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Small-scale brightenings (SBs) are commonly observed in the transition region (TR) that separates the solar chromosphere from the corona. These brightenings, omnipresent in active region patches known as "moss" regions, could potentially contribute to the heating of active region plasma. In this study, we investigate the properties of SB events in a moss region and their associated chromospheric dynamics, which could provide insights into the underlying generation mechanisms of the SBs. We analyzed the data sets obtained by coordinated observations using the Interface Region Imaging Spectrograph and the Goode Solar Telescope at Big Bear Solar Observatory. We studied 131 SB events in our region of interest and found that 100 showed spatial and temporal matches with the dynamics observed in the chromospheric H α images. Among these SBs, 98 of them were associated with spicules that are observed in H α images. Furthermore, detailed analysis revealed that one intense SB event corresponded to an Ellerman bomb (EB), while another SB event consisted of several recurring brightenings caused by a stream of falling plasma. We observed that H α far wings often showed flashes of strong brightening caused by the falling plasma, creating an H α spectral profile similar to an EB. However, 31 of the 131 investigated SB events showed no noticeable spatial and temporal matches with any apparent features in H α images. Our analysis indicated that the predominant TR SB events in moss regions are associated with chromospheric phenomena primarily caused by spicules. Most of these spicules display properties akin to dynamic fibrils. [ABSTRACT FROM AUTHOR]

Minifilaments are widespread small-scale structures in the solar atmosphere. To better understand their formation and eruption mechanisms, we investigate the entire life of a sigmoidal minifilament located below a large quiescent filament observed by Big Bear Solar Observatory/Goode Solar Telescope on 2015 August 3. The H α structure initially appears as a group of arched threads, then transforms into two J-shaped arcades, and finally forms a sigmoidal shape. Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly observations in 171 Å show that two coronal jets occur around the southern footpoint of the minifilament before the minifilament eruption. The minifilament eruption starts from the southern footpoint, then interacts with the overlying filament and fails. The aforementioned observational changes correspond to three episodes of flux cancellations observed by SDO/Helioseismic and Magnetic Imager. Unlike previous studies, the flux cancellation occurs between the polarity where the southern footpoint of the minifilament is rooted and an external polarity. We construct two magnetic field models before the eruption using the flux rope insertion method and find a hyperbolic flux tube above the flux cancellation site. The observation and modeling results suggest that the eruption is triggered by the external magnetic reconnection between the core field of the minifilament and the external fields due to flux cancellations. This study reveals a new triggering mechanism for minifilament eruptions and a new relationship between minifilament eruptions and coronal jets. [ABSTRACT FROM AUTHOR]

We consider small-scale jetlike events that might make the solar wind, as has been suggested in recent studies. We show that the events referred to as "coronal jets" and as "jetlets" both fall on a power-law distribution that also includes large-scale eruptions and spicule-sized features; all of the jetlike events could contribute to the solar wind. Based on imaging and magnetic field data, it is plausible that many or most of these events might form by the same mechanism: Magnetic flux cancelation produces small-scale flux ropes, often containing a cool-material minifilament. This minifilament/flux rope erupts and reconnects with adjacent open coronal field, along which "plasma jets" flow and contribute to the solar wind. The erupting flux ropes can contain twist that is transferred to the open field, and these become Alfvénic pulses that form magnetic switchbacks, providing an intrinsic connection between switchbacks and the production of the solar wind. [ABSTRACT FROM AUTHOR]

Magnetic reconnection in the lower atmosphere is a critical process in determining the chromospheric dynamics, such as Ellerman bombs and UV bursts. Because the heating of the atmosphere significantly depends on the ionization degree and plasma β, which varies with height, it is essential to diagnose the height at which the magnetic reconnection takes place. Multiwavelength spectropolarimetry is a powerful solution to fulfill this requirement. We verify the diagnostic capabilities and usefulness of near-infrared multiwavelength spectropolarimetric observations for understanding magnetic reconnection phenomena by synthesizing the Stokes vector from a realistic magnetohydrodynamic simulation. The analysis considers two magnetic reconnection regions occurring at different heights. In the case of magnetic reconnection at low altitude, both red- and blueshifted components originating from reconnection bidirectional flow are identified in the photospheric lines, Fe i 8468 Å, K i 7664 Å, and K i 7698 Å. In the case of magnetic reconnection at high altitudes, chromospheric lines, Ca ii 8498 Å and 8542 Å, show emission due to the heating that occurs at the upper part of the formation layer. These results suggest that multiwavelength spectropolarimetric observations are capable of distinguishing the height where magnetic reconnection occurs. [ABSTRACT FROM AUTHOR]

We present a new simulation setup using the MURaM radiative MHD code that allows the study of the formation of collisional polarity inversion lines (cPILs) in the photosphere and the coronal response including flares. In this scheme, we start with a bipolar sunspot configuration and set the spots on collision course by imposing the appropriate velocity field at the footpoints in the subphotospheric boundary. We produce different setups with the same initial spot separation by varying physical parameters such as the collision speed and minimum collision distance. While all setups lead to the formation of an EUV and X-ray sigmoid structure, only the cases with a close passing of the spots cause flares and mass eruptions. The energy release is in the 1–2 × 1031 erg range, putting the simulated flares into the upper C-class to lower M-class range of GOES X-ray 1–8 Å flux. While the setup with the more distant passing of the spots does not lead to a flare, the corona is nonetheless substantially heated, suggesting noneruptive energy-release mechanisms. We focus our discussion on two particular setups that differ in spot coherence and resulting cPIL length persistence. We find different timings in the transition from a sheared magnetic arcade to magnetic flux rope (MFR); the setup with a large length but shorter duration cPIL produces a MFR during the eruption, while the MFR is preexisting in the setup with a large length and longer duration cPIL. While both result in flares of comparable strength and the eruption of a coronal mass ejection, the setup with preexisting MFR (and embedded filament) leads to an MFR eruption with a larger mass content. [ABSTRACT FROM AUTHOR]

