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We analysed a GOES C6.2 class solar flare that occurred on 27 October 2003 at the heliographic position S20E29. Utilising a database developed from a simplified 3D magnetic loop model, we inferred a set of geometric and physical parameters to characterise the flare. This model, based on general properties of known solar flares and adjusted for instrument resolution, was used to replicate observations from the Nobeyama Radio Polarimeter (NoRP) spectra and the Nobeyama Radioheliograph (NoRH) brightness maps. We identified a possible range of parameters by comparing observed spectra and brightness distribution maps with modelled counterparts. The analysis involved computing a weighted mean of the one hundred best-fit model parameters, ranked by increasing χ 2 values.Across six discrete time intervals representing the gradual, impulsive, and decay phases of the flare, we observed slight variations in values for ten analysed parameters, with notable exceptions at the burst maxima, including an energy spectral index of approximately ∼ 2.34 , a photospheric magnetic field strength of around ∼ 2 308 G , and a non-thermal electron density of about ∼ 10 7.11 cm - 3 . In general, our results replicated the soft-hard-soft behaviour in the gradual-peak-decay phases of a solar flare. Therefore, we infer that the physical parameters (magnetic field strength, energy spectral index, and non-thermal electron density) were accurately recovered. However, fitting geometric parameters such as inclination, asymmetry, azimuth, loop radius, loop height, and footpoint separation proved challenging due to the limited resolution of the instruments, which affects the precise determination of loop-like flare geometries. [ABSTRACT FROM AUTHOR]

The accuracy of the M. Aschwanden's (2020) model for short-term (24 h) prediction of the maximum X-ray class of solar flares based on the power-law dependence on the energy of the potential magnetic field of active regions is checked and assessed. For this purpose, a sample of 275 flares (253 M-class and 22 X-class) in isolated active regions on the solar disk in 2010−2023 is analyzed. Magnetic field extrapolations are made in the nonlinear force-free and potential approximations using the GX Simulator based on photospheric vector magnetograms from the Helioseismic and Magnetic Imager (HMI) instrument onboard the Solar Dynamics Observatory (SDO). It is found that in 6% of cases, Aschwanden's model underestimates the predicted maximum flare class with respect to the observed one (maximal underestimation by 4.4 times). The accuracy of this model (the average ratio of the observed to predicted maximum flare class) is 0.31 ± 0.47. Four other statistical models are proposed, two of which, like Aschwanden's model, are based on the power-law dependence of the maximum flare class on the energy of potential magnetic field, and the other two are based on the power-law dependence on the free magnetic energy of active regions. These models give fewer (or no) underestimations of the maximum flare class, but two to three times lower forecast accuracy, ranging from 0.11 to 0.17. Additionally, based on the obtained statistical sample, estimates of the limiting X-ray class of solar flares are made. The five models give different limits ranging from ~X14 to ~X250. The realism of these values and the possibility of refining the models by expanding the sample of events is briefly discussed. [ABSTRACT FROM AUTHOR]

We present a novel deep Convolutional Neural Network model with one-against-rest approach (OAR-CNN) and modify the hybrid Convolutional Neural Network (H-CNN) model of Zheng et al. (2019) for multiclass flare prediction to forecast whether an active region generates multiclass flare within 24 h. Additionally, in the OAR-CNN and H-CNN models, we employ the decision strategies of majority voting and probability threshold, respectively, comparing the prediction outcomes of these two strategies. Our models undergo training and testing on the same 10 cross-validation datasets as employed by Zheng et al. (2019) , and then compare the results with previous studies using forecast verification metrics, with a focus on the true skill statistic (TSS). The major results are summarized as follows. (1) This is the first attempt to utilize the decision strategies of majority voting and probability threshold in the OAR-CNN model for multiclass solar flare prediction. (2) In both the OAR-CNN and H-CNN models, the predictive results with the probability threshold decision strategy are higher than those with majority voting across all six classes (i.e., No-flare, C, M, X, ⩾ C, and ⩾ M class), except for a slight decrease in the C class in the OAR-CNN model. (3) The OAR-CNN and modified H-CNN models with the probability threshold decision strategy demonstrate comparable statistical scores across all categories and outperform previous studies. (4) In the prediction of four-class flare, our proposed OAR-CNN model with the probability threshold decision strategy achieves relatively high mean TSS scores of 0.744, 0.429, 0.567, and 0.630 for No-flare, C, M, and X class, respectively, surpassing or comparable to results from prior studies. Furthermore, our model achieves high TSS scores of 0.744 ± 0.042 for ⩾ C–class and 0.764 ± 0.089 for ⩾ M-class predictions. [ABSTRACT FROM AUTHOR]

Context. The electron-cyclotron maser instability (ECMI) is a significant coherent radio emission mechanism widely utilized in various astrophysical radio phenomena. It is well known that the velocity anisotropic distribution of energetic electrons, which leads to an inverted perpendicular population in the vertical direction with ∂fb/∂v⊥ > 0, can provide the free energy necessary for the ECMI. Aims. The initial velocity distribution of energetic electrons leaving the acceleration region is typically isotropic or beam-like. However, as these energetic electrons travel along the magnetic field as fast electron beams (FEBs) in magnetic plasma, various velocity anisotropic distributions can emerge. In this paper, we examine the impact of temperature anisotropy formed by beam electrons traveling along a flare loop on the ECMI. Methods. By neglecting the energy loss of energetic electrons as they traverse the corona and invoking the conservation of energy and magnetic moments, we established the relationship between momentum dispersion and the magnetic field. Utilizing the magnetic field model of the flare loop, we calculated the evolution of momentum dispersion and the growth rates of the ECMI as FEBs precipitate along the flare loop. Results. The results demonstrate that the temperature anisotropy arising as FEBs descend along the flare loop significantly impacts the ECMI. The maximum growth rates of the excited modes exhibit a gradual increase initially and then decline rapidly after reaching a critical height for β0 = 0.2c and 0.15c. The results also show that the growth rates of the O2 mode are one order of magnitude smaller than those of the O1 and X2 modes. This indicates that the harmonic radiation is X-mode polarized. Notably, the temperature anisotropy of FEBs as they precipitate along the flare loop with different magnetic field models or at different heights has similar effects on the ECMI. [ABSTRACT FROM AUTHOR]

We present a statistical study on the relationship of solar dynamic events (solar flares and coronal mass ejections) with active regions during Solar Cycle 24 (December 2008–December 2019). Combining data from NOAA Space Weather Prediction Center and observations of Large Angle and Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO) spacecraft, we found that more than a half of the coronal mass ejections were generated inside active regions. Geostationary Operational Environmental Satellite (GOES) soft X-ray flare listing data completed our study showing that almost 83% of Solar Cycle 24 flares are connected with active regions. Finally, we summarize the details for the related phenomena into an online catalog based on a list of all 1533 active regions that produced at least one flare and/or coronal mass ejection during Solar Cycle 24 and explore their properties like flare class, coronal mass ejection speed, and angular width paying special attention to the most powerful and threatful to Earth solar events. [ABSTRACT FROM AUTHOR]

