Author name code: ding ADS astronomy entries on 2022-09-14 author:"Ding, Mingde D." ------------------------------------------------------------------------ Title: The Lyman-$\alpha$ Emission in a C1.4 Solar Flare Observed by the Extreme Ultraviolet Imager aboard Solar Orbiter Authors: Li, Ying; Li, Qiao; Song, De-Chao; Battaglia, Andrea Francesco; Xiao, Hualin; Krucker, Säm; Schühle, Udo; Li, Hui; Gan, Weiqun; Ding, M. D. Bibcode: 2022arXiv220806182L Altcode: The hydrogen Lyman-$\alpha$ (H {\sc i} Ly$\alpha$) emission during solar flares has rarely been studied in spatially resolved images and its physical origin has not been fully understood. In this paper, we present novel Ly$\alpha$ images for a C1.4 solar flare (SOL2021-08-20T22:00) from the Extreme Ultraviolet Imager aboard Solar Orbiter, together with multi-waveband and multi-perspective observations from the Solar Terrestrial Relations Observatory Ahead and the Solar Dynamics Observatory spacecraft. It is found that the Ly$\alpha$ emission has a good temporal correlation with the thermal emissions at 1--8 Å and 5--7 keV, indicating that the flaring Ly$\alpha$ is mainly produced by a thermal process in this small event. However, nonthermal electrons play a minor role in generating Ly$\alpha$ at flare ribbons during the rise phase of the flare, as revealed by the hard X-ray imaging and spectral fitting. Besides originating from flare ribbons, the Ly$\alpha$ emission can come from flare loops, likely caused by plasma heating and also cooling that happen in different flare phases. It is also found that the Ly$\alpha$ emission shows fairly similar features with the He {\sc ii} 304 Å emission in light curve and spatio-temporal variation along with small differences. These observational results improve our understanding of the Ly$\alpha$ emission in solar flares and also provide some insights for investigating the Ly$\alpha$ emission in stellar flares. Title: The Chinese Hα Solar Explorer (CHASE) mission: An overview Authors: Li, Chuan; Fang, Cheng; Li, Zhen; Ding, MingDe; Chen, PengFei; Qiu, Ye; You, Wei; Yuan, Yuan; An, MinJie; Tao, HongJiang; Li, XianSheng; Chen, Zhe; Liu, Qiang; Mei, Gui; Yang, Liang; Zhang, Wei; Cheng, WeiQiang; Chen, JianXin; Chen, ChangYa; Gu, Qiang; Huang, QingLong; Liu, MingXing; Han, ChengShan; Xin, HongWei; Chen, ChangZheng; Ni, YiWei; Wang, WenBo; Rao, ShiHao; Li, HaiTang; Lu, Xi; Wang, Wei; Lin, Jun; Jiang, YiXian; Meng, LingJie; Zhao, Jian Bibcode: 2022SCPMA..6589602L Altcode: 2022arXiv220505962L The Chinese Hα Solar Explorer (CHASE), dubbed "Xihe"—Goddess of the Sun, was launched on October 14, 2021 as the first solar space mission of China National Space Administration (CNSA). The CHASE mission is designed to test a newly developed satellite platform and to acquire the spectroscopic observations in the Hα waveband. The Hα Imaging Spectrograph (HIS) is the scientific payload of the CHASE satellite. It consists of two observational modes: raster scanning mode and continuum imaging mode. The raster scanning mode obtains full-Sun or region-of-interest spectral images from 6559.7 to 6565.9 å; and from 6567.8 to 6570.6 å with 0.024 å pixel spectral resolution and 1 min temporal resolution. The continuum imaging mode obtains photospheric images in continuum around 6689 å with the full width at half maximum of 13.4 å. The CHASE mission will advance our understanding of the dynamics of solar activity in the photosphere and chromosphere. In this paper, we present an overview of the CHASE mission including the scientific objectives, HIS instrument overview, data calibration flow, and first results of on-orbit observations. Title: Coronal loop kink oscillation periods derived from the information of density, magnetic field, and loop geometry Authors: Chen, G. Y.; Chen, L. Y.; Guo, Y.; Ding, M. D.; Chen, P. F.; Erdélyi, R. Bibcode: 2022A&A...664A..48C Altcode: Context. Coronal loop oscillations can be triggered by solar eruptions, for example, and are observed frequently by the Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO). The Helioseismic and Magnetic Imager (HMI) on board SDO offers us the opportunity to measure the photospheric vector magnetic field and carry out solar magneto-seismology (SMS).
Aims: By applying SMS, we aim to verify the consistency between the observed period and the one derived from the information of coronal density, magnetic field, and loop geometry, that is, the shape of the loop axis.
Methods: We analysed the data of three coronal loop oscillation events detected by SDO/AIA and SDO/HMI. First, we obtained oscillation parameters by fitting the observational data. Second, we used a differential emission measure (DEM) analysis to diagnose the temperature and density distribution along the coronal loop. Subsequently, we applied magnetic field extrapolation to reconstruct the three-dimensional magnetic field and then, finally, used the shooting method to compute the oscillation periods from the governing equation.
Results: The average magnetic field determined by magnetic field extrapolation is consistent with that derived by SMS. A new analytical solution is found under the assumption of exponential density profile and uniform magnetic field. The periods estimated by combining the coronal density and magnetic field distribution and the associated loop geometry are closest to the observed ones, and are more realistic than when the loop geometry is regarded as being semi-circular or having a linear shape.
Conclusions: The period of a coronal loop is sensitive to not only the density and magnetic field distribution but also the loop geometry. Title: Editorial Authors: Fang, Cheng; Ding, MingDe; Li, Chuan; Chen, PengFei; Zhang, Wei Bibcode: 2022SCPMA..6589601F Altcode: No abstract at ADS Title: Calibration procedures for the CHASE/HIS science data Authors: Qiu, Ye; Rao, ShiHao; Li, Chuan; Fang, Cheng; Ding, MingDe; Li, Zhen; Ni, YiWei; Wang, WenBo; Hong, Jie; Hao, Qi; Dai, Yu; Chen, PengFei; Wan, XiaoSheng; Xu, Zhi; You, Wei; Yuan, Yuan; Tao, HongJiang; Li, XianSheng; He, YuKun; Liu, Qiang Bibcode: 2022SCPMA..6589603Q Altcode: 2022arXiv220506075Q The Hα line is an important optical line in solar observations containing the information from the photosphere to the chromosphere. To study the mechanisms of solar eruptions and the plasma dynamics in the lower atmosphere, the Chinese Hα Solar Explorer (CHASE) was launched into a Sun-synchronous orbit on October 14, 2021. The scientific payload of the CHASE satellite is the Hα Imaging Spectrograph (HIS). The CHASE/HIS acquires, for the first time, seeing-free Hα spectroscopic observations with high spectral and temporal resolutions. It consists of two observational modes. The raster scanning mode provides full-Sun or region-of-interest spectra at Hα (6559.7-6565.9 Å) and Fe I (6567.8-6570.6 Å) wavebands. The continuum imaging mode obtains full-Sun photospheric images at around 6689 Å. In this paper, we present detailed calibration procedures for the CHASE/HIS science data, including the dark-field and flat-field correction, slit image curvature correction, wavelength and intensity calibration, and coordinate transformation. The higher-level data products can be directly used for scientific research. Title: Imaging and Spectroscopic Observations of the Dynamic Processes in Limb Solar Flares Authors: Yu, Ke; Li, Y.; Hong, Jie; Song, De-Chao; Ding, M. D. Bibcode: 2022ApJ...935....8Y Altcode: 2022arXiv220705925Y We investigate various dynamic processes including magnetic reconnection, chromospheric evaporation, and coronal rain draining in two limb solar flares through imaging and spectroscopic observations from the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory. In the early phase of the flares, a bright and dense loop-top structure with a cusp-like shape can be seen in multiwavelength images, which is cospatial with the hard X-ray 25-50 keV emission. In particular, intermittent magnetic reconnection downflows are detected in the time-space maps of AIA 304 Å. The reconnection downflows are manifested as redshifts on one half of the loops and blueshifts on the other half in the IRIS Si IV 1393.76 Å line due to a projection effect. The Si IV profiles exhibit complex features (say, multipeak) with a relatively larger width at the loop-top region. During the impulsive phase, chromospheric evaporation is observed in both AIA images and the IRIS Fe XXI 1354.08 Å line. Upward motions can be seen from AIA 131 Å images. The Fe XXI line is significantly enhanced and shows a good Gaussian shape. In the gradual phase, warm rains are observed as downward moving plasmas in AIA 304 Å images. Both the Si IV and Fe XXI lines show a relatively symmetric shape with a larger width around the loop top. These results provide observational evidence for various dynamic processes involved in the energy release process of solar flares and are crucial to the understanding of this process. Title: Investigating Overexpansion of a Coronal Mass Ejection on 2021 May 07 Authors: Wang, Bitao; Song, Hong-Qiang; Cheng, Xin; Ding, Mingde Bibcode: 2022cosp...44.2429W Altcode: Coronal Mass Ejection(CME) burst in 2021 May 07 shows a typical three-part structure and is associated with a X-class flare, which is continuously observed from 0 to 3 Rsun by SDO/AIA(0-1.3 Rsun) and MLSO/KCor(1.05-3 Rsun). This event provides opportunity to study the early evolution of this CME. We measure the aspect ratio of the leading front and bright core to find there is a lateral overexpansion through the early evolution of the CME. The minimum of the aspect ratio occurs simultaneously with the peak of the SO/STIX 4-10 keV soft X-ray burst of the flare. During the study we compare the observations of the bright core between EUV(AIA) and white-light(KCor), the thermal structure spread lower but wider than the dense structure, which may be caused by the heat originating from the current sheet below conduct along the magnetic field. What is more, the overexpansion of the CME make it encounter a jacent pseudo-stream(PS) in the north. The interaction between the CME and the PS causes many kinds of radio signals, i.e., a formal type II burst, a zero-drifted type II burst and a C-shaped type III burst. To explain the abundant radio signals, A possible scenario of the interaction is discussed. Title: Three-dimensional Structures and Heating Mechanism of Solar Microflares Authors: Li, Zhuofei; Cheng, Xin; Ding, Mingde; Chen, Feng; Chen, Jun Bibcode: 2022cosp...44.2558L Altcode: Small-scale solar activities in the solar atmosphere, e.g., microflares and campfires, are believed to be caused by magnetic reconnection, with high similarity to major solar flares. Nevertheless, their detailed three-dimensional (3D) structures and heating mechanisms are still unsolved. In this Letter, based on high-resolution 3D radiative magnetohydrodynamic simulation of the solar quiet region, conducted with MURaM code including the upper convection zone, photosphere, chromosphere, and low corona, we investigate the 3D magnetic structures and heating mechanism of three homologous microflares. It is found that the microflares originate from local hot plasma in the chromosphere. The heated plasma is almost co-spatial with the maximal heating rate normalized by the density. The 3D velocity field reveals a pair of low-speed converging flows toward and high-speed outflows moving away from the hot plasma. These features support that magnetic reconnection plays a critical role in heating the local chromospheric plasma to coronal temperature, giving rise to the observed microflares. The magnetic topology analysis further discloses that the reconnection is located near quasi-separators where both the current density and squashing factors are maximal although the specific topology varies from case to case. Title: Annihilation of Magnetic Islands at the Top of Solar Flare Loops Authors: Wang, Yulei; Lu, Quanming; Cheng, Xin; Ding, Mingde Bibcode: 2022cosp...44.1514W Altcode: The dynamics of magnetic reconnection in the solar current sheet (CS) is studied by high-resolution 2.5-dimensional MHD simulation. With the commencing of magnetic reconnection, a number of magnetic islands are formed intermittently and move quickly upward and downward along the CS. Upon collision with the semi-closed flux of the flare loops, the downflow islands cause a second reconnection with a rate comparable with that in the main CS. Though the time-integrated magnetic energy release is still dominated by the reconnection in the main CS, the second reconnection can release substantial magnetic energy, annihilating the main islands and generating secondary islands with various scales at the flare loop top. The distribution function of the flux of the secondary islands is found to follow a power law varying from $f(\psi)\sim \psi^{-1}$ (small scale) to $\psi^{-2}$ (large scale), which seems to be independent to background plasma $\beta$ and thermal conduction (TC). However, the spatial scale and the strength of the termination shocks driven by the main reconnection outflows or islands decrease if $\beta$ increases or if TC is included. We suggest that the annihilation of magnetic islands at the flare loop top, which is not included in the standard flare model, plays a nonnegligible role in releasing magnetic energy to heat flare plasma and accelerate particles. Title: Radiative Magnetohydrodynamic Simulation of the Confined Eruption of a Magnetic Flux Rope: Magnetic Structure and Plasma Thermodynamics Authors: Wang, Can; Chen, Feng; Ding, Mingde; Lu, Zekun Bibcode: 2022ApJ...933L..29W Altcode: 2022arXiv220614188W It is widely believed that magnetic flux ropes are the key structure of solar eruptions; however, their observable counterparts are not clear yet. We study a flare associated with flux rope eruption in a comprehensive radiative magnetohydrodynamic simulation of flare-productive active regions, especially focusing on the thermodynamic properties of the plasma involved in the eruption and their relation to the magnetic flux rope. The preexisting flux rope, which carries cold and dense plasma, rises quasi-statically before the onset of eruptions. During this stage, the flux rope does not show obvious signatures in extreme ultraviolet (EUV) emission. After the flare onset, a thin "current shell" is generated around the erupting flux rope. Moreover, a current sheet is formed under the flux rope, where two groups of magnetic arcades reconnect and create a group of postflare loops. The plasma within the "current shell," current sheet, and postflare loops are heated to more than 10 MK. The postflare loops give rise to abundant soft X-ray emission. Meanwhile, a majority of the plasma hosted in the flux rope is heated to around 1 MK, and the main body of the flux rope is manifested as a bright arch in cooler EUV passbands such as the AIA 171 Å channel. Title: Growth of a filament channel by intermittent small-scale magnetic reconnection Authors: Li, H. T.; Cheng, X.; Guo, J. H.; Yan, X. L.; Wang, L. F.; Zhong, Z.; Li, C.; Ding, M. D. Bibcode: 2022A&A...663A.127L Altcode: 2022arXiv220309110L Context. A filament channel (FC), a plasma volume where the magnetic field is primarily aligned with the polarity inversion line, is believed to be the pre-eruptive configuration of coronal mass ejections. Nevertheless, evidence for how the FC is formed is still elusive.
Aims: In this paper, we present a detailed study of the build-up of a FC in order to understand its formation mechanism.
Methods: The New Vacuum Solar Telescope (NVST) of the Yunnan Observatory and the Optical and Near-infrared Solar Eruption Tracer (ONSET) of Nanjing University, as well as the Atmospheric Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO), are used to study the growth process of the FC. Furthermore, we reconstruct the nonlinear force-free field (NLFFF) of the active region using the regularized Biot-Savart laws (RBSL) and the magnetofrictional method to reveal the three-dimensional (3D) magnetic field properties of the FC.
Results: We find that partial filament materials are quickly transferred to longer magnetic field lines formed by small-scale magnetic reconnection, as evidenced by dot-like Hα and extreme ultraviolet (EUV) brightenings and subsequent bidirectional outflow jets, as well as untwisting motions. The Hα and EUV bursts appear repeatedly at the same location and are closely associated with flux cancelation, which occurs between two small-scale opposite polarities and is driven by shearing and converging motions. The 3D NLFFF model reveals that the reconnection takes place in a hyperbolic flux tube that is located above the flux-cancelation site and below the FC.
Conclusions: The FC is gradually built up toward a twisted flux rope via a series of small-scale reconnection events that occur intermittently prior to the eruption.

Movie associated to Fig. 3 is available at https://www.aanda.org Title: Current-sheet Oscillations Caused by the Kelvin-Helmholtz Instability at the Loop Top of Solar Flares Authors: Wang, Yulei; Cheng, Xin; Ren, Zining; Ding, Mingde Bibcode: 2022ApJ...931L..32W Altcode: 2022arXiv220510361W Current sheets (CSs), long stretching structures of magnetic reconnection above solar flare loops, are usually observed to oscillate; their origins, however, are still puzzled at present. Based on a high-resolution 2.5D MHD simulation of magnetic reconnection, we explore the formation mechanism of CS oscillations. We find that large-amplitude transverse waves are excited by the Kelvin-Helmholtz instability at the highly turbulent cusp-shaped region. The perturbations propagate upward along the CS with a phase speed close to local Alfvén speed thus resulting in the CS oscillations we observe. Though the perturbations damp after propagating for a long distance, the CS oscillations are still detectable. In terms of detected CS oscillations, with a combination of differential emission measure techniques, we propose a new method for measuring the magnetic field strength of the CS and its distribution in height. Title: An approximate recipe of chromospheric radiative losses for solar flares Authors: Hong, J.; Carlsson, M.; Ding, M. D. Bibcode: 2022A&A...661A..77H Altcode: 2022arXiv220307630H Context. Radiative losses in the chromosphere are very important for the energy balance of the Sun. There have been efforts to make simple lookup tables for chromospheric radiative losses in the quiet Sun. During solar flares, the atmospheric conditions are quite different, and the currently available recipe is constructed from semi-empirical models. How these recipes work in flare conditions remains to be evaluated.
Aims: We aim to construct an approximate recipe of chromospheric radiative losses for solar flares.
Methods: We tabulate the optically thin radiative loss, escape probability, and ionization fraction using a grid of flare models from radiative hydrodynamic simulations as our dataset.
Results: We provide new lookup tables to calculate chromospheric radiative losses for flares. Compared with previous recipes, our recipe provides a better approximation of the detailed radiative losses for flares. Title: Three-dimensional Magnetic and Thermodynamic Structures of Solar Microflares Authors: Li, Z. F.; Cheng, X.; Chen, F.; Chen, J.; Ding, M. D. Bibcode: 2022ApJ...930L...7L Altcode: 2022arXiv220407762L Microflares, one of the small-scale solar activities, are believed to be caused by magnetic reconnection. Nevertheless, their three-dimensional (3D) magnetic structures, thermodynamic structures, and physical links to reconnection are unclear. In this Letter, based on a high-resolution 3D radiative magnetohydrodynamic simulation of the quiet Sun spanning from the upper convection zone to the corona, we investigate the 3D magnetic and thermodynamic structures of three homologous microflares. It is found that they originate from localized hot plasma embedded in the chromospheric environment at the height of 2-10 Mm above the photosphere and last for 3-10 minutes with released magnetic energy in the range of 1027-1028 erg. The heated plasma is almost cospatial with the regions where the heating rate per particle is maximal. The 3D velocity field reveals a pair of converging flows with velocities of tens of km s-1 moving toward and outflows with velocities of about 100 km s-1 moving away from the hot plasma. These features support magnetic reconnection playing a critical role in heating the localized chromospheric plasma to coronal temperature, giving rise to the observed microflares. The magnetic topology analysis further discloses that the reconnection region is located near quasi-separators where both current density and squashing factors are maximal although the specific topology may vary from a tether-cutting to fan-spine-like structure. Title: Quantifying the Magnetic Structure of a Coronal Shock Producing a Type II Radio Burst Authors: Su, W.; Li, T. M.; Cheng, X.; Feng, L.; Zhang, P. J.; Chen, P. F.; Ding, M. D.; Chen, L. J.; Guo, Y.; Wang, Y.; Li, D.; Zhang, L. Y. Bibcode: 2022ApJ...929..175S Altcode: 2022arXiv220311042S Type II radio bursts are thought to be produced by shock waves in the solar atmosphere. However, what magnetic conditions are needed for the generation of type II radio bursts is still a puzzling issue. Here, we quantify the magnetic structure of a coronal shock associated with a type II radio burst. Based on multiperspective extreme-ultraviolet observations, we reconstruct the three-dimensional (3D) shock surface. By using a magnetic field extrapolation model, we then derive the orientation of the magnetic field relative to the normal of the shock front (θ Bn) and the Alfvén Mach number (M A ) on the shock front. Combining the radio observations from the Nancay Radio Heliograph, we obtain the source region of the type II radio burst on the shock front. It is found that the radio burst is generated by a shock with M A ≳ 1.5 and a bimodal distribution of θ Bn. We also use the Rankine-Hugoniot relations to quantify the properties of the shock downstream. Our results provide a quantitative 3D magnetic structure condition of a coronal shock that produces a type II radio burst. Title: Groove Formation on Phobos from Orbital Ejecta of Stickney Crater Authors: Xi, X.; Ding, M.; Zhu, M. -H. Bibcode: 2022LPICo2678.1062X Altcode: Orbital ejecta from Stickney Crater can produce Phobos' grooves, according to combined crater formation, ejecta trajectory, and groove formation calculations. Title: Ejecta Pattern of Oblique Impacts on the Moon from Numerical Simulation Authors: Luo, X. -Z.; Zhu, M. -H.; Ding, M. Bibcode: 2022LPICo2678.1133L Altcode: We use 3D numerical simulation to investigate the combined effect of impact angles and impactor diameters on the ejecta pattern of oblique impacts on the Moon. Title: Comparison of the energy spectra between pileup shock and converging shock Authors: Wang, X.; Feng, X.; Yan, Y.; Ding, M.; Lu, H.; Shan, H. Bibcode: 2022icrc.confE1361W Altcode: 2022PoS...395E1361W No abstract at ADS Title: Automated Crater Detection and Absolute Model Age Estimation for Craters in the South Pole-Aitken Basin of the Moon Authors: Cui, X. -L.; Ding, M.; Wang, G. Bibcode: 2022LPICo2678.1900C Altcode: We use a convolutional neural network to detect craters in the South Pole-Aitken basin, and estimate model ages of mid-sized craters with detection results. Title: Resolving Two Distinct Thermal X-Ray Components in a Compound Solar Flare Authors: Zhou, Zhenjun; Liu, Rui; Sun, Jianqing; Zhang, Jie; Ding, Mingde; Cheng, Xin; Wang, Yuming; Yu, Xiaoyu; Liu, Lijuan; Cui, Jun Bibcode: 2022ApJ...925..132Z Altcode: 2021arXiv211204652Z X-ray emission provides the most direct diagnostics of the energy release process in solar flares. Occasionally, a superhot X-ray source is found to be above hot flare loops of ~10 MK temperature. While the origin of the superhot plasma is still elusive, it has conjured up an intriguing image of in situ plasma heating near the reconnection site high above the flare loops, in contrast to the conventional picture of chromospheric evaporation. Here we investigate an extremely long duration solar flare, in which EUV images show two distinct flare loop systems that appear successively along a Γ-shaped polarity inversion line (PIL). When both flare loop systems are present, the hard X-ray spectrum is found to be well fitted by combining a hot component (T e ~ 12 MK) and a superhot component (T e ~ 30 MK). Associated with a fast coronal mass ejection (CME), the superhot X-ray source is located at the top of the flare arcade that appears earlier, straddling and extending along the long "arm" of the Γ-shaped PIL. Associated with a slow CME, the hot X-ray source is located at the top of the flare arcade that appears later and sits astride the short "arm" of the Γ-shaped PIL. Aided by observations from a different viewing angle, we are able to verify that the superhot X-ray source is above the hot one in projection, but the two sources belong to different flare loop systems. Thus, this case study provides a stereoscopic observation explaining the coexistence of superhot and hot X-ray-emitting plasmas in solar flares. Title: Diagnosing the Optically Thick/Thin Features Using the Intensity Ratio of Si IV Resonance Lines in Solar Flares Authors: Zhou, Yi-An; Hong, Jie; Li, Y.; Ding, M. D. Bibcode: 2022ApJ...926..223Z Altcode: 2022arXiv220105305Z In the optically thin regime, the intensity ratio of the two Si IV resonance lines (1394 and 1403 Å) are theoretically the same as the ratio of their oscillator strengths, which is exactly 2. Here, we study the ratio of the integrated intensity of the Si IV lines (R = ∫I 1394(λ)dλ/∫I 1403(λ)dλ) and the ratio of intensity at each wavelength point (r(Δλ) = I 1394(Δλ)/I 1403(Δλ)) in two solar flares observed by the Interface Region Imaging Spectrograph. We find that at flare ribbons, the ratio R ranges from 1.8 to 2.3 and would generally decrease when the ribbons sweep across the slit position. In addition, the distribution of r(Δλ) shows a descending trend from the blue wing to the red wing. In loop cases, the Si IV line presents a wide profile with a central reversal. The ratio R deviates little from 2, but the ratio r(Δλ) can vary from 1.3 near the line center to greater than 2 in the line wings. Hence we conclude that in flare conditions, the ratio r(Δλ) varies across the line, due to the variation of the opacity at the line center and line wings. We notice that, although the ratio r(Δλ) could present a value that deviates from 2 as a result of the opacity effect near the line center, the ratio R is still close to 2. Therefore, caution should be taken when using the ratio of the integrated intensity of the Si IV lines to diagnose the opacity effect. Title: Multiwavelength and Dual-perspective Observations of Eruption and Untwisting of Two Homologous Magnetic Flux Ropes Authors: Song, De-Chao; Li, Y.; Su, Y.; Ding, M. D.; Gan, W. Q. Bibcode: 2021ApJ...922..238S Altcode: 2021arXiv210911187S In this paper, we present a detailed morphological, kinematic, and thermal analysis of two homologous magnetic flux ropes (MFRs) from NOAA 11515 on 2012 July 8-9. The study is based on multiwavelength and dual-perspective imaging observations from the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory Ahead spacecraft, which can reveal the structure and evolution of the two MFRs. We find that both of the MFRs show up in multiple passbands and their emissions mainly consist of a cold component peaking at a temperature of ~0.4-0.6 MK and a hot component peaking at ~7-8 MK. The two MFRs exhibit erupting, expanding, and untwisting motions that manifest distinctive features from two different viewpoints. Their evolution can be divided into two stages-a fast-eruption stage with speeds of about 105-125 km s-1 for MFR-1 and 50-65 km s-1 for MFR-2-and a slow-expansion (or untwisting) stage with speeds of about 10-35 km s-1 for MFR-1 and 10-30 km s-1 for MFR-2 in the plane of the sky. We also find that during the two-stage evolution, the high-temperature features mainly appear in the interface region between MFRs and ambient magnetic structures and also in the center of MFRs, which suggests that some heating processes take place in such places as magnetic reconnection and plasma compression. These observational results indicate that the eruption and untwisting processes of MFRs are coupled with the heating process, among which an energy conversion exists. Title: Annihilation of Magnetic Islands at the Top of Solar Flare Loops Authors: Wang, Yulei; Cheng, Xin; Ding, Mingde; Lu, Quanming Bibcode: 2021ApJ...923..227W Altcode: 2021arXiv211008526W The dynamics of magnetic reconnection in the solar current sheet (CS) is studied by high-resolution 2.5-dimensional MHD simulation. With the commencing of magnetic reconnection, a number of magnetic islands are formed intermittently and move quickly upward and downward along the CS. Upon collision with the semi-closed flux of the flare loops, the downflow islands cause a second reconnection with a rate comparable with that in the main CS. Though the time-integrated magnetic energy release is still dominated by the reconnection in the main CS, the second reconnection can release substantial magnetic energy, annihilating the main islands and generating secondary islands with various scales at the flare loop top. The distribution function of the flux of the secondary islands is found to follow a power law varying from $f\left(\psi \right)\sim {\psi }^{-1}$ (small scale) to ψ -2 (large scale), which seems to be independent to background plasma β and thermal conduction (TC). However, the spatial scale and the strength of the termination shocks driven by the main reconnection outflows or islands decrease if β increases or if TC is included. We suggest that the annihilation of magnetic islands at the flare loop top, which is not included in the standard flare model, plays a nonnegligible role in releasing magnetic energy to heat flare plasma and accelerate particles. Title: Revisiting the Spectral Features of Ellerman Bombs and UV Bursts. I. Radiative Hydrodynamic Simulations Authors: Hong, Jie; Li, Ying; Ding, M. D.; Hao, Qi Bibcode: 2021ApJ...921...50H Altcode: 2021arXiv210802699H Ellerman bombs (EBs) and UV bursts are both small-scale solar activities that occur in active regions. They are now believed to form at different heights in the lower atmosphere. In this paper, we use one-dimensional radiative hydrodynamic simulations to calculate various line profiles in response to heating in different atmospheric layers. We confirm that heating in the upper photosphere to the lower chromosphere can generate spectral features of typical EBs, while heating in the mid to upper chromosphere can generate spectral features of typical UV bursts. The intensity evolution of the Hα line wing in EBs shows a rise-plateau pattern, while that of the Si IV 1403 Å line center in UV bursts shows a rise-fall pattern. However, the predicted enhancement of the FUV continuum near 1400 Å for EBs is rarely reported and requires further observations to check it. With two heating sources or an extended heating source in the atmosphere, both EB and UV burst features could be reproduced simultaneously. Title: Data-constrained Magnetohydrodynamic Simulation of a Long-duration Eruptive Flare Authors: Guo, Yang; Zhong, Ze; Ding, M. D.; Chen, P. F.; Xia, Chun; Keppens, Rony Bibcode: 2021ApJ...919...39G Altcode: 2021arXiv210615080G We perform a zero-β magnetohydrodynamic simulation for the C7.7 class flare initiated at 01:18 UT on 2011 June 21 using the Message Passing Interface Adaptive Mesh Refinement Versatile Advection Code (MPI-AMRVAC). The initial condition for the simulation involves a flux rope, which we realize through the regularized Biot-Savart laws, whose parameters are constrained by observations from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and the Extreme Ultraviolet Imager (EUVI) on the twin Solar Terrestrial Relations Observatory (STEREO). This data-constrained initial state is then relaxed to a force-free state by the magnetofrictional module in MPI-AMRVAC. The further time-evolving simulation results reproduce the eruption characteristics obtained by SDO/AIA 94 Å, 304 Å, and STEREO/EUVI 304 Å observations fairly well. The simulated flux rope possesses similar eruption direction, height range, and velocity to the observations. In particular, the two phases of slow evolution and fast eruption are reproduced