Sunspot light bridges (LBs) exhibit a wide range of short-lived phenomena in the chromosphere and transition region. In contrast, we use here data from the Multi-Application Solar Telescope (MAST), the Interface Region Imaging Spectrograph (IRIS), Hinode, the Atmospheric Imaging Assembly (AIA), and the Helioseismic and Magnetic Imager (HMI) to analyze the sustained heating over days in an LB in a regular sunspot. Chromospheric temperatures were retrieved from the MAST Ca ii and IRIS Mg ii lines by nonlocal thermodynamic equilibrium inversions. Line widths, Doppler shifts, and intensities were derived from the IRIS lines using Gaussian fits. Coronal temperatures were estimated through the differential emission measure, while the coronal magnetic field was obtained from an extrapolation of the HMI vector field. At the photosphere, the LB exhibits a granular morphology with field strengths of about 400 G and no significant electric currents. The sunspot does not fragment, and the LB remains stable for several days. The chromospheric temperature, IRIS line intensities and widths, and AIA 171 and 211 Å intensities are all enhanced in the LB with temperatures from 8000 K to 2.5 MK. Photospheric plasma motions remain small, while the chromosphere and transition region indicate predominantly redshifts of 5–20 km s−1 with occasional supersonic downflows exceeding 100 km s−1. The excess thermal energy over the LB is about 3.2 × 1026 erg and matches the radiative losses. It could be supplied by magnetic flux loss of the sunspot (7.5 × 1027 erg), kinetic energy from the increase in the LB width (4 × 1028 erg), or freefall of mass along the coronal loops (6.3 × 1026 erg). [ABSTRACT FROM AUTHOR]

We describe a new automated technique for active region emergence in coronal magnetic field models, based on the inversion of the electric field locally from a single line-of-sight magnetogram for each region. The technique preserves the arbitrary shapes of magnetic field distribution associated with individual active regions and incorporates emerging magnetic helicity (twist) in a parametrized manner through a noninductive electric field component. We test the technique with global magnetofrictional simulations of the coronal magnetic field during Solar Cycle 24 Maximum from 2011 June 1 to 2011 December 31. The active regions are determined in a fully automated and objective way using Spaceweather HMI Active Region Patch (SHARP) data. Our primary aim is to constrain two free parameters in the emergence algorithm: the duration of emergence and the twist parameter for each individual active region. While the duration has a limited effect on the resulting coronal magnetic field, changing the sign and amplitude of the twist parameters profoundly influences the amount of nonpotentiality generated in the global coronal magnetic field. We explore the possibility of constraining both the magnitude and sign of the twist parameter using estimates of the current helicity derived from vector magnetograms and supplied in the SHARP metadata for each region. Using the observed sign of twist for each region reduces the overall nonpotentiality in the corona, highlighting the importance of scatter in the emerging active region helicities. [ABSTRACT FROM AUTHOR]

In the solar corona, the free energy, i.e., the excess in magnetic energy over a ground-state potential field, forms the reservoir of energy that can be released during solar flares and coronal mass ejections. Such free energy provides a measure of the magnetic field nonpotentiality. Recent theoretical and observational studies indicate that the presence of nonpotential magnetic fields is imprinted into the structures of infrared, off-limb, coronal polarization. In this paper, we investigate the possibility of exploiting such observations for mapping and studying the accumulation and release of coronal free magnetic energy, with the goal of developing a new tool for identifying "hot spots" of coronal free energy such as those associated with twisted and/or sheared coronal magnetic fields. We applied forward modeling of infrared coronal polarimetry to three-dimensional models of nonpotential and potential magnetic fields. From these we defined a quantitative diagnostic of nonpotentiality that in the future could be calculated from a comparison of infrared, off-limb, coronal polarization observations from, e.g., the Coronal Multi-channel Polarimeter or the Daniel K. Inouye Solar Telescope, and the corresponding polarization signal forward-modeled from a potential field extrapolated from photospheric magnetograms. We considered the relative diagnostic potential of linear and circular polarization, and the sensitivities of these diagnostics to coronal density distributions and assumed boundary conditions of the potential field. Our work confirms the capacity of polarization measurements for diagnosing nonpotentiality and free energy in the solar corona. [ABSTRACT FROM AUTHOR]

Using Hα line core and off-band imaging data from the New Vacuum Solar Telescope in China, we present a high-resolution observation on the entire life cycle of a solar minifilament from its birth to its final eruption. We find that the minifilament originates from a series of cascade-like reconfigurations of chromospheric fine structures. During which, owing to strong photospheric shearing and converging flows near its polarity inversion line, basic short chromospheric fibrils first slowly coalesce to elongated dark threads, and then further create a longer filament channel in a "head-to-tail" linkage scenario. In this course, obvious magnetic flux cancelation simultaneously proceeds below it, and further facilitates its destabilization. In its onset phase, clear clues indicate that the minifilament first starts to rise without brightening signals; instead, after a slow-to-fast acceleration, obvious runaway reconnection soon takes over its final jet-like eruption. Besides, off-band observations further reveal that the formed minifilament has a possible flux-rope configuration, and chromospheric upflows that detected in its early forming phase persistently supplies cool plasma into its channel. This observation is consistent with earlier observations and supports the view that both miniature and large-scale filaments may share analogous formation and destabilization mechanisms. [ABSTRACT FROM AUTHOR]

We study the evolution of a minifilament eruption in a quiet region at the center of the solar disk and its impact on the ambient atmosphere. We used high spectral resolution imaging spectroscopy in Hα acquired by the echelle spectrograph of the Vacuum Tower Telescope, Tenerife, Spain; photospheric magnetic field observations from the Helioseismic Magnetic Imager; and UV/EUV imaging from the Atmospheric Imaging Assembly of the Solar Dynamics Observatory. The Hα line profiles were noise-stripped using principal component analysis and then inverted to produce physical and cloud model parameter maps. The minifilament formed between small-scale, opposite-polarity magnetic features through a series of small reconnection events, and it erupted within an hour after its appearance in Hα. Its development and eruption exhibited similarities to large-scale erupting filaments, indicating the action of common mechanisms. Its eruption took place in two phases, namely, a slow rise and a fast expansion, and it produced a coronal dimming, before the minifilament disappeared. During its eruption, we detected a complicated velocity pattern, indicative of a twisted, thread-like structure. Part of its material returned to the chromosphere, producing observable effects on nearby low-lying magnetic structures. Cloud model analysis showed that the minifilament was initially similar to other chromospheric fine structures, in terms of optical depth, source function, and Doppler width, but it resembled a large-scale filament on its course to eruption. High spectral resolution observations of the chromosphere can provide a wealth of information regarding the dynamics and properties of minifilaments and their interactions with the surrounding atmosphere. [ABSTRACT FROM AUTHOR]