We report for the first time the detection of thermal free–free emission from post-flare loops at 34 GHz in images from the Nobeyama Radioheliograph. We studied eight loops, seven of which were from regions with an extremely strong coronal magnetic field reported by Fedenev et al. Loop emission was observed in a wide range of wavelength bands, up to soft X-rays, confirming their multitemperature structure and was associated with noise storm emission in metric λ. The comparison of the 17 GHz emission with that at 34 GHz, 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 × 1010 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 × 106 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 1010 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 10–30 G. [ABSTRACT FROM AUTHOR]

Flare ribbons formed in solar two-ribbon flares after eruptions of prominences diverge in opposite directions from the polarity inversion line of the photospheric longitudinal magnetic field, sharply slowing down with time and distance from this line. Examples of such events are given, and the kinematics of flare ribbons is demonstrated. A comparison of the position of the ribbons with the distribution of the photospheric magnetic field shows that the separation of the ribbons slows down when they enter a region of a strong longitudinal field. A simple model of prominence eruption illustrates the kinematic features of the motion of the ribbons and the relation to the sources of the coronal magnetic field in the photosphere. [ABSTRACT FROM AUTHOR]

Solar flares will cause the change of the photospheric magnetic field and sunspot structure. However, the exact physical processes involved remain unclear. Here, we study the changes of photospheric magnetic field before and after an X1.5 flare caused by a circular filament eruption in the active region NOAA 13006. The magnetic field structure of this active region is a fan-spine structure with a circular polarity inversion line (PIL). We found that the sunspot structure contracted towards the PIL as a whole after the flare. The penumbra away from the PIL gradually disappears, and the umbra and penumbra near the PIL gradually enhances. By analysing the local magnetic field, the Lorentz force (LF), and the photospheric velocity field in these three regions, we find that the magnetic flux in the region of the disappearing penumbra converges and contracts towards the PIL, leading to an enhancement of the umbra, while the enhancement of the penumbra potentially indicates that this region may be the footpoint of a reconnected magnetic field system. We suggest that this contracting motion is driven by the horizontal LF. [ABSTRACT FROM AUTHOR]

Measurements of magnetic fields near seismic sources during solar flares are vital for understanding the dynamics of solar activity. We used spectropolarimetric observations of the X17.2/4B solar flare on 28 October 2003, over a wavelength interval of 43 Å, including the D3, D2, D1, and Ni I 5892.88 Å lines, to analyze the Stokes I ± V profiles. Effective magnetic fields within 0.5–1.5 kG were measured in the D1, D2, and D3 lines at different flare locations, with the photospheric Ni I 5892.88 Å line showing a weaker field of below 0.5 kG. The D3 line showed rapid plasma descents of up to 11 km/s, in contrast to the slower velocities within 2.3 km/s observed in other lines. The differing amplitudes in the I + V and I − V profiles indicated potential non-Zeeman polarization effects. Secondary Stokes V peaks were also detected up to 8 Å from the D3 emission core. Significant altitudinal inhomogeneity in the magnetic field strengths was detected, possibly indicating the local magnetic collapse, facilitating the Lorentz-force driven mechanism of the seismic source excitation. [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 research on solar flare predicting holds significant practical and scientific value for safeguarding human activities. Current solar flare prediction models have not fully considered important factors such as time step length, nor have they conducted a comparative analysis of the physical features in multiple models or explored the consistency in the importance of features. In this work, based on SHARP data from SDO, we build 9 machine learning-based solar flare prediction models for binary "Yes" or "No" class prediction within the next 24 hours, and study the impact of different time steps and other factors on the forecasting performance. The main results are as follows. (1) The predictive performance of eight deep learning models shows an increasing trend as the time step length increases, and the models perform the best at the length of 40. (2) In predicting solar flares of ≥C class and ≥M class, the True Skill Statistic(TSS) of deep learning models consistently outperforms that of baseline model. For the same model, the TSS for predicting ≥M class flares generally exceeds that for predicting ≥C class flares. (3) The Brier Skill Score (BSS) of deep learning models significantly surpasses that of baseline model in predicting ≥C class flares. However, the BSS scores of the nine models are comparable for predicting ≥M class flares. For the same model, the BSS for predicting ≥C class flares is generally higher than that for predicting ≥M class flares. (4) Through feature importance analysis of multiple models, the common features that consistently rank at the top and bottom are identified. [ABSTRACT FROM AUTHOR]

The study of evolution of magnetic field and electric currents in active regions of the Sun over a long-time interval is of interest for understanding the processes of energy accumulation and release in them, leading to various phenomena that affect space weather. In this study, based on the photospheric vector magnetograms of the helioseismic and magnetic imager instrument aboard the Solar Dynamics Observatory, the authors analyzed the evolution of a number of characteristics of the magnetic field and vertical electric current in three active regions, 11 158, 11 675, and 12 673, that produced M and X class flares, during the time from their origin in the eastern hemisphere, during passage through the solar disk, and until their disappearance near the western limb with a step of 2 h. The characteristics considered included: the power-law exponent of the probability density function of the absolute value of the vertical electric current density, the maximum absolute value of the vertical current density, the signed and unsigned total vertical currents and the unsigned total vertical and horizontal magnetic fluxes, the energy of the nonlinear force-free and potential magnetic fields, the free magnetic energy, and the number of islands with strong vertical current. Some regularities in the behavior of the characteristics considered are found, in particular, regarding the occurrence of solar flares. The correlation coefficients between pairs of these characteristics are calculated. Additionally, M. Aschwanden's approach is shown to be promising for predicting the maximum X-ray class of a flare by calculating the energy of the potential magnetic field in active regions. The results can be used to predict powerful solar flares. [ABSTRACT FROM AUTHOR]