by varying the density distribution in the light of the draining process of the filament material. Our data-constrained simulations also show other advantages, such as a large field of view (about 0.76 R). We study the twist of the magnetic flux rope and the decay index of the overlying field, and find that in this event, both the magnetic strapping force and the magnetic tension force are sufficiently weaker than the magnetic hoop force, thus allowing the successful eruption of the flux rope. We also find that the anomalous resistivity is necessary to keep the correct morphology of the erupting flux rope. Title: Extreme-ultraviolet Late Phase in Homologous Solar Flares from a Complex Active Region Authors: Zhong, Y.; Dai, Y.; Ding, M. D. Bibcode: 2021ApJ...916...37Z Altcode: 2021arXiv210510069Z Recent observations in extreme-ultraviolet (EUV) wavelengths reveal a new late phase in some solar flares, which is seen as a second peak in warm coronal emissions (~3 MK) several tens of minutes to a few hours after the soft X-ray peak. The origin of the EUV late phase (ELP) is explained by either a long-lasting cooling process in the long ELP loops or a delayed energy ejection into the ELP loops well after the main flare heating. Using the observations with the Solar Dynamics Observatory, we investigate the production of the ELP in six homologous flares (F1-F6) originating from a complex active region (AR), NOAA Active Region 11283, with an emphasis on the emission characteristics of the flares. It is found that the main production mechanism of the ELP changes from additional heating in flare F1 to long-lasting cooling in flares F3-F6, with both mechanisms playing a role in flare F2. The transition is evidenced by an abrupt decrease of the time lag of the ELP peak, and the long-lasting cooling process in the majority of the flares is validated by a positive correlation between the flare ribbon fluence and the ELP peak intensity. We attribute the change in ELP production mechanism to an enhancement of the envelope magnetic field above the AR, which facilitates a more prompt and energetic heating of the ELP loops. In addition, the last and the only confined flare F6 exhibits an extremely large ELP. The different emission pattern revealed in this flare may reflect a different energy partitioning inside the ELP loops, which is due to a different magnetic reconnection process. Title: Thermodynamic Evolution of Solar Flare Supra-arcade Downflows Authors: Li, Z. F.; Cheng, X.; Ding, M. D.; Reeves, Katharine K.; Kittrell, DeOndre; Weber, Mark; McKenzie, David E. Bibcode: 2021ApJ...915..124L Altcode: 2021arXiv210709215L Solar flares are rapid energy release phenomena that appear as bright ribbons in the chromosphere and high temperature loops in the corona, respectively. Supra-arcade Downflows (SADs) are plasma voids that first come out above the flare loops and then move quickly toward the flare loop top during the decay phase of the flare. In our work, we study 20 SADs appearing in three flares. By differential emission measure (DEM) analysis, we calculate the DEM weighted average temperature and emission measure of the front region and the main body of SADs. It is found that the temperatures of the SAD front and body tend to increase during the course of SADs flowing downwards. The relationship between the pressure and temperature fits well with the adiabatic equation for both the SAD front and body, suggesting that the heating of SADs is mainly caused by adiabatic compression. Moreover, we also estimate the velocities of SADs via the Fourier Local Correlation Tracking method and find that increase of the temperature of the SAD front presents a correlation with the decrease of the SAD kinetic energy, while the SAD body does not, implying that the viscous process may also heat the SAD front in spite of a limited role. Title: Revealing the structure of light pseudoscalar mesons at the electron–ion collider Authors: Arrington, J.; Gayoso, C. Ayerbe; Barry, P. C.; Berdnikov, V.; Binosi, D.; Chang, L.; Diefenthaler, M.; Ding, M.; Ent, R.; Frederico, T.; Furletova, Y.; Hobbs, T. J.; Horn, T.; Huber, G. M.; Kay, S. J. D.; Keppel, C.; Lin, H. -W.; Mezrag, C.; Montgomery, R.; Pegg, I. L.; Raya, K.; Reimer, P.; Richards, D. G.; Roberts, C. D.; Rodríguez-Quintero, J.; Romanov, D.; Salmè, G.; Sato, N.; Segovia, J.; Stepanov, P.; Tadepalli, A. S.; Trotta, R. L. Bibcode: 2021JPhG...48g5106A Altcode: 2021arXiv210211788A The questions of how the bulk of the Universe's visible mass emerges and how it is manifest in the existence and properties of hadrons are profound, and probe the heart of strongly interacting matter. Paradoxically, the lightest pseudoscalar mesons appear to be key to a further understanding of the emergent mass and structure mechanisms. These mesons, namely, the pion and kaon, are the Nambu–Goldstone boson modes of quantum chromodynamics (QCD). Unravelling their partonic structure and the interplay between emergent and Higgs-boson mass mechanisms is a common goal of three interdependent approaches—continuum QCD phenomenology, lattice-regularised QCD, and the global analysis of parton distributions—linked to experimental measurements of hadron structure. Experimentally, the anticipated electron–ion collider will enable a revolution in our ability to study pion and kaon structures, accessed by scattering from the 'meson cloud' of the proton through the Sullivan process. With the goal of enabling a suite of measurements that can address these questions, we examine key reactions that identify the critical detector-system requirements needed to map tagged pion and kaon cross-sections over a wide range of kinematics. The excellent prospects for extracting pion structural, functional, and form-factor data are outlined, and similar prospects for kaon structures are discussed in the context of a worldwide programme. The successful completion of the programme outlined herein will deliver deep, far-reaching insights into the emergence of pions and kaons, their properties, and their role as QCD's Goldstone boson modes. Title: Evolution of the Toroidal Flux of CME Flux Ropes During Eruption Authors: Xing, C.; Cheng, X.; Ding, M. Bibcode: 2021AAS...23832206X Altcode: Coronal mass ejections (CMEs) are large-scale explosions of the coronal magnetic field. It is believed that magnetic reconnection significantly builds up the core structure of CMEs, a magnetic flux rope, during the eruption. However, the quantitative evolution of the flux rope, particularly its toroidal flux, is still unclear. In this work, we study the evolution of the toroidal flux of the CME flux rope for four events. The toroidal flux is estimated as the magnetic flux in the footpoint region of the flux rope, which is identified by a method that simultaneously takes the coronal dimming and the hook of the flare ribbon into account. We find that the toroidal flux of the CME flux rope for all four events shows a two-phase evolution: a rapid increasing phase followed by a decreasing phase. We further compare the evolution of the toroidal flux with that of the Geostationary Operational Environmental Satellites soft X-ray flux and find that they are basically synchronous in time, except that the peak of the former is somewhat delayed. The results suggest that the toroidal flux of the CME flux rope may be first quickly built up by the reconnection mainly taking place in the sheared overlying field and then reduced by the reconnection among the twisted field lines within the flux rope, as enlightened by a recent 3D magnetohydrodynamic simulation of CMEs. Title: Exploring the Nature of EUV Waves in a Radiative Magnetohydrodynamic Simulation Authors: Wang, Can; Chen, Feng; Ding, Mingde Bibcode: 2021ApJ...911L...8W Altcode: 2021arXiv210310326W Coronal extreme-ultraviolet (EUV) waves are large-scale disturbances propagating in the corona, whose physical nature and origin have been discussed for decades. We report the first three-dimensional (3D) radiative magnetohydrodynamic simulation of a coronal EUV wave and the accompanying quasi-periodic wave trains. The numerical experiment is conducted with the MURaM code and simulates the formation of solar active regions through magnetic flux emergence from the convection zone to the corona. The coronal EUV wave is driven by the eruption of a magnetic flux rope that also gives rise to a C-class flare. It propagates in a semicircular shape with an initial speed ranging from about 550 to 700 km s-1, which corresponds to an average Mach number (relative to fast magnetoacoustic waves) of about 1.2. Furthermore, the abrupt increase of the plasma density, pressure, and tangential magnetic field at the wave front confirms fast-mode shock nature of the coronal EUV wave. Quasi-periodic wave trains with a period of about 30 s are found as multiple secondary wavefronts propagating behind the leading wave front and ahead of the erupting magnetic flux rope. We also note that the true wave front in the 3D space can be very inhomogeneous; however, the line-of-sight integration of EUV emission significantly smoothes the sharp structures in 3D and leads to a more diffuse wave front. Title: Crustal Annulus of Impact Basins Controlled by Regional Thermal State of the Moon Authors: Ding, M.; Zhu, M. -H. Bibcode: 2021LPI....52.1097D Altcode: We recognize that the PKT impact basins show no crustal annulus that commonly exists for the FHT basins. We attribute the different crustal annulus to regional thermal state and use impact dynamic and viscoelastic models to test this hypothesis. Title: The role of non-axisymmetry of magnetic flux rope in constraining solar eruptions Authors: Zhong, Ze; Guo, Yang; Ding, M. D. Bibcode: 2021NatCo..12.2734Z Altcode: 2021arXiv210507339Z Whether a solar eruption is successful or failed depends on the competition between different components of the Lorentz force exerting on the flux rope that drives the eruption. The present models only consider the strapping force generated by the background magnetic field perpendicular to the flux rope and the tension force generated by the field along the flux rope. Using the observed magnetic field on the photosphere as a time-matching bottom boundary, we perform a data-driven magnetohydrodynamic simulation for the 30 January 2015 confined eruption and successfully reproduce the observed solar flare without a coronal mass ejection. Here we show a Lorentz force component, resulting from the radial magnetic field or the non-axisymmetry of the flux rope, which can essentially constrain the eruption. Our finding contributes to the solar eruption model and presents the necessity of considering the topological structure of a flux rope when studying its eruption behaviour. Title: From Nonlinear Force-free Field Models to Data-driven Magnetohydrodynamic Simulations Authors: Guo, Yang; Chen, P. F.; Keppens, Rony; Xia, Chun; Ding, Mingde; Yang, Kai; Zhong, Ze Bibcode: 2021cosp...43E1777G Altcode: To study the origin, structures, and dynamics of various solar activities, such as flares, prominences/filaments, and coronal mass ejections, we have to know the 3D magnetic field in the solar corona. Since many static phenomena in the corona live in a low beta environment, they can be modelled as a force-free state. We have implemented a new nonlinear force-free field (NLFFF) algorithm in the Message Passing Interface Adaptive Mesh Refinement Versatile Advection Code (MPI-AMRVAC), which could construct an NLFFF model in both Cartesian and spherical coordinate systems, and in all uniform, adaptive mesh refinement, and stretched grids. The NLFFF models have been applied to observations to study the magnetic structures of flux ropes, the coronal emission in extreme ultraviolet lines and the morphology of flare ribbons. To further study the dynamic eruption of a magnetic flux rope, we have developed a data-driven magnetohydrodynamic (MHD) model using the zero-beta MHD equations. The NLFFF model is served as the initial condition, and the time series of observed magnetic field and velocity field provide the boundary conditions. This model can reproduce the evolution of a magnetic flux rope in its dynamic eruptive phase. We also find that a data-constrained boundary condition, where the bottom boundary is fixed to the initial values, reproduces a similar simulation result as the data-driven simulation. The data-driven MHD model has also been applied to study a failed eruption, where the torus instability, kink instability, and additional components of Lorentz forces are studied in detail. Title: The magnetic flux rope structure of a triangulated solar filament Authors: Guo, Yang; Chen, P. F.; Keppens, Rony; Xia, Chun; Ding, Mingde; Xu, Yu Bibcode: 2021cosp...43E1734G Altcode: We construct a magnetic flux rope model for a prominence observed at 01:11 UT on 2011 June 21 in AR 11236 using the following methods, triangulation from multi perspective observations, the flux rope embedding method, the regularized Biot-Savart laws, and the magnetofrictional method. First, the prominence path is reconstructed with the triangulation with 304 Å images observed by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory (SDO) and by the Extreme Ultraviolet Imager on board the twin Solar Terrestrial Relations Observatory. Then, a flux rope is constructed with the regularized Biot-Savart laws using the information of its axis. Next, it is embedded into a potential magnetic field computed from the photospheric radial magnetic field observed by the Helioseismic and Magnetic Imager on board SDO. The combined magnetic field is finally relaxed by the magnetofrictional method to reach a nonlinear force-free state. It is found that both models constructed by the regularized Biot-Savart laws and after the magnetofrictional relaxation coincide with the 304 Å images. The distribution of magnetic dips coincides with part of the prominence material, and the quasi-separatrix layers wrap the magnetic flux ropes, displaying hyperbolic flux tube structures. These models have the advantages of constructing magnetic flux ropes in the higher atmosphere and weak magnetic field regions, which could be used as initial conditions for magnetohydrodynamic simulations of coronal mass ejections. Title: Higgs modulation of emergent mass as revealed in kaon and pion parton distributions Authors: Cui, Z. -F.; Ding, M.; Gao, F.; Raya, K.; Binosi, D.; Chang, L.; Roberts, C. D.; Rodríguez-Quintero, J.; Schmidt, S. M. Bibcode: 2021EPJA...57....5C Altcode: 2020arXiv200614075C Strangeness was discovered roughly seventy years ago, lodged in a particle now known as the kaon, K. Kindred to the pion, π ; both states are massless in the absence of Higgs-boson couplings. Kaons and pions are Nature's most fundamental Nambu-Goldstone modes. Their properties are largely determined by the mechanisms responsible for emergent mass in the standard model, but modulations applied by the Higgs are crucial to Universe evolution. Despite their importance, little is known empirically about K and π structure. This study delivers the first parameter-free predictions for all K distribution functions (DFs) and comparisons with the analogous π distributions, i.e. the one-dimensional maps that reveal how the light-front momentum of these states is shared amongst the gluons and quarks from which they are formed. The results should stimulate improved analyses of existing data and motivate new experiments sensitive to all K and π DFs. Title: Diquark correlations in hadron physics: Origin, impact and evidence Authors: Barabanov, M. Yu.; Bedolla, M. A.; Brooks, W. K.; Cates, G. D.; Chen, C.; Chen, Y.; Cisbani, E.; Ding, M.; Eichmann, G.; Ent, R.; Ferretti, J.; Gothe, R. W.; Horn, T.; Liuti, S.; Mezrag, C.; Pilloni, A.; Puckett, A. J. R.; Roberts, C. D.; Rossi, P.; Salmé, G.; Santopinto, E.; Segovia, J.; Syritsyn, S. N.; Takizawa, M.; Tomasi-Gustafsson, E.; Wein, P.; Wojtsekhowski, B. B. Bibcode: 2021PrPNP.11603835B Altcode: 2020arXiv200807630B The last decade has seen a marked shift in how the internal structure of hadrons is understood. Modern experimental facilities, new theoretical techniques for the continuum bound-state problem and progress with lattice-regularised QCD have provided strong indications that soft quark+quark (diquark) correlations play a crucial role in hadron physics. For example, theory indicates that the appearance of such correlations is a necessary consequence of dynamical chiral symmetry breaking, viz. a corollary of emergent hadronic mass that is responsible for almost all visible mass in the universe; experiment has uncovered signals for such correlations in the flavour-separation of the proton's electromagnetic form factors; and phenomenology suggests that diquark correlations might be critical to the formation of exotic tetra- and penta-quark hadrons. A broad spectrum of such information is evaluated herein, with a view to consolidating the facts and therefrom moving toward a coherent, unified picture of hadron structure and the role that diquark correlations might play. Title: Spectroscopic Observations of High-speed Downflows in a C1.7 Solar Flare Authors: Zhou, Yi-An; Li, Y.; Ding, M. D.; Hong, Jie; Yu, Ke Bibcode: 2020ApJ...904...95Z Altcode: 2020arXiv200906158Z In this paper, we analyze the high-resolution UV spectra for a C1.7 solar flare (SOL2017-09-09T06:51) observed by the Interface Region Imaging Spectrograph (IRIS). We focus on the spectroscopic observations at the locations where the cool lines of Si IV 1402.8 Å (∼104.8 K) and C II 1334.5/1335.7 Å (∼104.4 K) reveal significant redshifts with Doppler velocities up to ∼150 km s-1. These redshifts appear in the rise phase of the flare, then increase rapidly, reach the maximum in a few minutes, and proceed into the decay phase. Combining the images from IRIS and Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we propose that the redshifts in the cool lines are caused by the downflows in the transition region and upper chromospheric layers, which likely result from a magnetic reconnection leading to the flare. In addition, the cool Si IV and C II lines show gentle redshifts (a few tens of km s-1) at some other locations, which manifest some distinct features from the above locations. This is supposed to originate from a different physical process. Title: The Kinematic Evolution of Erupting Structures in Confined Solar Flares Authors: Huang, Z. W.; Cheng, X.; Ding, M. D. Bibcode: 2020ApJ...904L...2H Altcode: 2020arXiv201111929H In this Letter, we study the kinematic properties of ascending hot blobs associated with confined flares. Taking advantage of high-cadence extreme-ultraviolet images provided by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we find that for the 26 events selected here, the hot blobs are first impulsively accelerated outward, but then quickly slow down to motionlessness. Their velocity evolution is basically synchronous with the temporal variation of the Geostationary Operational Environmental Satellite soft X-ray flux of the associated flares, except that the velocity peak precedes the soft X-ray peak by minutes. Moreover, the duration of the acceleration phase of the erupting blobs is moderately correlated with that of the flare rise phase. For nine of the 26 cases, the erupting blobs even appear minutes prior to the onset of the associated flares. Our results show that a fraction of confined flares also involve the eruption of a magnetic flux rope, which sometimes is formed and heated prior to the flare onset. We suggest that the initiation and development of these confined flares are similar to that of eruptive ones, and the main difference may lie in the background field constraint, which is stronger for the former than for the latter. Title: The Lyα Emission in Solar Flares. I. A Statistical Study on Its Relationship with the 1-8 ŠSoft X-Ray Emission Authors: Jing, Zhichen; Pan, Wuqi; Yang, Yukun; Song, Dechao; Tian, Jun; Li, Y.; Cheng, X.; Hong, Jie; Ding, M. D. Bibcode: 2020ApJ...904...41J Altcode: 2020arXiv200910358J We statistically study the relationship between the Lyα and 1-8 Å soft X-ray (SXR) emissions from 658 M- and X-class solar flares observed by the Geostationary Operational Environmental Satellite during 2006-2016. Based on the peak times of the two wave band emissions, we divide the flares into three types. Type I (III) has an earlier (a later) peak time in the Lyα emission than that in the SXR emission, while type II has nearly the same peak time (within the time resolution of 10 s) between the Lyα and SXR emissions. In these 658 flares, we find that there are 505 (76.8%) type I flares, 10 (1.5%) type II flares, and 143 (21.7%) type III flares, and that the three types appear to have no dependence on the flare duration, flare location, or solar cycle. Besides the main peak, the Lyα emission of the three type flares also shows sub-peaks which can appear in the impulsive or gradual phase of the flare. It is found that the main-peak (for type I) and subpeak (for type III) emissions of Lyα that appear in the impulsive phase follow the Neupert effect in general. This indicates that such Lyα emissions are related to the nonthermal electron beam heating. While the main-peak (for type III) and subpeak (for type I) emissions of Lyα that appear in the gradual phase are supposed to be primarily contributed by the thermal plasma that cools down. Title: Evolution of the Toroidal Flux of CME Flux Ropes during Eruption Authors: Xing, C.; Cheng, X.; Ding, M. D. Bibcode: 2020Innov...100059X Altcode: 2020arXiv201110750X Coronal mass ejections (CMEs) are large-scale explosions of the coronal magnetic field. It is believed that magnetic reconnection significantly builds up the core structure of CMEs, a magnetic flux rope, during the eruption. However, the quantitative evolution of the flux rope, particularly its toroidal flux, is still unclear. In this paper, we study the evolution of the toroidal flux of the CME flux rope for four events. The toroidal flux is estimated as the magnetic flux in the footpoint region of the flux rope, which is identified by a method that simultaneously takes the coronal dimming and the hook of the flare ribbon into account. We find that the toroidal flux of the CME flux rope for all four events shows a two-phase evolution: a rapid increasing phase followed by a decreasing phase. We further compare the evolution of the toroidal flux with that of the Geostationary Operational Environmental Satellites soft X-ray flux and find that they are basically synchronous in time, except that the peak of the former is somewhat delayed. The results suggest that the toroidal flux of the CME flux rope may be first quickly built up by the reconnection mainly taking place in the sheared overlying field and then reduced by the reconnection among the twisted field lines within the flux rope, as enlightened by a recent 3D magnetohydrodynamic simulation of CMEs. Title: The Magnetic Topology and Eruption Mechanism of a Multiple-ribbon Flare Authors: Qiu, Ye; Guo, Yang; Ding, Mingde; Zhong, Ze Bibcode: 2020ApJ...901...13Q Altcode: 2020arXiv200808866Q Multiple-ribbon flares are usually complex in their magnetic topologies and eruption mechanisms. In this paper, we investigate an X2.1 flare (SOL2015-03-11T16:22) that occurred in active region 12297 near the center of the solar disk by both potential and nonlinear force-free field models extrapolated with the data observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We calculate the three-dimensional squashing degree distribution. The results reveal that there are two flux ropes in this active region covered by a large-scale hyperbolic flux tube (HFT), which is the intersection of quasi-separatrix layers with a null point embedded in it. When the background magnetic field diminishes due to the separation of the northwest dipole and the flux cancellation, the central flux rope rises up, forming the two brightest central ribbons. It then squeezes the overlying HFT structure to generate further brightenings. This very energetic flare with a complex shape is accompanied by a coronal mass ejection (CME). We adopt the simplified line-tied force-balance equation of the current ring model and assign the observed value of the decay index to the equation to simulate the acceleration profile of the CME in the early stage. It is found that the path with an inclination of 45° from radial best fits the profile of the actual acceleration. Title: Spectral Diagnostics of Solar Photospheric Bright Points Authors: Hao, Q.; Fang, C.; Ding, M. D.; Li, Z.; Cao, Wenda Bibcode: 2020ApJ...900..130H Altcode: 2020arXiv200709675H 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. Title: Non-LTE Calculations of the Mg I 12.32 μm Line in a Flaring Atmosphere Authors: Hong, Jie; Bai, Xianyong; Li, Ying; Ding, M. D.; Deng, Yuanyong Bibcode: 2020ApJ...898..134H Altcode: 2020arXiv200606108H The infrared Mg I lines near 12 μm are a pair of emission lines that are magnetically sensitive and have been used to measure solar magnetic fields. Here we calculate the response of the Mg I 12.32 μm line during a flare and find that in our modeling this line has a complicated behavior. At the beginning of the flare heating, this line shows an intensity dimming at the line center. The intensity then increases when heating continues, with increasing contributions from the heated layers in the chromosphere. The line formation height and the line width also increase as a result. As for the polarized line profiles, we find that flare heating tends to decrease the Zeeman splitting width and attenuates the Stokes V lobe intensity. The wider features in the Stokes V profiles are more pronounced during flare heating, which should be considered when performing magnetic field inversions. Title: What Determines Solar Flares Producing Interplanetary Type III Radio Bursts? Authors: Kou, Y. K.; Jing, Z. C.; Cheng, X.; Pan, W. Q.; Liu, Y.; Li, C.; Ding, M. D. Bibcode: 2020ApJ...898L..24K Altcode: 2020arXiv200703852K Energetic electrons accelerated by solar flares often give rise to type III radio bursts at a broad wave band and even interplanetary type III bursts (IT3) if the wavelength extends to a decameter-kilometer. In this Letter, we investigate the probability of the flares that produce IT3, based on the sample of 2272 flares above M-class observed from 1996 to 2016. It is found that only 49.6% of the flares are detected to be accompanied with IT3. The duration, peak flux, and fluence of the flares with and without IT3 both present power-law distributions in the frequency domain, but the corresponding spectral indices for the former (2.06 ± 0.17, 2.04 ± 0.18, and 1.55 ± 0.09) are obviously smaller than that for the latter (2.82 ± 0.22, 2.51 ± 0.19, and 2.40 ± 0.09), showing that the flares with IT3 have longer durations and higher peak fluxes. We further examine the relevance of coronal mass ejections (CMEs) to the two groups of flares. It is found that 58% (655 of 1127) of the flares with IT3 but only 19% (200 of 1078) of the flares without IT3 are associated with CMEs, and that the associated CMEs for the flares with IT3 are inclined to be wider and faster. This indicates that CMEs may also play a role in producing IT3, speculatively facilitating the escape of accelerated electrons from the low corona to the interplanetary space. Title: New Accurate Atomic Data for Iron-group Elements for Astrophysics Applications Authors: Concepcion Mairey, F.; Belmonte, M.; Clear, C.; Liggins, F.; Pickering, J.; Ding, M. Bibcode: 2020AAS...23624706C Altcode: Modern observations of astrophysical spectra are in many cases of higher quality than those observed within a laboratory setting. This can result in the possibility of inaccurate conclusions being drawn. There is a need in astrophysics for improved atomic data for light and heavy elements across the spectrum, from IR to vacuum-UV (VUV). Accurate measurements of spectral line wavelengths and oscillator strengths are required particularly for use in stellar models and chemical abundance calculations, and in surveys such as Gaia-ESO or APOGEE and future surveys. Laboratory astrophysicists aim to measure the atomic data most useful for astronomers. Atomic data of iron-group elements are particularly important due to their high abundance and line-rich spectra. Our high-resolution Fourier Transform Spectrometry (FTS) group at Imperial College London, UK, has, supported by STFC, been providing accurate atomic data for use in astrophysics. Recent results include new Fe I oscillator strengths for use in Galactic surveys (Belmonte et al. 2017). There has been an order of magnitude improvement in atomic data for Co III (Smillie et al. 2016) and in the accuracy of energy levels and transition wavelengths for Mn II (Liggins, PhD Physics, Imperial College, 2018) and Ni II (Clear, PhD Physics, Imperial College, 2018). The first high-resolution measurements of UV transition wavelengths of Cr III are being used as wavelength standards (Smillie et al. 2008). Using new spectra recorded at the National Institute of Standards and Technology (NIST) and in the VUV at Imperial College, analysis is underway for accurate wavelengths and atomic energy levels in Mn I and Fe III. The study of the hyperfine structure of Co II is also ongoing using FTS data. Title: IRIS Si IV Line Profiles at Flare Ribbons as Indications of Chromospheric Condensation Authors: Yu, Ke; Li, Y.; Ding, M. D.; Li, D.; Zhou, Yi-An; Hong, Jie Bibcode: 2020ApJ...896..154Y Altcode: 2020arXiv200502029Y We present temporal variations of the Si IV line profiles at the flare ribbons in three solar flares observed by the Interface Region Imaging Spectrograph. In the M1.1 flare on 2014 September 6 and the X1.6 flare on 2014 September 10, the Si IV line profiles evolve from wholly redshifted to red-wing enhanced with the flare development. However, in the B1.8 flare on 2016 December 2, the Si IV line profiles are wholly redshifted throughout the flare evolution. We fit the wholly redshifted line profiles with a single-Gaussian function, but fit the red-asymmetric ones with a double-Gaussian function to deduce the corresponding Doppler velocities. In addition, we find that hard X-ray emission above 25 keV shows up in the two large flares, implying a nonthermal electron beam heating. In the microflare, there only appears weak hard X-ray emission up to 12 keV, indicative of a mostly thermal heating. We interpret the redshifts or red asymmetries of the Si IV line at the ribbons in the three flares as spectral manifestations of chromospheric condensation. We propose that whether the line appears to be wholly redshifted or red-asymmetric depends on the heating mechanisms, as well as on the propagation of the condensation. Title: Initiation and Early Kinematic Evolution of Solar Eruptions Authors: Cheng, X.; Zhang, J.; Kliem, B.; Török, T.; Xing, C.; Zhou, Z. J.; Inhester, B.; Ding, M. D. Bibcode: 2020ApJ...894...85C Altcode: 2020arXiv200403790C We investigate the initiation and early evolution of 12 solar eruptions, including six active-region hot channel and six quiescent filament eruptions, which were well observed by the Solar Dynamics Observatory, as well as by the Solar Terrestrial Relations Observatory for the latter. The sample includes one failed eruption and 11 coronal mass ejections, with velocities ranging from 493 to 2140 km s-1. A detailed analysis of the eruption kinematics yields the following main results. (1) The early evolution of all events consists of a slow-rise phase followed by a main-acceleration phase, the height-time profiles of which differ markedly and can be best fit, respectively, by a linear and an exponential function. This indicates that different physical processes dominate in these phases, which is at variance with models that involve a single process. (2) The kinematic evolution of the eruptions tends to be synchronized with the flare light curve in both phases. The synchronization is often but not always close. A delayed onset of the impulsive flare phase is found in the majority of the filament eruptions (five out of six). This delay and its trend to be larger for slower eruptions favor ideal MHD instability models. (3) The average decay index at the onset heights of the main acceleration is close to the threshold of the torus instability for both groups of events (although, it is based on a tentative coronal field model for the hot channels), suggesting that this instability initiates and possibly drives the main acceleration. Title: Erratum: "Modeling the IRIS Lines During a Flare. I. The Blue-wing Enhancement in the Mg II k Line" (2020, ApJ, 890, 115) Authors: Hong, Jie; Li, Ying; Ding, M. D.; Zhou, Yu-Hao Bibcode: 2020ApJ...894..160H Altcode: No abstract at ADS Title: Control Parameters on Gravitational Signature of Large Impact Craters on the Moon Authors: Ding, M.; Soderblom, J. M.; Nimmo, F.; Bierson, C. J.; Zuber, M. T. Bibcode: 2020LPI....51.1329D Altcode: The lack of statistical distinction in the Bouguer anomaly between large FHT and SPA craters is due to competing effects of thermal state and crustal thickness. Title: Quantifying the Toroidal Flux of Preexisting Flux Ropes of Coronal Mass Ejections Authors: Xing, C.; Cheng, X.; Qiu, Jiong; Hu, Qiang; Priest, E. R.; Ding, M. D. Bibcode: 2020ApJ...889..125X Altcode: 2019arXiv191210623X In past decades, much progress has been achieved in understanding the origin and evolution of coronal mass ejections (CMEs). In situ observations of the counterparts of CMEs, especially magnetic clouds (MCs) near the Earth, have provided measurements of the structure and total flux of CME flux ropes. However, it has been difficult to measure these properties in an erupting CME flux rope, in particular in a preexisting flux rope. In this work, we propose a model to estimate the toroidal flux of a preexisting flux rope by subtracting the flux contributed by magnetic reconnection during the eruption from the flux measured in the MC. The flux contributed by the reconnection is derived from geometric properties of two-ribbon flares based on a quasi-2D reconnection model. We then apply the model to four CME/flare events and find that the ratio of toroidal flux in the preexisting flux rope to that in the associated MC lies in the range 0.40-0.88. This indicates that the toroidal flux of the preexisting flux rope makes an important contribution to that of the CME flux rope and is usually at least as large as the flux arising from the eruption process for the selected events. Title: Modeling the IRIS Lines During a Flare. I. The Blue-wing Enhancement in the Mg II k Line Authors: Hong, Jie; Li, Ying; Ding, M. D.; Zhou, Yu-Hao Bibcode: 2020ApJ...890..115H Altcode: 2020arXiv200105677H The Interface Region Imaging Spectrograph (IRIS) Mg II k line serves as a very good tool to diagnose the heating processes in solar flares. Recent studies have shown that apart from the usual red asymmetries that are interpreted as the result of condensation downflows, this line could also show a blue-wing enhancement. To investigate how such a blue asymmetry is formed, we perform a grid of radiative hydrodynamic simulations and calculate the corresponding line profiles. We find that such a spectral feature is likely to originate from the upward plasma motion in the upper chromosphere. However, the formation region that is responsible for the blue-wing enhancement could be located in an evaporation region, in an upward-moving blob, and even an upward-moving condensation region. We discuss how the electron beam parameters affect these different dynamics of the atmosphere. Title: Formation and Eruption of a Mini-sigmoid Originating in Coronal Hole Authors: Huang, Z. W.; Cheng, X.; Su, Y. N.; Liu, T.; Ding, M. D. Bibcode: 2019ApJ...887..130H Altcode: 2019arXiv191210404H In this paper, we study in detail the evolution of a mini-sigmiod originating in a cross-equatorial coronal hole, where the magnetic field is mostly open and seriously distinct from the closed background field above active-region sigmoids. The source region first appeared as a bipole, which subsequently experienced a rapid emergence followed by a long-term decay. Correspondingly, the coronal structure initially appeared as arc-like loops, then gradually sheared and transformed into continuously sigmoidal loops, mainly owing to flux cancellation near the polarity inversion line. The temperature of J-shaped and sigmoidal loops is estimated to be about 2.0 × 106 K, greater than that of the background coronal hole. Using the flux-rope insertion method, we further reconstruct the nonlinear force-free fields that well reproduces the transformation of the potential field into a sigmoidal field. The fact that the sheared and sigmoidal loops are mainly concentrated at around the high-Q region implies that the reconnection most likely takes place there to form the sigmoidal field and heat the plasma. Moreover, the twist of sigmoidal field lines is estimated to be around 0.8, less than the values derived for the sigmoids from active regions. However, the sigmoidal flux may quickly enter an unstable regime at the very low corona (<10 Mm) due to the open background field. The results suggest that the mini-sigmoid, at least the one in our study, has the same formation and eruption process as the large-scale one, but is significantly influenced by the overlying flux. Title: Particle Acceleration at the Pileup Collision of the Twin Shock Authors: Wang, Xin; Giacalone, Joe; Yan, Yihua; Ding, Mingde; Li, Chuan; Lu, Hong; Shan, Hao Bibcode: 2019ApJ...885...66W Altcode: Ground-level enhancement (GLE) events are often associated with large gradual solar events such as fast coronal mass ejections (CMEs), but not all fast CMEs lead to GLE events. Is there a type of coordinated CME that could produce GLEs with larger intensity and higher energies than those in the normal fast isolated CMEs? Here we propose a twin-shock scenario driven by the twin CME coordinately, in which the posterior shock catches up with the preceding shock and has a pileup collision. In the present study, we chose the first GLE event of the solar cycle 24 occurring on 2012 May 17 as an example to investigate the probable association with the twin-shock scenario. We use a dynamic Monte Carlo method to examine the energy spectrum with relevance to the GLE event. In the twin-shock scenario, the seed energetic particles produced by the normal preceding shock can be injected into the posterior shock for reacceleration efficiently. As a result, we obtain the detailed energy spectrum of the solar energetic particles (SEPs) with different behaviors at the related episodes of the twin-shock evolution. Therefore, we predict that the pileup collision of the twin shock would dominate a concave energy spectral slope in the 2012 May 17 SEP event. Title: The Magnetic Flux Rope Structure of a Triangulated Solar Filament Authors: Guo, Yang; Xu, Yu; Ding, M. D.; Chen, P. F.; Xia, Chun; Keppens, Rony Bibcode: 2019ApJ...884L...1G Altcode: Solar magnetic flux ropes are core structures driving solar activities. We construct a magnetic flux rope for a filament/prominence observed at 01:11 UT on 2011 June 21 in AR 11236 with a combination of state-of-the-art methods, including triangulation from multiperspective observations, the flux rope embedding method, the regularized Biot-Savart laws, and the magnetofrictional method. First, the path of the filament is reconstructed via the triangulation with 304 Å images observed by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory (SDO) and by the Extreme Ultraviolet Imager on board the twin Solar Terrestrial Relations Observatory. Then, a flux rope is constructed with the regularized Biot-Savart laws using the information of its axis. Next, it is embedded into a potential magnetic field computed from the photospheric radial magnetic field observed by the Helioseismic and Magnetic Imager on board SDO. The combined magnetic field is finally relaxed by the magnetofrictional method to reach a nonlinear force-free state. It is found that both models constructed by the regularized Biot-Savart laws and after the magnetofrictional relaxation coincide with the 304 Å images. The distribution of magnetic dips coincides with part of the filament/prominence material, and the quasi-separatrix layers wrap the magnetic flux ropes, displaying hyperbolic flux tube structures. These models have the advantages of constructing magnetic flux ropes in the higher atmosphere and weak magnetic field regions, which could be used as initial conditions for magnetohydrodynamic simulations of coronal mass ejections. Title: Measurement of the cosmic ray proton spectrum from 40 GeV to 100 TeV with the DAMPE satellite Authors: An, Q.; Asfandiyarov, R.; Azzarello, P.; Bernardini, P.; Bi, X. J.; Cai, M. S.; Chang, J.; Chen, D. Y.; Chen, H. F.; Chen, J. L.; Chen, W.; Cui, M. Y.; Cui, T. S.; Dai, H. T.; D'Amone, A.; De Benedittis, A.; De Mitri, I.; Di Santo, M.; Ding, M.; Dong, T. K.; Dong, Y. F.; Dong, Z. X.; Donvito, G.; Droz, D.; Duan, J. L.; Duan, K. K.; D'Urso, D.; Fan, R. R.; Fan, Y. Z.; Fang, F.; Feng, C. Q.; Feng, L.; Fusco, P.; Gallo, V.; Gan, F. J.; Gao, M.; Gargano, F.; Gong, K.; Gong, Y. Z.; Guo, D. Y.; Guo, J. H.; Guo, X. L.; Han, S. X.; Hu, Y. M.; Huang, G. S.; Huang, X. Y.; Huang, Y. Y.; Ionica, M.; Jiang, W.; Jin, X.; Kong, J.; Lei, S. J.; Li, S.; Li, W. L.; Li, X.; Li, X. Q.; Li, Y.; Liang, Y. F.; Liang, Y. M.; Liao, N. H.; Liu, C. M.; Liu, H.; Liu, J.; Liu, S. B.; Liu, W. Q.; Liu, Y.; Loparco, F.; Luo, C. N.; Ma, M.; Ma, P. X.; Ma, S. Y.; Ma, T.; Ma, X. Y.; Marsella, G.; Mazziotta, M. N.; Mo, D.; Niu, X. Y.; Pan, X.; Peng, W. X.; Peng, X. Y.; Qiao, R.; Rao, J. N.; Salinas, M. M.; Shang, G. Z.; Shen, W. H.; Shen, Z. Q.; Shen, Z. T.; Song, J. X.; Su, H.; Su, M.; Sun, Z. Y.; Surdo, A.; Teng, X. J.; Tykhonov, A.; Vitillo, S.; Wang, C.; Wang, H.; Wang, H. Y.; Wang, J. Z.; Wang, L. G.; Wang, Q.; Wang, S.; Wang, X. H.; Wang, X. L.; Wang, Y. F.; Wang, Y. P.; Wang, Y. Z.; Wang, Z. M.; Wei, D. M.; Wei, J. J.; Wei, Y. F.; Wen, S. C.; Wu, D.; Wu, J.; Wu, L. B.; Wu, S. S.; Wu, X.; Xi, K.; Xia, Z. Q.; Xu, H. T.; Xu, Z. H.; Xu, Z. L.; Xu, Z. Z.; Xue, G. F.; Yang, H. B.; Yang, P.; Yang, Y. Q.; Yang, Z. L.; Yao, H. J.; Yu, Y. H.; Yuan, Q.; Yue, C.; Zang, J. J.; Zhang, F.; Zhang, J. Y.; Zhang, J. Z.; Zhang, P. F.; Zhang, S. X.; Zhang, W. Z.; Zhang, Y.; Zhang, Y. J.; Zhang, Y. L.; Zhang, Y. P.; Zhang, Y. Q.; Zhang, Z.; Zhang, Z. Y.; Zhao, H.; Zhao, H. Y.; Zhao, X. F.; Zhou, C. Y.; Zhou, Y.; Zhu, X.; Zhu, Y.; Zimmer, S. Bibcode: 2019SciA....5.3793A Altcode: 2019arXiv190912860A The precise measurement of the spectrum of protons, the most abundant component of the cosmic radiation, is necessary to understand the source and acceleration of cosmic rays in the Milky Way. This work reports the measurement of the cosmic ray proton fluxes with kinetic energies from 40 GeV to 100 TeV, with two and a half years of data recorded by the DArk Matter Particle Explorer (DAMPE). This is the first time an experiment directly measures the cosmic ray protons up to ~100 TeV with a high statistics. The measured spectrum confirms the spectral hardening found by previous experiments and reveals a softening at ~13.6 TeV, with the spectral index changing from ~2.60 to ~2.85. Our result suggests the existence of a new spectral feature of cosmic rays at energies lower than the so-called knee, and sheds new light on the origin of Galactic cosmic rays. Title: The Response of the Lyα Line in Different Flare Heating Models Authors: Hong, Jie; Li, Ying; Ding, M. D.; Carlsson, Mats Bibcode: 2019ApJ...879..128H Altcode: 2019arXiv190513356H The solar Lyα line is the strongest line in the ultraviolet waveband, and is greatly enhanced during solar flares. Here we present radiative hydrodynamic simulations of solar flares under different heating models, and calculate the response of this line taking into account nonequilibrium ionization of hydrogen and partial frequency redistribution. We find that in nonthermal heating models, the Lyα line can show a red or blue asymmetry corresponding to the chromospheric evaporation or condensation, respectively. The asymmetry may change from red to blue if the electron beam flux is large enough to produce a significant chromospheric condensation region. In the Lyα intensity light curve, a dip appears when the change of asymmetry occurs. In thermal models, the Lyα line intensity peaks quickly and then falls, and the profile has an overall red asymmetry, which is similar to the profiles from heating by a soft electron beam. The Lyα profile shows a single red peak at the end of thermal heating, and the whole line is formed in a very small height range. Title: Solar Energetic particles at pileup collisions of the multiple-shock Authors: Wang, X.; Yan, Y.; Ding, M. Bibcode: 2019ICRC...36.1168W Altcode: 2019PoS...358.1168W No abstract at ADS Title: Different Signatures of Chromospheric Evaporation in Two Solar Flares Observed with IRIS Authors: Li, Y.; Ding, M. D.; Hong, J.; Li, H.; Gan, W. Q. Bibcode: 2019ApJ...879...30L Altcode: 2019arXiv190509709L We present different signatures of chromospheric evaporation in two solar flares observed by the Interface Region Imaging Spectrograph (IRIS). In the B1.6 flare on 2016 December 6 (SOL2016-12-06T10:40), the transition region Si IV line and the chromospheric C II and Mg II lines show blueshifts with low velocities up to 20 km s-1 at the flare loop footpoints in the rise phase, indicative of a gentle chromospheric evaporation. While in the C1.6 flare on 2015 December 19 (SOL2015-12-19T10:51), the Si IV, C II, and Mg II lines exhibit redshifts with velocities from several to tens of km s-1 at the footpoints, which might suggest an explosive chromospheric evaporation. Explosive evaporation has been observed in many flares that were captured by IRIS; however, gentle evaporation, especially manifested as blueshifts in the cool Si IV, C II, and Mg II lines, has scarcely been reported. Our results bring some new insights into chromospheric evaporation in the IRIS era. Title: Chinese Sunspot Drawings and Their Digitization - (I) Parameter Archives Authors: Lin, G. H.; Wang, X. F.; Liu, S.; Yang, X.; Zhu, G. F.; Deng, Y. Y.; Ji, H. S.; Zhou, T. H.; Sun, L. N.; Feng, Y. L.; Liu, Z. Z.; Tao, J. P.; Ben, M. X.; Lin, J.; Ding, M. D.; Li, Z.; Zheng, S.; Zeng, S. G.; He, H. L.; Zeng, X. Y.; Shu, Y.; Sun, X. B. Bibcode: 2019SoPh..294...79L Altcode: 2019arXiv190412316L Based on the Chinese historical sunspots drawings, a data set consisting of the scanned images and all their digitized parameters from 1925 to 2015 have been constructed. In this paper, we briefly describe the developmental history of sunspots drawings in China. This paper describes the preliminary handling processes that start from the initial scanning to the extraction of parameters, and finally summarizes the general features of this data set. It is the first systematic project in the Chinese solar-physics community to digitize the historical series of sunspot drawings. Our data set fills in an almost 90-year historical gap, which originates from an area spanning 60 degrees from east to west and 50 degrees from north to south and has no continuous and detailed digital sunspot observation information. Being complementary to other sunspot observations in the world, our data set provides abundant information to the long-term solar activity cycles research. Title: 2.5 m wide-field and high-resolution telescope Authors: Fang, Cheng; Gu, Bozhong; Yuan, Xiangyan; Ding, Mingde; Chen, Pengfei; Dai, Zigao; Li, Xiangdong; Shi, Yong; Xie, Jiwei; Bai, Jinming; Qu, Zhongquan; Hao, Qi; Quo, Yang; Cheng, Xin; Li, Zhen Bibcode: 2019SSPMA..49e9603F Altcode: 2019SSPMA..49e9603C No abstract at ADS Title: A Statistical Study of the Magnetic Imprints of X-class Solar Flares Authors: Lu, Zekun; Cao, Weiguang; Jin, Gaoxiang; Zhang, Yining; Ding, Mingde; Guo, Yang Bibcode: 2019ApJ...876..133L Altcode: 2018arXiv180308310L Magnetic imprints, the rapid and irreversible evolution of photospheric magnetic fields as feedback from flares in the corona, have been confirmed by many previous studies. These studies showed that the horizontal field will permanently increase at the polarity inversion line (PIL) after eruptions, indicating that a more horizontal geometry of the photospheric magnetic field is produced. In this study, we analyze 20 X-class flares since the launch of the Solar Dynamics Observatory in 15 active regions with heliographic angles no greater than 45°. We observe clear magnetic imprints in 16 flares, whereas 4 flares are exceptional. The imprint regions of the horizontal field are located not only at the PIL but also at sunspot penumbra with strong vertical fields. Making use of the observed mass and speed of the corresponding coronal mass ejections (CMEs), we find that the CMEs with larger momenta are associated with stronger magnetic imprints. Furthermore, a linear relationship, with a Kendall’s Tau-b coefficient 0.54, between the CME momentum and the change of Lorentz force, is revealed. Based on that, we quantify the back reaction time to be ∼70 s, with a 90% confidence interval from about 50 to 90 s. Title: Erratum: “Observations of Turbulent Magnetic Reconnection within a Solar Current Sheet” (2018, ApJ, 866, 64) Authors: Cheng, X.; Li, Y.; Wan, L. F.; Ding, M. D.; Chen, P. F.; Zhang, J.; Liu, J. J. Bibcode: 2019ApJ...874..108C Altcode: No abstract at ADS Title: The on-orbit calibration of DArk Matter Particle Explorer Authors: Ambrosi, G.; An, Q.; Asfandiyarov, R.; Azzarello, P.; Bernardini, P.; Cai, M. S.; Caragiulo, M.; Chang, J.; Chen, D. Y.; Chen, H. F.; Chen, J. L.; Chen, W.; Cui, M. Y.; Cui, T. S.; Dai, H. T.; D'Amone, A.; De Benedittis, A.; De Mitri, I.; Ding, M.; Di Santo, M.; Dong, J. N.; Dong, T. K.; Dong, Y. F.; Dong, Z. X.; Droz, D.; Duan, K. K.; Duan, J. L.; D'Urso, D.; Fan, R. R.; Fan, Y. Z.; Fang, F.; Feng, C. Q.; Feng, L.; Fusco, P.; Gallo, V.; Gan, F. J.; Gao, M.; Gao, S. S.; Gargano, F.; Garrappa, S.; Gong, K.; Gong, Y. Z.; Guo, J. H.; Hu, Y. M.; Huang, G. S.; Huang, Y. Y.; Ionica, M.; Jiang, D.; Jiang, W.; Jin, X.; Kong, J.; Lei, S. J.; Li, S.; Li, X.; Li, W. L.; Li, Y.; Liang, Y. F.; Liang, Y. M.; Liao, N. H.; Liu, C. M.; Liu, H.; Liu, J.; Liu, S. B.; Liu, W. Q.; Liu, Y.; Loparco, F.; Ma, M.; Ma, P. X.; Ma, S. Y.; Ma, T.; Ma, X. Q.; Ma, X. Y.; Marsella, G.; Mazziotta, M. N.; Mo, D.; Niu, X. Y.; Pan, X.; Peng, X. Y.; Peng, W. X.; Qiao, R.; Rao, J. N.; Salinas, M. M.; Shang, G. Z.; Shen, W. H.; Shen, Z. Q.; Shen, Z. T.; Song, J. X.; Su, H.; Su, M.; Sun, Z. Y.; Surdo, A.; Teng, X. J.; Tian, X. B.; Tykhonov, A.; Vitillo, S.; Wang, C.; Wang, H.; Wang, H. Y.; Wang, J. Z.; Wang, L. G.; Wang, Q.; Wang, S.; Wang, X. H.; Wang, X. L.; Wang, Y. F.; Wang, Y. P.; Wang, Y. Z.; Wang, Z. M.; Wen, S. C.; Wei, D. M.; Wei, J. J.; Wei, Y. F.; Wu, D.; Wu, J.; Wu, L. B.; Wu, S. S.; Wu, X.; Xi, K.; Xia, Z. Q.; Xin, Y. L.; Xu, H. T.; Xu, Z. H.; Xu, Z. L.; Xu, Z. Z.; Xue, G. F.; Yang, H. B.; Yang, P.; Yang, Y. Q.; Yang, Z. L.; Yao, H. J.; Yu, Y. H.; Yuan, Q.; Yue, C.; Zang, J. J.; Zhang, D. L.; Zhang, F.; Zhang, J. B.; Zhang, J. Y.; Zhang, J. Z.; Zhang, L.; Zhang, P. F.; Zhang, S. X.; Zhang, W. Z.; Zhang, Y.; Zhang, Y. J.; Zhang, Y. Q.; Zhang, Y. L.; Zhang, Y. P.; Zhang, Z.; Zhang, Z. Y.; Zhao, H.; Zhao, H. Y.; Zhao, X. F.; Zhou, C. Y.; Zhou, Y.; Zhu, X.; Zhu, Y.; Zimmer, S. Bibcode: 2019APh...106...18A Altcode: 2019arXiv190702173A The DArk Matter Particle Explorer (DAMPE), a satellite-based cosmic ray and gamma-ray detector, was launched on December 17, 2015, and began its on-orbit operation on December 24, 2015. In this work we document the on-orbit calibration procedures used by DAMPE and report the calibration results of the Plastic Scintillator strip Detector (PSD), the Silicon-Tungsten tracKer-converter (STK), the BGO imaging calorimeter (BGO), and the Neutron Detector (NUD). The results are obtained using Galactic cosmic rays, bright known GeV gamma-ray sources, and charge injection into the front-end electronics of each sub-detector. The determination of the boundary of the South Atlantic Anomaly (SAA), the measurement of the live time, and the alignments of the detectors are also introduced. The calibration results demonstrate the stability of the detectors in almost two years of the on-orbit operation. Title: Transition from Circular-ribbon to Parallel-ribbon Flares Associated with a Bifurcated Magnetic Flux Rope Authors: Zhong, Z.; Guo, Y.; Ding, M. D.; Fang, C.; Hao, Q. Bibcode: 2019ApJ...871..105Z Altcode: 2018arXiv181210223Z Magnetic flux ropes play a key role in triggering solar flares in the solar atmosphere. In this paper, we investigate the evolution of NOAA Active Region 12268 within 36 hr from 2015 January 29 to 30, during which a flux rope was formed and three M-class and three C-class flares were triggered without coronal mass ejections. During the evolution of the active region, the flare emission seen in the Hα and ultraviolet wavebands changed from a circular shape (plus an adjacent conjugated ribbon and a remote ribbon) to three relatively straight and parallel ribbons. Based on a series of reconstructed nonlinear force-free fields, we find sheared or twisted magnetic field lines and a large-scale quasi-separatrix layer (QSL) associated with 3D null points in a quadrupolar magnetic field. These features always existed and constantly evolved during the 2 days. The twist of the flux rope was gradually accumulated, eventually leading to its instability. Around the flux rope, there were some topological structures, including a bald patch, a hyperbolic flux tube, and a torus QSL. We discuss how the particular magnetic structure and its evolution produce the flare emission. In particular, the bifurcation of the flux rope can explain the transition of the flares from circular to parallel ribbons. We propose a two-stage evolution of the magnetic structure and its associated flares. In the first stage, sheared arcades under the dome-like large-scale QSL were gradually transformed into a flux rope through magnetic reconnection, which produced the circular-ribbon flare. In the second stage, the flux rope bifurcated to form the three relatively straight and parallel flare ribbons. Title: Solar Magnetic Flux Rope Eruption Simulated by a Data-driven Magnetohydrodynamic Model Authors: Guo, Yang; Xia, Chun; Keppens, Rony; Ding, M. D.; Chen, P. F. Bibcode: 2019ApJ...870L..21G Altcode: 2018arXiv181210030G The combination of magnetohydrodynamic (MHD) simulation and multi-wavelength observations is an effective way to study the mechanisms of magnetic flux rope eruption. We develop a data-driven MHD model using the zero-β approximation. The initial condition is provided by a nonlinear force-free field derived from the magneto-frictional method based on vector magnetic field observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. The bottom boundary uses observed time series of the vector magnetic field and the vector velocity derived by the Differential Affine Velocity Estimator for Vector Magnetograms. We apply the data-driven model to active region 11123 observed from 06:00 UT on 2010 November 11 to about 2 hr later. The evolution of the magnetic field topology coincides with the flare ribbons observed in the 304 and 1600 Å wavebands by the Atmospheric Imaging Assembly. The morphology, propagation path, and propagation range of the flux rope are comparable with the observations in 304 Å. We also find that a data-constrained boundary condition, where the bottom boundary is fixed to the initial values, reproduces a similar simulation result. This model can reproduce the evolution of a magnetic flux rope in its dynamic eruptive phase. Title: Observations of Turbulent Magnetic Reconnection within a Solar Current Sheet Authors: Cheng, X.; Li, Y.; Wan, L. F.; Ding, M. D.; Chen, P. F.; Zhang, J.; Liu, J. J. Bibcode: 2018ApJ...866...64C Altcode: 2018arXiv180806071C Magnetic reconnection is a fundamental physical process in various astrophysical, space, and laboratory environments. Many pieces of evidence for magnetic reconnection have been uncovered. However, its specific processes that could be fragmented and turbulent have been short of direct observational evidence. Here, we present observations of a super-hot current sheet during the SOL2017-09-10T X8.2-class solar flare that display the fragmented and turbulent nature of magnetic reconnection. As bilateral plasmas converge toward the current sheet, significant plasma heating and nonthermal motions are detected therein. Two oppositely directed outflow jets are intermittently expelled out of the fragmenting current sheet, whose intensity shows a power-law distribution in the spatial frequency domain. The intensity and velocity of the sunward outflow jets also display a power-law distribution in the temporal frequency domain. The length-to-width ratio of current sheet is estimated to be larger than the theoretical threshold and thus ensures its occurrence. The observations therefore suggest that fragmented and turbulent magnetic reconnection occurs in the long stretching current sheet. Title: Extremely Large Extreme-ultraviolet Late Phase Powered by Intense Early Heating in a Non-eruptive Solar Flare Authors: Dai, Yu; Ding, Mingde; Zong, Weiguo; Yang, Kai E. Bibcode: 2018ApJ...863..124D Altcode: 2018arXiv180701315D We analyzed and modeled an M1.2 non-eruptive solar flare on 2011 September 9. The flare exhibited a strong late-phase peak of extreme-ultraviolet (EUV) warm coronal emissions (∼3 MK), with peak emission over 1.3 times that of the main flare peak. Multiple flare ribbons are observed, whose evolution indicates a two-stage energy release process. A nonlinear force-free field extrapolation reveals the existence of a magnetic null point, a fan-spine structure, and two flux ropes embedded in the fan dome. Magnetic reconnections involved in the flare are driven by the destabilization and rise of one of the flux ropes. In the first stage, the fast ascending flux rope drives reconnections at the null point and the surrounding quasi-separatrix layer (QSL), while in the second stage, reconnection mainly occurs between the two legs of the field lines stretched by the eventually stopped flux rope. The late-phase loops are mainly produced by the first-stage QSL reconnection, while the second-stage reconnection is responsible for the heating of main flaring loops. The first-stage reconnection is believed to be more powerful, leading to an extremely strong EUV late phase. We find that the delayed occurrence of the late-phase peak is mainly due to the long cooling process of the long late-phase loops. Using the model enthalpy-based thermal evolution of loops, we model the EUV emissions from a late-phase loop. The modeling reveals a peak heating rate of 1.1 erg cm-3 s-1 for the late-phase loop, which is obviously higher than previous values. Title: Project of a New 2.5m Solar Telescope Authors: Fang, Cheng; Chen, P. F.; Li, Zhen; Cao, Wenda; Hao, Qi; Ding, Mingde; Gu, Baizhong; Yuan, Xiangyan Bibcode: 2018cosp...42E1041F Altcode: A project of a 2.5m Solar Telescope has been worked out in China. It is the first facility in the world with a special innovation design and can conduct both high-resolution solar observations and the large-field of view (FOV) night survey. Its scientific objectives cover solar physics and time-domain astronomy, all of which are recent hot topics in astronomy. As a large on-axis solar telescope in the world with a larger FOV (7') than all the large solar telescopes operating at present, it can provide unprecedented high-resolution solar imaging and magnetic field data, which can help us obtain breakthrough achievements on the study of solar active regions and solar eruptions. The data are also very useful for the study of space weather. As a large telescope in China, the telescope is very unique in continuously monitoring short-time transient events by filling the gap in the specific time-zone. Moreover, the telescope can make essential contribution to training the graduate and undergraduate students majoring in observational astronomy. Title: Ellerman bombs vs. UV bursts Authors: Fang, Cheng; Li, Zhen; Hong, Jie; Ding, Mingde Bibcode: 2018cosp...42E1040F Altcode: Ellerman bombs (EBs) are small brightening events in the solar lower atmosphere. Most authors found that the temperature increase of EBs around the temperature minimum region is in the range of 600K-3000K. With recent IRIS observations, people found UV brightening events called UV bursts or IRIS bombs. Some authors proposed that the temperature increase of EBs could be more than 10000K, and the UV bursts could, in some cases, related to EBs. Using non-LTE semi-empirical modeling, we investigate the line profiles, continuum emission and the radiative losses for the EBs, and compare them with observations. Our result indicates that if the EB maximum temperature attains more than 10000K around the temperature minimum region, then the resulted Hα and Ca II 8542 Å line profiles and the continuum emission would be much stronger than that of EB observations. Moreover, due to the high radiative losses, the high temperature EB would have a very short lifetime, which is not comparable with the observations. Thus, our study does not support the proposal that the EB temperatures are higher than 10000K. Our non-LTE calculation also indicates that the UV bursts are not the same as the EBs. The former are probably at the higher layer of the solar atmosphere, while the latter are at the lower atmosphere with lower temperature. Title: Project of a New 2.5m Solar Telescope Authors: Fang, Cheng; Chen, P. F.; Li, Zhen; Cao, Wenda; Hao, Qi; Ding, Mingde; Gu, Baizhong; Yuan, Xiangyan Bibcode: 2018cosp...42E1042F Altcode: A project of a 2.5m Solar Telescope has been worked out in China. It is the first facility in the world with a special innovation design and can conduct both high-resolution solar observations and the large-field of view (FOV) night survey. Its scientific objectives cover solar physics and time-domain astronomy, all of which are recent hot topics in astronomy. As a large on-axis solar telescope in the world with a larger FOV (7') than all the large solar telescopes operating at present, it can provide unprecedented high-resolution solar imaging and magnetic field data, which can help us obtain breakthrough achievements on the study of solar active regions and solar eruptions. The data are also very useful for the study of space weather. As a large telescope in China, the telescope is very unique in continuously monitoring short-time transient events by filling the gap in the specific time-zone. Moreover, the telescope can make essential contribution to training the graduate and undergraduate students majoring in observational astronomy. Title: Observations of white-light flares in NOAA active region 11515: high occurrence rate and relationship with magnetic transients Authors: Song, Y. L.; Tian, H.; Zhang, M.; Ding, M. D. Bibcode: 2018A&A...613A..69S Altcode: 2018arXiv180104371S
Aims: There are two goals in this study. One is to investigate how frequently white-light flares (WLFs) occur in a flare-productive active region (NOAA active region 11515). The other is to investigate the relationship between WLFs and magnetic transients (MTs).
Methods: We used the high-cadence (45 s) full-disk continuum filtergrams and line-of-sight magnetograms taken by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) to identify WLFs and MTs, respectively. Images taken by the Atmospheric Imaging Assembly (AIA) on board SDO were also used to show the flare morphology in the upper atmosphere.
Results: We found at least 20 WLFs out of a total of 70 flares above C class (28.6%) in NOAA active region 11515 during its passage across the solar disk (E45°-W45°). Each of these WLFs occurred in a small region, with a short duration of about 5 min. The enhancement of the white-light continuum intensity is usually small, with an average enhancement of 8.1%. The 20 WLFs we observed were found along an unusual configuration of the magnetic field that was characterized by a narrow ribbon of negative field. Furthermore, the WLFs were found to be accompanied by MTs, with radical changes in magnetic field strength (or even a sign reversal) observed during the flare. In contrast, there is no obvious signature of MTs in the 50 flares without white-light enhancements.
Conclusions: Our results suggest that WLFs occur much more frequently than previously thought, with most WLFs being fairly weak enhancements. This may explain why WLFs are reported rarely. Our observations also suggest that MTs and WLFs are closely related and appear cospatial and cotemporal, when considering HMI data. A greater enhancement of WL emission is often accompanied by a greater change in the line-of-sight component of the unsigned magnetic field. Considering the close relationship between MTs and WLFs, many previously reported flares with MTs may be WLFs.