In this study we determine scaling relationships of observed solar flares that can be used to predict upper limits of the Geostationary Orbiting Earth Satellite (GOES)–class magnitude of solar flares. The flare prediction scheme is based on the scaling of the slowly varying potential energy Ep(t), which is extrapolated in time over an interval of Δt ≤ 24 hr. The observed scaling of the dissipated energy Ediss scales with the potential field energy as. In addition, the observed scaling relationship of the flare volume, , the multi-thermal energy, Eth ∝ V0.76, the flare emission measure , the EM-weighted temperature Tw, and the GOES flux, , allows us then to predict an upper limit of the GOES-class flare magnitude in the extrapolated time window. We find a good correlation (cross-correlation coefficient (CCC) ≈ 0.7) between the observed and predicted GOES-class flare magnitudes (in 172 X- and M-class events). This is the first algorithm that employs observed scaling laws of physical flare parameters to predict GOES flux upper limits, an important capability that complements previous flare prediction methods based on machine-learning algorithms used in space-weather forecasting. [ABSTRACT FROM AUTHOR]

We suggest a generalized definition of self-organized criticality (SOC) systems: SOC is a critical state of a nonlinear energy dissipation system that is slowly and continuously driven toward a critical value of a system-wide instability threshold, producing scale-free, fractal-diffusive, and intermittent avalanches with power law-like size distributions. We develop here a macroscopic description of SOC systems that provides an equivalent description of the complex microscopic fine structure, in terms of fractal-diffusive transport (FD-SOC). Quantitative values for the size distributions of SOC parameters (length scales L, time scales T, waiting times Δt, fluxes F, and fluences or energies E) are derived from first principles, using the scale-free probability conjecture, N(L)dL∝L–d, for Euclidean space dimension d. We apply this model to astrophysical SOC systems, such as lunar craters, the asteroid belt, Saturn ring particles, magnetospheric substorms, radiation belt electrons, solar flares, stellar flares, pulsar glitches, soft gamma-ray repeaters, black-hole objects, blazars, and cosmic rays. The FD-SOC model predicts correctly the size distributions of 8 out of these 12 astrophysical phenomena, and indicates non-standard scaling laws and measurement biases for the others. [ABSTRACT FROM AUTHOR]

We study a granular-sized magnetic flux emergence event that occurred in NOAA 11024 in 2009 July. The observations were made with the CRISP spectropolarimeter at the Swedish 1 m Solar Telescope achieving a spatial resolution of 0.″14. Simultaneous full Stokes observations of the two photospheric Fe I lines at 630.2 nm and the chromospheric Ca II 854.2 nm line allow us to describe in detail the emergence process across the solar atmosphere. We report here on three-dimensional (3D) semi-spherical bubble events, where instead of simple magnetic footpoints, we observe complex semi-circular feet straddling a few granules. Several phenomena occur simultaneously, namely, abnormal granulation, separation of opposite-polarity legs, and brightenings at chromospheric heights. However, the most characteristic signature in these events is the observation of a dark bubble in filtergrams taken in the wings of the Ca II 854.2 nm line. There is a clear coincidence between the emergence of horizontal magnetic field patches and the formation of the dark bubble. We can infer how the bubble rises through the solar atmosphere as we see it progressing from the wings to the core of Ca II 854.2 nm. In the photosphere, the magnetic bubble shows mean upward Doppler velocities of 2 km s–1 and expands at a horizontal speed of 4 km s–1. In about 3.5 minutes it travels some 1100 km to reach the mid chromosphere, implying an average ascent speed of 5.2 km s–1. The maximum separation attained by the magnetic legs is 6.″6. From an inversion of the observed Stokes spectra with the SIR code, we find maximum photospheric field strengths of 480 G and inclinations of nearly 90° in the magnetic bubble interior, along with temperature deficits of up to 250 K at log τ = –2 and above. To aid the interpretation of the observations, we carry out 3D numerical simulations of the evolution of a horizontal, untwisted magnetic flux sheet injected in the convection zone, using the Bifrost code. The computational domain spans from the upper convection zone to the lower corona. In the modeled chromosphere, the rising flux sheet produces a large, cool, magnetized bubble. We compare this bubble with the observed ones and find excellent agreement, including similar field strengths and velocity signals in the photosphere and chromosphere, temperature deficits, ascent speeds, expansion velocities, and lifetimes. [ABSTRACT FROM AUTHOR]

In this work we investigate the properties of penumbral transients observed in the upper photospheric and chromospheric region above a sunspot penumbra using two-dimensional spectroscopic observations of the Ca II 854.21 nm line with a 5 s cadence. In our 30 minutes of observations, we identify several penumbral-micro jets (PMJs) with cotemporal observations from Dunn Solar Telescope/IBIS and Hinode/SOT. We find that the line profiles of these PMJ events show emission in the two wings of the line (±0.05 nm), but little modification of the line core. These are reminiscent of the line profiles of Ellerman bombs observed in plage and network regions. Furthermore, we find evidence that some PMJ events have a precursor phase starting 1 minute prior to the main brightening that might indicate initial heating of the plasma prior to an acoustic or bow shock event. With the IBIS data, we also find several other types of transient brightenings with timescales of less than 1 minute that are not clearly seen in the Hinode/SOT data. The spectral profiles and other characteristics of these events are significantly different from those of PMJs. The different appearances of all these transients are an indicator of the general complexity of the chromospheric magnetic field and underscore the highly dynamic behavior above sunspots. It also highlights the care that is needed in interpreting broadband filter images of chromospheric lines, which may conceal very different spectral profiles, and the underlying physical mechanisms at work. [ABSTRACT FROM AUTHOR]

The presence of photospheric magnetic reconnection has long been thought to give rise to short and impulsive events, such as Ellerman bombs (EBs) and Type II spicules. In this article, we combine high-resolution, high-cadence observations from the Interferometric BIdimensional Spectrometer and Rapid Oscillations in the Solar Atmosphere instruments at the Dunn Solar Telescope, National Solar Observatory, New Mexico, with co-aligned Solar Dynamics Observatory Atmospheric Imaging Assembly and Hinode Solar Optical Telescope (SOT) data to observe small-scale events situated within an active region. These data are then compared with state-of-the-art numerical simulations of the lower atmosphere made using the MURaM code. It is found that brightenings, in both the observations and the simulations, of the wings of the Hα line profile, interpreted as EBs, are often spatially correlated with increases in the intensity of the Fe I λ6302.5 line core. Bipolar regions inferred from Hinode/SOT magnetic field data show evidence of flux cancellation associated, co-spatially, with these EBs, suggesting that magnetic reconnection could be a driver of these high-energy events. Through the analysis of similar events in the simulated lower atmosphere, we are able to infer that line profiles analogous to the observations occur co-spatially with regions of strong opposite-polarity magnetic flux. These observed events and their simulated counterparts are interpreted as evidence of photospheric magnetic reconnection at scales observable using current observational instrumentation. [ABSTRACT FROM AUTHOR]

