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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]

Light bridges (LBs) are narrow structures dividing sunspot umbra, and their role in active region evolution is yet to be explored. We investigated the magnetic structure of the two LBs: a narrow LB (with width ∼810 km) and a considerably wider LB (2475 km) in the active region NOAA 12371. We employed: (1) the high-spatial-resolution spectropolarimetric data obtained by the Near InfraRed Imaging Spectropolarimeter (NIRIS) of the 1.6 m Goode Solar Telescope (GST) for studying the magnetic structure at the photosphere, and (2) the nonlinear force-free field (NLFFF) models, extrapolated from both the photospheric magnetogram from GST/NIRIS and from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, for studying the three-dimensional (3D) magnetic structure on a larger scale. Our observations reveal the presence of a field-free (or, more precisely, weak-field) region and the different velocity structures inside the two LBs. Analysis of the 3D NLFFF model shows a low-lying magnetic canopy as well as the enhanced current system above the LBs. The substantial difference between the LBs and the umbrae is found in the overall magnetic topology in that the field lines emanating from the two LBs are more twisted than that from the neighboring umbrae. [ABSTRACT FROM AUTHOR]

Obtaining high-quality magnetic and velocity fields through Stokes inversion is crucial in solar physics. In this paper, we present a new deep learning method, named Stacked Deep Neural Networks (SDNN), for inferring line-of-sight (LOS) velocities and Doppler widths from Stokes profiles collected by the Near InfraRed Imaging Spectropolarimeter (NIRIS) on the 1.6 m Goode Solar Telescope (GST) at the Big Bear Solar Observatory (BBSO). The training data for SDNN are prepared by a Milneâ€"Eddington (ME) inversion code used by BBSO. We quantitatively assess SDNN, comparing its inversion results with those obtained by the ME inversion code and related machine-learning (ML) algorithms such as multiple support vector regression, multilayer perceptrons, and a pixel-level convolutional neural network. Major findings from our experimental study are summarized as follows. First, the SDNN-inferred LOS velocities are highly correlated to the ME-calculated ones with the Pearson productâ€"moment correlation coefficient being close to 0.9 on average. Second, SDNN is faster, while producing smoother and cleaner LOS velocity and Doppler width maps, than the ME inversion code. Third, the maps produced by SDNN are closer to ME’s maps than those from the related ML algorithms, demonstrating that the learning capability of SDNN is better than those of the ML algorithms. Finally, a comparison between the inversion results of ME and SDNN based on GST/NIRIS and those from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory in flare-prolific active region NOAA 12673 is presented. We also discuss extensions of SDNN for inferring vector magnetic fields with empirical evaluation. [ABSTRACT FROM AUTHOR]

In this study, we analyze high-spatial-resolution (0.″24) magnetograms and high-spatial-resolution (0.″10) Hα off-band (± 0.8 Å) images taken by the 1.6 m Goode Solar Telescope to investigate the magnetic properties associated with small-scale ejections in a coronal hole boundary region from a statistical perspective. With one and a half hours of optical observations under excellent seeing, we focus on the magnetic structure and evolution by tracking the magnetic features with the Southwest Automatic Magnetic Identification Suite (SWAMIS). The magnetic field at the studied coronal hole boundary is dominated by negative polarity with flux cancellations at the edges of the negative unipolar cluster. In a total of 1250 SWAMIS-detected magnetic cancellation events, ∼39% are located inside the coronal hole with an average flux cancellation rate of 2.0 × 1018 Mx Mm−2 hr−1, and ∼49% are located outside the coronal hole with an average flux cancellation rate of 8.8 × 1017 Mx Mm−2 hr−1. We estimated that the magnetic energy released due to flux cancellation inside the coronal hole is six times more than that outside the coronal hole. Flux cancellation accounts for ∼9.5% of the total disappearance of magnetic flux. Other forms of its disappearance are mainly due to fragmentation of unipolar clusters or merging with elements of the same polarity. We also observed a number of significant small-scale ejections associated with magnetic cancellations at the coronal hole boundary that have corresponding EUV brightenings. [ABSTRACT FROM AUTHOR]