The movie is available at http://www.aanda.org Title: Non-LTE Calculations of the Fe I 6173 Å Line in a Flaring Atmosphere Authors: Hong, Jie; Ding, M. D.; Li, Ying; Carlsson, Mats Bibcode: 2018ApJ...857L...2H Altcode: 2018arXiv180309912H The Fe I 6173 Å line is widely used in the measurements of vector magnetic fields by instruments including the Helioseismic and Magnetic Imager (HMI). We perform non-local thermodynamic equilibrium calculations of this line based on radiative hydrodynamic simulations in a flaring atmosphere. We employ both a quiet-Sun atmosphere and a penumbral atmosphere as the initial one in our simulations. We find that, in the quiet-Sun atmosphere, the line center is obviously enhanced during an intermediate flare. The enhanced emission is contributed from both radiative backwarming in the photosphere and particle beam heating in the lower chromosphere. A blue asymmetry of the line profile also appears due to an upward mass motion in the lower chromosphere. If we take a penumbral atmosphere as the initial atmosphere, the line has a more significant response to the flare heating, showing a central emission and an obvious asymmetry. The low spectral resolution of HMI would indicate some loss of information, but the enhancement and line asymmetry are still kept. By calculating polarized line profiles, we find that the Stokes I and V profiles can be altered as a result of flare heating. Thus the distortion of this line has a crucial influence on the magnetic field measured from this line, and one should be cautious in interpreting the magnetic transients observed frequently in solar flares. Title: Probing the Production of Extreme-ultraviolet Late-phase Solar Flares Using the Model Enthalpy-based Thermal Evolution of Loops Authors: Dai, Yu; Ding, Mingde Bibcode: 2018ApJ...857...99D Altcode: 2018arXiv180307210D Recent observations in extreme-ultraviolet (EUV) wavelengths reveal an EUV late phase in some solar flares that is characterized by a second peak in warm coronal emissions (∼3 MK) several tens of minutes to a few hours after the soft X-ray (SXR) peak. Using the model enthalpy-based thermal evolution of loops (EBTEL), we numerically probe the production of EUV late-phase solar flares. Starting from two main mechanisms of producing the EUV late phase, i.e., long-lasting cooling and secondary heating, we carry out two groups of numerical experiments to study the effects of these two processes on the emission characteristics in late-phase loops. In either of the two processes an EUV late-phase solar flare that conforms to the observational criteria can be numerically synthesized. However, the underlying hydrodynamic and thermodynamic evolutions in late-phase loops are different between the two synthetic flare cases. The late-phase peak due to a long-lasting cooling process always occurs during the radiative cooling phase, while that powered by a secondary heating is more likely to take place in the conductive cooling phase. We then propose a new method for diagnosing the two mechanisms based on the shape of EUV late-phase light curves. Moreover, from the partition of energy input, we discuss why most solar flares are not EUV late flares. Finally, by addressing some other factors that may potentially affect the loop emissions, we also discuss why the EUV late phase is mainly observed in warm coronal emissions. Title: Two Types of Long-duration Quasi-static Evolution of Solar Filaments Authors: Xing, C.; Li, H. C.; Jiang, B.; Cheng, X.; Ding, M. D. Bibcode: 2018ApJ...857L..14X Altcode: 2018arXiv180401232X In this Letter, we investigate the long-duration quasi-static evolution of 12 pre-eruptive filaments (four active region (AR) and eight quiescent filaments), mainly focusing on the evolution of the filament height in 3D and the decay index of the background magnetic field. The filament height in 3D is derived through two-perspective observations of Solar Dynamics Observatory (SDO) and Solar TErrestrial RElations Observatory (STEREO). The coronal magnetic field is reconstructed using the potential field source surface model. A new finding is that the filaments we studied show two types of long-duration evolution: one type comprises a long-duration static phase and a short, slow rise phase with a duration of less than 12 hr and a speed of 0.1-0.7 km s-1, while the other one only presents a slow rise phase but with an extremely long duration of more than 60 hr and a smaller speed of 0.01-0.2 km s-1. At the moment approaching the eruption, the decay index of the background magnetic field at the filament height is similar for both AR and quiescent filaments. The average value and upper limit are ∼0.9 and ∼1.4, close to the critical index of torus instability. Moreover, the filament height and background magnetic field strength are also found to be linearly and exponentially related with the filament length, respectively. Title: Observations of the CME shocks and the production of SEPs Authors: Xu, Zigong; li, Chuan; Ding, Mingde Bibcode: 2018EGUGA..20.7722X Altcode: The acceleration mechanism of SEPs is still a hot debated problem especially during the onset of large SEPs. Here, we present a case study on 2011-08-09 SEP event to clarify its acceleration source and an 3D statistical study including 22 SEPs in order to know where and how the particles are released. Based on the remote-sensing and in-situ observations as well as the DSA theory, we found that the theoretical particle spectrum was comparable to a observational one and a weak correlation was obtained between the electrons producing HXR and the in-situ observed electrons. These results indicated that the coronal shock wave was the potential accelerator of SEPs. In statistical study, we obtained the electrons and protons solar release and compared to the 3D CME structures and HXR observation from RHESSI. It is found the electrons are released 15 minutes earlier than the protons and both electrons and protons release are in decay phase of flare eruptions. The simulations of SEP propagation tell us that the time difference of the electrons and protons release probably arises from their scattering propagation in interplanetary space. Title: Period Increase and Amplitude Distribution of Kink Oscillation of Coronal Loop Authors: Su, W.; Guo, Y.; Erdélyi, R.; Ning, Z. J.; Ding, M. D.; Cheng, X.; Tan, B. L. Bibcode: 2018NatSR...8.4471S Altcode: 2018arXiv180306848S Coronal loops exist ubiquitously in the solar atmosphere. These loops puzzle astronomers over half a century. Solar magneto-seismology (SMS) provides a unique way to constrain the physical parameters of coronal loops. Here, we study the evolution of oscillations of a coronal loop observed by the Atmospheric Imaging Assembly (AIA). We measure geometric and physical parameters of the loop oscillations. In particular, we find that the mean period of the oscillations increased from 1048 to 1264 s during three oscillatory cycles. We employ the differential emission measure method and apply the tools of SMS. The evolution of densities inside and outside the loop is analyzed. We found that an increase of density inside the loop and decrease of the magnetic field strength along the loop are the main reasons for the increase in the period during the oscillations. Besides, we also found that the amplitude profile of the loop is different from a profile would it be a homogeneous loop. It is proposed that the distribution of magnetic strength along the loop rather than density stratification is responsible for this deviation. The variation in period and distribution of amplitude provide, in terms of SMS, a new and unprecedented insight into coronal loop diagnostics. Title: Unambiguous Evidence of Filament Splitting-induced Partial Eruptions Authors: Cheng, X.; Kliem, B.; Ding, M. D. Bibcode: 2018ApJ...856...48C Altcode: 2018arXiv180204932C Coronal mass ejections are often considered to result from the full eruption of a magnetic flux rope (MFR). However, it is recognized that, in some events, the MFR may release only part of its flux, with the details of the implied splitting not completely established due to limitations in observations. Here, we investigate two partial eruption events including a confined and a successful one. Both partial eruptions are a consequence of the vertical splitting of a filament-hosting MFR involving internal reconnection. A loss of equilibrium in the rising part of the magnetic flux is suggested by the impulsive onset of both events and by the delayed onset of reconnection in the confined event. The remaining part of the flux might be line-tied to the photosphere in a bald patch (BP) separatrix surface, and we confirm the existence of extended BP sections for the successful eruption. The internal reconnection is signified by brightenings in the body of one filament and between the rising and remaining parts of both filaments. It evolves quickly into the standard current sheet reconnection in the wake of the eruption. As a result, regardless of being confined or successful, both eruptions produce hard X-ray sources and flare loops below the erupting but above the surviving flux, as well as a pair of flare ribbons enclosing the latter. Title: Observationally quantified reconnection providing a viable mechanism for active region coronal heating Authors: Yang, Kai E.; Longcope, Dana W.; Ding, M. D.; Guo, Yang Bibcode: 2018NatCo...9..692Y Altcode: 2018arXiv180206206Y The heating of the Sun's corona has been explained by several different mechanisms including wave dissipation and magnetic reconnection. While both have been shown capable of supplying the requisite power, neither has been used in a quantitative model of observations fed by measured inputs. Here we show that impulsive reconnection is capable of producing an active region corona agreeing both qualitatively and quantitatively with extreme-ultraviolet observations. We calculate the heating power proportional to the velocity difference between magnetic footpoints and the photospheric plasma, called the non-ideal velocity. The length scale of flux elements reconnected in the corona is found to be around 160 km. The differential emission measure of the model corona agrees with that derived using multi-wavelength images. Synthesized extreme-ultraviolet images resemble observations both in their loop-dominated appearance and their intensity histograms. This work provides compelling evidence that impulsive reconnection events are a viable mechanism for heating the corona. Title: Spectroscopic Observations of a Current Sheet in a Solar Flare Authors: Li, Y.; Xue, J. C.; Ding, M. D.; Cheng, X.; Su, Y.; Feng, L.; Hong, J.; Li, H.; Gan, W. Q. Bibcode: 2018ApJ...853L..15L Altcode: 2018arXiv180103631L The current sheet is believed to be the region of energy dissipation via magnetic reconnection in solar flares. However, its properties, for example, the dynamic process, are not fully understood. Here, we report a current sheet in a solar flare (SOL2017-09-10T16:06) that was clearly observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory as well as the EUV Imaging Spectrometer on Hinode. The high-resolution imaging and spectroscopic observations show that the current sheet is mainly visible in high-temperature (>10 MK) passbands, particularly in the Fe XXIV 192.03 Å line with a formation temperature of ∼18 MK. The hot Fe XXIV 192.03 Å line exhibits very large nonthermal velocities up to 200 km s-1 in the current sheet, suggesting that turbulent motions exist there. The largest turbulent velocity occurs at the edge of the current sheet, with some offset with the strongest line intensity. At the central part of the current sheet, the turbulent velocity is negatively correlated with the line intensity. From the line emission and turbulent features we obtain a thickness in the range of 7-11 Mm for the current sheet. These results suggest that the current sheet has internal fine and dynamic structures that may help the magnetic reconnection within it proceed efficiently. Title: A circular white-light flare with impulsive and gradual white-light kernels Authors: Hao, Q.; Yang, K.; Cheng, X.; Guo, Y.; Fang, C.; Ding, M. D.; Chen, P. F.; Li, Z. Bibcode: 2017NatCo...8.2202H Altcode: 2017arXiv171207279H White-light flares are the flares with emissions visible in the optical continuum. They are thought to be rare and pose the most stringent requirements in energy transport and heating in the lower atmosphere. Here we present a nearly circular white-light flare on 10 March 2015 that was well observed by the Optical and Near-infrared Solar Eruption Tracer and Solar Dynamics Observatory. In this flare, there appear simultaneously both impulsive and gradual white-light kernels. The generally accepted thick-target model would be responsible for the impulsive kernels but not sufficient to interpret the gradual kernels. Some other mechanisms including soft X-ray backwarming or downward-propagating Alfvén waves, acting jointly with electron beam bombardment, provide a possible interpretation. However, the origin of this kind of white-light kernel is still an open question that induces more observations and researches in the future to decipher it. Title: Spectroscopic Observations of Magnetic Reconnection and Chromospheric Evaporation in an X-shaped Solar Flare Authors: Li, Y.; Kelly, M.; Ding, M. D.; Qiu, J.; Zhu, X. S.; Gan, W. Q. Bibcode: 2017ApJ...848..118L Altcode: 2017arXiv170808586L We present observations of distinct UV spectral properties at different locations during an atypical X-shaped flare (SOL2014-11-09T15:32) observed by the Interface Region Imaging Spectrograph (IRIS). In this flare, four chromospheric ribbons appear and converge at an X-point where a separator is anchored. Above the X-point, two sets of non-coplanar coronal loops approach laterally and reconnect at the separator. The IRIS slit was located close to the X-point, cutting across some of the flare ribbons and loops. Near the location of the separator, the Si IV 1402.77 Å line exhibits significantly broadened line wings extending to 200 km s-1 with an unshifted line core. These spectral features suggest the presence of bidirectional flows possibly related to the separator reconnection. While at the flare ribbons, the hot Fe xxi 1354.08 Å line shows blueshifts and the cool Si IV 1402.77 Å, C II 1335.71 Å, and Mg II 2803.52 Å lines show evident redshifts up to a velocity of 80 km s-1, which are consistent with the scenario of chromospheric evaporation/condensation. Title: RADYN Simulations of Non-thermal and Thermal Models of Ellerman Bombs Authors: Hong, Jie; Carlsson, Mats; Ding, M. D. Bibcode: 2017ApJ...845..144H Altcode: 2017arXiv170705514H Ellerman bombs (EBs) are brightenings in the Hα line wings that are believed to be caused by magnetic reconnection in the lower atmosphere. To study the response and evolution of the chromospheric line profiles, we perform radiative hydrodynamic simulations of EBs using both non-thermal and thermal models. Overall, these models can generate line profiles that are similar to observations. However, in non-thermal models we find dimming in the Hα line wings and continuum when the heating begins, while for the thermal models dimming occurs only in the Hα line core, and with a longer lifetime. This difference in line profiles can be used to determine whether an EB is dominated by non-thermal heating or thermal heating. In our simulations, if a higher heating rate is applied, then the Hα line will be unrealistically strong and there are still no clear UV burst signatures. Title: The Formation and Early Evolution of a CME and the Associated Shock on 2014 January 8 Authors: Wan, Linfeng; Cheng, Xin; Shi, Tong; Su, Wei; Ding, Mingde Bibcode: 2017SPD....4820606W Altcode: We study the formation and early evolution of a limb coronal mass ejection (CME) and its associated shock wave that occurred on 2014 January 8. The extreme ultraviolet (EUV) images provided by AIA on board \textit{Solar Dynamics Observatory} disclose that the CME first appears as a bubble-like structure. Subsequently, its expansion forms the CME and causes a quasi-circular EUV wave. Both the CME and the wave front are clearly visible at all of the AIA EUV passbands. Through a detailed kinematical analysis, it is found that the expansion of the CME undergoes two phases: a first phase with a strong but transient lateral over-expansion followed by a second phase with a self-similar expansion. The temporal evolution of the expansion velocity coincides very well with the variation of the 25--50 keV hard X-ray (HXR) flux of the associated flare, which indicates that magnetic reconnection most likely plays an important role in driving the expansion. Moreover, we find that, when the velocity of the CME reaches $\sim$600 km s$^{-1}$, the EUV wave starts to evolve into a shock wave, which is evidenced by the appearance of a type II radio burst. Interestingly, we also notice an unusual solar radio signal at $\sim$4 GHz that is similar to the pattern of a type II radio burst but drifts to higher frequencies at a rate of $\sim$0.3 MHz per second during about 7 minutes. Its derived density is $\sim$5$\times$10$^{10}$ cm$^{-3}$ and increases slowly with time. Joint imaging observations of HXR and EUV help to locate the loop-top region and calculate its thermal proprieties, including slowly increasing densities ($\sim$5$\times$10$^{10}$ cm$^{-3}$) and temperatures ($\sim$14 MK). The similar results obtained from two different ways above imply the possibility of this scenario: plasma blobs that are ejected along the current sheet via magnetic reconnection collide with underlying flare loops that are undergoing chromospheric evaporation. Finally, we also study the thermal properties of the CME and the EUV wave. We find that the plasma in the CME leading front and the wave front has a temperature of $\sim$2 MK, while that in the CME core region and the flare region has a much higher temperature of $\ge$8 MK. Title: Origin and Structures of Solar Eruptions I: Magnetic Flux Rope Authors: Cheng, Xin; Guo, Yang; Ding, MingDe Bibcode: 2017ScChD..60.1383C Altcode: 2017ScChE..60.1383C; 2017arXiv170508198C Coronal mass ejections (CMEs) and solar flares are the large-scale and most energetic eruptive phenomena in our solar system and able to release a large quantity of plasma and magnetic flux from the solar atmosphere into the solar wind. When these high-speed magnetized plasmas along with the energetic particles arrive at the Earth, they may interact with the magnetosphere and ionosphere, and seriously affect the safety of human high-tech activities in outer space. The travel time of a CME to 1 AU is about 1-3 days, while energetic particles from the eruptions arrive even earlier. An efficient forecast of these phenomena therefore requires a clear detection of CMEs/flares at the stage as early as possible. To estimate the possibility of an eruption leading to a CME/flare, we need to elucidate some fundamental but elusive processes including in particular the origin and structures of CMEs/flares. Understanding these processes can not only improve the prediction of the occurrence of CMEs/flares and their effects on geospace and the heliosphere but also help understand the mass ejections and flares on other solar-type stars. The main purpose of this review is to address the origin and early structures of CMEs/flares, from multi-wavelength observational perspective. First of all, we start with the ongoing debate of whether the pre-eruptive configuration, i.e., a helical magnetic flux rope (MFR), of CMEs/flares exists before the eruption and then emphatically introduce observational manifestations of the MFR. Secondly, we elaborate on the possible formation mechanisms of the MFR through distinct ways. Thirdly, we discuss the initiation of the MFR and associated dynamics during its evolution toward the CME/flare. Finally, we come to some conclusions and put forward some prospects in the future. Title: Using observations of slipping velocities to test the hypothesis that reconnection heats the active region corona Authors: Yang, Kai; Longcope, Dana; Guo, Yang; Ding, Mingde Bibcode: 2017SPD....4830301Y Altcode: Numerous proposed coronal heating mechanisms have invoked magnetic reconnection in some role. Testing such a mechanism requires a method of measuring magnetic reconnection coupled with a prediction of the heat delivered by reconnection at the observed rate. In the absence of coronal reconnection, field line footpoints move at the same velocity as the plasma they find themselves in. The rate of coronal reconnection is therefore related to any discrepancy observed between footpoint motion and that of the local plasma — so-called slipping motion. We propose a novel method to measure this velocity discrepancy by combining a sequence of non-linear force-free field extrapolations with maps of photospheric velocity. We obtain both from a sequence of vector magnetograms of an active region (AR). We then propose a method of computing the coronal heating produced under the assumption the observed slipping velocity was due entirely to coronal reconnection. This heating rate is used to predict density and temperature at points along an equilibrium loop. This, in turn, is used to synthesize emission in EUV and SXR bands. We perform this analysis using a sequence of HMI vector magnetograms of a particular AR and compare synthesized images to observations of the same AR made by SDO. We also compare differential emission measure inferred from those observations to that of the modeled corona. Title: Origin and structures of solar eruptions II: Magnetic modeling Authors: Guo, Yang; Cheng, Xin; Ding, MingDe Bibcode: 2017ScChD..60.1408G Altcode: 2017arXiv170605769G The topology and dynamics of the three-dimensional magnetic field in the solar atmosphere govern various solar eruptive phenomena and activities, such as flares, coronal mass ejections, and filaments/prominences. We have to observe and model the vector magnetic field to understand the structures and physical mechanisms of these solar activities. Vector magnetic fields on the photosphere are routinely observed via the polarized light, and inferred with the inversion of Stokes profiles. To analyze these vector magnetic fields, we need first to remove the 180° ambiguity of the transverse components and correct the projection effect. Then, the vector magnetic field can be served as the boundary conditions for a force-free field modeling after a proper preprocessing. The photospheric velocity field can also be derived from a time sequence of vector magnetic fields. Three-dimensional magnetic field could be derived and studied with theoretical force-free field models, numerical nonlinear force-free field models, magnetohydrostatic models, and magnetohydrodynamic models. Magnetic energy can be computed with three-dimensional magnetic field models or a time series of vector magnetic field. The magnetic topology is analyzed by pinpointing the positions of magnetic null points, bald patches, and quasi-separatrix layers. As a well conserved physical quantity, magnetic helicity can be computed with various methods, such as the finite volume method, discrete flux tube method, and helicity flux integration method. This quantity serves as a promising parameter characterizing the activity level of solar active regions. Title: Particle Acceleration in Two Converging Shocks Authors: Wang, Xin; Giacalone, Joe; Yan, Yihua; Ding, Mingde; Wang, Na; Shan, Hao Bibcode: 2017ApJ...842...74W Altcode: Observations by spacecraft such as ACE, STEREO, and others show that there are proton spectral “breaks” with energy E br at 1-10 MeV in some large CME-driven shocks. Generally, a single shock with the diffusive acceleration mechanism would not predict the “broken” energy spectrum. The present paper focuses on two converging shocks to identify this energy spectral feature. In this case, the converging shocks comprise one forward CME-driven shock on 2006 December 13 and another backward Earth bow shock. We simulate the detailed particle acceleration processes in the region of the converging shocks using the Monte Carlo method. As a result, we not only obtain an extended energy spectrum with an energy “tail” up to a few 10 MeV higher than that in previous single shock model, but also we find an energy spectral “break” occurring on ∼5.5 MeV. The predicted energy spectral shape is consistent with observations from multiple spacecraft. The spectral “break,” then, in this case is caused by the interaction between the CME shock and Earth’s bow shock, and otherwise would not be present if Earth were not in the path of the CME.

This work is supported by the Xinjiang Natural Science Foundation No. 2014211A069. Title: Observations of a Coronal Shock Wave and the Production of Solar Energetic Particles Authors: Xu, Z. G.; Li, C.; Ding, M. D. Bibcode: 2017ApJ...840...38X Altcode: We present a study that clarifies the acceleration source/mechanism of the solar energetic particle (SEP) event on 2011 August 9. Based on the assumption of scatter-free propagation of charged particles along the interplanetary magnetic field, the solar particle release times of the electrons and protons are derived and both found to be in the decay phase of the flare emission. Furthermore, we compare the peak-flux spectra of the in situ particles and the remote-sensing hard X-ray photons and find a weak correlation between them. In particular, we note that an extreme ultraviolet shock wave, presumed to be a signature of coronal mass ejection (CME) shock front on the solar surface, and an associated type II radio burst were observed alongside this event. Under the framework of diffusive shock acceleration, the derived shock compression ratio can accelerate particles with a theoretical spectral index γ ={2.14}-0.02+0.01, which is comparable to the observational index of ∼2.0. Our results appear to support the notion that the coronal shock wave was most likely responsible for the SEP event. Specifically, we find that the electrons were released in a low coronal site at ∼0.58 solar radii, and protons were released when the CME-driven shock propagated to ∼1.38 solar radii. The multi-spacecraft observations, in addition, reveal the connection between the acceleration of shock waves and the release of SEPs. Title: Multi-wavelength Spectral Analysis of Ellerman Bombs Observed by FISS and IRIS Authors: Hong, Jie; Ding, M. D.; Cao, Wenda Bibcode: 2017ApJ...838..101H Altcode: 2017arXiv170304268H Ellerman bombs (EBs) are a kind of solar activity that is suggested to occur in the lower solar atmosphere. Recent observations using the Interface Region Imaging Spectrograph (IRIS) show connections between EBs and IRIS bombs (IBs), which imply that EBs might be heated to a much higher temperature (8 × 104 K) than previous results. Here we perform a spectral analysis of EBs simultaneously observed by the Fast Imaging Solar Spectrograph and IRIS. The observational results show clear evidence of heating in the lower atmosphere, indicated by the wing enhancement in Hα, Ca II 8542 Å, and Mg II triplet lines and also by brightenings in images of the 1700 Å and 2832 Å ultraviolet continuum channels. Additionally, the intensity of the Mg II triplet line is correlated with that of Hα when an EB occurs, suggesting the possibility of using the triplet as an alternative way to identify EBs. However, we do not find any signal in IRIS hotter lines (C II and Si IV). For further analysis, we employ a two-cloud model to fit the two chromospheric lines (Hα and Ca II 8542 Å) simultaneously, and obtain a temperature enhancement of 2300 K for a strong EB. This temperature is among the highest of previous modeling results, albeit still insufficient to produce IB signatures at ultraviolet wavelengths. Title: Plasma Brightenings in a Failed Solar Filament Eruption Authors: Li, Y.; Ding, M. D. Bibcode: 2017ApJ...838...15L Altcode: 2017arXiv170205136L Failed filament eruptions are solar eruptions that are not associated with coronal mass ejections. In a failed filament eruption, the filament materials usually show some ascending and falling motions as well as generating bright EUV emissions. Here we report a failed filament eruption (SOL2016-07-22) that occurred in a quiet-Sun region observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. In this event, the filament spreads out but gets confined by the surrounding magnetic field. When interacting with the ambient magnetic field, the filament material brightens up and flows along the magnetic field lines through the corona to the chromosphere. We find that some materials slide down along the lifting magnetic structure containing the filament and impact the chromosphere, and through kinetic energy dissipation, cause two ribbon-like brightenings in a wide temperature range. There is evidence suggesting that magnetic reconnection occurs between the filament magnetic structure and the surrounding magnetic fields where filament plasma is heated to coronal temperatures. In addition, thread-like brightenings show up on top of the erupting magnetic fields at low temperatures, which might be produced by an energy imbalance from a fast drop of radiative cooling due to plasma rarefaction. Thus, this single event of a failed filament eruption shows the existence of a variety of plasma brightenings that may be caused by completely different heating mechanisms. Title: Imaging Observations of Magnetic Reconnection in a Solar Eruptive Flare Authors: Li, Y.; Sun, X.; Ding, M. D.; Qiu, J.; Priest, E. R. Bibcode: 2017ApJ...835..190L Altcode: 2016arXiv161209417L Solar flares are among the most energetic events in the solar atmosphere. It is widely accepted that flares are powered by magnetic reconnection in the corona. An eruptive flare is usually accompanied by a coronal mass ejection, both of which are probably driven by the eruption of a magnetic flux rope (MFR). Here we report an eruptive flare on 2016 March 23 observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The extreme-ultraviolet imaging observations exhibit the clear rise and eruption of an MFR. In particular, the observations reveal solid evidence of magnetic reconnection from both the corona and chromosphere during the flare. Moreover, weak reconnection is observed before the start of the flare. We find that the preflare weak reconnection is of tether-cutting type and helps the MFR to rise slowly. Induced by a further rise of the MFR, strong reconnection occurs in the rise phases of the flare, which is temporally related to the MFR eruption. We also find that the magnetic reconnection is more of 3D-type in the early phase, as manifested in a strong-to-weak shear transition in flare loops, and becomes more 2D-like in the later phase, as shown by the apparent rising motion of an arcade of flare loops. Title: Comparison of the energy spectra between single shock and converging double-shock Authors: Wang, X.; Yin, Y.; Ding, M.; Wang, N.; Shan, H. Bibcode: 2017ICRC...35..102W Altcode: 2017PoS...301..102W No abstract at ADS Title: Energy spectral properties of the twin-CME driven shocks Authors: Wang, X.; Yin, Y.; Ding, M.; Wang, N.; Shan, H. Bibcode: 2017ICRC...35..100W Altcode: 2017PoS...301..100W No abstract at ADS Title: Observation of Magnetic Reconnection at a 3D Null Point Associated with a Solar Eruption Authors: Sun, J. Q.; Zhang, J.; Yang, K.; Cheng, X.; Ding, M. D. Bibcode: 2016ApJ...830L...4S Altcode: 2016arXiv160906787S Magnetic null has long been recognized as a special structure serving as a preferential site for magnetic reconnection (MR). However, the direct observational study of MR at null-points is largely lacking. Here, we show the observations of MR around a magnetic null associated with an eruption that resulted in an M1.7 flare and a coronal mass ejection. The Geostationary Operational Environmental Satellites X-ray profile of the flare exhibited two peaks at ∼02:23 UT and ∼02:40 UT on 2012 November 8, respectively. Based on the imaging observations, we find that the first and also primary X-ray peak was originated from MR in the current sheet (CS) underneath the erupting magnetic flux rope (MFR). On the other hand, the second and also weaker X-ray peak was caused by MR around a null point located above the pre-eruption MFR. The interaction of the null point and the erupting MFR can be described as a two-step process. During the first step, the erupting and fast expanding MFR passed through the null point, resulting in a significant displacement of the magnetic field surrounding the null. During the second step, the displaced magnetic field started to move back, resulting in a converging inflow and subsequently the MR around the null. The null-point reconnection is a different process from the current sheet reconnection in this flare; the latter is the cause of the main peak of the flare, while the former is the cause of the secondary peak of the flare and the conspicuous high-lying cusp structure. Title: Direct imaging of a classical solar eruptive flare Authors: Li, Y.; Sun, X. D.; Ding, M. D.; Qiu, J.; Priest, E. R.; Longcope, D. W. Bibcode: 2016usc..confE..21L Altcode: Solar flares are the most energetic events in the solar system that have a potential hazard on Earth. Although a standard model for the eruptive flare accompanied by a coronal mass ejection has been outlined and elaborated for decades, some key aspects are still under debate, such as what drives the eruption, what is the role of magnetic reconnection, and how the flare loops evolve. Here we present an excellent event exhibiting nearly all the key elements involved in the standard flare model. Using extreme-ultraviolet imaging observations, we detect the unambiguous rise and eruption of a magnetic flux rope, solid evidence for magnetic reconnection, and evident slipping and rising motions in flare loops. Modeled coronal magnetic field supports the interpretation of a pre-existing flux rope that persists after the eruption with reduced twist. This flare, from the observational view, shows a clear and comprehensive picture of how a classical solar eruptive flare occurs and evolves, and helps to clarify some of the controversial topics in the standard flare model. Title: Investigating the Conditions of the Formation of a Type II Radio Burst on 2014 January 8 Authors: Su, W.; Cheng, X.; Ding, M. D.; Chen, P. F.; Ning, Z. J.; Ji, H. S. Bibcode: 2016ApJ...830...70S Altcode: 2016arXiv160905633S It is believed that type II radio bursts are generated by shock waves. In order to understand the generation conditions of type II radio bursts, we analyze the physical parameters of a shock front. The type II radio burst we selected was observed by the Siberian Solar Radio Telescope (SSRT) and Learmonth radio station and was associated with a limb coronal mass ejection (CME) occurring on 2014 January 8 observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The evolution of the CME in the inner corona presents a double-layered structure that propagates outward. We fit the outer layer (OL) of the structure with a partial circle and divide it into seven directions from -45° to 45° with an angular separation of 15°. We measure the OL speed along the seven directions and find that the speed in the direction of -15° with respect to the central direction is the fastest. We use the differential emission measure method to calculate the physical parameters at the OL at the moment when the type II radio burst was initiated, including the temperature (T), emission measure (EM), temperature ratio ({T}d/{T}{{u}}), compression ratio (X), and Alfvén Mach number (M A). We compare the quantities X and M A to those obtained from band-splitting in the radio spectrum, and find that this type II radio burst is generated at a small region of the OL that is located at the sector in the 45° direction. The results suggest that the generation of type II radio bursts (shocks) requires larger values of X and M A rather than simply a higher speed of the disturbance. Title: The Formation and Early Evolution of a Coronal Mass Ejection and its Associated Shock Wave on 2014 January 8 Authors: Wan, Linfeng; Cheng, Xin; Shi, Tong; Su, Wei; Ding, M. D. Bibcode: 2016ApJ...826..174W Altcode: 2016arXiv160501132W In this paper, we study the formation and early evolution of a limb coronal mass ejection (CME) and its associated shock wave that occurred on 2014 January 8. The extreme ultraviolet (EUV) images provided by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory disclose that the CME first appears as a bubble-like structure. Subsequently, its expansion forms the CME and causes a quasi-circular EUV wave. Interestingly, both the CME and the wave front are clearly visible at all of the AIA EUV passbands. Through a detailed kinematical analysis, it is found that the expansion of the CME undergoes two phases: a first phase with a strong but transient lateral over-expansion followed by a second phase with a self-similar expansion. The temporal evolution of the expansion velocity coincides very well with the variation of the 25-50 keV hard X-ray flux of the associated flare, which indicates that magnetic reconnection most likely plays an important role in driving the expansion. Moreover, we find that, when the velocity of the CME reaches ∼600 km s-1, the EUV wave starts to evolve into a shock wave, which is evidenced by the appearance of a type II radio burst. The shock’s formation height is estimated to be ∼0.2 R sun, which is much lower than the height derived previously. Finally, we also study the thermal properties of the CME and the EUV wave. We find that the plasma in the CME leading front and the wave front has a temperature of ∼2 MK, while that in the CME core region and the flare region has a much higher temperature of ≥8 MK. Title: The Characteristics of the Footpoints of Solar Magnetic Flux Ropes during Eruptions Authors: Cheng, X.; Ding, M. D. Bibcode: 2016ApJS..225...16C Altcode: 2016arXiv160504047C We investigate the footpoints of four erupted magnetic flux ropes (MFRs) that appear as sigmoidal hot channels prior to the eruptions in the Atmospheric Imaging Assembly high temperature passbands. The simultaneous Helioseismic and Magnetic Imager observations disclose that one footpoint of the MFRs originates in the penumbra or penumbra edge with a stronger magnetic field, while the other originates in the moss region with a weaker magnetic field. The significant deviation of the axes of the MFRs from the main polarity inversion lines and associated filaments suggests that the MFRs have ascended to a high altitude, thus becoming distinguishable from the source sigmoidal active regions. Further, with the eruption of the MFRs, the average inclination angle and direct current at the footpoints with stronger magnetic fields tend to decrease, which is suggestive of a straightening and untwisting of the magnetic field in the MFR legs. Moreover, the associated flare ribbons also display an interesting evolution. They initially appear as sporadic brightenings at the two footpoints of the MFRs and in the regions below, and then quickly extend to two slender sheared J-shaped ribbons with the two hooks corresponding to the two ends of the MFRs. Finally, the straight parts of the two ribbons separate from each other, evolving into two widened parallel ones. These features mostly conform to and support the recently proposed three-dimensional standard coronal