To better understand a preferred magnetic field configuration and its evolution during coronal mass ejection (CME) events, we investigated the spatial and temporal evolution of photospheric magnetic fields in the active region NOAA 9236 that produced eight flare-associated CMEs during the time period of 2000 November 23-26. The time variations of the total magnetic helicity injection rate and the total unsigned magnetic flux are determined and examined not only in the entire active region but also in some local regions such as the main sunspots and the CME-associated flaring regions using SOHO/MDI magnetogram data. As a result, we found that (1) in the sunspots, a large amount of positive (right-handed) magnetic helicity was injected during most of the examined time period, (2) in the flare region, there was a continuous injection of negative (left-handed) magnetic helicity during the entire period, accompanied by a large increase of the unsigned magnetic flux, and (3) the flaring regions were mainly composed of emerging bipoles of magnetic fragments in which magnetic field lines have substantially favorable conditions for making reconnection with large-scale, overlying, and oppositely directed magnetic field lines connecting the main sunspots. These observational findings can also be well explained by some MHD numerical simulations for CME initiation (e.g., reconnection-favored emerging flux models). We therefore conclude that reconnection-favored magnetic fields in the flaring emerging flux regions play a crucial role in producing the multiple flare-associated CMEs in NOAA 9236. [ABSTRACT FROM AUTHOR]

In previous works, we introduced a nonlinear force-free method that self-consistently calculates the instantaneous budgets of freemagnetic energy and relative magnetic helicity in solar active regions (ARs). Calculation is expedient and practical, using only a single vector magnetogram per computation. We apply this method to a time series of 600 high-cadence vector magnetograms of the eruptive NOAA AR 11158 acquired by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory over a five-day observing interval. Besides testing our method extensively, we use it to interpret the dynamical evolution in the AR, including eruptions. We find thatthe AR builds large budgets of both free magnetic energy and relative magnetic helicity, sufficient to power many more eruptions than the ones it gave within the interval of interest. For each of these major eruptions, we finderuption-related decreases and subsequent free-energy and helicity budgets that are consistent with the observed eruption (flare and coronal mass ejection (CME)) sizes. In addition, we find that (1) evolution in the AR is consistent with the recently proposed (free) energy-(relative) helicity diagram of solar ARs, (2) eruption-related decreases occur before the flare and the projected CME-launch times, suggesting that CME progenitors precede flares, and(3) self terms of free energy and relative helicity most likely originate from respective mutual terms, following a progressive mutual-to-self conversion pattern that most likely stems from magnetic reconnection. This results in the non-ideal formation of increasingly helical pre-eruption structures and instigates further research on the triggeringof solar eruptions with magnetic helicity firmly placed in the eruption cadre. [ABSTRACT FROM AUTHOR]

We present results from a study of 54 polar X-ray jets that were observed in coronal X-ray movies from the X-ray Telescope on Hinode and had simultaneous coverage in movies of the cooler transition region (T ∼ 105 K) taken in the He II 304Å band of the Atmospheric Imaging Assembly (AIA) on Solar Dynamics Observatory. These dual observations verify the standard-jet/blowout-jet dichotomy of polar X-ray jets previously found primarily from XRT movies alone. In accord with models of blowout jets and standard jets, the AIA 304Å movies show a cool(T ∼ 105 K) component in nearly all blowout X-ray jets and in a small minority of standard X-ray jets, obvious lateral expansion in blowout X-ray jets but none in standard X-ray jets, and obvious axial rotation in both blowout X-ray jets and standard X-ray jets. In our sample, the number of turns of axial rotation in the cool-component standard X-ray jets is typical of that in the blowout X-ray jets, suggesting that the closed bipolar magnetic field in the jet base has substantial twist not only in all blowout X-ray jets but also in many standard X-ray jets. We pointout that our results for the dichotomy, lateral expansion, and axial rotation of X-ray jets add credence to published speculation that type-II spicules areminiature analogs of X-ray jets, are generated by granule-size emerging bipoles, and thereby carry enough energy to power the corona and solar wind. [ABSTRACT FROM AUTHOR]

It has been found previously, by measuring from active-region magnetograms a proxy of the free energy in the active region's magnetic field, (1) that there is a sharp upper limit to the free energy the field can hold that increases with the amount of magnetic field in the active region, the active region's magnetic flux content, and (2) that most active regions are near this limit when their field explodes in a coronal mass ejection/flare eruption. That is, explosive active regions are concentrated in a main-sequence path bordering the free-energy-limit line in (flux content, free- energy proxy) phase space. Here, we present evidence that specifies the underlying magnetic condition that gives rise to the free-energy limit and the accompanying main sequence of explosive active regions. Using a suitable free-energy proxy measured from vector magnetograms of 44 active regions, we find evidence that (1) in active regions at and near their free-energy limit, the ratio of magnetic-shear free energy to the non-free magnetic energy the potential field would have is of the order of one in the core field, the field rooted along the neutral line, and (2) this ratio is progressively less in active regions progressively farther below their free-energy limit. Evidently, most active regions in which this core-field energy ratio is much less than one cannot be triggered to explode; as this ratio approaches one, most active regions become capable of exploding; and when this ratio is one, most active regions are compelled to explode. [ABSTRACT FROM AUTHOR]

The properties of the polarity inversion line (PIL) in solar active regions (ARs) are strongly correlated to flare occurrences. The PIL mask, enclosing the PIL areas, has shown significant potential for improving machine-learning-based flare prediction models. In this study, an unsupervised machine-learning algorithm, Kernel Principle Component Analysis (KPCA), is adopted to directly derive features from the PIL mask and difference PIL mask, and use those features to classify ARs into two categories—non-strong flaring ARs and strong-flaring (M-class and above flares) ARs—for time-in-advance from one hour to 72 hr at a 1 hr cadence. The two best features are selected from the KPCA results to develop random-forest classifiers for predicting flares, and the models are then evaluated and compared to similar models based on the R value and difference R value. The results show that the features derived from the PIL masks by KPCA are effective in predicting flare occurrence, with overall better Fisher ranking scores and similar predictive statistics as the R value characteristics. [ABSTRACT FROM AUTHOR]