In this work, we provide a detailed analysis of the broad-band temporal and spectral properties of the blazar Ton 599 by using observations from the Fermi Large Area Telescope (LAT) and Swift X-Ray Telescope (XRT)/Ultraviolet–Optical Telescope (UVOT), during its brightest γ-ray flaring. The one-day bin γ-ray light curve exhibits multiple substructures with asymmetric and symmetric profiles. Notably, the γ-ray light curve shows a maximum flux of |$\rm 3.63 \times 10^{-6}\, photon\, cm^{-2}\, s^{-1}$| on MJD 59954.50, which is the highest flux ever observed from this source. The correlation between the γ-ray flux and γ-ray spectral indices suggests a moderate 'harder when brighter' trend. Taking the γ-ray light curve as the reference, a strong correlation is observed with X-ray, optical, and UV energies. Additionally, the γ-rays and optical/UV emission exhibit higher variability compared with X-rays. To understand the parameter variation during the active state of the source, we conducted a statistical broad-band spectral modelling of the source in 10 flux intervals of equal duration. A one-zone leptonic model involving synchrotron, synchrotron-self-Compton, and external Compton processes successfully reproduces the broad-band spectral energy distribution (SED) in each of these flux intervals. We observed that flux variation during the active state is associated mainly with variation in the magnetic field and particle spectral indices. [ABSTRACT FROM AUTHOR]

We consider the M1.1-class solar flare occurred on July 5, 2012, at UT. This event is unique in that a helioseismic perturbation was detected in it despite its low hard X-ray flux in the 25–50 keV energy band and its very soft hard X-ray spectrum. As a rule, most of the known sunquakes have been detected in solar flares with large hard X-ray fluxes at high energies (at least up to 100–300 keV). The event under consideration contradicts the popular hypothesis about the generation of sunquakes by beams of accelerated high-energy electrons. An analysis of the available RHESSI X-ray spectra shows that they can be explained in two ways. The X-ray spectrum in the 25–50 keV energy band is explained by a power-law distribution of accelerated electrons with an power-law index of 7–9 or by the presence of a superhot plasma with a temperature –60 MK. In both cases, we are dealing with electrons of relatively low energies that either were responsible for the sunquake generation or should be considered as a secondary (accompanying) phenomenon with respect to the true cause of the photospheric perturbation. The results of a joint analysis of the X-ray and microwave spectra are presented for the first time for a helioseismically active solar flare. Our analysis shows that the spectra in both ranges can be well explained by the emission of a superhot magnetized plasma and not by accelerated electrons with a soft spectrum. However, the explanation of the spectra when considering partially magnetically trapped accelerated electrons is also possible. We have estimated the parameters of the thermal plasma, accelerated electrons, and energy fluxes of various types. We analyze the dynamics of ultraviolet and X-ray emission sources. We also present an analysis of the magnetic field structure based on vector magnetograms and the nonlinear force-free coronal magnetic field extrapolation. We discuss the mechanisms for the generation of the helioseismic perturbation during the solar flare under consideration. An eruptive process could probably be both primary and secondary causes of the sunquake. The appearance of a superhot plasma in the corona could give rise to propagating thermal fronts into the lower layers of the solar atmosphere, where helioseismic waves are excited. Our analysis does not allow the possibility of the sunquake generation by accelerated electrons with a soft spectrum to be ruled out either. [ABSTRACT FROM AUTHOR]

Image super-resolution is an important subject in image processing and recognition. Here, we present an attention-aided convolutional neural network for solar image super-resolution. Our method, named SolarCNN, aims to enhance the quality of line-of-sight (LOS) magnetograms of solar active regions (ARs) collected by the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO). The ground-truth labels used for training SolarCNN are the LOS magnetograms collected by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. Solar ARs consist of strong magnetic fields in which magnetic energy can suddenly be released to produce extreme space-weather events, such as solar flares, coronal mass ejections, and solar energetic particles. SOHO/MDI covers Solar Cycle 23, which is stronger with more eruptive events than Cycle 24. Enhanced SOHO/MDI magnetograms allow for better understanding and forecasting of violent events of space weather. Experimental results show that SolarCNN improves the quality of SOHO/MDI magnetograms in terms of the structural similarity index measure, Pearson's correlation coefficient, and the peak signal-to-noise ratio. [ABSTRACT FROM AUTHOR]

Statistical relations between the geomagnetic Dst index, cosmic ray variations, and solar wind characteristics are compared for Forbush decreases associated with: (i) coronal mass ejections from active regions (AR-CMEs) accompanied by solar flares, (ii) filament eruptions outside active regions, (iii) corotating interaction regions (CIRs) caused by high-speed streams from coronal holes, (iv) mixed events induced by two or more solar sources. Relationships of geomagnetic indices and parameters of cosmic rays and the solar wind are also compared between sporadic events with or without magnetic clouds (MCs) and between Solar Cycles (SCs) 23 and 24. The results reveal that interplanetary disturbances originated by AR-CMEs associated with an MC are most geoeffective and cause powerful geomagnetic storms, while CIRs create only moderate and weak storms. Sporadic and recurrent events differ in values of the Dst index and southward component of the magnetic field, as well as in the relationship between them. For sporadic events, geomagnetic activity is more affected by the presence or absence of an MC than by the type of solar source. Interplanetary disturbances associated with AR-CMEs are more effective in SC 23 while those associated with other types of solar sources have approximately the same geoeffectiveness in both SCs. [ABSTRACT FROM AUTHOR]

Here, we performed a statistical analysis of the parameters describing the transverse component of the longitudinal magnetic field gradient (∇ ⊥B z) in the active regions of the AR – the AR-averaged transverse component of the longitudinal magnetic field gradient <∇ ⊥B z >, the average transverse component of the longitudinal magnetic field gradient in the vicinity of a point with its maximum value , the maximum value of the transverse component of the longitudinal magnetic field gradient between pairs of spots in the AR max(∇ ⊥B z) sp. For calculating the denoted parameters, magnetograms of the Bz -components of the magnetic field vector at the level of the Sun's photosphere obtained by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) are used. The statistical sample contains data on 75 ARs. The values of the analyzed parameters obtained over the time of monitoring are compared with the level of flare productivity of the AR (the flare index FI). It is shown that: 1. The ⟨ ∇ ⊥ B z ¯ ⟩ -FI dependence (the overhead line denotes averaging over the time of monitoring the AR) is quadratic with a correlation coefficient k = 0.54 while the spread in values of the quantity ⟨ ∇ ⊥ B z ¯ ⟩ is small (lying within a a range of 0.08-0.12 G km-1 for the overwhelming majority of the studied regions) and differs very little for regions with low and high flare productivity, which may be explained by the dependence of ∇ ⊥B z on the area of the AR. 2. The dependence ⟨ max ∇ ⊥ B z ¯ ⟩ -FI is quasilinear with a correlation coefficient k = 0.61. 3. The dependence max ∇ ⊥ B z ¯ -FI is linear with a correlation coefficient k = 0.63. 4. The threshold values of the analyzed parameters are found: for ⟨ ∇ ⊥ B z ¯ ⟩ ≥ 0.078 G km-1, for ⟨ max ∇ ⊥ B z ¯ ⟩ ≥ 0.983 G km-1, and for the parameter max(∇ ⊥B z) sp ≥ 0.118G km-1. For lower values of the analyzed parameters, no x-ray flares were recorded in any of the ARs of the analyzed sample. [ABSTRACT FROM AUTHOR]