Multiwavelength bright points (BPs) are taken to be cross sections of magnetic flux tubes extending from the surface of the photosphere upward to the higher photosphere. We aim to study the characteristics of isolated multiwavelength BPs using the cotemporal and cospatial TiO band and Hα line wings from the Goode Solar Telescope at Big Bear Solar Observatory. A deep-learning method, based on Track Region-based Convolutional Neural Networks, is proposed to detect, segment, and match the BPs across multiple wavelength observations, including the TiO, Hα + 1 Å, Hα − 1 Å, Hα + 0.8 Å, and Hα − 0.8 Å line wings. Based on the efficient detection and matching result with a precision of 0.98, 1283 groups of BPs matched in all five wavelengths are selected for statistics analysis. The characteristic values of the BPs observed at the same red and blue line wings are averaged. For the BPs of the TiO, averaged Hα ± 1 Å, and averaged Hα ± 0.8 Å line wings, the mean equivalent diameters are 162 ± 32, 254 ± 33, and 284 ± 28 km, respectively. The maximum intensity contrasts are 1.11 ± 0.09, 1.05 ± 0.03, and 1.05 ± 0.02 , respectively. The mean eccentricities are 0.65 ± 0.14, 0.63 ± 0.11, and 0.65 ± 0.11, respectively. Moreover, the characteristic ratios of each Hα ± 1 Å and Hα ± 0.8 Å BP to its corresponding TiO BP are derived. Hα ± 1 Å and Hα ± 0.8 Å line wings BPs show 60% and 80% increases compared to TiO BPs, respectively. With increasing height, most BPs almost keep their shapes. This work is helpful for modeling the three-dimensional structure of flux tubes. [ABSTRACT FROM AUTHOR]

In this paper, we report the EUV late phase for the M1.8 class flare on 2012 July 5 in the active region (AR) 11515. The late phase is shown by the prominent appearance of EUV emission at 131 Å of two additional flare loop systems (flare arcades 2 and 3, as named in this paper) other than the main flare loop (flare arcade 1), as observed by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO). Three sets of flare arcades connect four flare ribbons, which forms an asymmetric quadrupole magnetic field configuration. While the emission from flare arcade 2, linking the pair of secondary flare ribbons, and arcade 3, linking one of the main flare ribbons and one of the secondary flare ribbons, conjointly contributes to the EUV late phase, their heating mechanisms are quite different. While the brightening of flare arcade 2 is the result of disturbance created by the eruption of EUV hot channels to the overlying coronal magnetic field, the heating of flare arcade 3 was closely associated with two rapid contractions of the overlying filament threads during the partial eruption of the filament. The contractions are discernible in He i 10830 Å images and have signatures in the EUV wavelengths of AIA. The two rapid contractions are the result of a sudden drop in magnetic pressure after the eruption of two hot channels. Clear evidence suggests that magnetic reconnection may occur between the contracting filament threads and the low-lying magnetic field. [ABSTRACT FROM AUTHOR]

Through the use of the high-resolution spectral data and the broadband imaging obtained with the Goode Solar Telescope at the Big Bear Solar Observatory on 2013 June 6, the spectra of three typical photospheric bright points (PBPs) have been analyzed. Based on the Hα and Ca ii 8542 Å line profiles, as well as the TiO continuum emission, for the first time, the non-LTE semiempirical atmospheric models for the PBPs are computed. The attractive characteristic is the temperature enhancement in the lower photosphere. The temperature enhancement is about 200–500 K at the same column mass density as in the atmospheric model of the quiet-Sun. The total excess radiative energy of a typical PBP is estimated to be 1 × 1027–2 × 1027 erg, which can be regarded as the lower limit energy of the PBPs. The radiation flux in the visible continuum for the PBPs is about 5.5 × 1010 erg cm−2 s−1. Our result also indicates that the temperature in the atmosphere above PBPs is close to that of a plage. It gives clear evidence that PBPs may contribute significantly to the heating of the plage atmosphere. Using our semiempirical atmospheric models, we estimate self-consistently the average magnetic flux density B in the PBPs. It is shown that the maximum value is about 1 kG, and it decreases toward both higher and lower layers, reminding us of the structure of a flux tube between photospheric granules. [ABSTRACT FROM AUTHOR]