We present the statistical analysis of 33 flare-related coronal jets, and discuss the link between the jet and the flare properties in these events. We selected jets that were observed between 2010 and 2016 by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO) that are temporally and spatially associated with flares observed by the Reuven Ramaty High Energy Solar Spectrometric Imager (RHESSI). For each jet, we calculated the jet duration and projected velocity in the plane of sky. The jet duration distribution has a median of 18.8 minutes. The projected velocities are between 31 and 456 km s−1 , with a median at 210 km s−1. For each associated flare, we performed X-ray imaging and spectroscopy and identify nonthermal emission. Nonthermal emission was detected in only 1/4 of the events considered. We did not find a clear correlation between the flare thermal energy or soft X-ray (SXR) peak flux and the jet velocity or jet duration. There is no preferential time delay between the flare and the jet. The X-ray emission is generally located at the base of the jet. The analysis presented in this paper suggests that the flare and jet are part of the same explosive event, that the jet is driven by the propagation of an Alfvénic perturbation, and that the energy partition between flare and jets varies substantially from one event to another. [ABSTRACT FROM AUTHOR]

We compare the coronal magnetic energy and helicity of two solar active regions (ARs), prolific in major eruptive (AR 11158) and confined (AR 12192) flaring, and analyze the potential of deduced proxies to forecast upcoming flares. Based on nonlinear force-free (NLFF) coronal magnetic field models with a high degree of solenoidality, and applying three different computational methods to investigate the coronal magnetic helicity, we are able to draw conclusions with a high level of confidence. Based on real observations of two solar ARs we checked trends regarding the potential eruptivity of the active-region corona, as suggested earlier in works that were based on numerical simulations, or solar observations. Our results support that the ratio of current-carrying to total helicity, , shows a strong ability to indicate the eruptive potential of a solar AR. However, does not seem to be indicative for the magnitude or type of an upcoming flare (confined or eruptive). Interpreted in the context of earlier observational studies, our findings furthermore support that the total relative helicity normalized to the magnetic flux at the NLFF model's lower boundary, , represents no indicator for the eruptivity. [ABSTRACT FROM AUTHOR]

One of the main goals of solar physics is the timely identification of eruptive active regions. Space missions such as Solar Orbiter or future space weather forecasting missions would largely benefit from this achievement. Our aim is to produce a relatively simple technique that can provide real-time indications or predictions that an active region will produce an eruption. We expand on the theoretical work of Pagano et al. that was able to distinguish eruptive from non-eruptive active regions. From this, we introduce a new operational metric that uses a combination of observed line-of-sight magnetograms, 3D data-driven simulations, and the projection of the 3D simulations forward in time. Results show that the new metric correctly distinguishes active regions as eruptive when observable signatures of eruption have been identified and as non-eruptive when there are no observable signatures of eruption. After successfully distinguishing eruptive from non-eruptive active regions we illustrate how this metric may be used in a “real-time” operational sense were three levels of warning are categorized. These categories are: high risk (red), medium risk (amber), and low risk (green) of eruption. Through considering individual cases, we find that the separation into eruptive and non-eruptive active regions is more robust the longer the time series of observed magnetograms used to simulate the build up of magnetic stress and free magnetic energy within the active region. Finally, we conclude that this proof of concept study delivers promising results where the ability to categorize the risk of an eruption is a major achievement. [ABSTRACT FROM AUTHOR]

We use Interface Region Imaging Spectrograph (IRIS) spacecraft data to study a group of Chromospheric ultraviolet bursts (UVBs) associated with an active region. We classify the UVBs into two types: smaller ones that can only be measured once by the scanning slit, and larger UVBs that are measured twice by the slit. The UVBs’ optically thin Si iv 1402.77 Å line profiles are studied intensively. By fitting the smaller UVBs’ lines with 1–2 Gaussians, we obtain a variety of line-of-sight flow measurements that hint various 3D orientations of small-scale magnetic reconnections, each associated with a UVB. The larger UVBs are, however, unique in a way that they each have two sets of measurements at two slit locations. This makes it possible to unambiguously detect two oppositely directed heated flows jetting out of a single UVB, a signature of magnetic reconnection operating at the heart of the UVB. Here, we report on the first of such an observation. Additionally, all the optically thin Si iv 1402.77 Å line profiles from those UVBs consistently demonstrate excessive broadening, an order of magnitude larger than would be expected from thermal broadening, suggesting that those small-scale reconnections could be driven by large scale (macroscale) turbulence in the active region. [ABSTRACT FROM AUTHOR]

A more accurate analytical solution of the vertical-current approximation nonlinear force-free field (VCA3-NLFFF) model is presented that includes, besides the radial (Br) and azimuthal (Bφ) magnetic field components, a poloidal component as well. This new analytical solution is of second-order accuracy in the divergence-freeness condition and of third-order accuracy in the force-freeness condition. We reanalyze the sample of 173 GOES M- and X-class flares observed with the Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO). The new code reproduces helically twisted loops with a low winding number below the kink instability consistently, avoiding unstable, highly twisted structures of the Gold–Hoyle flux rope type. The magnetic energies agree within with the Wiegelmann (W-NLFFF) code. The time evolution of the magnetic field reveals multiple, intermittent energy buildup and releases in most flares, contradicting both the Rosner–Vaiana model (with gradual energy storage in the corona) and the principle of timescale separation (τflare ≪ τstorage) postulated in self-organized criticality models. The mean dissipated flare energy is found to amount to 7% ± 3% of the potential energy, or 60% ± 26% of the free energy, a result that can be used for predicting flare magnitudes based on the potential field of active regions. [ABSTRACT FROM AUTHOR]

It is well established that solar flares and coronal mass ejections (CMEs) are powered by the free magnetic energy stored in volumetric electric currents in the corona, predominantly in active regions (ARs). Much effort has been made to search for eruption-related signatures from magnetic field observed mostly in the photosphere; and the signatures are further employed for predicting flares and CMEs. The parameters in the Space-weather HMI Active Region Patches (SHARP) data from the Solar Dynamics Observatory/HMI observation of vector magnetic field are designed and generated for this purpose. In this paper, we report research done on modification of these SHARP parameters with an attempt to improve flare prediction. The newly modified parameters are weighed heavily by magnetic polarity inversion lines (PIL) with high magnetic gradient, as suggested by Schrijver, by multiplying the parameters with a PIL mask. We demonstrate that the number of the parameters that can well discriminate erupted and nonerupted ARs increases significantly by a factor of two, in comparison with the original parameters. This improvement suggests that the high-gradient PILs are tightly related with solar eruption that agrees with previous studies. This also provides new data that possess potential to improve the machine-learning-based solar flare prediction models. [ABSTRACT FROM AUTHOR]