Solar flares – the most prominent manifestation of solar activity – typically manifest themselves as a single or a set of luminous arcs (magnetic flux tubes) rooted in regions of opposite polarity in the photosphere. However, a careful analysis of archival data from the Hinode satellite reveals occasional surprising cases of flaring arcs whose footpoints belong to regions of the same polarity or to areas without any appreciable magnetic field. Despite the counterintuitive nature of this phenomenon, it can be reasonably interpreted in the framework of the so-called 'topological model' of magnetic reconnection, whereby a magnetic null point is formed owing to a specific superposition of influences from remote sources rather than by local current systems. As a result, the energy release propagates along the separator of a flipping two-dome structure rather than along a fixed magnetic field line. Therefore, the luminous arc no longer needs to be associated immediately with the magnetic sources. Here, we report observational cases of the above-mentioned type and provide a theoretical model and numerical simulations. [ABSTRACT FROM AUTHOR]

It is widely accepted that X-ray emission in luminous active galactic nuclei (AGNs) originates from hot corona. To prevent the corona from overcooling by strong X-ray emission, steady heating to the corona is essential, for which the most promising mechanisms is the magnetic reconnection. Detailed studies of the coupled disc and corona, in the frame of magnetic field transferring accretion-released energy from the disc to the corona, reveal that the thermal electrons can only produce X-ray spectrum with |$\Gamma _{\rm 2-10\, keV}\gt 2.1$|⁠ , which is an inevitable consequence of the radiative coupling of the thermal corona and disc. In this work, we develop the magnetic-reconnection-heated corona model by taking into account the potential non-thermal electrons accelerated in the magnetic reconnection process, in addition to the thermal electrons. We show that the features of the structure and spectrum of the coupled disc and corona can be affected by the fraction of magnetic energy allocated to thermal electrons. Furthermore, we investigate the effects of the power-law index and energy range of non-thermal electrons and the magnetic field on the spectrum. It is found that the X-ray spectrum from the Comptonization of the hybrid electrons can be flatter than that from thermal electrons only, in agreement with observations. By comparing with the observed hard X-ray data, we suggest that a large fraction (⁠|$\gt 40~{{\ \rm per\ cent}}$|⁠) of the magnetic energy be allocated to the non-thermal electrons in the luminous and flat X-ray spectrum AGNs. [ABSTRACT FROM AUTHOR]

Chromospheric oscillations can give us insight into the physical environment in the solar atmosphere, both in quiet Sun and flaring conditions. Many authors have reported increases in the prevalence of 3-minute oscillations which are thought to be excited by events which impact the chromosphere such as flares. In this study, we utilized the Ca  ii 8542 Å line to study the oscillatory behaviour of the chromosphere in an active region which underwent two B-class flares. We analysed oscillations in both intensity and velocity, and found different behaviours in both. Intensity oscillations were most prevalent over the umbrae of sunspots and magnetic pores in the active region, and the extent of the area which contained significant oscillations was found to decrease when comparing times after the flares to before. By measuring the evolution of the magnetic field, we found that this could be because the areas surrounding the umbrae were becoming more 'penumbral' with an increase to the magnetic field inclination. Velocity oscillations were found across the active region both before and after the flares but were observed clearly in areas which were brightened by the second flare. By comparing to EUV imaging, it was seen that strong chromospheric velocity oscillations with 3–4-minute periods occurred at the same time and location as a flare loop cooling 30 min after the second flare peak. This could be evidence of disturbances in the loop exciting a response from the chromosphere at its acoustic cut-off frequency. [ABSTRACT FROM AUTHOR]

Solar eruptions, such as flares and coronal mass ejections, have serious impacts on the Earth's environment and human activity. Predicting the eruptions and therefore avoiding their damages is a main goal in the research community. Heliospheric current sheet (HCS), the boundary dividing the positive and negative magnetic fields that open up to the interplanetary space, is believed to be related with solar eruptions. The HCS consists of two types of boundaries: the Hale boundary and the non-Hale boundary, and the former is suggested in previous studies to be the region producing most of the solar flares. In this work, we explore relationships between Hale boundary, flares, and property of active regions (ARs). The HCS is determined from the magnetic field in the heliosphere calculated from the observed photospheric magnetic field using a potential field source surface model. We have analysed 8075 flares of C-class and above and 1652 ARs in the period from May 2010 to May 2019 in Solar Cycle 24. The following results are obtained: (1) 5957 flares (⁠|$74\pm 1~{{\ \rm per\ cent}}$|⁠) and 1212 ARs (⁠|$73\pm 2~{{\ \rm per\ cent}}$|⁠) are in the HCS; (2) among them, 70  per cent (⁠|$\pm 1~{{\ \rm per\ cent}}$|⁠) flares and 57  per cent (⁠|$\pm 3~{{\ \rm per\ cent}}$|⁠) ARs are in the Hale boundary; and (3) big ARs tend to emerge in the Hale boundary. We conclude that the HCS is related tightly to solar activity, and the Hale boundary is a region favoured to produce flares and to host large ARs. [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]

We present a detailed analysis of a partial eruption of a sigmoid filament lying along the polarity inversion line (PIL) of the small active region (AR) NOAA 12734 (with an area of 1.44 × 10 3 square megameters). The active filament was rooted in a dipole sunspot of the AR. The eruption was associated with a C1.3 flare and subsequent large-scale coronal disturbances. During its solar disk passage before the flare, the AR had the following characteristics: (1) Most of the time, the magnetic field lines in the AR showed a sigmoidal structure ('L1') in the low corona and arc-shaped loops (i.e., 'L2') in the upper atmosphere. (2) An 'X'-shaped structure was formed between the original 'S'-shaped magnetic loop ('L1') and the newly rising one ('L3') between the main positive and negative magnetic polarities of the sunspots, and the intersection point of flux ropes 'L1' and 'L3' corresponds well with the area where the initial extreme-ultraviolet (EUV) 1600 Å brightening of the flare occurred. (3) The AR disobeyed the hemispherical helicity rule and had magnetic twist and writhe of the same signs, i.e., its magnetic helicity/current helicity were positive in the northern hemisphere. (4) Sustained magnetic emergence and cancellation occurred before the flare. Therefore, the magnetic reconnection of highly twisted helical flux ropes under the confinement of the overlying magnetic fields is probably responsible for the partial eruption of the filament. [ABSTRACT FROM AUTHOR]

Rapid magnetic reconnection was experimentally observed to occur within a microsecond timescale, leading to the field-reversed configuration (FRC) comprising a single closed magnetic field structure. The compressibility of the plasma within the FRC played a crucial role in accelerating the coalescence process, making it considerably faster than the resistive process. A strong similarity between the observed experimental results and those of the coalescence of solar flare loops indicates that astrophysical phenomena and laboratory plasmas are governed by a common physics. [ABSTRACT FROM AUTHOR]