mass ejection/flare model, I.e., the twisted MFR eruption stretches and leads to the reconnection of the overlying field that transits from a strong to weak shear with increasing height. Title: A few perspectives of solar physics research in China - current status and future Authors: Wang, Jingxiu; Ding, Mingde; Ji, Haisheng; Deng, Yuanyong; Liu, Yu; Liu, Zhong; Qu, Zhongquan; Wang, Huaning; Xia, Lidong; Yan, Yihua Bibcode: 2016AsJPh..25..461W Altcode: Solar physics research as an important discipline in astrophysics in China aims at improving the understanding of origin and variation of solar magnetic field and magnetic activity, and founding the basis for forecast of disastrous space weather. The current review is focused on the solar physics research in China in recent three years. Highlights in scientific research in solar magnetism, magnetic activity, coronal plasma, and space weather forecast are briefly summarized. Key advances in instrument development are reported in some necessary details. Future tendency and working direction are considered and discussed. Title: Quantifying the Topology and Evolution of a Magnetic Flux Rope Associated with Multi-flare Activities Authors: Yang, Kai; Guo, Yang; Ding, M. D. Bibcode: 2016ApJ...824..148Y Altcode: 2016arXiv160407502Y Magnetic flux ropes (MFRs) play an important role in solar activities. The quantitative assessment of the topology of an MFR and its evolution is crucial for a better understanding of the relationship between the MFR and associated activities. In this paper, we investigate the magnetic field of active region (AR) 12017 from 2014 March 28-29, during which time 12 flares were triggered by intermittent eruptions of a filament (either successful or confined). Using vector magnetic field data from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, we calculate the magnetic energy and helicity injection in the AR, and extrapolate the 3D magnetic field with a nonlinear force-free field model. From the extrapolations, we find an MFR that is cospatial with the filament. We further determine the configuration of this MFR from the closed quasi-separatrix layer (QSL) around it. Then, we calculate the twist number and the magnetic helicity for the field lines composing the MFR. The results show that the closed QSL structure surrounding the MFR becomes smaller as a consequence of flare occurrence. We also find that the flares in our sample are mainly triggered by kink instability. Moreover, the twist number varies more sensitively than other parameters with the occurrence of flares. Title: Ultra-Narrow Negative Flare Front Observed in Helium-10830 Å Using the1.6m New Solar Telescope Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su, Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho, Kyuhyoun; Cho, Kyung-Suk; Gary, Dale E.; Wang, Haimin Bibcode: 2016SPD....47.0633X Altcode: Solar flares are sudden flashes of brightness on the Sun and are often associated with coronal mass ejections and solar energetic particles that have adverse effects on the near-Earth environment. By definition, flares are usually referred to as bright features resulting from excess emission. Using the newly commissioned 1.6-m New Solar Telescope at Big Bear Solar Observatory, we show a striking “negative” flare with a narrow but unambiguous “dark” moving front observed in He I 10830 Å, which is as narrow as 340 km and is associated with distinct spectral characteristics in Hα and Mg II lines. Theoretically, such negative contrast in He I 10830 Å can be produced under special circumstances by nonthermal electron collisions or photoionization followed by recombination. Our discovery, made possible due to unprecedented spatial resolution, confirms the presence of the required plasma conditions and provides unique information in understanding the energy release and radiative transfer in solar flares. Title: Spectroscopic Diagnostics of Solar Magnetic Flux Ropes Using Iron Forbidden Line Authors: Cheng, X.; Ding, M. D. Bibcode: 2016ApJ...823L...4C Altcode: 2016arXiv160500195C In this Letter, we present Interface Region Imaging Spectrograph Fe xxi 1354.08 Å forbidden line emission of two magnetic flux ropes (MFRs) that caused two fast coronal mass ejections with velocities of ≥1000 km s-1 and strong flares (X1.6 and M6.5) on 2014 September 10 and 2015 June 22, respectively. The extreme-ultraviolet images at the 131 and 94 Å passbands provided by the Atmospheric Imaging Assembly on board Solar Dynamics Observatory reveal that both MFRs initially appear as suspended hot channel-like structures. Interestingly, part of the MFRs is also visible in the Fe xxi 1354.08 forbidden line, even prior to the eruption, e.g., for the SOL2014-09-10 event. However, the line emission is very weak and that only appears at a few locations but not the whole structure of the MFRs. This implies that the MFRs could be comprised of different threads with different temperatures and densities, based on the fact that the formation of the Fe xxi forbidden line requires a critical temperature (∼11.5 MK) and density. Moreover, the line shows a non-thermal broadening and a blueshift in the early phase. It suggests that magnetic reconnection at that time has initiated; it not only heats the MFR and, at the same time, produces a non-thermal broadening of the Fe xxi line but also produces the poloidal flux, leading to the ascension of the MFRs. Title: Observations of an X-shaped Ribbon Flare in the Sun and Its Three-dimensional Magnetic Reconnection Authors: Li, Y.; Qiu, J.; Longcope, D. W.; Ding, M. D.; Yang, K. Bibcode: 2016ApJ...823L..13L Altcode: 2016arXiv160501833L We report evolution of an atypical X-shaped flare ribbon that provides novel observational evidence of three-dimensional (3D) magnetic reconnection at a separator. The flare occurred on 2014 November 9. High-resolution slit-jaw 1330 Å images from the Interface Region Imaging Spectrograph reveal four chromospheric flare ribbons that converge and form an X-shape. Flare brightening in the upper chromosphere spreads along the ribbons toward the center of the “X” (the X-point), and then spreads outward in a direction more perpendicular to the ribbons. These four ribbons are located in a quadrupolar magnetic field. Reconstruction of magnetic topology in the active region suggests the presence of a separator connecting to the X-point outlined by the ribbons. The inward motion of flare ribbons in the early stage therefore indicates 3D magnetic reconnection between two sets of non-coplanar loops that approach laterally, and reconnection proceeds downward along a section of vertical current sheet. Coronal loops are also observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory confirming the reconnection morphology illustrated by ribbon evolution. Title: Observations of an X-shaped Ribbon Flare and Its Three-dimensional Magnetic Reconnection with IRIS and SDO Authors: Li, Ying; Qiu, Jiong; Longcope, Dana; Ding, Mingde Bibcode: 2016SPD....4730206L Altcode: We report evolution of an atypical X-shaped flare ribbon which provides novel observational evidence of three-dimensional (3D) magnetic reconnection at a separator. The flare occurred on 2014 November 9, and high-resolution slit-jaw 1330 images from IRIS reveal four chromospheric flare ribbons that converge and form an X-shape. These four ribbons are located in a quadrupolar magnetic field. Reconstruction of magnetic topology in the active region suggests the presence of a separator connecting to the X-point outlined by the ribbons. The inward motion of flare ribbons, as well as coronal loops observed by the SDO/AIA, indicates 3D magnetic reconnection between two sets of non-coplanar loops that approach laterally, and the reconnection proceeds downward to a very low height. We also study spectra of Si IV, C II, and Mg II observed with the IRIS slit, which cuts across the flare ribbons near the X-point. We have found two distinct types of line profiles. At the flare ribbon, all the lines show evident redshifts with a velocity up to 50 km/s, and the redshifts are well correlated with the line intensity and width. These redshifts suggest chromospheric condensation caused by impulsive energy deposition from the separator reconnection. While right outside the flare ribbon, the lines exhibit unshifted, symmetric, yet broadened profiles; in particular, the Si IV line is significantly broadened at the far wing. The line broadening persists for 20 minutes till after the end of the flare. The distinct spectral features near the X-point indicate different dynamics associated with the separator reconnection. Title: Target Porosity Controls Crater Residual Bouguer Anomaly in the Lunar Highlands Authors: Ding, M.; Soderblom, J. M.; Zuber, M. T.; Bierson, C. J.; Nimmo, F.; Milbury, C. Bibcode: 2016LPI....47.1359D Altcode: We spatially map crater RBA, and find that the large-scale spatial variations in crater RBA can be explained by variations in regional porosity. Title: Bidirectional Outflows as Evidence of Magnetic Reconnection Leading to a Solar Microflare Authors: Hong, Jie; Ding, M. D.; Li, Ying; Yang, Kai; Cheng, Xin; Chen, Feng; Fang, Cheng; Cao, Wenda Bibcode: 2016ApJ...820L..17H Altcode: 2016arXiv160300941H Magnetic reconnection is a rapid energy release process that is believed to be responsible for flares on the Sun and stars. Nevertheless, such flare-related reconnection is mostly detected to occur in the corona, while there have been few studies concerning the reconnection in the chromosphere or photosphere. Here, we present both spectroscopic and imaging observations of magnetic reconnection in the chromosphere leading to a microflare. During the flare peak time, chromospheric line profiles show significant blueshifted/redshifted components on the two sides of the flaring site, corresponding to upflows and downflows with velocities of ±(70-80) km s-1, comparable with the local Alfvén speed as expected by the reconnection in the chromosphere. The three-dimensional nonlinear force-free field configuration further discloses twisted field lines (a flux rope) at a low altitude, cospatial with the dark threads in He I 10830 Å images. The instability of the flux rope may initiate the flare-related reconnection. These observations provide clear evidence of magnetic reconnection in the chromosphere and show the similar mechanisms of a microflare to those of major flares. Title: Ultra-narrow Negative Flare Front Observed in Helium-10830 Å Using the 1.6 m New Solar Telescope Authors: Xu, Yan; Cao, Wenda; Ding, Mingde; Kleint, Lucia; Su, Jiangtao; Liu, Chang; Ji, Haisheng; Chae, Jongchul; Jing, Ju; Cho, Kyuhyoun; Cho, Kyungsuk; Gary, Dale; Wang, Haimin Bibcode: 2016ApJ...819...89X Altcode: 2016arXiv160104729X Solar flares are sudden flashes of brightness on the Sun and are often associated with coronal mass ejections and solar energetic particles that have adverse effects on the near-Earth environment. By definition, flares are usually referred to as bright features resulting from excess emission. Using the newly commissioned 1.6 m New Solar Telescope at Big Bear Solar Observatory, we show a striking “negative” flare with a narrow but unambiguous “dark” moving front observed in He I 10830 Å, which is as narrow as 340 km and is associated with distinct spectral characteristics in Hα and Mg II lines. Theoretically, such negative contrast in He I 10830 Å can be produced under special circumstances by nonthermal electron collisions or photoionization followed by recombination. Our discovery, made possible due to unprecedented spatial resolution, confirms the presence of the required plasma conditions and provides unique information in understanding the energy release and radiative transfer in astronomical objects. Title: Response of chromospheric lines to different periodic non-thermal electron beams Authors: Cheng, Jianxia; Ding, Mingde Bibcode: 2016IAUS..320..239C Altcode: Solar flares produce radiations in very broad wavelengths. Spectra can supply us abundant information about the local plasma, such as temperature, density, mass motion and so on. Strong chromospheric lines, like the most studied Hα and Ca II 8542 Å lines are formed under conditions of departures from local thermodynamic equilibrium in the lower atmosphere subject to flare heating. Understanding how these lines are formed is very useful for us to correctly interpret the observations. In this paper, we try to figure out the response of chromospheric lines heated by different periodic non-thermal electron beams. Our results are based on radiative hydrodynamic simulations. We vary the periods of electron beam injection from 1.25 s to 20 s. We compare the response times to different heating parameters. Possible explanations are discussed. Title: Toward an Accurate Prediction of the Arrival Time of Geomagnetic-Effective Coronal Mass Ejections Authors: Shi, T.; Wang, Y.; Wan, L.; Cheng, X.; Ding, M.; Zhang, J. Bibcode: 2015AGUFMSH51B2447S Altcode: Accurately predicting the arrival of coronal mass ejections (CMEs) to the Earth based on remote images is of critical significance for the study of space weather. Here we make a statistical study of 21 Earth-directed CMEs, specifically exploring the relationship between CME initial speeds and transit times. The initial speed of a CME is obtained by fitting the CME with the Graduated Cylindrical Shell model and is thus free of projection effects. We then use the drag force model to fit results of the transit time versus the initial speed. By adopting different drag regimes, i.e., the viscous, aerodynamics, and hybrid regimes, we get similar results, with a least mean estimation error of the hybrid model of 12.9 hr. CMEs with a propagation angle (the angle between the propagation direction and the Sun-Earth line) larger than their half-angular widths arrive at the Earth with an angular deviation caused by factors other than the radial solar wind drag. The drag force model cannot be reliably applied to such events. If we exclude these events in the sample, the prediction accuracy can be improved, i.e., the estimation error reduces to 6.8 hr. This work suggests that it is viable to predict the arrival time of CMEs to the Earth based on the initial parameters with fairly good accuracy. Thus, it provides a method of forecasting space weather 1-5 days following the occurrence of CMEs. Title: Chromospheric Evaporation in an X1.0 Flare on 2014 March 29 Observed with IRIS and EIS Authors: Li, Y.; Ding, M. D.; Qiu, J.; Cheng, J. X. Bibcode: 2015ApJ...811....7L Altcode: 2015arXiv150803927L Chromospheric evaporation refers to dynamic mass motions in flare loops as a result of rapid energy deposition in the chromosphere. These motions have been observed as blueshifts in X-ray and extreme-ultraviolet (EUV) spectral lines corresponding to upward motions at a few tens to a few hundreds of km s-1. Past spectroscopic observations have also revealed a dominant stationary component, in addition to the blueshifted component, in emission lines formed at high temperatures (∼10 MK). This is contradictory to evaporation models predicting predominant blueshifts in hot lines. The recently launched Interface Region Imaging Spectrograph (IRIS) provides high-resolution imaging and spectroscopic observations that focus on the chromosphere and transition region in the UV passband. Using the new IRIS observations, combined with coordinated observations from the EUV Imaging Spectrometer, we study the chromospheric evaporation process from the upper chromosphere to the corona during an X1.0 flare on 2014 March 29. We find evident evaporation signatures, characterized by Doppler shifts and line broadening, at two flare ribbons that are separating from each other, suggesting that chromospheric evaporation takes place in successively formed flaring loops throughout the flare. More importantly, we detect dominant blueshifts in the high-temperature Fe xxi line (∼10 MK), in agreement with theoretical predictions. We also find that, in this flare, gentle evaporation occurs at some locations in the rise phase of the flare, while explosive evaporation is detected at some other locations near the peak of the flare. There is a conversion from gentle to explosive evaporation as the flare evolves. Title: The Response of Chromosphere Lines Heated by Different Periodic Impulsive Non-thermal Electrons Authors: Cheng, Jianxia; Ding, Mingde Bibcode: 2015IAUGA..2254236C Altcode: Solar flares involve in radiations in very broad wavelengths. Spectra can supply us abundant information about local plasma, such as temperature, density, mass motion and so on. Strong chromosphere lines, like the most studied Hα and Ca II 8542 Å are formed under conditions of non-local thermal equilibrium which represent the response of the lower atmosphere to flare heating. Understanding the line formation is very useful for us to correctly interpret the observations and evolution of line intensities and profiles. Numerical simulation is a very important way to interpret observational data and understand the flare mechanisms. In this paper, we try to figure out the response of chromospheric lines heated by different periodic impulsive non-thermal electrons. Our results are based on radiative hydrodynamic simulations. We vary the non-thermal heating periods from 1.25s to 20s. Different non-thermal electron fluxes are also considered in our simulations. We compare the response times for different heating parameters. Line centers and line wings also show different signatures. Possible explanations are discussed in the paper.This work is supported by NSFC under grants 11303073 ,11373023 and 11103008. This research is also supported by the open project of Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences. Title: A Two-ribbon White-light Flare Associated with a Failed Solar Eruption Observed by ONSET, SDO, and IRIS Authors: Cheng, X.; Hao, Q.; Ding, M. D.; Liu, K.; Chen, P. F.; Fang, C.; Liu, Y. D. Bibcode: 2015ApJ...809...46C Altcode: 2015arXiv150702109C Two-ribbon brightenings are one of the most remarkable characteristics of an eruptive solar flare and are often used to predict the occurrence of coronal mass ejections (CMEs). Nevertheless, it was recently called into question whether all two-ribbon flares are eruptive. In this paper, we investigate a two-ribbon-like white-light (WL) flare that is associated with a failed magnetic flux rope (MFR) eruption on 2015 January 13, which has no accompanying CME in the WL coronagraph. Observations by the Optical and Near-infrared Solar Eruption Tracer and the Solar Dynamics Observatory reveal that with the increase of the flare emission and the acceleration of the unsuccessfully erupting MFR, two isolated kernels appear at the WL 3600 Å passband and quickly develop into two elongated ribbon-like structures. The evolution of the WL continuum enhancement is completely coincident in time with the variation of Fermi hard X-ray 26-50 keV flux. An increase of continuum emission is also clearly visible at the whole FUV and NUV passbands observed by the Interface Region Imaging Spectrograph. Moreover, in one WL kernel, the Si iv, C ii, and Mg ii h/k lines display significant enhancement and non-thermal broadening. However, their Doppler velocity pattern is location-dependent. At the strongly bright pixels, these lines exhibit a blueshift, while at moderately bright ones, the lines are generally redshifted. These results show that the failed MFR eruption is also able to produce a two-ribbon flare and high-energy electrons that heat the lower atmosphere, causing the enhancement of the WL and FUV/NUV continuum emissions and chromospheric evaporation. Title: Slow Patchy Extreme-ultraviolet Propagating Fronts Associated with Fast Coronal Magneto-acoustic Waves in Solar Eruptions Authors: Guo, Y.; Ding, M. D.; Chen, P. F. Bibcode: 2015ApJS..219...36G Altcode: Using the high spatiotemporal resolution extreme ultraviolet (EUV) observations of the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we conduct a statistical study of the observational properties of the coronal EUV propagating fronts. We find that it might be a universal phenomenon for two types of fronts to coexist in a large solar eruptive event. It is consistent with the hybrid model of EUV propagating fronts, which predicts that coronal EUV propagating fronts consist of both a fast magneto-acoustic wave and a nonwave component. We find that the morphologies, propagation behaviors, and kinematic features of the two EUV propagating fronts are completely different from each other. The fast magneto-acoustic wave fronts are almost isotropic. They travel continuously from the flaring region across multiple magnetic polarities to global distances. On the other hand, the slow nonwave fronts appear as anisotropic and sequential patches of EUV brightening. Each patch propagates locally in the magnetic domains where the magnetic field lines connect to the bottom boundary and stops at the magnetic domain boundaries. Within each magnetic domain, the velocities of the slow patchy nonwave component are an order of magnitude lower than that of the fast-wave component. However, the patches of the slow EUV propagating front can jump from one magnetic domain to a remote one. The velocities of such a transit between different magnetic domains are about one-third to one-half of those of the fast-wave component. The results show that the velocities of the nonwave component, both within one magnetic domain and between different magnetic domains, are highly nonuniform due to the inhomogeneity of the magnetic field in the lower atmosphere. Title: On the Nature of the EUV Late Phase of Solar Flares Authors: Li, Ying; Ding, Mingde; Guo, Yang; Dai, Yu Bibcode: 2015IAUGA..2258380L Altcode: The extreme-ultraviolet (EUV) late phase of solar flares is a second peak of warm coronal emissions (e.g., Fe XVI) for many minutes to a few hours after the GOES soft X-ray peak. It was first observed by the EUV Variability Experiment on board the Solar Dynamics Observatory (SDO). The late-phase emission originates from a second set of longer loops (late-phase loops) that are higher than the main flaring loops. It is suggested to be caused by either additional heating or long-lasting cooling. In this paper, we study the role of long-lasting cooling and additional heating in producing the EUV late phase using the enthalpy based thermal evolution of loops model. We find that a long cooling process in late-phase loops can well explain the presence of the EUV late-phase emission, but we cannot exclude the possibility of additional heating in the decay phase. Moreover, we provide two preliminary methods based on the UV and EUV emissions from the Atmospheric Imaging Assembly on board SDO to determine whether or not additional heating plays a role in the late-phase emission. Using nonlinear force-free field modeling, we study the magnetic configuration of the EUV late phase. It is found that the late phase can be generated either in hot spine field lines associated with a magnetic null point or in large-scale magnetic loops of multipolar magnetic fields. In this paper, we also discuss why the EUV late phase is usually observed in warm coronal emissions and why the majority of flares do not exhibit an EUV late phase. Title: Predicting the Arrival Time of Coronal Mass Ejections with the Graduated Cylindrical Shell and Drag Force Model Authors: Shi, Tong; Wang, Yikang; Wan, Linfeng; Cheng, Xin; Ding, Mingde; Zhang, Jie Bibcode: 2015ApJ...806..271S Altcode: 2015arXiv150500884S Accurately predicting the arrival of coronal mass ejections (CMEs) to the Earth based on remote images is of critical significance for the study of space weather. In this paper, we make a statistical study of 21 Earth-directed CMEs, specifically exploring the relationship between CME initial speeds and transit times. The initial speed of a CME is obtained by fitting the CME with the Graduated Cylindrical Shell model and is thus free of projection effects. We then use the drag force model to fit results of the transit time versus the initial speed. By adopting different drag regimes, i.e., the viscous, aerodynamics, and hybrid regimes, we get similar results, with a least mean estimation error of the hybrid model of 12.9 hr. CMEs with a propagation angle (the angle between the propagation direction and the Sun-Earth line) larger than their half-angular widths arrive at the Earth with an angular deviation caused by factors other than the radial solar wind drag. The drag force model cannot be reliably applied to such events. If we exclude these events in the sample, the prediction accuracy can be improved, i.e., the estimation error reduces to 6.8 hr. This work suggests that it is viable to predict the arrival time of CMEs to the Earth based on the initial parameters with fairly good accuracy. Thus, it provides a method of forecasting space weather 1-5 days following the occurrence of CMEs. Title: Extreme ultraviolet imaging of three-dimensional magnetic reconnection in a solar eruption Authors: Sun, J. Q.; Cheng, X.; Ding, M. D.; Guo, Y.; Priest, E. R.; Parnell, C. E.; Edwards, S. J.; Zhang, J.; Chen, P. F.; Fang, C. Bibcode: 2015NatCo...6.7598S Altcode: 2015NatCo...6E7598S; 2015arXiv150608255S Magnetic reconnection, a change of magnetic field connectivity, is a fundamental physical process in which magnetic energy is released explosively, and it is responsible for various eruptive phenomena in the universe. However, this process is difficult to observe directly. Here, the magnetic topology associated with a solar reconnection event is studied in three dimensions using the combined perspectives of two spacecraft. The sequence of extreme ultraviolet images clearly shows that two groups of oppositely directed and non-coplanar magnetic loops gradually approach each other, forming a separator or quasi-separator and then reconnecting. The plasma near the reconnection site is subsequently heated from ~1 to >=5 MK. Shortly afterwards, warm flare loops (~3 MK) appear underneath the hot plasma. Other observational signatures of reconnection, including plasma inflows and downflows, are unambiguously revealed and quantitatively measured. These observations provide direct evidence of magnetic reconnection in a three-dimensional configuration and reveal its origin. Title: On the 2012 October 23 Circular Ribbon Flare: Emission Features and Magnetic Topology Authors: Yang, Kai; Guo, Yang; Ding, M. D. Bibcode: 2015ApJ...806..171Y Altcode: 2015arXiv150502914Y Circular ribbon flares are usually related to spine-fan type magnetic topology containing null points. In this paper, we investigate an X-class circular ribbon flare on 2012 October 23, using the multiwavelength data from the Solar Dynamics Observatory, Hinode, and RHESSI. In Ca ii H emission, the flare showed three ribbons with two highly elongated ones inside and outside a quasi-circular one, respectively. A hot channel was displayed in the extreme-ultraviolet emissions that infers the existence of a magnetic flux rope. Two hard X-ray (HXR) sources in the 12-25 keV energy band were located at the footpoints of this hot channel. Using a nonlinear force-free magnetic field extrapolation, we identify three topological structures: (1) a three-dimensional null point, (2) a flux rope below the fan of the null point, and (3) a large-scale quasi-separatrix layer (QSL) induced by the quadrupolar-like magnetic field of the active region. We find that the null point is embedded within the large-scale QSL. In our case, all three identified topological structures must be considered to explain all the emission features associated with the observed flare. Besides, the HXR sources are regarded as the consequence of the reconnection within or near the border of the flux rope. Title: A Type II Radio Burst without a Coronal Mass Ejection Authors: Su, W.; Cheng, X.; Ding, M. D.; Chen, P. F.; Sun, J. Q. Bibcode: 2015ApJ...804...88S Altcode: 2015arXiv150300861S Type II radio bursts are thought to be a signature of coronal shocks. In this paper, we analyze a short-lived type II burst that started at 07:40 UT on 2011 February 28. By carefully checking white-light images, we find that the type II radio burst is not accompanied by a coronal mass ejection, only by a C2.4 class flare and narrow jet. However, in the EUV images provided by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we find a wave-like structure that propagated at a speed of ∼600 km s-1 during the burst. The relationship between the type II radio burst and the wave-like structure is, in particular, explored. For this purpose, we first derive the density distribution under the wave by the differential emission measure method, which is used to restrict the empirical density model. We then use the restricted density model to invert the speed of the shock that produces the observed frequency drift rate in the dynamic spectrum. The inverted shock speed is similar to the speed of the wave-like structure. This implies that the wave-like structure is most likely a coronal shock that produces the type II radio burst. We also examine the evolution of the magnetic field in the flare-associated active region and find continuous flux emergence and cancellation taking place near the flare site. Based on these facts, we propose a new mechanism for the formation of the type II radio burst, i.e., the expansion of the strongly inclined magnetic loops after reconnecting with a nearby emerging flux acts as a piston to generate the shock wave. Title: Imaging and Spectroscopic Diagnostics on the Formation of Two Magnetic Flux Ropes Revealed by SDO/AIA and IRIS Authors: Cheng, X.; Ding, M. D.; Fang, C. Bibcode: 2015ApJ...804...82C Altcode: 2015arXiv150207801C Helical magnetic flux rope (MFR) is a fundamental structure of coronal mass ejections (CMEs) and has been discovered recently to exist as a sigmoidal channel structure prior to its eruption in the EUV high-temperature passbands of the Atmospheric Imaging Assembly (AIA). However, when and where the MFR is built up are still elusive. In this paper, we investigate two MFRs (MFR1 and MFR2) in detail, whose eruptions produced two energetic solar flares and CMEs on 2014 April 18 and 2014 September 10, respectively. The AIA EUV images reveal that for a long time prior to their eruption, both MFR1 and MFR2 are under formation, which is probably through magnetic reconnection between two groups of sheared arcades driven by the shearing and converging flows in the photosphere near the polarity inversion line. At the footpoints of the MFR1, the Interface Region Imaging Spectrograph Si iv, C ii, and Mg ii lines exhibit weak to moderate redshifts and a non-thermal broadening in the pre-flare phase. However, a relatively large blueshift and an extremely strong non-thermal broadening are found at the formation site of the MFR2. These spectral features consolidate the proposition that the reconnection plays an important role in the formation of MFRs. For the MFR1, the reconnection outflow may propagate along its legs, penetrating into the transition region and the chromosphere at the footpoints. For the MFR2, the reconnection probably takes place in the lower atmosphere and results in the strong blueshift and non-thermal broadening for the Mg ii, C ii, and Si iv lines. Title: Magnetohydrodynamic Seismology of a Coronal Loop System by the First Two Modes of Standing Kink Waves Authors: Guo, Y.; Erdélyi, R.; Srivastava, A. K.; Hao, Q.; Cheng, X.; Chen, P. F.; Ding, M. D.; Dwivedi, B. N. Bibcode: 2015ApJ...799..151G Altcode: 2014arXiv1411.7095G We report the observation of the first two harmonics of the horizontally polarized kink waves excited in a coronal loop system lying southeast of AR 11719 on 2013 April 11. The detected periods of the fundamental mode (P 1), its first overtone (P 2) in the northern half, and that in the southern one are 530.2 ± 13.3, 300.4 ± 27.7, and 334.7 ± 22.1 s, respectively. The periods of the first overtone in the two halves are the same considering uncertainties in the measurement. We estimate the average electron density, temperature, and length of the loop system as (5.1 ± 0.8) × 108 cm-3, 0.65 ± 0.06 MK, and 203.8 ± 13.8 Mm, respectively. As a zeroth-order estimation, the magnetic field strength, B = 8.2 ± 1.0 G, derived by the coronal seismology using the fundamental kink mode matches with that derived by a potential field model. The extrapolation model also shows the asymmetric and nonuniform distribution of the magnetic field along the coronal loop. Using the amplitude profile distributions of both the fundamental mode and its first overtone, we observe that the antinode positions of both the fundamental mode and its first overtone shift toward the weak field region along the coronal loop. The results indicate that the density stratification and the temperature difference effects are larger than the magnetic field variation effect on the period ratio. On the other hand, the magnetic field variation has a greater effect on the eigenfunction of the first overtone than the density stratification does for this case. Title: Variations in Crustal Structure, Lithospheric Flexural Strength, and Isostatic Compensation Mechanisms of Mars Authors: Ding, M.; Lin, J.; Zuber, M. T. Bibcode: 2014AGUFM.P51E3991D Altcode: We analyze gravity and topography of Mars to investigate the spatial variations in crustal thickness, lithospheric strength, and mechanisms of support of prominent topographic features on Mars. The latest gravity model JGMRO110c (released in 2012) from the Mars Reconnaissance Orbiter mission has a spatial block size resolution of ~97 km (corresponding to degree-110), enabling us to resolve crustal structures at higher spatial resolution than those determined from previous degree-80 and 85 gravity models [Zuber et al., 2000; McGovern et al., 2002, 2004; Neumann et al., 2004; Belleguic et al., 2005]. Using the latest gravity data, we first inverted for a new version of crustal thickness model of Mars assuming homogeneous crust and mantle densities of 2.9 and 3.5 g/cm3. We calculated "isostatic" topography for the Airy local isostatic compensation mechanism, and "non-isostatic" topography after removing the isostatic part. We find that about 92% of the Martian surface is in relatively isostatic state, indicating either relatively small lithospheric strength and/or small vertical loading. Relatively isostatic regions include the hemispheric dichotomy, Hellas and Argyre Planitia, Noachis and Arabia Terra, and Terra Cimmeria. In contrast, regions with significant amount of non-isostatic topography include the Olympus, Ascraeus, Arsia, Pavonis, Alba, and Elysium Mons, Isidis Planitia and Valles Marineris. Their relatively large "non-isostatc topography" implies relatively strong lithospheric strength and large vertical loading. Spectral analysis of the admittance and correlation relationship between gravity and topography were conducted for the non-isostatic regions using the localized spectra method [Wieczorek and Simons, 2005, 2007] and thin-shell lithospheric flexural approximation [Forsyth, 1985; McGovern et al., 2002, 2004]. The best-fitting models reveal significant variations in the effective lithospheric thickness with the greatest values for the Olympus Mon, Valles Marineris, and Isidis Planitia; reduced values for the Ascraeus, Arsis, and Pavonis Mons; and smallest values for the Alba and Elysium Mons. Our models also suggest that there could be significant sub-surface loading underneath the Olympus, Ascraeus, Arsia, and Pavonis Mons, and Isidis Planitia. Title: On the Nature of the Extreme-ultraviolet Late Phase of Solar Flares Authors: Li, Y.; Ding, M. D.; Guo, Y.; Dai, Y. Bibcode: 2014ApJ...793...85L Altcode: 2014arXiv1407.6041L The extreme-ultraviolet (EUV) late phase of solar flares is a second peak of warm coronal emissions (e.g., Fe XVI) for many minutes to a few hours after the GOES soft X-ray peak. It was first observed by the EUV Variability Experiment on board the Solar Dynamics Observatory (SDO). The late-phase emission originates from a second set of longer loops (late-phase loops) that are higher than the main flaring loops. It is suggested to be caused by either additional heating or long-lasting cooling. In this paper, we study the role of long-lasting cooling and additional heating in producing the EUV late phase using the enthalpy based thermal evolution of loops model. We find that a long cooling process in late-phase loops can well explain the presence of the EUV late-phase emission, but we cannot exclude the possibility of additional heating in the decay phase. Moreover, we provide two preliminary methods based on the UV and EUV emissions from the Atmospheric Imaging Assembly on board SDO to determine whether or not additional heating plays a role in the late-phase emission. Using nonlinear force-free field modeling, we study the magnetic configuration of the EUV late phase. It is found that the late phase can be generated either in hot spine field lines associated with a magnetic null point or in large-scale magnetic loops of multipolar magnetic fields. In this paper, we also discuss why the EUV late phase is usually observed in warm coronal emissions and why the majority of flares do not exhibit an EUV late phase. Title: Recurrent Coronal Jets Induced by Magnetic Emergence in the Solar Atmosphere Authors: Guo, Y.; Démoulin, P.; Schmieder, B.; Ding, M. D.; Vargas Domínguez, S.; Liu, Y. Bibcode: 2014RMxAC..44...45G Altcode: Jets are part of the observed phenomenology in the solar corona. They are thought to be a consequence of magnetic reconnection but the physics involved is not completely understood. We study some recurrent jetting events with unprecedented temporal and spatial resolutions. Title: Spectral Observations of Ellerman Bombs and Fitting with a Two-cloud Model Authors: Hong, Jie; Ding, M. D.; Li, Ying; Fang, Cheng; Cao, Wenda Bibcode: 2014ApJ...792...13H Altcode: 2014arXiv1407.3048H 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. Title: Reciprocatory magnetic reconnection in a coronal bright point Authors: Zhang, Q. M.; Chen, P. F.; Ding, M. D.; Ji, H. S. Bibcode: 2014A&A...568A..30Z Altcode: 2014arXiv1406.5328Z Context. Coronal bright points (CBPs) are small-scale and long-duration brightenings in the lower solar corona. They are often explained in terms of magnetic reconnection.
Aims: We aim to study the substructures of a CBP and clarify the relationship among the brightenings of different patches inside the CBP.
Methods: The event was observed by the X-ray Telescope (XRT) aboard the Hinode spacecraft on 2009 August 22-23.
Results: The CBP showed repeated brightenings (or CBP flashes). During each of the two successive CBP flashes, that is, weak and strong flashes that were separated by ~2 hr, the XRT images revealed that the CBP was composed of two chambers, patches A and B. During the weak flash, patch A brightened first, and patch B brightened ~2 min later. During the transition, the right leg of a large-scale coronal loop drifted from the right side of the CBP to the left side. During the strong flash, patch B brightened first, and patch A brightened ~2 min later. During the transition, the right leg of the large-scale coronal loop drifted from the left side of the CBP to the right side. In each flash, the rapid change of the connectivity of the large-scale coronal loop is strongly suggestive of the interchange reconnection.