Solar flares originate from magnetically active regions (ARs) but not all solar ARs give rise to a flare. Therefore, the challenge of solar flare prediction benefits from an intelligent computational analysis of physics-based properties extracted from AR observables, most commonly line-of-sight or vector magnetograms of the active region photosphere. For the purpose of flare forecasting, this study utilizes an unprecedented 171 flare-predictive AR properties, mainly inferred by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO/HMI) in the course of the European Union Horizon 2020 FLARECAST project. Using two different supervised machine-learning methods that allow feature ranking as a function of predictive capability, we show that (i) an objective training and testing process is paramount for the performance of every supervised machine-learning method; (ii) most properties include overlapping information and are therefore highly redundant for flare prediction; (iii) solar flare prediction is still—and will likely remain—a predominantly probabilistic challenge. [ABSTRACT FROM AUTHOR]

We search for outliers in extreme events of statistical size distributions of astrophysical data sets, motivated by the Dragon-King hypothesis of Sornette, which suggests that the most extreme events in a statistical distribution may belong to a different population, and thus may be generated by a different physical mechanism, in contrast to the strict power-law behavior of self-organized criticality models. Identifying such disparate outliers is important for space weather predictions. Possible physical mechanisms to produce such outliers could be generated by sympathetic flaring. However, we find that Dragon-King events are not common in solar and stellar flares, identified in 4 out of 25 solar and stellar flare data sets only. Consequently, small, large, and extreme flares are essentially scale-free and can be modeled with a single physical mechanism. In very large data sets (N ≳ 104) we find significant deviations from ideal power laws in almost all data sets. Nevertheless, the fitted power-law slopes constrain physical scaling laws in terms of flare areas and volumes, which have the highest nonlinearity in their scaling laws. [ABSTRACT FROM AUTHOR]

Coronal jets occur frequently on the Sun, and may contribute significantly to the solar wind. With the suite of instruments available now, we can observe these phenomena in greater detail than ever before. Modeling and simulations can assist further with understanding the dynamic processes involved, but previous studies tended to consider only one mechanism (e.g., emergence or rotation) for the origin of the jet. In this study we model a series of idealized archetypal jet configurations and follow the evolution of the coronal magnetic field. This is a step toward understanding these idealized situations before considering their observational counterparts. Several simple situations are set up for the evolution of the photospheric magnetic field: a single parasitic polarity rotating or moving in a circular path; as well as opposite polarity pairs involved in flyby (shearing), cancellation or emergence; all in the presence of a uniform, open background magnetic field. The coronal magnetic field is evolved in time using a magnetofrictional relaxation method. While magnetofriction cannot accurately reproduce the dynamics of an eruptive phase, the structure of the coronal magnetic field, as well as the buildup of electric currents and free magnetic energy are instructive. Certain configurations and motions produce a flux rope and allow the significant buildup of free energy, reminiscent of the progenitors of so-called blowout jets, whereas other, simpler configurations are more comparable to the standard jet model. The next stage is a comparison with observed coronal jet structures and their corresponding photospheric evolution. [ABSTRACT FROM AUTHOR]

We present results of three-dimensional MHD simulations of recurrent eruptions in emerging flux regions. The initial numerical setup is the same as that in the work by Syntelis et al. Here, we perform a parametric study on the magnetic field strength (B0) of the emerging field. The kinetic energy of the produced ejective eruptions in the emerging flux region ranges from 1026 to 1028 erg, reaching up to the energies of small coronal mass ejections. The kinetic and magnetic energies of the eruptions scale linearly in a logarithmic plot. We find that the eruptions are triggered earlier for higher B0 and that B0 is not directly correlated to the frequency of occurrence of the eruptions. Using large numerical domains, we show the initial stage of the partial merging of two colliding erupting fields. The partial merging occurs partly by the reconnection between the field lines of the following and the leading eruption at the interface between them. We also find that tether-cutting reconnection of the field lines of the leading eruption underneath the following eruption magnetically links the two eruptions. Shocks develop inside the leading eruption during the collision. [ABSTRACT FROM AUTHOR]

Recent studies have revealed that solar jets involving minifilament eruptions may be initiated under the well-known magnetic-breakout mechanism. Before or just at the onset of those jets, there should be a current sheet, where breakout magnetic reconnection takes place, between open fields and the outside of the jet-base arcade carrying a minifilament in its core. In this paper we present a jet produced by the eruption of two minifilaments lying at the jet base. A current sheet is directly detected near the jet base before the onset of the eruption, suggesting the magnetic-breakout mechanism. However, we further find that the current sheet undergoes a transition. The current sheet first shortens to zero in length, but then lengthens toward an orthogonal direction relative to its initial orientation. The change of the current sheet gives rise to a reversal of the breakout reconnection, as the inflow and outflow regions before the transition become the outflow and inflow regions after the transition, respectively. We therefore propose that this observation provides evidence for the so-called oscillatory reconnection that is defined by a series of reconnection reversals but not yet proven to exist in the real plasma environment of the solar atmosphere. [ABSTRACT FROM AUTHOR]

Jets (transient/collimated plasma ejections) occur frequently throughout the solar corona and contribute mass/energy to the corona and solar wind. By combining numerical simulations and high-resolution observations, we have made substantial progress recently on determining the energy buildup and release processes in these jets. Here we describe a study of 27 equatorial coronal-hole jets using Solar Dynamics Observatory/Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager observations on 2013 June 27–28 and 2014 January 8–10. Out of 27 jets, 18 (67%) are associated with mini-filament ejections; the other nine (33%) do not show mini-filament eruptions but do exhibit mini-flare arcades and other eruptive signatures. This indicates that every jet in our sample involved a filament-channel eruption. From the complete set of events, six jets (22%) are apparently associated with tiny flux-cancellation events at the polarity inversion line, and two jets (7%) are associated with sympathetic eruptions of filaments from neighboring bright points. Potential-field extrapolations of the source-region photospheric magnetic fields reveal that all jets originated in the fan-spine topology of an embedded bipole associated with an extreme ultraviolet coronal bright point. Hence, all our jets are in agreement with the breakout model of solar eruptions. We present selected examples and discuss the implications for the jet energy buildup and initiation mechanisms. [ABSTRACT FROM AUTHOR]