Recent observations demonstrated that emerging flux regions, which constitute the early stage of solar active regions, consist of emergence of numerous small-scale magnetic elements. They in turn interact, merge, and form mature sunspots. However, observations of fine magnetic structures on photosphere with subarcsecond resolution are very rare due to limitations of observing facilities. In this work, taking advantage of the high resolution of the 1.6 m Goode Solar Telescope, we jointly analyze vector magnetic fields, continuum images, and Hα observations of NOAA AR 12665 on 2017 July 13, with the goal of understanding the signatures of small-scale flux emergence, as well as their atmospheric responses as they emerge through multiple heights in the photosphere and chromosphere. Under such a high resolution of 0.″1–0.″2, our results confirm two kinds of small-scale flux emergence: magnetic flux sheet emergence associated with the newly forming granules, and the traditional magnetic flux loop emergence. With direct imaging in the broadband TiO, we observe that both types of flux emergence are associated with darkening of granular boundaries, while only flux sheets elongate granules along the direction of emerging magnetic fields and expand laterally. With a life span of 10 ∼ 15 minutes, the total emerged vertical flux is on the order of 1018 Mx for both types of emergence. The magnitudes of the vertical and horizontal fields are comparable in the flux sheets, while the former is stronger in flux loops. Hα observations reveal transient brightenings in the wings in the events of magnetic loop emergence, which are most probably the signatures of Ellerman bombs. [ABSTRACT FROM AUTHOR]

Observations with the Goode Solar Telescope (GST) are presented here showing that the emergence of 1.91 × 1018 Mx of new magnetic flux occurred at the edge of a filamentary light bridge (LB). This emergence was accompanied by brightness enhancement of a photospheric overturning convection cell (OCC) at the endpoints of the emerging magnetic structure. We present an analysis of the origin and the dynamics of this event using high-resolution GST Fe i 1564.85 nm vector magnetic field data, TiO photospheric, and Hα chromospheric images. The emerged structure was 1.5 × 0.3 Mm in size at the peak of development and lasted for 17 minutes. Doppler observations showed presence of systematic upflows before the appearance of the magnetic field signal and downflows during the decay phase. Changes in the orientation of the associated transverse fields, determined from the differential angle, suggest the emergence of a twisted magnetic structure. A fan-shaped jet was observed to be spatially and temporally correlated with the endpoint of the OCC intruding into the LB. Our data suggest that the emerging fields may have reconnected with the magnetic fields in the vicinity of the LB, which could lead to the formation of the jet. Our observation is the first report of flux emergence within a granular LB with evidence in the evolution of vector magnetic field, as well as photosphere convection motions, and supports the idea that the impulsive jets above the LB are caused by magnetic reconnection. [ABSTRACT FROM AUTHOR]

We studied flux emergence events of sub-granular scale in a solar active region. The New Solar Telescope (NST) of the Big Bear Solar Observatory made it possible to clearly observe the photospheric signature of flux emergence with very high spatial (0″.11 at 7057 Å) and temporal (15 s) resolution. From TiO observations with the pixel scale of 0″.0375, we found several elongated granule-like features (GLFs) stretching from the penumbral filaments of a sunspot at a relatively high speed of over 4 km s-1. After a slender arched darkening appeared at the tip of a penumbral filament, a bright point (BP) developed and quickly moved away from the filament, forming and stretching a GLF. The size of a GLF was approximately 0″.5 wide and 3″ long. The moving BP encountered nearby structures after several minutes of stretching, and the well-defined elongated shape of the GLF faded away. Magnetograms from the Solar Dynamics Cfeervafory/Helioseismic and Magnetic Imager and NST/InfraRed Imaging Magnetograph revealed that those GLFs are photospheric indicators of small-scale flux emergence, and their disappearance is related to magnetic cancellation. From two well-observed events, we describe detailed development of the substructures of GLFs and different cancellation processes that each of the two GLFs underwent. [ABSTRACT FROM AUTHOR]