Conclusions: For the first time we found reciprocatory reconnection in the CBP, which means that reconnected loops in the outflow region of the first reconnection process serve as the inflow of the second reconnection process.

Movies associated with Figs. 2 and 5 are available in electronic form at http://www.aanda.org Title: Formation of a Double-decker Magnetic Flux Rope in the Sigmoidal Solar Active Region 11520 Authors: Cheng, X.; Ding, M. D.; Zhang, J.; Sun, X. D.; Guo, Y.; Wang, Y. M.; Kliem, B.; Deng, Y. Y. Bibcode: 2014ApJ...789...93C Altcode: 2014arXiv1405.4923C In this paper, we address the formation of a magnetic flux rope (MFR) that erupted on 2012 July 12 and caused a strong geomagnetic storm event on July 15. Through analyzing the long-term evolution of the associated active region observed by the Atmospheric Imaging Assembly and the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory, it is found that the twisted field of an MFR, indicated by a continuous S-shaped sigmoid, is built up from two groups of sheared arcades near the main polarity inversion line a half day before the eruption. The temperature within the twisted field and sheared arcades is higher than that of the ambient volume, suggesting that magnetic reconnection most likely works there. The driver behind the reconnection is attributed to shearing and converging motions at magnetic footpoints with velocities in the range of 0.1-0.6 km s-1. The rotation of the preceding sunspot also contributes to the MFR buildup. Extrapolated three-dimensional non-linear force-free field structures further reveal the locations of the reconnection to be in a bald-patch region and in a hyperbolic flux tube. About 2 hr before the eruption, indications of a second MFR in the form of an S-shaped hot channel are seen. It lies above the original MFR that continuously exists and includes a filament. The whole structure thus makes up a stable double-decker MFR system for hours prior to the eruption. Eventually, after entering the domain of instability, the high-lying MFR impulsively erupts to generate a fast coronal mass ejection and X-class flare; while the low-lying MFR remains behind and continuously maintains the sigmoidicity of the active region. Title: On the Relationship Between a Hot-channel-like Solar Magnetic Flux Rope and its Embedded Prominence Authors: Cheng, X.; Ding, M. D.; Zhang, J.; Srivastava, A. K.; Guo, Y.; Chen, P. F.; Sun, J. Q. Bibcode: 2014ApJ...789L..35C Altcode: 2014arXiv1406.4196C A magnetic flux rope (MFR) is a coherent and helical magnetic field structure that has recently been found likely to appear as an elongated hot channel prior to a solar eruption. In this Letter, we investigate the relationship between the hot channel and the associated prominence through analysis of a limb event on 2011 September 12. In the early rise phase, the hot channel was initially cospatial with the prominence. It then quickly expanded, resulting in a separation of the top of the hot channel from that of the prominence. Meanwhile, they both experienced an instantaneous morphology transformation from a Λ shape to a reversed-Y shape and the top of these two structures showed an exponential increase in height. These features are a good indication of the occurrence of kink instability. Moreover, the onset of kink instability is found to coincide in time with the impulsive enhancement of flare emission underneath the hot channel, suggesting that ideal kink instability likely also plays an important role in triggering fast flare reconnection besides initiating the impulsive acceleration of the hot channel and distorting its morphology. We conclude that the hot channel is most likely the MFR system and the prominence only corresponds to the cool materials that are collected in the bottom of the helical field lines of the MFR against gravity. Title: Dynamic Evolution of an X-shaped Structure above a Trans-equatorial Quadrupole Solar Active Region Group Authors: Sun, J. Q.; Cheng, X.; Guo, Y.; Ding, M. D.; Li, Y. Bibcode: 2014ApJ...787L..27S Altcode: 2014arXiv1405.1563S In the solar corona, magnetic reconnection usually takes place at the singular configuration of the magnetic field, in particular near a magnetic null, owing to its high susceptibility to perturbations. In this Letter, we report a rare X-shaped structure, encompassing a magnetic null, above a trans-equatorial quadrupole active region group that is well observed by the Atmospheric Imaging Assembly (AIA). The observations show that this X-shaped structure is visible in all AIA EUV passbands and stably exists for days. However, possibly induced by flare activities at the northern part of the quadrupole active region group, the X-shaped structure starts to destabilize while a jet erupts near its center at ~15:05 UT on 2013 October 7. Through nonlinear force-free field modeling, we identify a magnetic null that is above the quadrupole polarities and well corresponds to the X-shaped structure. After the jet eruption, the temperature and emission measure of the plasma near the X-shaped structure rise from ~2.3 MK and ~1.2 × 1027 cm-5 at 15:01 UT to ~5.4 MK and ~3.7 × 1027 cm-5 at 15:36 UT, respectively, revealed by the differential emission measure analysis, indicating that magnetic reconnection most likely takes place there to heat the plasma. Moreover, the height of the null increases ~10 Mm, which is most likely due to the partial opening of the field lines near the fan surface that makes the null underneath rise to seek a new equilibrium. Title: On the Nature of the EUV Late Phase of Solar Flares Authors: Li, Ying; Ding, Mingde; Guo, Yang; Dai, Yu Bibcode: 2014AAS...22411103L Altcode: The EUV late phase of solar flares is a second peak of the warm coronal emissions (e.g., Fe XVI) many minutes to a few hours after the GOES soft X-ray peak, which was first observed by the EUV Variability Experiment (EVE) on board the Solar Dynamics Observatory (SDO). The late phase emission originates from a second set of longer loops (late phase loops) that are higher than the main flaring loops. It is explained as being caused by either additional heating or long-lasting cooling. In this paper, we study the role of long-lasting cooling and additional heating in producing the EUV late phase using the "enthalpy-based thermal evolution of loops'" (EBTEL) experiments. We find that the long cooling process in late phase loops, which definitely exists, can sufficiently explain the emission property of the EUV late phase; meanwhile, we cannot exclude the role of an additional heating of these loops that possibly occurs. Moreover, we provide two preliminary methods based on the UV and EUV emissions from the Atmospheric Imaging Assembly (AIA) on board SDO to determine whether an additional heating plays some role or not in the late phase emission. Through the nonlinear force-free field modeling, we study the magnetic configuration related to the EUV late phase. It is found that the late phase can be generated either in hot spine field lines associated with a magnetic null point or large-scale magnetic loops in multipolar magnetic fields. In this paper, we also explain why the EUV late phase appears most obviously in the warm coronal emissions and why the majority of flares do not exhibit an EUV late phase. Title: Differential Emission Measure Analysis of A Limb Solar Flare on 2012 July 19 Authors: Sun, J. Q.; Cheng, X.; Ding, M. D. Bibcode: 2014ApJ...786...73S Altcode: 2014arXiv1403.6202S We perform Differential Emission Measure (DEM) analysis of an M7.7 flare that occurred on 2012 July 19 and was well observed by the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamic Observatory. Using the observational data with unprecedented high temporal and spatial resolution from six AIA coronal passbands, we calculate the DEM of the flare and derive the time series of maps of DEM-weighted temperature and emission measure (EM). It is found that, during the flare, the highest EM region is located in the flare loop top with a value varying between ~8.4 × 1028 cm-5 and ~2.5 × 1030 cm-5. The temperature there rises from ~8 MK at about 04:40 UT (the initial rise phase) to a maximum value of ~13 MK at about 05:20 UT (the hard X-ray peak). Moreover, we find a hot region that is above the flare loop top with a temperature even up to ~16 MK. We also analyze the DEM properties of the reconnection site. The temperature and density there are not as high as that in the loop top and the flux rope, indicating that the main heating may not take place inside the reconnection site. In the end, we examine the dynamic behavior of the flare loops. Along the flare loop, both the temperature and the EM are the highest in the loop top and gradually decrease toward the footpoints. In the northern footpoint, an upward force appears with a biggest value in the impulsive phase, which we conjecture originates from chromospheric evaporation. Title: Heating and Dynamics of Two Flare Loop Systems Observed by AIA and EIS Authors: Li, Y.; Qiu, J.; Ding, M. D. Bibcode: 2014ApJ...781..120L Altcode: 2013arXiv1312.5056L We investigate heating and evolution of flare loops in a C4.7 two-ribbon flare on 2011 February 13. From Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) imaging observations, we can identify two sets of loops. Hinode/EUV Imaging Spectrometer (EIS) spectroscopic observations reveal blueshifts at the feet of both sets of loops. The evolution and dynamics of the two sets are quite different. The first set of loops exhibits blueshifts for about 25 minutes followed by redshifts, while the second set shows stronger blueshifts, which are maintained for about one hour. The UV 1600 observation by AIA also shows that the feet of the second set of loops brighten twice. These suggest that continuous heating may be present in the second set of loops. We use spatially resolved UV light curves to infer heating rates in the few tens of individual loops comprising the two loop systems. With these heating rates, we then compute plasma evolution in these loops with the "enthalpy-based thermal evolution of loops" model. The results show that, for the first set of loops, the synthetic EUV light curves from the model compare favorably with the observed light curves in six AIA channels and eight EIS spectral lines, and the computed mean enthalpy flow velocities also agree with the Doppler shift measurements by EIS. For the second set of loops modeled with twice-heating, there are some discrepancies between modeled and observed EUV light curves in low-temperature bands, and the model does not fully produce the prolonged blueshift signatures as observed. We discuss possible causes for the discrepancies. Title: Tracking the Evolution of a Coherent Magnetic Flux Rope Continuously from the Inner to the Outer Corona Authors: Cheng, X.; Ding, M. D.; Guo, Y.; Zhang, J.; Vourlidas, A.; Liu, Y. D.; Olmedo, O.; Sun, J. Q.; Li, C. Bibcode: 2014ApJ...780...28C Altcode: 2013arXiv1310.6782C The magnetic flux rope (MFR) is believed to be the underlying magnetic structure of coronal mass ejections (CMEs). However, it remains unclear how an MFR evolves into and forms the multi-component structure of a CME. In this paper, we perform a comprehensive study of an extreme-ultraviolet (EUV) MFR eruption on 2013 May 22 by tracking its morphological evolution, studying its kinematics, and quantifying its thermal property. As EUV brightenings begin, the MFR starts to rise slowly and shows helical threads winding around an axis. Meanwhile, cool filamentary materials descend spirally down to the chromosphere. These features provide direct observational evidence of intrinsically helical structure of the MFR. Through detailed kinematical analysis, we find that the MFR evolution has two distinct phases: a slow rise phase and an impulsive acceleration phase. We attribute the first phase to the magnetic reconnection within the quasi-separatrix layers surrounding the MFR, and the much more energetic second phase to the fast magnetic reconnection underneath the MFR. We suggest that the transition between these two phases is caused by the torus instability. Moreover, we identify that the MFR evolves smoothly into the outer corona and appears as a coherent structure within the white-light CME volume. The MFR in the outer corona was enveloped by bright fronts that originated from plasma pile-up in front of the expanding MFR. The fronts are also associated with the preceding sheath region followed by the outmost MFR-driven shock. Title: Twist Accumulation and Topology Structure of a Solar Magnetic Flux Rope Authors: Guo, Y.; Ding, M. D.; Cheng, X.; Zhao, J.; Pariat, E. Bibcode: 2013ApJ...779..157G Altcode: 2013arXiv1311.1883G To study the buildup of a magnetic flux rope before a major flare and coronal mass ejection (CME), we compute the magnetic helicity injection, twist accumulation, and topology structure of the three-dimensional (3D) magnetic field, which is derived by the nonlinear force-free field model. The Extreme-ultraviolet Imaging Telescope on board the Solar and Heliospheric Observatory observed a series of confined flares without any CME before a major flare with a CME at 23:02 UT on 2005 January 15 in active region NOAA 10720. We derive the vector velocity at eight time points from 18:27 UT to 22:20 UT with the differential affine velocity estimator for vector magnetic fields, which were observed by the Digital Vector Magnetograph at Big Bear Solar Observatory. The injected magnetic helicity is computed with the vector magnetic and velocity fields. The helicity injection rate was (- 16.47 ± 3.52) × 1040 Mx2 hr-1. We find that only about 1.8% of the injected magnetic helicity became the internal helicity of the magnetic flux rope, whose twist increasing rate was -0.18 ± 0.08 Turns hr-1. The quasi-separatrix layers (QSLs) of the 3D magnetic field are computed by evaluating the squashing degree, Q. We find that the flux rope was wrapped by QSLs with large Q values, where the magnetic reconnection induced by the continuously injected magnetic helicity further produced the confined flares. We suggest that the flux rope was built up and heated by the magnetic reconnection in the QSLs. Title: Production of the Extreme-ultraviolet Late Phase of an X Class Flare in a Three-stage Magnetic Reconnection Process Authors: Dai, Y.; Ding, M. D.; Guo, Y. Bibcode: 2013ApJ...773L..21D Altcode: 2013arXiv1307.4866D We report on observations of an X class flare on 2011 September 6 by the instruments on board the Solar Dynamics Observatory. The flare occurs in a complex active region with multiple polarities. The Extreme-Ultraviolet (EUV) Variability Experiment observations in the warm coronal emission reveal three enhancements, the third of which corresponds to an EUV late phase. The three enhancements have a one-to-one correspondence to the three stages in flare evolution identified by the spatially resolved Atmospheric Imaging Assembly observations, which are characterized by a flux rope eruption, a moderate filament ejection, and the appearance of EUV late phase loops, respectively. The EUV late phase loops are spatially and morphologically distinct from the main flare loops. Multi-channel analysis suggests the presence of a continuous but fragmented energy injection during the EUV late phase resulting in the warm corona nature of the late phase loops. Based on these observational facts, we propose a three-stage magnetic reconnection scenario to explain the flare evolution. Reconnections in different stages involve different magnetic fields but show a casual relationship between them. The EUV late phase loops are mainly produced by the least energetic magnetic reconnection in the last stage. Title: Recurrent coronal jets induced by repetitively accumulated electric currents Authors: Guo, Y.; Démoulin, P.; Schmieder, B.; Ding, M. D.; Vargas Domínguez, S.; Liu, Y. Bibcode: 2013A&A...555A..19G Altcode: 2013arXiv1305.0902G Context. Jets of plasma are frequently observed in the solar corona. A self-similar recurrent behavior is observed in a fraction of them.
Aims: Jets are thought to be a consequence of magnetic reconnection; however, the physics involved is not fully understood. Therefore, we study some jet observations with unprecedented temporal and spatial resolutions.
Methods: The extreme-ultraviolet (EUV) jets were observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory (SDO). The Helioseismic and Magnetic Imager (HMI) on board SDO measured the vector magnetic field, from which we derive the magnetic flux evolution, the photospheric velocity field, and the vertical electric current evolution. The magnetic configuration before the jets is derived by the nonlinear force-free field extrapolation.
Results: Three EUV jets recurred in about one hour on 17 September 2010 in the following magnetic polarity of active region 11106. We derive that the jets are above a pair of parasitic magnetic bipoles that are continuously driven by photospheric diverging flows. The interaction drove the buildup of electric currents, which we observed as elongated patterns at the photospheric level. For the first time, the high temporal cadence of the HMI allows the evolution of such small currents to be followed. In the jet region, we found that the integrated absolute current peaks repetitively in phase with the 171 Å flux evolution. The current buildup and its decay are both fast, about ten minutes each, and the current maximum precedes the 171 Å also by about ten minutes. Then, the HMI temporal cadence is marginally fast enough to detect such changes.
Conclusions: The photospheric current pattern of the jets is found to be associated with the quasi-separatrix layers deduced from the magnetic extrapolation. From previous theoretical results, the observed diverging flows are expected to continuously build such currents. We conclude that the magnetic reconnection occurs periodically, in the current layer created between the emerging bipoles and the large-scale active region field. The periodic magnetic reconnection induced the observed recurrent coronal jets and the decrease of the vertical electric current magnitude.

Two movies are available in electronic form at http://www.aanda.org Title: Tests and applications of nonlinear force-free field extrapolations in spherical geometry Authors: Guo, Y.; Ding, M. D. Bibcode: 2013IAUS..294..553G Altcode: We test a nonlinear force-free field (NLFFF) optimization code in spherical geometry with an analytical solution from Low and Lou. The potential field source surface (PFSS) model is served as the initial and boundary conditions where observed data are not available. The analytical solution can be well recovered if the boundary and initial conditions are properly handled. Next, we discuss the preprocessing procedure for the noisy bottom boundary data, and find that preprocessing is necessary for NLFFF extrapolations when we use the observed photospheric magnetic field as bottom boundaries. Finally, we apply the NLFFF model to a solar area where four active regions interacting with each other. An M8.7 flare occurred in one active region. NLFFF modeling in spherical geometry simultaneously constructs the small and large scale magnetic field configurations better than the PFSS model does. Title: Radiative Hydrodynamic Simulations of He I 10830 Å Authors: Cheng, J. X.; Ding, M. D.; Fang, C. Bibcode: 2013IAUS..294..545C Altcode: We study the properties of the He I 10830 Å line under the bombardment of nonthermal electron beam. Using radiative hydrodynamics method, we obtain the line profiles from different model atmospheres by varying the electron beam fluxes. Below a certain electron flux, the spectra become much more absorptions as nonthemal flux increases. Above the threshold, the spectral intensities increase as the flux goes up. We also investigate the temporal evolution of the spectra under the nonthermal heating. For weak electron flux, the profiles evolve from weak absorptions to strong absorptions. For strong nonthermal heating, the profiles can be significantly changed from absorptions to emissions. The spectra also show red or blue asymmetries. Title: Heating and Dynamics of Two Flare Loop Systems Observed by AIA and EIS Authors: Li, Ying; Qiu, J.; Ding, M. Bibcode: 2013SPD....4420005L Altcode: We focus on investigating heating and evolution of flare loops. A C4.7 two-ribbon flare on 2011 February 13 is analyzed and modeled. From AIA imaging observations, we can identify two sets of loops in this event. EIS spectroscopic observations reveal blueshifts at the feet of both sets of loops during the impulsive phase. However, the dynamic evolutions of the two sets of loops are quite different. The first set of loops exhibits blueshifts 10 km/s) for about 25 minutes followed by redshifts, while the second set shows stronger blueshifts 20 km/s) which are maintained for about 1 hour. The long-lasting blueshifts in the second set of loops are indicative of continuous heating. The UV 1600 Å observation by AIA also shows that the feet of the loops brighten twice with 15 minutes apart. The first set of loops, on the other hand, brighten only once in the UV band. We construct heating functions of the two sets of loops using spatially resolved UV light curves at their footpoints, and model plasma evolution in these loops with the EBTEL model. The results show that, for the first set of loops, the synthetic EUV light curves from the model compare favorably with the observed light curves in six AIA channels and eight EIS spectral lines, and the computed mean enthalpy flow velocities also agree with the Doppler shifts measured in EIS lines. For the second set of loops modeled with twice-heating, there are some discrepancies between modeled and observed EUV light curves at low-temperature lines, and the model does not fully reproduce the prolonged blueshift signatures as observed. The prominent discrepancies between model and observations for the second set of loops may be caused by non-uniform heating localized especially at the loop footpoints which cannot be modeled by the 0D EBTEL model, or by unresolved fine flaring strands in the loops with quite different heating rates and profiles. Title: Solar Flare Hard X-ray Spikes Observed by RHESSI: a Statistical Study Authors: Cheng, Jianxia; Qiu, J.; Ding, M.; Wang, H. Bibcode: 2013SPD....4440205C Altcode: Hard X-ray (HXR) spikes refer to fine time structures on timescales of seconds to milliseconds in high-energy HXR emission profiles during solar flare eruptions. We present a preliminary statistical investigation of temporal and spectral properties of HXR spikes. Using a three-sigma spike selection rule, we detected 184 spikes in 94 out of 322 flares with significant counts at given photon energies, which were detected from demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). About one fifth of these spikes are also detected at photon energies higher than 100 keV. The statistical properties of the spikes are as follows. (1) HXR spikes are produced in both impulsive flares and long-duration flares with nearly the same occurrence rates. Ninety percent of the spikes occur during the rise phase of the flares, and about 70% occur around the peak times of the flares. (2) The time durations of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not dependent on photon energies. The spikes exhibit symmetric time profiles with no significant difference between rise and decay times.(3) Among the most energetic spikes, nearly all of them have harder count spectra than their underlying slow-varying components. There is also a weak indication that spikes exhibiting time lags in high-energy emissions tend to have harder spectra than spikes with time lags in low-energy emissions. Title: Investigating Two Successive Flux Rope Eruptions in a Solar Active Region Authors: Cheng, X.; Zhang, J.; Ding, M. D.; Olmedo, O.; Sun, X. D.; Guo, Y.; Liu, Y. Bibcode: 2013ApJ...769L..25C Altcode: 2013arXiv1304.7532C We investigate two successive flux rope (FR1 and FR2) eruptions resulting in two coronal mass ejections (CMEs) on 2012 January 23. Both flux ropes (FRs) appeared as an EUV channel structure in the images of high temperature passbands of the Atmospheric Imaging Assembly prior to the CME eruption. Through fitting their height evolution with a function consisting of linear and exponential components, we determine the onset time of the FR impulsive acceleration with high temporal accuracy for the first time. Using this onset time, we divide the evolution of the FRs in the low corona into two phases: a slow rise phase and an impulsive acceleration phase. In the slow rise phase of FR1, the appearance of sporadic EUV and UV brightening and the strong shearing along the polarity inverse line indicates that the quasi-separatrix-layer reconnection likely initiates the slow rise. On the other hand, for FR2, we mainly contribute its slow rise to the FR1 eruption, which partially opened the overlying field and thus decreased the magnetic restriction. At the onset of the impulsive acceleration phase, FR1 (FR2) reaches the critical height of 84.4 ± 11.2 Mm (86.2 ± 13.0 Mm) where the decline of the overlying field with height is fast enough to trigger the torus instability. After a very short interval (~2 minutes), the flare emission began to enhance. These results reveal the compound activity involving multiple magnetic FRs and further suggest that the ideal torus instability probably plays the essential role of initiating the impulsive acceleration of CMEs. Title: The Driver of Coronal Mass Ejections in the Low Corona: A Flux Rope Authors: Cheng, X.; Zhang, J.; Ding, M. D.; Liu, Y.; Poomvises, W. Bibcode: 2013ApJ...763...43C Altcode: 2012arXiv1211.6524C Recent Solar Dynamic Observatory observations reveal that coronal mass ejections (CMEs) consist of a multi-temperature structure: a hot flux rope and a cool leading front (LF). The flux rope first appears as a twisted hot channel in the Atmospheric Imaging Assembly (AIA) 94 Å and 131 Å passbands. The twisted hot channel initially lies along the polarity inversion line and then rises and develops into a semi-circular flux-rope-like structure during the impulsive acceleration phase of CMEs. In the meantime, the rising hot channel compresses the surrounding magnetic field and plasma, which successively stack into the CME LF. In this paper, we study in detail two well-observed CMEs that occurred on 2011 March 7 and 2011 March 8, respectively. Each of them is associated with an M-class flare. Through a kinematic analysis we find that (1) the hot channel rises earlier than the first appearance of the CME LF and the onset of the associated flare and (2) the speed of the hot channel is always faster than that of the LF, at least in the field of view of AIA. Thus, the hot channel acts as a continuous driver of the CME formation and eruption in the early acceleration phase. Subsequently, the two CMEs in white-light images can be well reproduced by the graduated cylindrical shell flux rope model. These results suggest that the pre-existing flux rope plays a key role in CME initiation and formation. Title: Differential Emission Measure Analysis of Multiple Structural Components of Coronal Mass Ejections in the Inner Corona Authors: Cheng, X.; Zhang, J.; Saar, S. H.; Ding, M. D. Bibcode: 2012ApJ...761...62C Altcode: 2012arXiv1210.7287C In this paper, we study the temperature and density properties of multiple structural components of coronal mass ejections (CMEs) using differential emission measure (DEM) analysis. The DEM analysis is based on the six-passband EUV observations of solar corona from the Atmospheric Imaging Assembly on board the Solar Dynamic Observatory. The structural components studied include the hot channel in the core region (presumably the magnetic flux rope of the CME), the bright loop-like leading front (LF), and coronal dimming in the wake of the CME. We find that the presumed flux rope has the highest average temperature (>8 MK) and density (~1.0 × 109 cm-3), resulting in an enhanced emission measure over a broad temperature range (3 <= T(MK) <= 20). On the other hand, the CME LF has a relatively cool temperature (~2 MK) and a narrow temperature distribution similar to the pre-eruption coronal temperature (1 <= T(MK) <= 3). The density in the LF, however, is increased by 2%-32% compared with that of the pre-eruption corona, depending on the event and location. In coronal dimmings, the temperature is more broadly distributed (1 <= T(MK) <= 4), but the density decreases by ~35%-~40%. These observational results show that: (1) CME core regions are significantly heated, presumably through magnetic reconnection; (2) CME LFs are a consequence of compression of ambient plasma caused by the expansion of the CME core region; and (3) the dimmings are largely caused by the plasma rarefaction associated with the eruption. Title: Spectral diagnostics of the heating and dynamics of the solar chromosphere Authors: Ding, M. Bibcode: 2012IAUSS...6E.306D Altcode: The solar chromosphere is a highly dynamic and inhomogeneous layer in which spectral lines are formed under non-LTE conditions making the spectral diagnostics relatively difficult. In particular, during solar flares, the chromosphere is heated by non-thermal electrons (or heat conduction) from the corona, which produce hard X-ray emission and result in enhancements in chromospheric line emission, UV emission and possibly white-light emission. The chromosphere is also believed to be the energy release site of some small-scale activities like Ellerman bombs. Here, I make a brief review of the recent progress in diagnosing the heating and dynamics of the chromosphere in various solar activities using multi-wavelength line and continuum observations by especially ground-based telescopes. Title: Evolution and DEM analysis of Early Structures of Coronal Mass Ejections by SDO-AIA Authors: Cheng, X.; Zhang, J.; Ding, M. Bibcode: 2012AGUFMSH41A2096C Altcode: Recent Solar Dynamic Observatory observations reveal that some coronal mass ejections (CMEs) consist of multi-temperature structures: a hot channel and a cool leading edge (LE). The channel first appears as a twisted structure in the Atmospheric Imaging Assembly 94 ? and 131 ? passbands. The twisted hot channel initially lies along the polarity inversion line and then gradually develops into the less twisted semicircular structure during the impulsive phase of CME acceleration. In the meantime, the hot channel compresses the surrounding magnetic field and plasma, which successively stack into the CME leading front. Further kinematic analysis shows that, (1) the hot channel rises earlier than the first appearance of the CME leading edge and the onset of the associated flare; (2) the speed of the hot channel is always larger than that of the leading edge, at least in the field of view of AIA. It seems that the hot channel acts as a continuous driver of the CME formation and acceleration in the early acceleration phase, like the behaviors of flux rope in CME models and simulations. Moreover, through the differential emission measure analysis, it is confirmed that the channel is dominated by the plasma with high temperature (>8 MK) and high density (109cm-3), while the leading edge and dimming region are mainly dominated by the low temperature plasma (~2-3MK). Title: Modeling Magnetic Field Structure of a Solar Active Region Corona Using Nonlinear Force-free Fields in Spherical Geometry Authors: Guo, Y.; Ding, M. D.; Liu, Y.; Sun, X. D.; DeRosa, M. L.; Wiegelmann, T. Bibcode: 2012ApJ...760...47G Altcode: 2012arXiv1210.0998G We test a nonlinear force-free field (NLFFF) optimization code in spherical geometry using an analytical solution from Low and Lou. Several tests are run, ranging from idealized cases where exact vector field data are provided on all boundaries, to cases where noisy vector data are provided on only the lower boundary (approximating the solar problem). Analytical tests also show that the NLFFF code in the spherical geometry performs better than that in the Cartesian one when the field of view of the bottom boundary is large, say, 20° × 20°. Additionally, we apply the NLFFF model to an active region observed by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (SDO) both before and after an M8.7 flare. For each observation time, we initialize the models using potential field source surface (PFSS) extrapolations based on either a synoptic chart or a flux-dispersal model, and compare the resulting NLFFF models. The results show that NLFFF extrapolations using the flux-dispersal model as the boundary condition have slightly lower, therefore better, force-free, and divergence-free metrics, and contain larger free magnetic energy. By comparing the extrapolated magnetic field lines with the extreme ultraviolet (EUV) observations by the Atmospheric Imaging Assembly on board SDO, we find that the NLFFF performs better than the PFSS not only for the core field of the flare productive region, but also for large EUV loops higher than 50 Mm. Title: Quadrature Observations of Wave and Non-wave Components and their Decoupling in an Extreme-ultraviolet Wave Event Authors: Dai, Y.; Ding, M. D.; Chen, P. F.; Zhang, J. Bibcode: 2012ApJ...759...55D Altcode: 2012arXiv1208.4539D We report quadrature observations of an extreme-ultraviolet (EUV) wave event on 2011 January 27 obtained by the Extreme Ultraviolet Imager on board the Solar Terrestrial Relations Observatory, and the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Two components are revealed in the EUV wave event. A primary front is launched with an initial speed of ~440 km s-1. It appears that significant emission enhancement occurs in the hotter channel while deep emission reduction occurs in the cooler channel. When the primary front encounters a large coronal loop system and slows down, a secondary, much fainter, front emanates from the primary front with a relatively higher starting speed of ~550 km s-1. Afterward, the two fronts propagate independently with increasing separation. The primary front finally stops at a magnetic separatrix, while the secondary front travels farther until it fades out. In addition, upon the arrival of the secondary front, transverse oscillations of a prominence are triggered. We suggest that the two components are of different natures. The primary front belongs to a non-wave coronal mass ejection (CME) component, which can be reasonably explained with the field-line stretching model. The multi-temperature behavior may be caused by considerable heating due to nonlinear adiabatic compression on the CME frontal loop. As for the secondary front, it is most likely a linear fast-mode magnetohydrodynamic wave that propagates through a medium of the typical coronal temperature. X-ray and radio data provide us with complementary evidence in support of the above scenario. Title: Solar flare hard X-ray spikes observed by RHESSI: a statistical study Authors: Cheng, J. X.; Qiu, J.; Ding, M. D.; Wang, H. Bibcode: 2012A&A...547A..73C Altcode: 2012arXiv1210.7027C Context. Hard X-ray (HXR) spikes refer to fine time structures on timescales of seconds to milliseconds in high-energy HXR emission profiles during solar flare eruptions.
Aims: We present a preliminary statistical investigation of temporal and spectral properties of HXR spikes.
Methods: Using a three-sigma spike selection rule, we detected 184 spikes in 94 out of 322 flares with significant counts at given photon energies, which were detected from demodulated HXR light curves obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). About one fifth of these spikes are also detected at photon energies higher than 100 keV.