High-resolution spectra of an Ellerman burst (EB) sampling the Hα and the Ca ii 8542 Å lines obtained with the Fast Imaging Solar Spectrograph (FISS) installed on the 1.6 m Goode Solar Telescope at the Big Bear Solar Observatory are compared with synthetic line profiles constructed using the RH code for nonlocal thermodynamical equilibrium radiative transfer. The EB heating is modeled by a local temperature hump above the quiet-Sun temperature. Our first finding is that FISS Hα and Ca ii 8542 Å intensity profiles cannot be reproduced simultaneously by a single hump model as far as the hump is thicker than ≥100 km. Simultaneous reproduction of both line profiles is possible when the EB temperature enhancement is confined to a layer as thin as ≤20 km in the photosphere where the Hα wing response is high and that of the Ca ii 8542 Å is not. Moreover, when we examine the EB spectra at different times, we find that the EB at a time of weaker appearance is located at lower heights, ∼50 km, and moves upward to ∼120 km at the time of maximum intensity. Complementary calculations of the Na i D1 and Mg i b2 lines as well as that of UV continuum at 1600 and 1700 Å with the deduced EB atmosphere are also performed to test the result, which allows us to discuss the shortcomings of this plane–parallel static model atmosphere for understanding the physical properties of EBs. [ABSTRACT FROM AUTHOR]

Active regions (ARs) that exhibit compact polarity inversion lines (PILs) are known to be very flare productive. However, the physical mechanisms behind this statistical inference have not been demonstrated conclusively. We show that such PILs can occur owing to the collision between two emerging flux tubes nested within the same AR. In such multipolar ARs, the flux tubes may emerge simultaneously or sequentially, each initially producing a bipolar magnetic region (BMR) at the surface. During each flux tube’s emergence phase, the magnetic polarities can migrate such that opposite polarities belonging to different BMRs collide, resulting in shearing and cancellation of magnetic flux. We name this process “collisional shearing” to emphasize that the shearing and flux cancellation develop owing to the collision. Collisional shearing is a process different from the known concept of flux cancellation occurring between polarities of a single bipole, a process that has been commonly used in many numerical models. High spatial and temporal resolution observations from the Solar Dynamics Observatory for two emerging ARs, AR 11158 and AR 12017, show the continuous cancellation of up to 40% of the unsigned magnetic flux of the smallest BMR, which occurs at the collisional PIL for as long as the collision persists. The flux cancellation is accompanied by a succession of solar flares and CMEs, products of magnetic reconnection along the collisional PIL. Our results suggest that the quantification of magnetic cancellation driven by collisional shearing needs to be taken into consideration in order to improve the prediction of solar energetic events and space weather. [ABSTRACT FROM AUTHOR]

Using the 135 s cadence of the photospheric vector data provided by the Helioseismic and Magnetic Imager telescope on board the Solar Dynamic Observatory, we examined the time evolution of magnetic helicity fluxes across the photosphere during 16 flares with the energy class lower than M5.0. During the flare, in four out of 16 events, we found impulsive changes in the helicity fluxes. This indicates that even the flare with less energy could be associated with anomalistic transportation of the magnetic helicity across the photosphere. Accompanying the impulsive helicity fluxes, the poynting fluxes across the photosphere evolved from positive to negative. As such, the transportations of magnetic energy across the photosphere were toward the solar interior during these flares. In each of the four events, the impulsive change in the helicity flux was always mainly contributed by an abrupt change in the horizontal velocity field on a sunspot located near the flaring polarity inversion line. The velocity field on each sunspot shows either an obvious vortex pattern or a shearing pattern relative to the magnetic polarity, which tended to relax the magnetic twist or shear in the corona. During these flares, an abrupt change in the Lorentz force acting on these sunspots was found. The rotational motions and shearing motions of these sunspots always had the same directions with the resultant Lorentz forces. These results support the view that the impulsive helicity transportation during the flare could be driven by the change in the Lorentz force applied on the photosphere. [ABSTRACT FROM AUTHOR]

Coronal jets and bright points occur prolifically in predominantly unipolar magnetic regions, such as coronal holes (CHs), where they appear above minority-polarity intrusions. Intermittent low-level reconnection and explosive, high-energy-release reconnection above these intrusions are thought to generate bright points and jets, respectively. The magnetic field above the intrusions possesses a spine-fan topology with a coronal null point. The movement of magnetic flux by surface convection adds free energy to this field, forming current sheets and inducing reconnection. We conducted three-dimensional magnetohydrodynamic simulations of moving magnetic elements as a model for coronal jets and bright points. A single minority-polarity concentration was subjected to three different experiments: a large-scale surface flow that sheared part of the separatrix surface only, a large-scale surface flow that also sheared part of the polarity inversion line surrounding the minority flux, and the latter flow setup plus a “flyby” of a majority-polarity concentration past the moving minority-polarity element. We found that different bright-point morphologies, from simple loops to sigmoids, were created. When only the field near the separatrix was sheared, steady interchange reconnection modulated by quasi-periodic, low-intensity bursts of reconnection occurred, suggestive of a bright point with periodically varying intensity. When the field near the polarity inversion line was strongly sheared, on the other hand, filament channels repeatedly formed and erupted via the breakout mechanism, explosively increasing the interchange reconnection and generating nonhelical jets. The flyby produced even more energetic and explosive jets. Our results explain several key aspects of CH bright points and jets, and the relationships between them. [ABSTRACT FROM AUTHOR]

The relative magnetic helicity is a quantity that is often used to describe the level of entanglement of non-isolated magnetic fields, such as the magnetic field of solar active regions. The aim of this paper is to investigate how different kinds of photospheric boundary flows accumulate relative magnetic helicity in the corona and if and how well magnetic-helicity-related quantities identify the onset of an eruption. We use a series of three-dimensional, parametric magnetohydrodynamic simulations of the formation and eruption of magnetic flux ropes. All the simulations are performed on the same grid, using the same parameters, but they are characterized by different driving photospheric flows, i.e., shearing, convergence, stretching, and peripheral- and central- dispersion flows. For each of the simulations, the instant of the onset of the eruption is carefully identified by using a series of relaxation runs. We find that magnetic energy and total relative helicity are mostly injected when shearing flows are applied at the boundary, while the magnetic energy and helicity associated with the coronal electric currents increase regardless of the kind of photospheric flows. We also find that, at the onset of the eruptions, the ratio between the non-potential magnetic helicity and the total relative magnetic helicity has the same value for all the simulations, suggesting the existence of a threshold in this quantity. Such a threshold is not observed for other quantities as, for example, those related to the magnetic energy. [ABSTRACT FROM AUTHOR]