We present results from the investigation of 5 minute umbral oscillations in a single-polarity sunspot of active region NOAA 12132. The spectra of TiO, Hα, and 304 Å are used for corresponding atmospheric heights from the photosphere to lower corona. Power spectrum analysis at the formation height of Hα – 0.6 Å to the Hα center resulted in the detection of 5 minute oscillation signals in intensity interpreted as running waves outside the umbral center, mostly with vertical magnetic field inclination >15°. A phase-speed filter is used to extract the running wave signals with speed vph > 4 km s−1, from the time series of Hα – 0.4 Å images, and found twenty-four 3 minute umbral oscillatory events in a duration of one hour. Interestingly, the initial emergence of the 3 minute umbral oscillatory events are noticed closer to or at umbral boundaries. These 3 minute umbral oscillatory events are observed for the first time as propagating from a fraction of preceding running penumbral waves (RPWs). These fractional wavefronts rapidly separate from RPWs and move toward the umbral center, wherein they expand radially outwards suggesting the beginning of a new umbral oscillatory event. We found that most of these umbral oscillatory events develop further into RPWs. We speculate that the waveguides of running waves are twisted in spiral structures and hence the wavefronts are first seen at high latitudes of umbral boundaries and later at lower latitudes of the umbral center. [ABSTRACT FROM AUTHOR]

On 2012 June 17, we observed the evolution of a C-class flare associated with the eruption of a filament near a large sunspot in the active region NOAA 11504. We obtained high spatial resolution filtergrams using the 1.6 m New Solar Telescope at the Big Bear Solar Observatory in broadband TiO at 706 nm (bandpass: 10 Å) and He I 10830 Å narrow band (bandpass: 0.5 Å, centered 0.25 Å to the blue). We analyze the spatio-temporal behavior of the He I 10830 Å data, which were obtained over a 90″×90″ field of view with a cadence of 10 s. We also analyze simultaneous data from the Atmospheric Imaging Assembly and Extreme Ultraviolet Variability Experiment instruments on board the Solar Dynamics Observatory spacecraft, and data from the Reuven Ramaty High Energy Solar Spectroscopic Imager and GOES spacecrafts. Non-thermal effects are ignored in this analysis. Several quantitative aspects of the data, as well as models derived using the “0D” enthalpy-based thermal evolution of loops model code, indicate that the triplet states of the 10830 Å multiplet are populated by photoionization of chromospheric plasma followed by radiative recombination. Surprisingly, the He II 304 Å line is reasonably well matched by standard emission measure calculations, along with the C IV emission which dominates the Atmosphere Imaging Assembly 1600 Å channel during flares. This work lends support to some of our previous work combining X-ray, EUV, and UV data of flares to build models of energy transport from corona to chromosphere. [ABSTRACT FROM AUTHOR]

We study the Hα and Ca II 8542 Å line spectra of four typical Ellerman bombs (EBs) in the active region NOAA 11765 on 2013 June 6, observed with the Fast Imaging Solar Spectrograph installed at the 1.6 m New Solar Telescope at Big Bear Solar Observatory. Considering that EBs may occur in a restricted region in the lower atmosphere, and that their spectral lines show particular features, we propose a two-cloud model to fit the observed line profiles. The lower cloud can account for the wing emission, and the upper cloud is mainly responsible for the absorption at line center. After choosing carefully the free parameters, we get satisfactory fitting results. As expected, the lower cloud shows an increase of the source function, corresponding to a temperature increase of 400-1000 K in EBs relative to the quiet Sun. This is consistent with previous results deduced from semi-empirical models and confirms that local heating occurs in the lower atmosphere during the appearance of EBs. We also find that the optical depths can increase to some extent in both the lower and upper clouds, which may result from either direct heating in the lower cloud, or illumination by an enhanced radiation on the upper cloud. The velocities derived from this method, however, are different from those obtained using the traditional bisector method, implying that one should be cautious when interpreting this parameter. The two-cloud model can thus be used as an efficient method to deduce the basic physical parameters of EBs. [ABSTRACT FROM AUTHOR]