xi and Ca xvii. The Fe xii 195.12 Å emission line is used as a proxy for the rest component of 193.51 Å. The observation highlights the risks of using narrow wavelength windows for spectrometer observations when observing highly-dynamic solar phenomena. The consequences of large Doppler shifts for ultraviolet solar spectrometers, including the upcoming Multi-slit Solar Explorer (MUSE) mission, are discussed. [ABSTRACT FROM AUTHOR]

We present an in-depth study of a large and long duration (>1.3 d) X-ray flare observed on an RS CVn-type eclipsing binary system SZ Psc using observations from Swift observatory. In the 0.35–10 keV energy band, the peak luminosity is estimated to be 4.2 × 1033  |$\rm {erg}~\rm {s}^{-1}$|⁠. The quiescent corona of SZ Psc was observed ∼5.67 d after the flare using Swift observatory, and also ∼1.4 yr after the flare using the XMM–Newton satellite. The quiescent corona is found to consist of three temperature plasma: 4, 13, and 48 MK. High-resolution X-ray spectral analysis of the quiescent corona of SZ Psc suggests that the high first ionization potential (FIP) elements are more abundant than the low-FIP elements. The time-resolved X-ray spectroscopy of the flare shows a significant variation in the flare temperature, emission measure, and abundance. The peak values of temperature, emission measure, and abundances during the flare are estimated to be 199 ± 11 MK, 2.13 ± 0.05 ×  1056 cm−3, 0.66 ± 0.09  |$\rm {Z}_{\odot }$|⁠ , respectively. Using the hydrodynamic loop modelling, we derive the loop length of the flare as 6.3 ± 0.5 ×  1011 cm, whereas the loop pressure and density at the flare peak are derived to be 3.5 ± 0.7 ×  103 dyn cm−2 and 8 ± 2 ×  1010 cm−3, respectively. The total magnetic field to produce the flare is estimated to be 490 ± 60 G. The large magnetic field at the coronal height is supposed to be due to the presence of an extended convection zone of the subgiant and the high orbital velocity. [ABSTRACT FROM AUTHOR]

We analyze various tracers of magnetic activity for 23 solar twins which are characterized by significant scatter of lithium abundance in their atmospheres. A level of coronal and chromospheric activity has been studied from available X-ray and UV-archival data. It gives us a chance to compare coronae of solar twins of various ages with the solar case. We found a scatter in the X-ray to bolometric luminosity ratio LX/Lbol by several orders of magnitude, which exists in these stars along with a significant spread in Li abundance. This may link the surface activity of stars with phenomena at the base of their convective zones. The TESS data allowed us to reveal rotation modulation of stellar brightness associated with starspots. For some twins of our samples, periods of axial rotation are detected around 6 days, i.e. these stars rotate almost 4 times faster than the contemporary Sun. This indicates their relative youth. Flare activity of solar twins is discovered in the TESS data; we showed existence of various kinds of flares, and present temporal profiles for some of them. We obtained the energy about of 8 × 1033 erg for the largest flare of our samples, lasting longer than 4 h. In addition, we discuss also magnetic fields and exoplanets, orbiting these stars. [ABSTRACT FROM AUTHOR]

As a frequent and energetic particle accelerator, our Sun provides us with an excellent astrophysical laboratory for understanding the fundamental process of particle acceleration. The exploitation of radiative diagnostics from electrons has shown that acceleration operates on sub-second time scales in a complex magnetic environment, where direct electric fields, wave turbulence, and shock waves all must contribute, although precise details are severely lacking. Ions were assumed to be accelerated in a similar manner to electrons, but γ-ray imaging confirmed that emission sources are spatially separated from X-ray sources, suggesting distinctly different acceleration mechanisms. Current X-ray and γ-ray spectroscopy provides only a basic understanding of accelerated particle spectra and the total energy budgets are therefore poorly constrained. Additionally, the recent detection of relativistic ion signatures lasting many hours, without an electron counterpart, is an enigma. We propose a single platform to directly measure the physical conditions present in the energy release sites and the environment in which the particles propagate and deposit their energy. To address this fundamental issue, we set out a suite of dedicated instruments that will probe both electrons and ions simultaneously to observe; high (seconds) temporal resolution photon spectra (4 keV – 150 MeV) with simultaneous imaging (1 keV – 30 MeV), polarization measurements (5–1000 keV) and high spatial and temporal resolution imaging spectroscopy in the UV/EUV/SXR (soft X-ray) regimes. These instruments will observe the broad range of radiative signatures produced in the solar atmosphere by accelerated particles. [ABSTRACT FROM AUTHOR]

Reconnection of magnetic field lines represents a universal process of releasing the stored energy of the magnetic field and its transformation into the thermal energy of plasma and the energy of accelerated charged particles. The initialization and proceeding of the magnetic reconnection process are essentially related with the dynamics of a spatially localized region of strong plasma currents—the current sheet. The two most studied cosmic magnetoplasma systems containing current sheets are the tail region of the Earth's magnetosphere and the regions with closely spaced, elongated magnetic field lines of opposite polarity in the solar corona (in particular, the rays of coronal streamers and the eruptive flares). However, whereas the main source of information for the Earth's magnetosphere about the structure and dynamics of current sheets are numerous direct measurements of satellite missions, then, for the solar corona, some characteristics of the current sheet can be restored on the basis of remote observations of quasi-periodic oscillations. As a consequence, to clarify the possible mechanisms responsible for these oscillations, it seems relevant to compare the properties of oscillations of the current sheet of the Earth's magnetosphere and current sheets of the solar corona. This work is devoted to such a comparative analysis; it contains a brief overview of the available information about the quasi-periodic dynamics of the magnetospheric current sheet and discusses the probable interpretation of this dynamics in terms and parameters of observations of quasi-periodic processes in current sheets of the solar corona. [ABSTRACT FROM AUTHOR]