We developed a solar flare prediction model using a deep neural network (DNN) named Deep Flare Net (DeFN). This model can calculate the probability of flares occurring in the following 24 hr in each active region, which is used to determine the most likely maximum classes of flares via a binary classification (e.g., ≥M class versus 5 observation images taken during 2010–2015 by the Solar Dynamic Observatory, we automatically detected sunspots and calculated 79 features for each region, to which flare occurrence labels of X-, M-, and C-class were attached. We adopted the features used in Nishizuka et al. (2017) and added some features for operational prediction: coronal hot brightening at 131 Å (T ≥ 107 K) and the X-ray and 131 Å intensity data 1 and 2 hr before an image. For operational evaluation, we divided the database into two for training and testing: the data set in 2010–2014 for training, and the one in 2015 for testing. The DeFN model consists of deep multilayer neural networks formed by adapting skip connections and batch normalizations. To statistically predict flares, the DeFN model was trained to optimize the skill score, i.e., the true skill statistic (TSS). As a result, we succeeded in predicting flares with TSS = 0.80 for ≥M-class flares and TSS = 0.63 for ≥C-class flares. Note that in usual DNN models, the prediction process is a black box. However, in the DeFN model, the features are manually selected, and it is possible to analyze which features are effective for prediction after evaluation. [ABSTRACT FROM AUTHOR]

Solid evidence of magnetic reconnection is rarely reported within sunspots, the darkest regions with the strongest magnetic fields and lowest temperatures in the solar atmosphere. Using the world’s largest solar telescope, the 1.6 m Goode Solar Telescope, we detect prevalent reconnection through frequently occurring fine-scale jets in the Hα line wings at light bridges, the bright lanes that may divide the dark sunspot core into multiple parts. Many jets have an inverted Y-shape, shown by models to be typical of reconnection in a unipolar field environment. Simultaneous spectral imaging data from the Interface Region Imaging Spectrograph show that the reconnection drives bidirectional flows up to 200 km s−1, and that the weakly ionized plasma is heated by at least an order of magnitude up to ∼80,000 K. Such highly dynamic reconnection jets and efficient heating should be properly accounted for in future modeling efforts of sunspots. Our observations also reveal that the surge-like activity previously reported above light bridges in some chromospheric passbands such as the Hα core has two components: the ever-present short surges likely to be related to the upward leakage of magnetoacoustic waves from the photosphere, and the occasionally occurring long and fast surges that are obviously caused by the intermittent reconnection jets. [ABSTRACT FROM AUTHOR]

Small, impulsive jets commonly occur throughout the solar corona, but are especially visible in coronal holes. Evidence is mounting that jets are part of a continuum of eruptions that extends to much larger coronal mass ejections and eruptive flares. Because coronal-hole jets originate in relatively simple magnetic structures, they offer an ideal testbed for theories of energy buildup and release in the full range of solar eruptions. We analyzed an equatorial coronal-hole jet observed by the Solar Dynamics Observatory (SDO)/AIA on 2014 January 9 in which the magnetic-field structure was consistent with the embedded-bipole topology that we identified and modeled previously as an origin of coronal jets. In addition, this event contained a mini-filament, which led to important insights into the energy storage and release mechanisms. SDO/HMI magnetograms revealed footpoint motions in the primary minority-polarity region at the eruption site, but show negligible flux emergence or cancellation for at least 16 hr before the eruption. Therefore, the free energy powering this jet probably came from magnetic shear concentrated at the polarity inversion line within the embedded bipole. We find that the observed activity sequence and its interpretation closely match the predictions of the breakout jet model, strongly supporting the hypothesis that the breakout model can explain solar eruptions on a wide range of scales. [ABSTRACT FROM AUTHOR]

On 2016 September 20, the Interface Region Imaging Spectrograph observed an active region during its earliest emerging phase for almost 7 hr. The Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory observed continuous emergence of small-scale magnetic bipoles with a rate of ∼1016 Mx s−1. The emergence of magnetic fluxes and interactions between different polarities lead to the frequent occurrence of ultraviolet (UV) bursts, which exhibit as intense transient brightenings in the 1400 Å images. In the meantime, discrete small patches with the same magnetic polarity tend to move together and merge, leading to the enhancement of the magnetic fields and thus the formation of pores (small sunspots) at some locations. The spectra of these UV bursts are characterized by the superposition of several chromospheric absorption lines on the greatly broadened profiles of some emission lines formed at typical transition region temperatures, suggesting heating of the local materials to a few tens of thousands of kelvin in the lower atmosphere by magnetic reconnection. Some bursts reveal blue- and redshifts of ∼100 km s−1 at neighboring pixels, indicating the spatially resolved bidirectional reconnection outflows. Many such bursts appear to be associated with the cancellation of magnetic fluxes with a rate of the order of ∼1015 Mx s−1. We also investigate the three-dimensional magnetic field topology through a magnetohydrostatic model and find that a small fraction of the bursts are associated with bald patches (magnetic dips). Finally, we find that almost all bursts are located in regions of large squashing factor at the height of ∼1 Mm, reinforcing our conclusion that these bursts are produced through reconnection in the lower atmosphere. [ABSTRACT FROM AUTHOR]

Numerous observations have revealed that power-law distributions are ubiquitous in energetic solar processes. Hard X-rays, soft X-rays, extreme ultraviolet radiation, and radio waves all display power-law frequency distributions. Since magnetic reconnection is the driving mechanism for many energetic solar phenomena, it is likely that reconnection events themselves display such power-law distributions. In this work, we perform numerical simulations of the solar corona driven by simple convective motions at the photospheric level. Using temperature changes, current distributions, and Poynting fluxes as proxies for heating, we demonstrate that energetic events occurring in our simulation display power-law frequency distributions, with slopes in good agreement with observations. We suggest that the braiding-associated reconnection in the corona can be understood in terms of a self-organized criticality model driven by convective rotational motions similar to those observed at the photosphere. [ABSTRACT FROM AUTHOR]