Results: The statistical properties of the spikes are as follows. (1) HXR spikes are produced in both impulsive flares and long-duration flares with nearly the same occurrence rates. Ninety percent of the spikes occur during the rise phase of the flares, and about 70% occur around the peak times of the flares. (2) The time durations of the spikes vary from 0.2 to 2 s, with the mean being 1.0 s, which is not dependent on photon energies. The spikes exhibit symmetric time profiles with no significant difference between rise and decay times. (3) Among the most energetic spikes, nearly all of them have harder count spectra than their underlying slow-varying components. There is also a weak indication that spikes exhibiting time lags in high-energy emissions tend to have harder spectra than spikes with time lags in low-energy emissions. Title: Analysis and Modeling of Two Flare Loops Observed by AIA and EIS Authors: Li, Y.; Qiu, J.; Ding, M. D. Bibcode: 2012ApJ...758...52L Altcode: 2012arXiv1208.5440L We analyze and model an M1.0 flare observed by SDO/AIA and Hinode/EIS to investigate how flare loops are heated and evolve subsequently. The flare is composed of two distinctive loop systems observed in extreme ultraviolet (EUV) images. The UV 1600 Å emission at the feet of these loops exhibits a rapid rise, followed by enhanced emission in different EUV channels observed by the Atmospheric Imaging Assembly (AIA) and the EUV Imaging Spectrometer (EIS). Such behavior is indicative of impulsive energy deposit and the subsequent response in overlying coronal loops that evolve through different temperatures. Using the method we recently developed, we infer empirical heating functions from the rapid rise of the UV light curves for the two loop systems, respectively, treating them as two big loops with cross-sectional area of 5'' by 5'', and compute the plasma evolution in the loops using the EBTEL model. We compute the synthetic EUV light curves, which, with the limitation of the model, reasonably agree with observed light curves obtained in multiple AIA channels and EIS lines: they show the same evolution trend and their magnitudes are comparable by within a factor of two. Furthermore, we also compare the computed mean enthalpy flow velocity with the Doppler shift measurements by EIS during the decay phase of the two loops. Our results suggest that the two different loops with different heating functions as inferred from their footpoint UV emission, combined with their different lengths as measured from imaging observations, give rise to different coronal plasma evolution patterns captured both in the model and in observations. Title: Understanding the white-light flare on 2012 March 9: evidence of a two-step magnetic reconnection Authors: Hao, Q.; Guo, Y.; Dai, Y.; Ding, M. D.; Li, Z.; Zhang, X. Y.; Fang, C. Bibcode: 2012A&A...544L..17H Altcode: 2012arXiv1211.1751H
Aims: We attempt to understand the white-light flare (WLF) that was observed on 2012 March 9 with a newly constructed multi-wavelength solar telescope called the Optical and Near-infrared Solar Eruption Tracer (ONSET).
Methods: We analyzed WLF observations in radio, Hα, white-light, ultraviolet, and X-ray bands. We also studied the magnetic configuration of the flare via the nonlinear force-free field (NLFFF) extrapolation and the vector magnetic field observed by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO).
Results: Continuum emission enhancement clearly appeared at the 3600 Å and 4250 Å bands, with peak contrasts of 25% and 12%, respectively. The continuum emission enhancement closely coincided with the impulsive increase in the hard X-ray emission and a microwave type III burst at 03:40 UT. We find that the WLF appeared at one end of either the sheared or twisted field lines or both. There was also a long-lasting phase in the Hα and soft X-ray bands after the white-light emission peak. In particular, a second, yet stronger, peak appeared at 03:56 UT in the microwave band.
Conclusions: This event shows clear evidence that the white-light emission was caused by energetic particles bombarding the lower solar atmosphere. A two-step magnetic reconnection scenario is proposed to explain the entire process of flare evolution, i.e., the first-step magnetic reconnection between the field lines that are highly sheared or twisted or both, and the second-step one in the current sheet, which is stretched by the erupting flux rope. The WLF is supposed to be triggered in the first-step magnetic reconnection at a relatively low altitude. Title: ONSET-A New Multi-Wavelength Solar Telescope Authors: Fang, C.; Chen, P. F.; Ding, M. D.; Dai, Y.; Li, Z. Bibcode: 2012EAS....55..349F Altcode: A new multi-wavelength solar telescope, Optical and Near-infrared Solar Eruption Tracer (ONSET), is constructed by Nanjing University, being run in cooperation with Yunnan Astronomical Observatory. ONSET is able to observe the Sun in three wavelength windows: He I 10830 Å, Hα and white-light at 3600 Å or 4250 Å. Full-disk or partial solar images with a field of 10 arcmin at three wavelengths can be obtained nearly simultaneously. It is designed to trace solar eruptions with high spatial and temporal resolutions. This telescope has been installed at a new solar observing site near the Fuxian Lake, Yunnan Province. The site is located at E102N24, with an altitude of 1722 m. The seeing is stable and very nice. We give a brief description of the scientific objectives and the basic structure of ONSET. Some preliminary results are also shown. Title: Kinematics and Differential Emission Measure of the Flux Rope during Coronal Mass Ejections Authors: Ding, M. D.; Cheng, X.; Zhang, J. Bibcode: 2012EAS....55..287D Altcode: Recent Solar Dynamic Observatory observations reveal that some coronal mass ejections (CMEs) consist of multi-temperature structures: a hot channel and a cool leading edge (LE). The channel first appears as a twisted loop and lies along the polarity inversion line; it then gradually develops into the less twisted semicircular structure during the impulsive phase of CME acceleration. In the meantime, the hot channel compresses the surrounding magnetic field and plasma, which successively stack into the CME leading front. Here, we study two well observed CMEs, which occurred on 2010 November 3 and 2011 March 8, respectively. We find that, the hot channel rises earlier than the first appearance of the CME leading front and the onset of the associated flare, and that the speed of the hot channel is always larger than that of the leading front. Further differential emission measure analysis shows that the channel often has significant emission at high temperatures. We thus conclude that the observed hot channel is likely to be magnetic flux rope that acts as a driver of the CME acceleration in the early phase. Title: Silicon Detector Telescope for proton detection in electron scattering reactions at MAMI Authors: Makek, M.; Achenbach, P.; Ayerbe Gayoso, C.; Baumann, D.; Bernauer, J. C.; Böhm, R.; Bosnar, D.; Denig, A.; Ding, M.; Distler, M. O.; Doria, L.; Friedrich, J.; Friščić, I.; Gómez Rodríguez de La Paz, M.; Merkel, H.; Müller, U.; Nungesser, L.; Pochodzalla, J.; Potokar, M.; Seimetz, M.; Sánchez Majos, S.; Schlimme, B. S.; Širca, S.; Walcher, Th.; Weinriefer, M. Bibcode: 2012NIMPA.673...82M Altcode: A new Silicon Detector Telescope has been constructed and installed within the experimental facility of the A1 collaboration at Mainz Microtron, with the goal to detect low-energy protons. It consists of seven silicon layers for energy and angle measurement and a plastic scintillator for triggering purposes. The detector subtends a solid angle up to 88 msr, depending on the distance from the target and covers the proton kinetic energy range of 25-41 MeV with the mean energy resolution σE=0.47MeV, operating at 500 kHz. Digital signal processing methods applied for energy reconstruction have been important for keeping the acceptable energy resolution at high counting rates. The Silicon Detector Telescope has been successfully used in double and triple coincidence measurements along with the magnetic spectrometers of the A1 collaboration. Title: Two Types of Magnetic Reconnection in Coronal Bright Points and the Corresponding Magnetic Configuration Authors: Zhang, Q. M.; Chen, P. F.; Guo, Y.; Fang, C.; Ding, M. D. Bibcode: 2012ApJ...746...19Z Altcode: Coronal bright points (CBPs) are long-lived small-scale brightenings in the solar corona. They are generally explained by magnetic reconnection. However, the corresponding magnetic configurations are not well understood. We carry out a detailed multi-wavelength analysis of two neighboring CBPs on 2007 March 16, observed in soft X-ray (SXR) and EUV channels. It is seen that the SXR light curves present quasi-periodic flashes with an interval of ~1 hr superposed over the long-lived mild brightenings, suggesting that the SXR brightenings of this type of CBPs might consist of two components: one is the gentle brightenings and the other is the CBP flashes. It is found that the strong flashes of the bigger CBP are always accompanied by SXR jets. The potential field extrapolation indicates that both CBPs are covered by a dome-like separatrix surface, with a magnetic null point above. We propose that the repetitive CBP flashes, as well as the recurrent SXR jets, result from the impulsive null-point reconnection, while the long-lived brightenings are due to the interchange reconnection along the separatrix surface. Although the EUV images at high-temperature lines resemble the SXR appearance, the 171 Å and 195 Å channels reveal that the blurry CBP in SXR consists of a cusp-shaped loop and several separate bright patches, which are explained to be due to the null-point reconnection and the separatrix reconnection, respectively. Title: Evolution of Hard X-Ray Sources and Ultraviolet Solar Flare Ribbons for a Confined Eruption of a Magnetic Flux Rope Authors: Guo, Y.; Ding, M. D.; Schmieder, B.; Démoulin, P.; Li, H. Bibcode: 2012ApJ...746...17G Altcode: 2011arXiv1111.1790G We study the magnetic field structures of hard X-ray (HXR) sources and flare ribbons of the M1.1 flare in active region NOAA 10767 on 2005 May 27. We have found in a nonlinear force-free field extrapolation over the same polarity inversion line, a small pre-eruptive magnetic flux rope located next to sheared magnetic arcades. RHESSI and the Transition Region and Coronal Explorer (TRACE) observed this confined flare in the X-ray bands and ultraviolet (UV) 1600 Å bands, respectively. In this event magnetic reconnection occurred at several locations. It first started at the location of the pre-eruptive flux rope. Then, the observations indicate that magnetic reconnection occurred between the pre-eruptive magnetic flux rope and the sheared magnetic arcades more than 10 minutes before the flare peak. This implies the formation of the larger flux rope, as observed with TRACE. Next, HXR sources appeared at the footpoints of this larger flux rope at the peak of the flare. The associated high-energy particles may have been accelerated below the flux rope in or around a reconnection region. Still, the close spatial association between the HXR sources and the flux rope footpoints favors an acceleration within the flux rope. Finally, a topological analysis of a large solar region, including active regions NOAA 10766 and 10767, shows the existence of large-scale Quasi-Separatrix Layers (QSLs) before the eruption of the flux rope. No enhanced emission was found at these QSLs during the flare, but the UV flare ribbons stopped at the border of the closest large-scale QSL. Title: Investigation of the Formation and Separation of an Extreme-ultraviolet Wave from the Expansion of a Coronal Mass Ejection Authors: Cheng, X.; Zhang, J.; Olmedo, O.; Vourlidas, A.; Ding, M. D.; Liu, Y. Bibcode: 2012ApJ...745L...5C Altcode: 2011arXiv1112.4540C We address the nature of EUV waves through direct observations of the formation of a diffuse wave driven by the expansion of a coronal mass ejection (CME) and its subsequent separation from the CME front. The wave and the CME on 2011 June 7 were well observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Following the solar eruption onset, marked by the beginning of the rapid increasing of the CME velocity and the X-ray flux of accompanying flare, the CME exhibits a strong lateral expansion. During this impulsive expansion phase, the expansion speed of the CME bubble increases from 100 km s-1 to 450 km s-1 in only six minutes. An important finding is that a diffuse wave front starts to separate from the front of the expanding bubble shortly after the lateral expansion slows down. Also a type II burst is formed near the time of the separation. After the separation, two distinct fronts propagate with different kinematic properties. The diffuse front travels across the entire solar disk, while the sharp front rises up, forming the CME ejecta with the diffuse front ahead of it. These observations suggest that the previously termed EUV wave is a composite phenomenon and driven by the CME expansion. While the CME expansion is accelerating, the wave front is cospatial with the CME front, thus the two fronts are indiscernible. Following the end of the acceleration phase, the wave moves away from the CME front with a gradually increasing distance between them. Title: Evidence of two-stage magnetic reconnection in the 2005 January 15 X2.6 flare Authors: Wang, Pu; Li, Yixuan; Ding, Mingde; Ji, Haisheng; Wang, Haimin Bibcode: 2011NewA...16..470W Altcode: We analyze in detail the X2.6 flare that occurred on 2005 January 15 in the NOAA AR 10720 using multiwavelength observations. There are several interesting properties of the flare that reveal possible two-stage magnetic reconnection similar to that in the physical picture of tether-cutting, where the magnetic fields of two separate loop systems reconnect at the flare core region, and subsequently a large flux rope forms, erupts, and breaks open the overlying arcade fields. The observed manifestations include: (1) remote H α brightenings appear minutes before the main phase of the flare; (2) separation of the flare ribbons has a slow and a fast phase, and the flare hard X-ray emission appears in the later fast phase; (3) rapid transverse field enhancement near the magnetic polarity inversion line (PIL) is found to be associated with the flare. We conclude that the flare occurrence fits the tether-cutting reconnection picture in a special way, in which there are three flare ribbons outlining the sigmoid configuration. We also discuss this event in the context of what was predicted by Hudson et al. (2008), where the Lorentz force near the flaring PIL drops after the flare and consequently the magnetic field lines there turn to be more horizontal as we observed. Title: Spectroscopic Analysis of Interaction between an Extreme-ultraviolet Imaging Telescope Wave and a Coronal Upflow Region Authors: Chen, F.; Ding, M. D.; Chen, P. F.; Harra, L. K. Bibcode: 2011ApJ...740..116C Altcode: 2011arXiv1107.5630C We report a spectroscopic analysis of an EUV Imaging Telescope (EIT) wave event that occurred in active region 11081 on 2010 June 12 and was associated with an M2.0 class flare. The wave propagated nearly circularly. The southeastern part of the wave front passed over an upflow region near a magnetic bipole. Using EUV Imaging Spectrometer raster observations for this region, we studied the properties of plasma dynamics in the wave front, as well as the interaction between the wave and the upflow region. We found a weak blueshift for the Fe XII λ195.12 and Fe XIII λ202.04 lines in the wave front. The local velocity along the solar surface, which is deduced from the line-of-sight velocity in the wave front and the projection effect, is much lower than the typical propagation speed of the wave. A more interesting finding is that the upflow and non-thermal velocities in the upflow region are suddenly diminished after the transit of the wave front. This implies a significant change of magnetic field orientation when the wave passed. As the lines in the upflow region are redirected, the velocity along the line of sight is diminished as a result. We suggest that this scenario is more in accordance with what was proposed in the field-line stretching model of EIT waves. Title: A Comparative Study of Confined and Eruptive Flares in NOAA AR 10720 Authors: Cheng, X.; Zhang, J.; Ding, M. D.; Guo, Y.; Su, J. T. Bibcode: 2011ApJ...732...87C Altcode: 2011arXiv1103.2323C We investigate the distinct properties of two types of flares: eruptive flares associated with coronal mass ejections (CMEs) and confined flares without CMEs. Our study sample includes nine M- and X-class flares, all from the same active region (AR), six of which are confined and three others which are eruptive. The confined flares tend to be more impulsive in the soft X-ray time profiles and show slenderer shapes in the Extreme-ultraviolet Imaging Telescope 195 Å images, while the eruptive ones are long-duration events and show much more extended brightening regions. The location of the confined flares is closer to the center of the AR, while the eruptive flares are at the outskirts. This difference is quantified by the displacement parameter, which is the distance between the AR center and the flare location; the average displacement of the six confined flares is 16 Mm, while that of the eruptive ones is as large as 39 Mm. Further, through nonlinear force-free field extrapolation, we find that the decay index of the transverse magnetic field in the low corona (~10 Mm) is larger for eruptive flares than for confined ones. In addition, the strength of the transverse magnetic field over the eruptive flare sites is weaker than it is over the confined ones. These results demonstrate that the strength and the decay index of the background magnetic field may determine whether or not a flare is eruptive or confined. The implication of these results on CME models is discussed in the context of torus instability of the flux rope. Title: Observing Flux Rope Formation During the Impulsive Phase of a Solar Eruption Authors: Cheng, X.; Zhang, J.; Liu, Y.; Ding, M. D. Bibcode: 2011ApJ...732L..25C Altcode: 2011arXiv1103.5084C Magnetic flux ropes are believed to be an important structural component of coronal mass ejections (CMEs). While there exists much observational evidence of flux ropes after the eruption, e.g., as seen in remote-sensing coronagraph images or in situ solar wind data, the direct observation of flux ropes during CME impulsive phase has been rare. In this Letter, we present an unambiguous observation of a flux rope still in the formation phase in the low corona. The CME of interest occurred above the east limb on 2010 November 3 with footpoints partially blocked. The flux rope was seen as a bright blob of hot plasma in the Atmospheric Imaging Assembly (AIA) 131 Å passband (peak temperature ~11 MK) rising from the core of the source active region, rapidly moving outward and stretching the surrounding background magnetic field upward. The stretched magnetic field seemed to curve-in behind the core, similar to the classical magnetic reconnection scenario in eruptive flares. On the other hand, the flux rope appeared as a dark cavity in the AIA 211 Å passband (2.0 MK) and 171 Å passband (0.6 MK) in these relatively cool temperature bands, a bright rim clearly enclosed the dark cavity. The bright rim likely represents the pileup of the surrounding coronal plasma compressed by the expanding flux rope. The composite structure seen in AIA multiple temperature bands is very similar to that in the corresponding coronagraph images, which consists of a bright leading edge and a dark cavity, commonly believed to be a flux rope. Title: Observing Flux Rope Formation During the Impulsive Phase of a Solar Eruption Authors: Cheng, Xin; Zhang, J.; Yang, L.; Ding, M. Bibcode: 2011SPD....42.2315C Altcode: 2011BAAS..43S.2315C Magnetic flux rope is believed to be an important structural component of coronal mass ejections (CMEs). While there exist much observational evidence of the flux rope post the eruption, e.g., as seen in remote-sensing coronagraph images or in-situ solar wind data, the direct observation of flux ropes during CME impulsive phase has been rare or non-exist. In this Letter, we present an unambiguous observation of a flux rope still in the formation phase in the low corona. The CME of interest occurred above the east limb on 2010 November 03 with footpoints partially blocked. The flux rope was seen as a blob of hot plasma in AIA 131 A passband (peak temperature 11 MK) rising from the core of the source active region, rapidly moving outward and stretching upward the surrounding background magnetic field. The stretched magnetic field seemed to curve-in, similar to the classical magnetic reconnection scenario in eruptive flares. The flux rope was also seen as a dark cavity in AIA 211 A passpand (2.0 MK) and 171 A passband (0.6 MK); in these relatively cool temperature bands, a bright rim clearly enclosed the dark cavity. The bright rim likely represents the pile-up of the surrounding coronal plasma compressed by the expanding flux rope. The composite structure seen in AIA multiple temperature bands is very similar to that in the corresponding coronagraph images, which consists of a bright leading edge and a dark cavity, commonly believed to be a flux rope. Title: Different Patterns of Chromospheric Evaporation in a Flaring Region Observed with Hinode/EIS Authors: Li, Y.; Ding, M. D. Bibcode: 2011ApJ...727...98L Altcode: 2010arXiv1011.4562L We investigate the chromospheric evaporation in the flare of 2007 January 16 using line profiles observed by the Exterme-UV Imaging Spectrometer on board Hinode. Three points at flare ribbons of different magnetic polarities are analyzed in detail. We find that the three points show different patterns of upflows and downflows in the impulsive phase of the flare. The spectral lines at the first point are mostly blueshifted, with the hotter lines showing a dominant blueshifted component over the stationary one. At the second point, however, only weak upflows are detected; instead, notable downflows appear at high temperatures (up to 2.5-5.0 MK). The third point is similar to the second one only in that it shows evidence of multi-component downflows. While the evaporated plasma falling back down as warm rain is a possible cause of the redshifts at the second and third points, the different patterns of chromospheric evaporation at the three points imply the existence of different heating mechanisms in the flaring active region. Title: Vector Magnetic Fields and Current Helicities in Coronal Holes and Quiet Regions Authors: Yang, Shuhong; Zhang, Jun; Li, Ting; Ding, Mingde Bibcode: 2011ApJ...726...49Y Altcode: 2010arXiv1011.0512Y In the solar photosphere, many properties of coronal holes (CHs) are not known, especially vector magnetic fields. Using observations from Hinode, we investigate vector magnetic fields, current densities, and current helicities in two CHs, and compare them with two normal quiet regions (QRs) for the first time. We find that the areas where large current helicities are located are mainly co-spatial with strong vertical and horizontal field elements both in shape and in location. In the CHs, horizontal magnetic fields, inclination angles, current densities, and current helicities are larger than those in the QRs. The mean vertical current density and current helicity in the CHs and QRs, averaged over all the observed areas including the CHs and QRs, are approximately 0.008 A m-2 and 0.005 G2 m-1, respectively. The mean current density in magnetic flux concentrations where the vertical fields are stronger than 100 G is as large as 0.012 ± 0.001 A m-2, consistent with that in the flare productive active regions. Our results imply that the magnetic fields, especially the strong fields, both in the CHs and in the QRs are nonpotential. Title: Driving mechanism of a failed eruption Authors: Guo, Y.; Ding, M. D.; Schmieder, B.; Li, H.; Törö, T.; Wiegelmann, T. Bibcode: 2011ASInC...2..307G Altcode: We find a magnetic flux rope before the M1.1 flare in active region 10767 on 2005 May 27 by a nonlinear force-free field extrapolation. TRACE observations of the filament eruption show that the erupting structure performed a writhing deformation and stopped rising at a certain height, suggesting that the flux rope converted some of its twist into writhe and was confined in the corona. After calculating the twist of the flux rope, we find that it was comparable to thresholds of the helical kink instability found in numerical simulations. We conclude that the activation and rise of the flux rope were triggered and initially driven by the kink instability. The decay index of the external magnetic field stays below the threshold for the torus instability within a long height range. The confinement of the eruption could be explained by the failure of the torus instability. Hard X-ray sources at the peak of the M1.1 flare coincided with the footpoints of the erupting helical structure, which indicates a high possibility that hard X-ray sources were produced more efficiently in the flux rope. Title: Driving Mechanism and Onset Condition of a Confined Eruption Authors: Guo, Y.; Ding, M. D.; Schmieder, B.; Li, H.; Török, T.; Wiegelmann, T. Bibcode: 2010ApJ...725L..38G Altcode: We study a confined eruption accompanied by an M1.1 flare in solar active region (AR) NOAA 10767 on 2005 May 27, where a pre-eruptive magnetic flux rope was reported in a nonlinear force-free field (NLFFF) extrapolation. The observations show a strong writhing motion of the erupting structure, suggesting that a flux rope was indeed present and converted some of its twist into writhe in the course of the eruption. Using the NLFFF extrapolation, we calculate the twist of the pre-eruptive flux rope and find that it is in very good agreement with thresholds of the helical kink instability found in numerical simulations. We conclude that the activation and rise of the flux rope were triggered and driven by the instability. Using a potential field extrapolation, we also estimate the height distribution of the decay index of the external magnetic field in the AR 1 hr prior to the eruption. We find that the decay index stays below the threshold for the torus instability for a significant height range above the erupting flux rope. This provides a possible explanation for the confinement of the eruption to the low corona. Title: Evidence of Explosive Evaporation in a Microflare Observed by Hinode/EIS Authors: Chen, F.; Ding, M. D. Bibcode: 2010ApJ...724..640C Altcode: 2010arXiv1009.3193C We present a detailed study of explosive chromospheric evaporation during a microflare which occurred on 2007 December 7 as observed with the Extreme-ultraviolet Imaging Spectrometer on board Hinode. We find temperature-dependent upflows for lines formed from 1.0 to 2.5 MK and downflows for lines formed from 0.05 to 0.63 MK in the impulsive phase of the flare. Both the line intensity and the nonthermal line width appear enhanced in most of the lines and are temporally correlated with the evaporation velocity. Our results are consistent with the numerical simulations of flare models, which take into account a strong nonthermal electron beam in producing the explosive chromospheric evaporation. The explosive evaporation observed in this microflare implies that the same dynamic processes may exist in events with very different magnitudes. Title: Spectroscopic Analysis of an EIT Wave/dimming Observed by Hinode/EIS Authors: Chen, F.; Ding, M. D.; Chen, P. F. Bibcode: 2010ApJ...720.1254C Altcode: 2010arXiv1009.3190C EUV Imaging Telescope (EIT) waves are a wavelike phenomenon propagating outward from the coronal mass ejection source region, with expanding dimmings following behind. We present a spectroscopic study of an EIT wave/dimming event observed by the Hinode/Extreme-ultraviolet Imaging Spectrometer. Although the identification of the wave front is somewhat affected by the pre-existing loop structures, the expanding dimming is well defined. We investigate the line intensity, width, and Doppler velocity for four EUV lines. In addition to the significant blueshift implying plasma outflows in the dimming region as revealed in previous studies, we find that the widths of all four spectral lines increase at the outer edge of the dimmings. We illustrate that this feature can be well explained by the field line stretching model, which claims that EIT waves are apparently moving brightenings that are generated by the successive stretching of the closed field lines. Title: Re-flaring of a Post-flare Loop System Driven by Flux Rope Emergence and Twisting Authors: Cheng, X.; Ding, M. D.; Guo, Y.; Zhang, J.; Jing, J.; Wiegelmann, T. Bibcode: 2010ApJ...716L..68C Altcode: 2010arXiv1005.1720C In this Letter, we study in detail the evolution of the post-flare loops on 2005 January 15 that occurred between two consecutive solar eruption events, both of which generated a fast halo coronal mass ejection (CME) and a major flare. The post-flare loop system, formed after the first CME/flare eruption, evolved rapidly, as manifested by the unusual accelerating rise motion of the loops. Through nonlinear force-free field models, we obtain the magnetic structure over the active region. It clearly shows that the flux rope below the loops also kept rising, accompanied with increasing twist and length. Finally, the post-flare magnetic configuration evolved to a state that resulted in the second CME/flare eruption. This is an event in which the post-flare loops can re-flare in a short period of ~16 hr following the first CME/flare eruption. The observed re-flaring at the same location is likely driven by the rapid evolution of the flux rope caused by the magnetic flux emergence and the rotation of the sunspot. This observation provides valuable information on CME/flare models and their prediction. Title: A Study of the Build-up, Initiation, and Acceleration of 2008 April 26 Coronal Mass Ejection Observed by STEREO Authors: Cheng, X.; Ding, M. D.; Zhang, J. Bibcode: 2010ApJ...712.1302C Altcode: 2010arXiv1002.4962C In this paper, we analyze the full evolution, from a few days prior to the eruption to the initiation, final acceleration, and propagation, of the coronal mass ejection (CME) that occurred on 2008 April 26 using the unprecedented high cadence and multi-wavelength observations by the Solar Terrestrial Relations Observatory. There existed frequent filament activities and EUV jets prior to the CME eruption for a few days. These activities were probably caused by the magnetic reconnection in the lower atmosphere driven by photospheric convergence motions, which were evident in the sequence of magnetogram images from the Michelson Doppler Imager on board the Solar and Heliospheric Observatory. The slow low-layer magnetic reconnection may be responsible for the storage of magnetic free energy in the corona and the formation of a sigmoidal core field or a flux rope leading to the eventual eruption. The occurrence of EUV brightenings in the sigmoidal core field prior to the rise of the flux rope implies that the eruption was triggered by the inner tether-cutting reconnection, but not the external breakout reconnection. During the period of impulsive acceleration, the time profile of the CME acceleration in the inner corona is found to be consistent with the time profile of the reconnection electric field inferred from the footpoint separation and the Reuven Ramaty High Energy Solar Spectroscopic Imager 15-25 keV hard X-ray flux curve of the associated flare. The full evolution of this CME can be described in four distinct phases: the build-up phase, initiation phase, main acceleration phase, and propagation phase. The physical properties and the transition between these phases are discussed, in an attempt to provide a global picture of CME dynamic evolution. Title: Radiative Hydrodynamic Simulation of the Continuum Emission in Solar White-Light Flares Authors: Cheng, J. X.; Ding, M. D.; Carlsson, Mats Bibcode: 2010ApJ...711..185C Altcode: It is believed that solar white-light flares (WLFs) originate in the lower chromosphere and upper photosphere. In particular, some recently observed WLFs show a large continuum enhancement at 1.56 μm where the opacity reaches its minimum. Therefore, it is important to clarify how the energy is transferred to the lower layers responsible for the production of WLFs. Based on radiative hydrodynamic simulations, we study the role of non-thermal electron beams in increasing the continuum emission. We vary the parameters of the electron beam and disk positions and compare the results with observations. The electron beam heated model can explain most of the observational white-light enhancements. For the most energetic WLFs observed so far, however, a very large electron beam flux and a high low-energy cutoff, which are possibly beyond the parameter space in our simulations, are required in order to reproduce the observed white-light emission. Title: A Statistical Study of the Post-impulsive-phase Acceleration of Flare-associated Coronal Mass Ejections Authors: Cheng, X.; Zhang, J.; Ding, M. D.; Poomvises, W. Bibcode: 2010ApJ...712..752C Altcode: 2010arXiv1002.4960C It is now generally accepted that the impulsive acceleration of a coronal mass ejection (CME) in the inner corona is closely correlated in time with the main energy release of the associated solar flare. In this paper, we examine in detail the post-impulsive-phase acceleration of a CME in the outer corona, which is the phase of evolution immediately following the main impulsive acceleration of the CME; this phase is believed to correspond to the decay phase of the associated flare. This observational study is based on a statistical sample of 247 CMEs that are associated with M- and X-class GOES soft X-ray flares from 1996 to 2006. We find that, from many examples of events, the CMEs associated with flares with long-decay time (or so-called long-duration flares) tend to have positive post-impulsive-phase acceleration, even though some of them have already obtained a high speed at the end of the impulsive acceleration but do not show a deceleration expected from the aerodynamic dragging of the background solar wind. On the other hand, the CMEs associated with flares of short-decay time tend to have significant deceleration. In the scattering plot of all events, there is a weak correlation between CME post-impulsive-phase acceleration and flare decay time. The CMEs deviated from the general trend are mostly slow or weak ones associated with flares of short-decay time; the deviation is caused by the relatively stronger solar wind dragging force for these events. The implications of our results on CME dynamics and CME-flare relations are discussed. Title: Coronal Mass Ejection Induced Outflows Observed with Hinode/EIS Authors: Jin, M.; Ding, M. D.; Chen, P. F.; Fang, C.; Imada, S. Bibcode: 2009ApJ...702...27J Altcode: We investigate the outflows associated with two halo coronal mass ejections (CMEs) that occurred on 2006 December 13 and 14 in NOAA 10930, using the Hinode/EIS observations. Each CME was accompanied by an EIT wave and coronal dimmings. Dopplergrams in the dimming regions are obtained from the spectra of seven EIS lines. The results show that strong outflows are visible in the dimming regions during the CME eruption at different heights from the lower transition region to the corona. It is found that the velocity is positively correlated with the photospheric magnetic field, as well as the magnitude of the dimming. We estimate the mass loss based on height-dependent EUV dimmings and find it to be smaller than the CME mass derived from white-light observations. The mass difference is attributed partly to the uncertain atmospheric model, and partly to the transition region outflows, which refill the coronal dimmings. Title: Formation Heights of Extreme Ultraviolet Lines in an Active Region Derived by Correlation of Doppler Velocity and Magnetic Field Authors: Guo, Y.; Ding, M. D.; Jin, M.; Wiegelmann, T. Bibcode: 2009ApJ...696.1526G Altcode: 2009arXiv0903.4343G We study the correlation heights, which indicate the formation height of Extreme Ultraviolet (EUV) lines in an active region using observations from the EUV Imaging Spectrometer and Solar Optical Telescope on board Hinode. The nonlinear force-free field optimization method is adopted to extrapolate the three-dimensional magnetic fields to higher layers. Three subregions with different characteristics are selected in the active region for this study. The results show that the formation heights in different subregions vary with their different magnetic fields or velocity patterns. After solving the line blending problem between the He II 256.32 Å and Si X 256.37 Å lines by the double Gaussian curve fitting, we find that the transition region lies higher in a strong magnetic area. In a preflare heating area, there possibly exist multithermal loops as implied by comparing the Doppler velocity and the magnetic field on the solar disk. Title: Energy Spectrum of the Electrons Accelerated by a Reconnection Electric Field: Exponential or Power Law? Authors: Liu, W. J.; Chen, P. F.; Ding, M. D.; Fang, C. Bibcode: 2009ApJ...690.1633L Altcode: 2008arXiv0809.1212L The direct current (DC) electric field near the reconnection region has been proposed as an effective mechanism to accelerate protons and electrons in solar flares. A power-law energy spectrum was generally claimed in the simulations of electron acceleration by the reconnection electric field. However in most of the literature, the electric and magnetic fields were chosen independently. In this paper, we perform test-particle simulations of electron acceleration in a reconnecting magnetic field, where both the electric and magnetic fields are adopted from numerical simulations of the MHD equations. It is found that the accelerated electrons present a truncated power-law energy spectrum with an exponential tail at high energies, which is analogous to the case of diffusive shock acceleration. The influences of reconnection parameters on the spectral feature are also investigated, such as the longitudinal and transverse components of the magnetic field and the size of the current sheet. It is suggested that the DC electric field alone might not be able to reproduce the observed single or double power-law distributions. Title: Loop-Like Hard X-Ray Emission in a 2005 January 20 Flare Authors: Jin, Meng; Ding, Mingde Bibcode: 2008PASJ...60..835J Altcode: We present analyses of hard X-ray (HXR) emission observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) of an X7.1 flare on 2005 January 20. Generally, this flare shows three HXR sources: two footpoint sources and a looptop one. During some period of the flare evolution, HXR emission from the legs of the loop was evident, and a loop-like HXR structure appeared, especially at 25-50keV, which is unusual, and has been rarely reported before. We consider this phenomenon to be observational evidence for chromospheric evaporation. To confirm this point, we calculated the height distribution of the energy deposition by an electron beam using different atmospheric models. The results suggest that, in order to engender the observed loop-like HXR emission in this flare, the coronal mass density and the electron-beam flux should be extremely high, which is a possible reason why loop-like HXR emission is rarely observed. Title: 3D Magnetic Field Configuration of the 2006 December 13 Flare Extrapolated with the Optimization Method Authors: Guo, Y.; Ding, M. D.; Wiegelmann, T.; Li, H. Bibcode: 2008ApJ...679.1629G Altcode: The photospheric vector magnetic field of the active region NOAA 10930 was obtained with the Solar Optical Telescope (SOT) on board the Hinode satellite with a very high spatial resolution (about 0.3''). Observations of the two-ribbon flare on 2006 December 13 in this active region provide us a good sample to study the magnetic field configuration related to the occurrence of the flare. Using the optimization method for nonlinear force-free field (NLFFF) extrapolation proposed by Wheatland et al. and recently developed by Wiegelmann, we derive the three-dimensional (3D) vector magnetic field configuration associated with this flare. The general topology can be described as a highly sheared core field and a quasi-potential envelope arch field. The core field clearly shows some dips supposed to sustain a filament. Free energy release in the flare, calculated by subtracting the energy contained in the NLFFF and the corresponding potential field, is 2.4 × 1031 ergs, which is ~2% of the preflare potential field energy. We also calculate the shear angles, defined as the angles between the NLFFF and potential field, and find that they become larger at some particular sites in the lower atmosphere, while they become significantly smaller in most places, implying that the whole configuration gets closer to the potential field after the flare. The Ca II H line images obtained with the Broadband Filter Imager (BFI) of the SOT and the 1600 Å images with the Transition Region and Coronal Explorer (TRACE) show that the preflare heating occurs mainly in the core field. These results provide evidence in support of the tether-cutting model of solar flares. Title: A complicated solar eruption event on 2003 October 26. Solar flare Authors: Ning, Zongjun; Ding, M. D.; Qiu, K. P.; Li, H.; Su, Y. N.; Fang, C. Bibcode: 2008Ap&SS.315...45N Altcode: 2008Ap&SS.tmp...73N We present multi-wavelength observations of a complicated solar eruption event to associate with an X1.2 flare and a Coronal Mass Ejection (CME) on 2003 October 26. The soft X-ray profile shows a possibility for occurrence of two flares with peaks around 06:20 and 07:00 UT. According to our observations, there are many evidences to show that they are corresponding to two energy releases. The first one produces type II, type III, moving type IV continua, a decimetric burst (DCIM) and strong emissions at H α, 195 and 1600 Å; While the second energy release only produces a group of RS-III bursts, DCIM and H α emissions. It appears that the first energy release is associated with a CME, while the second CME is quiet. Such observational difference between two energy releases is found indicating two magnetic reconnection processes occurrence with different plasma situation. Title: Simulations of the periodic flaring rate on YY Gem Authors: Gao, D. H.; Chen, P. F.; Ding, M. D.; Li, X. D. Bibcode: 2008MNRAS.384.1355G Altcode: 2008MNRAS.tmp..126G; 2007arXiv0712.2300G The binary YY Gem shows many interesting properties, one of which is the periodicity in its flaring rate. The period, which is about 48 +/- 3 min, was ever interpreted in terms of the oscillation of a filament. In this paper, we propose a new model to explain this phenomenon by means of 2.5D magnetohydrodynamic (MHD) numerical simulations. It is found that magnetic reconnection is induced as the coronal loops rooted on both stars inflate and approach each other, which is driven by the differential stellar rotation. The magnetic reconnection is modulated by fast-mode magnetoacoustic waves which are trapped between the surfaces of the two stars, so that the reconnection rate presents a periodic behaviour. With the typical parameters for the binary system, the observed period can be reproduced. We also derive an empirical formula to relate the period of the flaring rate to the coronal temperature and density, as well as the magnetic field. Title: Correlation and asymmetry between solar flare hard X-ray footpoints: a statistical study Authors: Jin, M.; Ding, M. D. Bibcode: 2007A&A...471..705J Altcode: Aims:We study the correlation and asymmetry between solar flare hard X-ray footpoint time profiles that can be used to constrain the energy release and particle acceleration processes in solar flares.