Solar eruptions, primarily manifested as solar flares, filament eruptions and coronal mass ejections, represent explosive releases of magnetic energy stored in the solar corona, with the potential to drive severe space weather. The initiation of solar eruptions remains an open question, leading to various theoretical models that are inferred from observations. However, these models are subjects of debate due to the absence of direct measurements of the three-dimensional (3D) magnetic fields in the corona. Numerical simulations, based on solving magnetohydrodynamics (MHD) equations that govern the macroscopic dynamics of solar corona, serve as a touchstone for testing these theoretical models. One early proposed model suggested that eruptions could be triggered by reconnection within a single sheared magnetic arcade, which is known as the tether-cutting reconnection model, but it was never confirmed through 3D MHD simulations until very recently. Consequently, two models have gained more popularity: one involving the eruption of a twisted magnetic flux rope (MFR) due to ideal instability (or loss of equilibrium), and the other known as the breakout eruption, which requires a quadrupolar configuration with a delicately located magnetic null point. Other mixed mechanisms, involving both ideal instability and reconnection, are also proposed in association with localized magnetic flux emergence. Now with the validation of the tether-cutting model, the fundamental mechanisms are boiled down to two types of models, one primarily based on the ideal instability of a pre-existing MFR, and the other based on the reconnection of sheared field lines with or without an MFR. Recently, the modelling of the birth of solar eruption using observed data-based MHD simulations has advanced rapidly, becoming a crucial research tool in the study of the initiation mechanisms. These realistic modellings reveal a higher level of complexity compared to all currently available theories and idealized models. [ABSTRACT FROM AUTHOR]

The observed primordial energy release of solar flare in the corona is explained by the mechanism of S. I. Syrovatskii, according to which the flare energy is accumulated in the current sheet. The flare release of the current sheet energy causes the observed manifestations of the flare, which are explained by the electrodynamical model of a solar flare proposed by I. M. Podgorny. According to this model, hard X-ray beam radiation on the solar surface is explained by the acceleration of electrons in field aligned currents caused by the Hall electric field in the current sheet. The study of the flare mechanism is impossible without performing magnetohydrodynamic (MHD) simulations above a real active region (AR), in which the calculation begins several days before the appearance of flares. When setting the problem, no assumptions were made about the flare mechanism. An absolutely implicit finite-difference scheme, conservative with respect to the magnetic flux, has been developed, which is implemented in the PERESVET code. MHD simulation in the real scale of time can only be carried out, thanks to parallel computations using compute unified device architecture (CUDA) technology. Methods have been developed that made it possible to stabilize the numerical instability arising near the boundary of the region. Calculation above AR 10365 for low viscosities (Rm=109{\rm{Rm}}=1{0}^{9}, Re=107{\rm{Re}}=1{0}^{7}, νArt Phoosphere=νMagn Art Phoosphere=10−4{\nu }_{\text{Art Phoosphere}}={\nu }_{\text{Magn Art Phoosphere}}=1{0}^{-4}) showed the appearance of a singular X-type line, in the vicinity of which a current sheet with accumulated magnetic energy for a flare can form. Also, by means of MHD simulation the appearance of singular lines above a real AR is shown, in which the magnetic field is a superposition of an X-type field and a diverging magnetic field. In such a superposition of configurations, even if the diverging field predominates, the formation of a current sheet is possible, which can explain the appearance of a flare of not very high power. The coincidence of the position of the source of the flare thermal X-ray radiation with the places of appearance of the current sheets confirms the mechanism of the solar flare, based on the accumulation of energy in the magnetic field of the current sheet.

Type III radio bursts are the result of plasma emission from mildly relativistic electron beams propagating from the low solar corona into the heliosphere where they can eventually be detected in situ if they align with the location of a heliospheric spacecraft. Here we observe a type III radio burst from 0.1 to 16 MHz using the Parker Solar Probe (PSP) FIELDS Radio Frequency Spectrometer (RFS) and from 20 to 80 MHz using the Low Frequency Array (LOFAR). This event was not associated with any detectable flare activity but was part of an ongoing type III and noise storm that occurred during PSP encounter 2. A deprojection of the LOFAR radio sources into 3D space shows that the type III radio burst sources were located on open magnetic field from 1.6 to 3 R ⊙ and originated from a near-equatorial active region around longitude E48°. Combining PSP/RFS observations with WIND/WAVES and Solar Terrestrial Relations Observatory (STEREO) WAVES, we reconstruct the type III radio source trajectory in the heliosphere interior to PSP’s position, assuming ecliptic confinement. An energetic electron enhancement is subsequently detected in situ at the STEREO A spacecraft at compatible times, although the onset and duration suggests the individual burst contributes a subset of the enhancement. This work shows relatively small-scale flux emergence in the corona can cause the injection of electron beams from the low corona into the heliosphere, without needing a strong solar flare. The complementary nature of combined ground and space-based radio observations, especially in the era of PSP, is also clearly highlighted by this study. [ABSTRACT FROM AUTHOR]