The main property of magnetic flux ropes in plasma is the shielding of electric currents. The second important property of force-free ropes is the need for external pressure, which prevents the loop from lateral expansion. With allowance for these factors, we calculated a force-free filament structure, in which the magnetic field on the axis significantly exceeds the external one. When a magnetic loop emerges into the rarefied solar atmosphere, the external pressure continuously drops, and at some limiting value, the longitudinal magnetic field of the loop turns to zero at the surface of the change of a sign of electric currents. This leads to a break in the azimuthal current on it and serves as a trigger for the excitation of plasma instability, the appearance of abnormal resistance with subsequent flare energy release and the generation of super-dreicer electric fields in elements of the thin magnetic structure with a diameter of ~100 km. Such fields can accelerate charged particles to hundreds of MeV on a path of several thousand km. The quasi-periodic mode of flare energy release in such a magnetic flux rope is discussed and a mechanism for ejection of accelerated particles from a loop magnetic trap upon entering the heliosphere is proposed. [ABSTRACT FROM AUTHOR]

We analyzed the dynamics of the reconnection and energy release processes of the X2.2 and X9.3 flares that occurred on September 6, 2017 in the active region NOAA 12673. We used SDO/AIA 1600 Å images together with SDO/HMI magnetograms. As a proxy for the flare energy release rate, we used the KRIM and RSTN microwave time profiles, GOES soft X-rays and its time derivative. Assuming that the chromospheric flare ribbons are located at the footpoints of magnetic field lines reconnecting in the corona and that the magnetic flux is conserved from the photosphere to the corona, we obtained magnetic reconnection fluxes and rates of its change. The cumulative positive and negative magnetic fluxes involved in the reconnection process were balanced. Temporal correlations are found between the calculated reconnection rate and the observed microwave emissions from for both events. An analysis of the total cumulative magnetic flux and SXR fluence of events showed that the magnitude of the magnetic flux involved in the reconnection process was greater in more energetic events and less in weaker ones. [ABSTRACT FROM AUTHOR]

For three events, the effect of eruptive processes in active regions on the characteristic of magnetic field oscillations in sunspot umbrae has been studied. Eruptive processes include solar flares and coronal mass ejections. The power spectra of oscillations of each analyzed parameter of the magnetic field in each sunspot of the studied active region (AR) are constructed, and, in general, for the entire AR. It was found that at certain stages the eruptive process has a noticeable effect on the power spectrum of oscillations of the magnetic field parameters in sunspot umbrae. It is shown that the power of the eruptive process, which is characterized by the X-ray flare, affects the features of the magnetic field oscillations in sunspot umbrae. The results obtained for ARs with eruptive processes are compared with the results for ARs without eruptive processes. The analysis of the influence of eruptive processes on the magnetic field oscillations in sunspot umbrae was carried out for the AR as a whole, and for some sunspots with the strongest response. For different events, the oscillation power spectrum was compared with the intensity maximum Amax, frequency fm, which accounts for the maximum Amax, and the frequency power spectrum width df. [ABSTRACT FROM AUTHOR]

In this paper we study the statistical relations between geomagnetic indices and the characteristics of cosmic rays and interplanetary disturbances for Forbush decreases associated with (a) coronal mass ejections from active regions accompanied by solar flares, (b) filament eruptions outside active regions, (c) high-speed streams from coronal holes, and (d) multiple sources. For sporadic Forbush decreases, the dependence of geomagnetic indices on cosmic ray and solar wind parameters in the presence or absence of a magnetic cloud is compared using statistical methods. The results show that (a) the highest geoeffectiveness is characteristic of interplanetary disturbances associated with solar matter eruptions from active regions in the presence of a magnetic cloud (and the lowest one for recurrent disturbances), (b) sporadic and recurrent events differ not only in the magnitude of geomagnetic indices and the southern component of the magnetic field but also in the nature of the relationship between them, (c) the geoeffectiveness of transient solar wind disturbances depends more strongly on the presence or absence of a magnetic cloud than on the type of solar source, and (d) for interplanetary disturbances associated with filament eruptions outside active regions, in the presence of a magnetic cloud, the geoeffectiveness depends only on the southern component of the magnetic field, while for other types of disturbances it depends on other solar wind parameters. [ABSTRACT FROM AUTHOR]

This paper is a study of the dynamics of the parameters describing the transverse component of the gradient of the longitudinal magnetic field ∇⊥Bz in active regions (AR) with different levels of flare productivity. Data obtained by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) has been used to analyze 75 ARs in the 24-th cycle of solar activity. ∇⊥Bz has been calculated using two approaches, modern and classical. In each case the parameters describing the quantity ∇⊥Bz in the AR are determined. For the modern approach, this includes the average of ∇⊥Bz over the AR, <∇⊥Bz> and the average value of ∇⊥Bz in the neighborhood of the point with its maximum value, ; for the classical approach, the maximum value of ∇⊥Bz between pairs of spots in the AR, . The dynamics of the chosen parameters are studied over the time of monitoring each of the regions of the analyzed sample. It is shown that: 1. the spread in values of ∇⊥Bz is small (for the overwhelming majority of studied regions it lies within a range of 0.08-0.12 G·km-1) and differs little for regions with low and high flare activity. 2. The numerical values of the parameter max(∇⊥Bz) and its dynamics in the overwhelming majority of examined cases are greater in regions with a higher level of flare activity. 3. The numerical values of the parameter max(∇⊥Bz) and its dynamics are greater in regions with higher levels of flare activity. 4. In the AR NOAA 11283 a stable rise in the magnitude of max (∇⊥Bz)sp was detected for approximately 19 h before the development of the first of a series of flares in high x-ray classes. [ABSTRACT FROM AUTHOR]

An actual problem today is the search for observed evidence of the existence of deep small-scale magnetic fields of the Sun. In this regard, the authors analyzed the theoretical criterion for separating the contributions to the solar surface magnetism of two qualitatively different mechanisms of a small-scale dynamo, the action of which is hidden in the depths of the solar convection zone (SCZ), proposed by Sokoloff and Khlystova [Astron. Nachr. 2010. 331. P. 82–87]. The first mechanism ensures the generation of small-scale magnetic fields due to the interaction of turbulent motions with the mean magnetic field (small-scale dynamo-1 of macroscopic MHD), while the second mechanism causes self-excitation of magnetic fluctuations due to turbulent pulsations of highly conductive plasma (diffusive small-scale dynamo-2 of classical MHD). The essence of the proposed criterion is that deep small-scale magnetic fields can lead under certain conditions to violations of Hale's and Joy's laws of observed magnetism on the surface of the Sun. Statistical analysis of these disturbances allows one to identify the differences in the evolution of the observed manifestations of two sources of small-scale fields since the contribution of two deep dynamo mechanisms to surface magnetism varies with the phase of the solar cycle in different ways. Such an important feature is the behavior of the percentage of anti-Hail groups of sunspots (in relation to the total number of sunspots) during the cycles. In the case of small-scale dynamo-1, the percentage of anti-Hale groups is independent of cycle phase, whereas the percentage of anti-Hale groups associated with diffusive small-scale dynamo-2 should reach its maximum value at solar minima. Therefore, the variations of magnetic anomalies make it possible to separate the meager contributions of two small-scale dynamo mechanisms to surface magnetism. In this connection, the task of identifying the markers of a small-scale dynamo in the solar depths from observations becomes relevant. With this in mind, we conducted an analysis of literature data of statistical studies of long series of observed violations of Hale's and Joy's laws, which can be caused by the presence of deep small-scale magnetic fluctuations of various origins. In particular, it was demonstrated in the work of Sokoloff, Khlystova, and Abramenko [Mon. Notic. Roy. Astron. Soc. 2015. 451. P. 1522–1527] on the basis of processing the data of different catalogs for the period 1917–2004 that the percentage of anti-Hale groups of spots increases during the minima of solar cycles. This testifies to the operation of a diffusive small-scale turbulent dynamo-2 within the SCZ, the efficiency of which becomes noticeable near the minima of the cycles, when the global toroidal magnetic field weakens. As a result of the authors' analysis of six magnetic active regions observed near the minima of the 24th and 25th solar cycles, characteristic violations of Hale's and Joy's laws were revealed, which may indicate the influence of a diffusive small-scale dynamo-2 on the evolution of these regions since it is this source that gives the most noticeable contribution in surface magnetism near cycles minima. [ABSTRACT FROM AUTHOR]

Impulsive solar energetic-particle (SEP) events were first distinguished as the streaming electrons that produce type III radio bursts as distinct from shock-induced type II bursts. They were then observed as the surprisingly enhanced 3He-rich SEP events, which were also found to have element enhancements rising smoothly with the mass-to-charge ratio A/Q through the elements, even up to Pb. These impulsive SEPs have been found to originate during magnetic reconnection in solar jets where open magnetic field lines allow energetic particles to escape. In contrast, impulsive solar flares are produced when similar reconnection involves closed field lines where energetic ions are trapped on closed loops and dissipate their energy as X-rays, γ-rays, and heat. Abundance enhancements that are power laws in A/Q can be used to determine Q values and hence the coronal source temperature in the events. Results show no evidence of heating, implying reconnection and ion acceleration occur early, rapidly, and at low density. Proton and He excesses that contribute their own power law may identify events with reacceleration of SEPs by shock waves driven by accompanying fast, narrow coronal mass ejections (CMEs) in many of the stronger jets. [ABSTRACT FROM AUTHOR]

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]

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]

We have analysed a solar event from 27 September 2021, which included a small GOES B-class flare, a compact and narrow-width CME, and radio type III bursts that appeared in groups. The long-duration, repeated metric type III burst emission indicates continuous electron acceleration at high altitudes. The flaring active region was surrounded by strong magnetic fields and large-scale loops, which guided the outflow of the CME plasmoid and hence the narrow, bullet-like appearance of the CME. Radio imaging and EUV observations confirmed the direction of particle propagation and the depletion of matter from the solar source region. We observed V-shaped type III burst emission lanes, which also explain the field configuration and suggest a possible location for repeated reconnection that occurred at a constant altitude. [ABSTRACT FROM AUTHOR]

Magnetic helicity has been used widely in the analysis and modelling of solar active regions. However, it is difficult to evaluate and interpret helicity in spherical geometry since coronal magnetic fields are rooted in the photosphere and helicity is susceptible to gauge choices. Recent work extended a geometrical definition of helicity from Cartesian to spherical domains, by interpreting helicity as the average, flux-weighted pairwise winding of magnetic-field lines. In this paper, by adopting the winding-based definition of helicity, we compute helicity and winding in spherical coordinates for SHARP (Spaceweather HMI Active Region Patches) magnetograms. This is compared with results obtained in Cartesian coordinates to quantitatively investigate the effect of spherical geometry. We find that the Cartesian approximations remain mostly valid, but for active regions with large spatial extents or strong field strengths (usually leading to flares and coronal mass ejections) there are significant deviations due to surface curvature that must be accounted for. [ABSTRACT FROM AUTHOR]

We simulate the evolution of reconnection in solar flares to study the influence of magnetic-field strength and thermal conduction on the dynamics of the magnetic-reconnection and energy-conversion processes. For this, we solve the 2.5D resistive magnetohydrodynamics (MHD) equations with thermal conduction on a domain that contains the chromosphere–corona interface. The flare is triggered at a null point where a Gaussian resistivity distribution is maximum, and further evolution is tracked. The parameter space considers magnetic-field strength [ B 0 ] between 22 G and 50 G, and thermal conductivity [ κ ] in the range from zero to 10 − 11 W m−1K − 7 / 2 . In this parameter space, we find that the magnetic field determines the reconnection rate, which can change by a 100% in the range of B 0 , whereas thermal conduction can induce a rate change of at most 10%. We also measure the evolution of magnetic, internal, and kinetic energies in a region just above the reconnection point and measure their interplay. For all simulations, magnetic energy dominates initially and relaxes on a time scale of about 20 seconds. In this interval, the magnetic energy drops by ≈ 50 % , whereas the internal energy grows by ≈ 100 % . During the process, part of the energy becomes kinetic, which pushes the reconnection jet upwards and is bigger for the bigger B 0 and smaller κ . [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]

The main goal of our research is to search for super-strong magnetic fields in active processes on the Sun. Our method is based on Stokes V spectro-polarimetry in a wide spectral range from −14 Å to +29 Å relative to the D3 He  i line. The object of the study is the area of a seismic source in the large solar flare on 2003 October 28 of the X17.2/4B class. The novelty of our study: we found characteristic spectral manifestations – secondary Stokes V peaks – far from D3 line, at distances of several angstroms. These secondary peaks have the following features: (a) their amplitudes reach 4 per cent – almost an order of magnitude more than previously detected in other flares, (b) in the general picture, the sign of the circular polarization changes when passing through the D3 centre, and (c) narrower spectral peculiarities were found at distances of +4.0, +6.3, and +8.5 Å, where the polarization sign also changes sharply. This unusual feature likely results from significant factors in the area of the seismic source, including substantial descending plasma velocities which exceed 400 km s−1 and, perhaps, super-strong magnetic fields. [ABSTRACT FROM AUTHOR]

The time evolution of high-energy synchrotron radiation generated in a relativistic pair plasma energized by reconnection of strong magnetic fields is investigated with 2D and 3D particle-in-cell (PIC) simulations. The simulations in this 2D/3D comparison study are conducted with the radiative PIC code OSIRIS, which self-consistently accounts for the synchrotron radiation reaction on the emitting particles, and enables us to explore the effects of synchrotron cooling. Magnetic reconnection causes compression of the plasma and magnetic field deep inside magnetic islands (plasmoids), leading to an enhancement of the flaring emission, which may help explain some astrophysical gamma-ray flare observations. Although radiative cooling weakens the emission from plasmoid cores, it facilitates additional compression there, further amplifying the magnetic field B and plasma density n , and thus partially mitigating this effect. Novel simulation diagnostics utilizing 2D histograms in the n - B space are developed and used to visualize and quantify the effects of compression. The n - B histograms are observed to be bounded by relatively sharp power-law boundaries marking clear limits on compression. Theoretical explanations for some of these compression limits are developed, rooted in radiative resistivity or 3D kinking instabilities. Systematic parameter-space studies with respect to guide magnetic field, system size, and upstream magnetization are conducted and suggest that stronger compression, brighter high-energy radiation, and perhaps significant quantum electrodynamic effects such as pair production, may occur in environments with larger reconnection-region sizes and higher magnetization, particularly when magnetic field strengths approach the critical (Schwinger) field, as found in magnetar magnetospheres. [ABSTRACT FROM AUTHOR]

Magnetic fields are responsible for a multitude of solar phenomena, including potentially destructive events such as solar flares and coronal mass ejections, with the number of such events rising as we approach the peak of the 11-year solar cycle in approximately 2025. High-precision spectropolarimetric observations are necessary to understand the variability of the Sun. The field of quantitative inference of magnetic field vectors and related solar atmospheric parameters from such observations has been investigated for a long time. In recent years, very sophisticated codes for spectropolarimetric observations have been developed. Over the past two decades, neural networks have been shown to be a fast and accurate alternative to classic inversion methods. However, most of these codes can be used to obtain point estimates of the parameters, so ambiguities, degeneracies, and uncertainties of each parameter remain uncovered. In this paper, we provide end-to-end inversion codes based on the simple Milne-Eddington model of the stellar atmosphere and deep neural networks to both parameter estimation and their uncertainty intervals. The proposed framework is designed in such a way that it can be expanded and adapted to other atmospheric models or combinations of them. Additional information can also be incorporated directly into the model. It is demonstrated that the proposed architecture provides high accuracy results, including a reliable uncertainty estimation, even in the multidimensional case. The models are tested using simulations and real data samples. [ABSTRACT FROM AUTHOR]

We present an analysis of the pre-limb eruptive X4.9 solar flare on February 25, 2014, by means of which we confirm a hypothesis of the two-stage energy release corresponding to two magnetic reconnection regimes in the flare impulsive phase. This flare is selected, firstly, because of its morphological peculiarities suggesting the presence of the two energy release stages. Secondly, the flare was very suitably located near the solar limb and it was well-observed by many instruments. We performed an analysis of multiwavelength observational data of this flare region to find a connection between changes of the photospheric magnetic field, morphology of hard and soft X-ray sources, dynamics of the photospheric optical emission sources, metric radio bursts, and kinematics of an eruptive structure. The simultaneous usage of the line-of-sight and vector Helioseismic Magnetic Imager (HMI) magnetograms allowed us to trace magnetic field changes during the flare impulsive phase with high temporal resolution. HMI filtergrams allowed to trace displacement of the photospheric emission sources, associated with the magnetic reconnection, with very high temporal resolution up to 2 s. Using all observational results, we argue that the found flare stages are characterized by the following magnetic reconnection regimes. The first stage is predominantly characterized by the three-dimensional zipping reconnection in the strong sheared magnetic field assuming the tether-cutting geometry. The second stage corresponds to the so-called "standard" model of eruptive flares with the quasi-two-dimensional reconnection below the eruptive flux-rope. All observational peculiarities of these two stages are discussed in details. [ABSTRACT FROM AUTHOR]

During 2017, when the Sun was moving toward the minimum phase of solar cycle 24, an exceptionally eruptive active region (AR) NOAA 12673 emerged on the Sun during August 28-September 10. During the highest activity level, the AR turned into a δ-type sunspot region, which manifests the most complex configuration of magnetic fields from the photosphere to the coronal heights. The AR 12673 produced four X-class and 27 M-class flares, along with numerous C-class flares, making it one of the most powerful ARs of solar cycle 24. Notably, it produced the largest flare of solar cycle 24, namely, the X9.3 event on 2017 September 6. In this work, we highlight the results of our comprehensive analysis involving multi-wavelength imaging and coronal magnetic field modeling to understand the evolution and eruptivity from AR 12673. We especially focus on the morphological, spectral and kinematical evolution of the two X-class flares on 6 September 2017. We explore various large- and small-scale magnetic field structures of the active region which are associated with the triggering and subsequent outbursts during the powerful solar transients. [ABSTRACT FROM AUTHOR]

Magnetic reconnection between neighboring magnetic field loops, the so-called interloop reconnection, is a common process to drive flares in the solar atmosphere. However, there is no direct evidence that a similar but less explosive process can take place on planets. The strong crustal fields on Mars generate plenty of magnetic loops in the near-Mars regions, providing a unique environment to research the interloop reconnection on a planet. Here, we report magnetic reconnection events between crustal field loops in the Martian ionosphere observed by Mars Atmosphere and Volatile EvolutioN (MAVEN) for the first time. During the current layer crossing, MAVEN recorded the characteristic signals of collisionless magnetic reconnection, including the Hall magnetic field, Alfvénic outflow, and electron energization. This finding implies that the interloop reconnection in the Martian ionosphere could contribute to the localized energy deposition and particle energization, which provides the seed source for aurora in the Martian atmosphere. [ABSTRACT FROM AUTHOR]

Supra-arcade downflows (SADs) are dark voids descending towards the post-reconnection flare loops and exhibit obvious variation in properties like width. However, due to the lack of further statistical studies, the mechanism behind such variations hitherto remains elusive. Here, we statistically investigated widths of 81 SADs observed in one flare by the Solar Dynamics Observatory (SDO). For each of SADs, six moments were selected with equal time intervals to measure their widths at different stages of their evolution. It is found that most SADs show a roughly monotonous width decrease during their descents, while some SADs with small initial widths can have complex evolutions. 3D reconstruction results based on SDO and Solar Terrestrial Relations Observatory Ahead (STEREO-A) images and thermal properties analysis reveal that differences in magnetic and plasma environments may result in that SADs in the north are overall wider than those in the south. Additionally, correlation analysis between the width and other parameters of SADs was further conducted and revealed that (1) SADs with different initial widths show no significant differences in their temperature and density evolution characteristics; (2) SADs with small initial widths usually appear in lower heights, where more frequent collisions between SADs could lead to their intermittent acceleration, width increment, and curved trajectories. These results indicate that SADs with different initial widths are produced the same way, while different environments (magnetic field or plasma) could affect their subsequent width evolutions. [ABSTRACT FROM AUTHOR]

Solar activity is often caused by the evolution of solar magnetic fields. Magnetic field parameters derived from photospheric vector magnetograms of solar active regions (ARs) have been used to analyze and forecast eruptive events, such as solar flares and coronal mass ejections. Unfortunately, the most recent Solar Cycle 24 was relatively weak with few large flares, though it is the only solar cycle in which consistent time-sequence vector magnetograms have been available through the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) since its launch in 2010. In this work, we look into another major instrument, namely the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO) from 1996 to 2010. The data archive of SOHO/MDI covers a more active Solar Cycle 23 with many large flares. However, SOHO/MDI only has line-of-sight (LOS) magnetograms. We propose a new deep learning method, named MagNet, to learn from combined LOS magnetograms, B x and B y , taken by SDO/HMI, along with H α observations collected by the Big Bear Solar Observatory (BBSO), and to generate synthetic vector components B x ′ and B y ′ of ARs. These generated vector components, together with observational LOS data, would form vector magnetograms for SOHO/MDI. In this way, we can expand the availability of vector magnetograms to the period from 1996 to present. Experimental results demonstrate the good performance of the MagNet method. To our knowledge, this is the first time that deep learning has been used to generate photospheric vector magnetograms of ARs for SOHO/MDI using SDO/HMI and H α data. [ABSTRACT FROM AUTHOR]

In this paper, we reanalyze the X1.7 class limb flare that occurred on 2013 May 13 (SOL2013-05-13T01:56 UT), concentrating on the energy-releasing process using microwave observations mainly made by Nobeyama and X-ray observations made by RHESSI. The analysis was carried out in the context of EUV observations made by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory. First, we complement the initiation process by showing that the initiation occurred together with material falling from a large-scale overlying prominence, a signature of drainage instability. The usual downward and upward motions of the microwave and X-ray sources are observed from their evolution. However, the microwave source's height shows a recurrent decrease and increase during its overall upward motion; it shows a kind of recurrent contraction and expansion. The time period of the recurrent contraction and expansion corresponds to the period of post-contraction oscillation of EUV loops, and the oscillatory motions are closely correlated with four microwave/hard X-ray peaks that unusually increased nonthermal emission levels by several times. X-ray spectra get hardened during the oscillation. In addition, the rapid contraction of magnetic loops located on the outside of the erupting flux rope occurs 5 minutes after the onset of the flare, showing that the contraction of the peripheral magnetic loops is more likely due to the vortex and sink flows generated by an upward erupting magnetic flux rope rather than a coronal implosion. The results can provide more insight into the physics of dynamic coronal magnetic field and particle acceleration during solar flares. [ABSTRACT FROM AUTHOR]

Solar flares result from the sudden release of energy deposited by subphotospheric motions into the magnetic field of the corona. The deposited energy accumulates secularly between events. One may interpret the observed event statistics as resulting from a state-dependent Poisson process in which the instantaneous flare rate is a function of the stress in the system and a flare becomes certain as the stress approaches a threshold set by the microphysics of the flare trigger. If the system is driven fast, and if the threshold is static and uniform globally, a cross-correlation is predicted between the size of a flare and the forward waiting time to the next flare. This cross-correlation is broadly absent from the Geostationary Operational Environmental Satellite (GOES) soft X-ray flare database. One also predicts higher cross-correlations in active regions where the shapes of the waiting time and size distributions match. Again, there is no evidence for such an association in the GOES data. The data imply at least one of the following: (i) the threshold at which a flare is triggered varies in time; (ii) the rate at which energy is driven into active regions varies in time; (iii) historical flare catalogs are incomplete; or (iv) the description of solar flares as resulting from a buildup and release of energy, once a threshold is reached, is incomplete. [ABSTRACT FROM AUTHOR]

Flare frequency distributions represent a key approach to addressing one of the largest problems in solar and stellar physics: determining the mechanism that counterintuitively heats coronae to temperatures that are orders of magnitude hotter than the corresponding photospheres. It is widely accepted that the magnetic field is responsible for the heating, but there are two competing mechanisms that could explain it: nanoflares or Alfvén waves. To date, neither can be directly observed. Nanoflares are, by definition, extremely small, but their aggregate energy release could represent a substantial heating mechanism, presuming they are sufficiently abundant. One way to test this presumption is via the flare frequency distribution, which describes how often flares of various energies occur. If the slope of the power law fitting the flare frequency distribution is above a critical threshold, α = 2 as established in prior literature, then there should be a sufficient abundance of nanoflares to explain coronal heating. We performed >600 case studies of solar flares, made possible by an unprecedented number of data analysts via three semesters of an undergraduate physics laboratory course. This allowed us to include two crucial, but nontrivial, analysis methods: preflare baseline subtraction and computation of the flare energy, which requires determining flare start and stop times. We aggregated the results of these analyses into a statistical study to determine that α = 1.63 ± 0.03. This is below the critical threshold, suggesting that Alfvén waves are an important driver of coronal heating. [ABSTRACT FROM AUTHOR]

In this paper, we study the evolution of the X5.4 flare (SOL2012-03-07T00:02) in NOAA Active Region 11429, focusing on its initiation mechanisms and back-reaction effects. To help our study, three-dimensional (3D) coronal magnetic field models are extrapolated from the photospheric magnetograms of the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory under the assumptions of nonlinear force-free field (NLFFF) and non-force-free field (non-FFF). We investigate the 3D magnetic structure and MHD kink instability, torus instability, and double-arc instability (DAI), and find that this flare is most likely triggered by the tether-cutting reconnection and the subsequent DAI. For the back-reactions of the flare, both NLFFF and non-FFF models clearly show an increase in horizontal magnetic field (B h ) and a decrease in inclination angle (ϕ) of the magnetic field near the polarity inversion line, from the photosphere up to a certain height (5 Mm and 8 Mm for non-FFF and NLFFF, respectively). In addition, the non-FFF model shows an enhancement of the downward Lorentz force acting on the photosphere, and the location of the enhancement spatially coincides with the location of the flare onset. The observed back-reaction is likely a consequence of magnetic reconnection. [ABSTRACT FROM AUTHOR]