Methods: We perform a statistical analysis of correlations and asymmetries of hard X-ray footpoints for 29 solar flares that were observed by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in 2002-2005. Using the CLEAN method, we derive the hard X-ray images and then extract light curves for each footpoint. We decompose the hard X-ray light curves into slowly-varying components and fast-varying components using the Fourier method and check their correlations separately.
Results: Our analysis indicates that the asymmetry between hard X-ray footpoints is a ubiquitous feature in solar flares. However, in most asymmetric cases, the correlations between the light curves of the two footpoints remain high. In particular, the correlation coefficients between the slowly-varying components are significantly higher than those between the fast-varying components. We discuss the causes and implications of the asymmetries and correlations between hard X-ray footpoints. Title: The Origin of Solar White-Light Flares Authors: Ding, M. D. Bibcode: 2007ASPC..368..417D Altcode: It is believed that solar white-light flares (WLFs) originate from the lower atmosphere. In particular, some recently observed WLFs show a very large continuum enhancement at 1.56 µ where the opacity reaches minimum. Therefore, it is important to make clear how the energy is transferred to the very low layers responsible for the production of WLFs. We study the possible mechanisms that may directly or indirectly help increase the continuum emission. We make calculations of model atmospheres and compare the results with observations. Title: Microwave Spectral Evolution of Solar Flares Authors: Ning, Zongjun; Ding, Mingde Bibcode: 2007PASJ...59..373N Altcode: We analyze the time evolution of microwave spectral indices for 103 solar flares observed by Nobeyama Radio Polarimeters (NoRP) between 1998 and 2005. The microwave spectral index, γ(t), is derived from the flux spectral index, δ(t). We assume a flux frequency dependence of the form S(ν,t) = F0νδ(t) in the optically thin part, i.e., above the turnover frequency, fmax. Here, fmax is defined as the frequency with the maximum flux among six discrete frequencies (i.e., 1, 2, 3.75, 9.4, 17, and 35GHz). Based on NoRP observations, we find two typical types of solar flares with various microwave spectral behavior. Type I displays a soft-hard-soft pattern, while the type II flare follows a soft-hard-harder behavior in the rise-peak-decay phases at 35GHz. Our statistical results of microwave flares confirm previous findings. Most microwave flares appear to be of type II (91 out of the present samples), which is different from the hard X-ray flares dominated by the soft-hard-soft property. Such a difference is an observational constraint for electron acceleration and transport theories in flare models. Title: Evolution of electron energy spectrum during solar flares Authors: Liu, W. J.; Chen, P. F.; Fang, C.; Ding, M. D. Bibcode: 2007AdSpR..39.1394L Altcode: Particle acceleration by direct current electric field in the current sheet has been extensively studied, in which an electric and a magnetic field are generally prescribed, and a power law distribution of the electron energy is obtained. Based on MHD numerical simulations of flares, this paper aims at investigating the time evolution of the electron energy spectrum during solar flares. It turns out that the model reproduces the soft-hard-hard spectral feature which was observed in some flares. Title: Diagnostics of the Heating Processes in Solar Flares Using Chromospheric Spectral Lines Authors: Cheng, J. X.; Ding, M. D.; Li, J. P. Bibcode: 2006ApJ...653..733C Altcode: 2006astro.ph..9674C We have calculated the Hα and Ca II 8542 Å line profiles based on four different atmospheric models, including the effects of nonthermal electron beams with various energy fluxes. These two lines have different responses to thermal and nonthermal effects, and can be used to diagnose the thermal and nonthermal heating processes. We apply our method to an X-class flare that occurred on 2001 October 19. We are able to identify quantitatively the heating effects during the flare eruption. We find that the nonthermal effects at the outer edge of the flare ribbon are more notable than that at the inner edge, while the temperature at the inner edge seems higher. On the other hand, the results show that nonthermal effects increase rapidly in the rise phase and decrease quickly in the decay phase, but the atmospheric temperature can still keep relatively high for some time after getting to its maximum. For the two kernels that we analyze, the maximum energy fluxes of the electron beams are ~1010 and 10 11 ergs cm-2 s-1, respectively. However, the atmospheric temperatures are not so high, i.e., lower than or slightly higher than that of the weak flare model F1 at the two kernels. We discuss the implications of the results for two-ribbon flare models. Title: Detection of Explosive Chromospheric Evaporation During a Two-Ribbon Flare Authors: Li, J. P.; Ding, M. D. Bibcode: 2006IAUJD...3E..15L Altcode: We present two-dimensional spectral observations of an X1.2 two-ribbon flare in which blue asymmetry of Hα profiles are found at one kernel for more than 1 min. It is interesting that this kernel seems to move along a flare loop seen on the Transition Region and Coronal Explorer (TRACE) images since its appearance. While on the loop footpoints, the Hα profiles are found to be red shifted. The statistical distribution and the spatial distribution of the blue asymmetry at the kernel are also studied. There seems to be a counterpart of the kernel on TRACE 1600 Å images. We calculate the moving speed of the kernel by measuring its moving front on TRACE 1600 images and find that it has good temporal correlation with the line-of-sight velocity calculated from the H_α profiles using a wing bisect method. The maximum speed of the kernel measured from TRACE images is ~ 200 , which is similar to the chromospheric evaporation speed deduced from soft X-ray lines. We suppose that this kernel may be the plasma evaporated from the chromosphere. If this is true, it will be the first time to get direct evidence that supports the scenario of chromospheric evaporation and condensation of solar flares in Hα line as we know. Title: Spectral Analysis of Ellerman Bombs Authors: Fang, C.; Tang, Y. H.; Xu, Z.; Ding, M. D.; Chen, P. F. Bibcode: 2006ApJ...643.1325F Altcode: By use of the high-resolution spectral data obtained with THEMIS on 2002 September 5, the characteristics of 14 well-observed Ellerman bombs (EBs) have been analyzed. Our results indicate that 9 of the 14 EBs are located near the longitudinal magnetic polarity inversion lines. Most of the EBs are accompanied by mass motions. The most obvious characteristic of the EB spectra is the two emission bumps at the two wings of both Hα and Ca II λ8542. For the first time both thermal and nonthermal semiempirical atmospheric models for the conspicuous and faint EBs are computed. In computing the nonthermal models, we assume that the electron beam resulting from magnetic reconnection is produced in the lower chromosphere. The reasons are that it requires much lower energies for the injected particles and that it gives rise to a more profound absorption at the Hα line center, in agreement with our observations. The common characteristic is the heating in the lower chromosphere and the upper photosphere. The temperature enhancement is about 600-1300 K in the thermal models. If the nonthermal effects are included, then the required temperature increase can be reduced by 100-300 K. These imply that the EBs could probably be produced by the magnetic reconnection in the solar lower atmosphere. The radiative and kinetic energies of the EBs are estimated, and the total energy is found to be 1026 to 5×1027 ergs. According to the characteristics of EBs, we tentatively suggest that EBs could be called ``submicroflares.'' Title: Footpoint Motion of the Continuum Emission in the 2002 September 30 White-Light Flare Authors: Chen, Q. R.; Ding, M. D. Bibcode: 2006ApJ...641.1217C Altcode: 2005astro.ph.12496C We present observations of the 2002 September 30 white-light flare, in which the optical continuum emission near the Hα line is enhanced by ~10%. The continuum emission exhibits a close temporal and spatial coincidence with the hard X-ray (HXR) footpoint observed by RHESSI. We find a systematic motion of the flare footpoint seen in the continuum emission; the motion history follows roughly that of the HXR source. This gives strong evidence that this white-light flare is powered by heating of nonthermal electrons. We note that the HXR spectrum in 10-50 keV is quite soft with γ~7 and there is no HXR emission above 50 keV. The magnetic configuration of the flaring region implies magnetic reconnection taking place at a relatively low altitude during the flare. Despite a very soft spectrum of the electron beam, its energy content is still sufficient to produce the heating in the lower atmosphere, where the continuum emission originates. This white-light flare highlights the importance of radiative back-warming to transport the energy below when direct heating by beam electrons is obviously impossible. Title: Assemblage of Diopside, Pyroxene, Akimotoite, and Ringwoodite in the Heavily Shocked Sixiangkou L6 Chondrite: Further Constraints of Shock Metamorphism Authors: Zhang, A.; Hsu, W.; Wang, R.; Ding, M. Bibcode: 2006LPI....37.1069Z Altcode: This study reports the occurrence of diopside, pyroxene, akimotoite, and ringwoodite in the shock-induced melt veins of Sixiangkou and discuss its implication to the shock metamorphism. Title: Periodicities in microwave radio bursts of solar flares Authors: Ning, Zongjun; Ding, M. D. Bibcode: 2006IAUS..233..421N Altcode: We analyze the periodicities for 294 solar flares observed by Nobeyama Radioheliograph over 6 discrete frequencies of 1, 2, 3.75, 9.4, 17 and 35 GHz during the solar cycle 23 between 1998 and 2004. Using Fourier analysis, we pick up the shortest period (T_{s}) for each event at each frequency channel. We find that T_{s} varies in the range of 0.25-0.62 s over 6 frequency channels. Title: Diagnostics of thermal and non-thermal processes in solar flares using chromospheric lines Authors: Ding, M. D.; Cheng, J. X.; Li, J. P. Bibcode: 2006IAUS..233..397D Altcode: The non-thermal electrons accelerated during solar flares can produce enhanced and broadened chromospheric lines when they precipitate into the chromosphere. In this paper, we propose a method to diagnose the non-thermal processes using two chromospheric lines, Hα and Ca II 8542 Å lines. First, we perform non-LTE calculations of these two lines for various (thermal) model atmospheres and (non-thermal) electron beams. Since the two lines have different sensitivities to the non-thermal electrons, a set of line spectra can uniquely determine a model atmosphere and an electron beam. We then apply this method to a solar flare for which we have observed two-dimensional spectra of the two lines. In particular, we examine the temporal variation of thermal vs. non-thermal effects in flare bright kernels, as well as the spatial variation across flare ribbons. The results show clearly that the non-thermal effects appear most obviously at the flare maximum, and preferentially at the outer edges of flare ribbons. The results are consistent with flare theoretical models. Title: Altitude Dependence of Hard X-Ray Spectra in Solar Flares Authors: Chen, Q. R.; Ding, M. D.; Chen, P. F. Bibcode: 2006IAUS..233..177C Altcode: We investigate the altitude dependence of hard X-ray (HXR) spectra in solar flares, i.e., whether the HXR spectra are related to the altitudes of reconnection sites. We assume that the reconnection altitude can be scaled by the distance between the two conjugate HXR footpoints in the flare. By searching the RHESSI flare list from 2002 to 2004, we find 42 solar flares below X-class that have enhanced 50-100 keV HXR emission and two well-resolved HXR footpoints at the nonthermal peak time. The preliminary results show that there is a weak correlation (∼ -0.31) between the HXR spectral index and the HXR footpoint distance. We further discuss the possible implications. Title: Occurrence of blue asymmetry in Halpha profiles of a two-ribbon flare Authors: Li, J. P.; Ding, M. D. Bibcode: 2006cosp...36.3085L Altcode: 2006cosp.meet.3085L We present two-dimensional spectral observations of an X1 2 two-ribbon flare in which blue asymmetry of H alpha profiles are seen at one kernel for more than 1 min It is interesting that this kernel seems to move along a flare loop seen on the emph Transition Region and Coronal Explorer emph TRACE images since its appearance While on the loop footpoints the H alpha profiles are found to be red shifted There seems to have counterpart of the kernel on emph TRACE 195 and 1600 AA images We calculate the moving speed of the kernel by measuring the moving front on emph TRACE AA 1600 images and find that it has good temporal correlation with the line-of-sight velocity calculated from the H alpha profiles using a wing bisect method The maximum speed of the kernel measured from emph TRACE 1600 AA images is sim 200 mbox km s -1 that is similar to the chromospheric evaporation speed calculated from soft X-ray lines We suppose that this kernel may be resulted from the material evaporated from the chromosphere If this is true it will be the first time to see direct evidence that supports the scenario of chromospheric evaporation and condensation in solar flares as we know Title: Bright points in UV continuum in a M9.1 flare Authors: Wang, L.; Fang, C.; Ding, M. D. Bibcode: 2006cosp...36.1547W Altcode: 2006cosp.meet.1547W Using the data of RHESSI SOHO MDI SOHO EIT and TRACE white light and 1700 AA images we analyze a gradual two-ribbon M9 1 solar flare occurred on 2004 July 22 We find some bright points appeared in white light and 1700 AA images alone the flare ribbons with the lifetime of several minutes Some of them have rapid movement in weak magnetic field regions with sharp contrast and small area We identify these short-lived brightenings as mainly at the least UV continuum enhancement Our primary result shows that the brightenings do not appear in the HXR main sources These new phenomena may provide a new challenge for the classical flare scenario Title: Statistical study of filament eruptions with emerging flux Authors: Xu, X. Y.; Fang, C.; Chen, P. F.; Ding, M. D. Bibcode: 2006cosp...36.1148X Altcode: 2006cosp.meet.1148X Observations indicate that solar coronal mass ejections CMEs are closely associated with reconnection-favored flux emergence which was explained in the emerging flux trigger mechanism for CMEs by Chen Shibata 2000 Using numerical simulations we made a parametric survey of the triggering factors the polarity orientation and the position of the emerging flux and the amount of the unsigned flux Xu Chen Fang 2005 A diagram is presented to show the eruption and non-eruption regimes in the parameter space In this paper a statistical study on the filament eruptions related to emerging flux is performed and it basically supports the theoretical results of our numerical simulation Our results suggest that whether a CME can be triggered depends on both the amount and the location of an emerging flux in addition to its polarity orientation The work provides useful information for the space weather forecast Title: Ellerman Bombs: Sub-Microflares in the Solar Atmosphere Authors: Fang, C.; Tang, Y. H.; Xu, Z.; Ding, M. D.; Chen, P. F. Bibcode: 2006apri.meet...32F Altcode: No abstract at ADS Title: A statistical study of the footpoint asymmetries in solar flares Authors: Ding, M. D.; Chen, Q. R. Bibcode: 2006cosp...36.2985D Altcode: 2006cosp.meet.2985D Solar flares often show two footpoints that appear to be asymmetric in particular in hard X-ray emissions The cause of such an asymmetry is not fully known In this work we study the footpoint asymmetries using a sample of flares observed by RHESSI that show clearly double footpoint hard X-ray sources We compare in detail the hard X-ray spectra the photon flux and the spectral index between each pair of footpoints A statistical result is drawn and compared with some recent theoretical computations Title: Investigation of Electron Energy Spectrum During Solar Flares Authors: Liu, W. J.; Chen, P. F.; Fang, C.; Ding, M. D. Bibcode: 2006cosp...36.2650L Altcode: 2006cosp.meet.2650L Observations indicated that the energy spectrum of non-thermal particles during the evolution of solar flares changes rapidly with the power index alternating from soft to hard and soft again Based on MHD numerical simulation of a flare with the typical prephase impulsive and decay phases we perform the test-particle simulations in order to investigate the time evolution of the electron spectrum The results are compared with observations in detail Title: Synthesis of CME-Associated Moreton and EIT Wave Features from MHD Simulations Authors: Chen, P. F.; Ding, M. D.; Fang, C. Bibcode: 2005SSRv..121..201C Altcode: Soft X-ray (SXR) waves, EIT waves, and Hα Moreton waves are all associated with coronal mass ejections (CMEs). The knowledge of the characteristics about these waves is crucial for the understanding of CMEs, and hence for the space weather researches. MHD numerical simulation is performed, with the consideration of the quiet Sun atmosphere, to investigate the CME/flare processes. On the basis of the numerical results, SXR, EUV, and Hα images of the eruption are synthesized, where SXR waves, EIT waves, and Hα Moreton waves are identified. It confirms that the EIT waves, which border the expanding dimmming region, are produced by the successive opening (or stretching) of the closed magnetic field lines. Hα Moreton waves are found to propagate outward synchronously with the SXR waves, lagging behind the latter spatially by ∼27 Mm in the simulated scenario. However, the EIT wave velocity is only a third of the Moreton wave velocity. The synthesized results also suggest that Hα± 0.45Å would be the best off-band for the detection of Hα Moreton waves. Title: Microwave type III bursts and pulsation groups Authors: Ning, Z.; Ding, M. D.; Wu, H. A.; Xu, F. Y.; Meng, X. Bibcode: 2005A&A...437..691N Altcode: We analyse 22 continuous microwave reverse slope (RS) type III and 21 microwave pulsation groups observed by the Purple Mountain Observatory (PMO) spectrometer over the frequency range of 4.5-7.5 GHz from 1999-2003. The RS type III groups have an average of 12 bursts per group, a mean duration of 0.1-2 s around 6 GHz, a mean quasi-period of 2.0 ± 0.6 s and a mean drift rate of 7.0 GHz s-1. The shortest quasi-period is measurable down to 0.05 s thanks to the high time resolution of this instrument. The microwave pulsation groups have an average of 22 events per group, a mean duration of 3-4 s and a mean quasi-period of 3.7 ± 1.1 s. The RS type III bursts have the same quasi-period as the upward type III bursts documented before. We discuss two possible origins of the quasi-periodicity of type III bursts. One is the quasi-periodic process in magnetic reconnection at the same site, and the other is spatially unresolved stochastic processes in solar flares. Title: Kinetic Properties of CMEs Corrected for the Projection Effect Authors: Yeh, Chin-Teh; Ding, M. D.; Chen, P. F. Bibcode: 2005SoPh..229..313Y Altcode: Observations of coronal mass ejections (CMEs) with coronagraphs are subject to a projection effect, which results in statistical errors in many properties of CMEs, such as the eruption speed and the angular width. In this paper, we develop a method to obtain the velocity and angular width distributions of CMEs corrected for the projection effect, and then re-examine the relationship between CMEs and the associated flares. We find that (1) the mean eruption speed is 792 km s−1 and the mean angular width is 59, compared to the values of 549 km s−1 and 77, respectively before the correction; (2) after the correction, the weak correlation between CME speeds and the GOES X-ray peak flux of the flares gets unexpectedly poorer; and (3) before correction, there is a weak correlation between the angular width and the speed of CMEs, whereas the correlation is absent after the correction. Title: Occurrence of a White-Light Flare Over a Sunspot and the Associated Change of Magnetic Flux Authors: Li, J. P.; Ding, M. D.; Liu, Y. Bibcode: 2005SoPh..229..115L Altcode: A white-light flare (WLF) on 10 March 2001 was well observed in the Hα line and the Ca II λ8542 line using the imaging spectrograph installed on the Solar Tower Telescope of Nanjing University. Three small sunspots appeared in the infrared continuum image. In one sunspot, the infrared continuum is enhanced by 4-6% compared to the preflare value, making the sunspot almost disappear in the continuum image for about 3 min. A hard X-ray (HXR) source appeared near the sunspot, the flux of which showed a good time correlation with the profile of the continuum emission. In the sunspot region, both positive and negative magnetic flux suffered a substantial change. We propose that electron precipitation followed by radiative back-warming may play the chief role in heating the sunspot. The temperature rise in the lower atmosphere and the corresponding energy requirement are estimated. The results show that the energy released in a typical WLF is sufficient to power the sunspot heating. Title: On the formation of the He I 10 830 Å line in a flaring atmosphere Authors: Ding, M. D.; Li, H.; Fang, C. Bibcode: 2005A&A...432..699D Altcode: We explore the formation of the He i 10 830 Å line in a flaring atmosphere, with special attention to the nonthermal effects of an electron beam. Using non-LTE calculations we obtain the line profiles from different model atmospheres. Without the nonthermal effects, the line changes from weak absorption in a cool atmosphere to emission in a hot and condensed atmosphere, as expected. However, the presence of an electron beam can significantly change the line strength, producing much stronger absorption and emission in these two cases. We find that in the nonthermal case, the collisional ionization of He i followed by recombinations becomes an important process in populating the triplet levels corresponding to the He i 10 830 Å line. These results suggest that the He i 10 830 Å line is also a potential diagnostic tool for nonthermal effects in solar or stellar flares. Title: Parametric Survey of Emerging Flux for Triggering CMEs Authors: Xu, X. Y.; Chen, P. F.; Fang, C.; Ding, M. D. Bibcode: 2005IAUS..226..217X Altcode: Observations suggest that solar coronal mass ejections (CMEs) are closely associated with reconnection-favored flux emergence, which was explained as the emerging flux trigger mechanism for CMEs by Chen and Shibata (2000) based on numerical simulations. This paper presents a parametric survey of the CME-triggering environment. Our numerical results show that whether the CMEs can be triggered depends on both the amount and the location of the emerging flux. The results are useful for space weather forecast. Title: On the impulsiveness of the Hα emission from a flaring atmosphere Authors: Ding, M. D. Bibcode: 2005AdSpR..35.1846D Altcode: Emissions from solar flares may reveal fast fluctuations, which can be attributed to small-scale injections of energetic electrons. In this paper, we perform numerical calculations of the Hα emission from a flaring atmosphere bombarded by a pulsating electron beam. We assume that the variation of the electron beam flux consists of two components: a fluctuation component and a background component. The results show that the amplitude of Hα fluctuations varies depending on the magnitude of the background flux of the electron beam. In the case of a higher background flux, the Hα fluctuations are more significant than in the case of a lower background flux. This result is compatible with the observations in which the Hα fluctuations appear preferentially near the hard X-ray maximum. Title: On the Relationship between the Continuum Enhancement and Hard X-Ray Emission in a White-Light Flare Authors: Chen, Q. R.; Ding, M. D. Bibcode: 2005ApJ...618..537C Altcode: 2004astro.ph.12171C We investigate the relationship between the continuum enhancement and the hard X-ray (HXR) emission of a white-light flare on 2002 September 29. By reconstructing the RHESSI HXR images in the impulsive phase, we find two bright conjugate footpoints (FPs) on the two sides of the magnetic neutral line. Using the thick-target model and assuming a low-energy cutoff of 20 keV, the energy fluxes of nonthermal electron beams bombarding FPs A and B are estimated to be 1.0×1010 and 0.8×1010 ergs cm-2 s-1, respectively. However, the continuum enhancement at the two FPs is not simply proportional to the electron beam flux. The continuum emission at FP B is relatively strong with a maximum enhancement of ~8% and correlates temporally well with the HXR profile; however, the continuum emission at FP A is less significant with an enhancement of only ~4%-5%, regardless of the relatively strong beam flux. By carefully inspecting the Hα line profiles, we ascribe such a contrast to different atmospheric conditions at the two FPs. The Hα line profile at FP B exhibits a relatively weak amplitude with a pronounced central reversal, while the profile at FP A is fairly strong without a visible central reversal. This indicates that in the early impulsive phase of the flare, the local atmosphere at FP A has been appreciably heated and the coronal pressure is high enough to prevent most high-energy electrons from penetrating into the deeper atmosphere; while at FP B, the atmosphere has not been fully heated, the electron beam can effectively heat the chromosphere and produce the observed continuum enhancement via the radiative back-warming effect. Title: Waiting Time Distribution of CMES Authors: Yeh, C. -T.; Ding, M. D.; Chen, P. F. Bibcode: 2005ASSL..320..171Y Altcode: 2005smp..conf..171Y No abstract at ADS Title: A CME and Related Phenomena on 2003 October 26 Authors: Ning, Zongjun; Fang, C.; Ding, M. D.; Yeh, C. -T.; Li, H.; Xu, Y. N.; Zhang, Y.; Tan, C. M. Bibcode: 2005IAUS..226..123N Altcode: We present the observational results of the solar bursts on the band of 1-80 GHz (NORH) associated with both a CME and a flare on Oct. 26 2003. This event shows two parts of radio bursts in the time profile. The first part is associated with an X1.2 flare. However, the following part seams related to both the flare and the CME, as the radio emission is enhanced while the H α is decreasing. Thus, these two parts of radio bursts may originate from different physical processes, i.e., flare and CME shock. A primary study is performed on the difference between this two parts. Title: Multiwavelength Analysis of an X1.6 Flare of 2001 October 19 Authors: Li, J. P.; Ding, M. D. Bibcode: 2004ApJ...606..583L Altcode: An X1.6 two-ribbon flare was observed on 2001 October 19 using the imaging spectrograph at the Solar Tower of Nanjing University. We obtained a time series of two-dimensional Hα line spectra. Combining the observations by the Nobeyama Radioheliograph (NoRH), the Transition Region and Coronal Explorer (TRACE), the Yohkoh soft X-ray telescope (SXT) and hard X-ray telescope (HXT), and the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory (SOHO), we performed a multiwavelength analysis for this flare in detail. The hard X-ray (HXR) and radio time profiles for this event have a double-peaked structure. Four Hα bright kernels exist, two of which are identified as the conjugate footpoints of a loop, while the other two are probably the footpoints of two different loops that are closely related. We compared the spatial distribution of the HXR sources with that of the Hα kernels at the two peak times. The results show quite asymmetric HXR emission in the Hα kernels. We ascribe this asymmetric behavior to the magnetic mirroring effect. In addition, we found that different heating mechanisms (nonthermal electron beam vs. heat conduction) may apply to different Hα kernels. We also derived the velocity field and found that the maximum velocity tends to occur at the outer edge of the flare ribbons. This is consistent with the general scenario of two-ribbon flare models. Title: Disappearance of a sunspot accompanying an M-Class flare Authors: Li, J. P.; Ding, M. D.; Liu, Y. Bibcode: 2004IAUS..223..119L Altcode: 2005IAUS..223..119L A white-light flare on 2001 March 10 was well observed in the Halpha line and Ca II λ8542 line using the imaging spectrograph installed in the Solar Tower Telescope of Nanjing University. Three small sunspots appeared in the infrared continuum image, one of which showed that the infrared continuum is enhanced by 4%-6% compared to the preflare value and it almost disappeared in the continuum image for about 3 min. Near the sunspot, there appeared a hard X-ray (HXR) source, the flux of which showed a good time correlation with the profile of the continuum emission. We propose that electron precipitation followed by radiative backwarming may play the chief role in heating the sunspot. Title: Two-dimensional spectroscopy of a white-light flare and its relationship with high-energy electrons Authors: Ding, M. D.; Chen, Q. R.; Fang, C. Bibcode: 2004cosp...35.2414D Altcode: 2004cosp.meet.2414D We perform a multi-wavelength study of a white-light flare of September 29, 2002 that was simultaneously observed by a ground-based imaging spectrograph and by the RHESSI. It is found that the enhanced continuum emission corresponds both spatially and temporally with the hard X-ray emission. There is a slight time delay between the peak of the continuum emission and that of the hard X-ray emission, which can be explained by the backwarming scenario, in which the chromosphere is directly heated by a beam of high-energy electrons and the photosphere is then heated through an enhanced radiation. We further deduce the electron flux from the hard X-ray spectra. The relationship between the electron flux and the continuum contrast is quantitatively compatible with the theoretical prediction. Title: Diagnostics of Non-thermal Particles in Solar Chromospheric Flares Authors: Fang, C.; Xu, Z.; Ding, M. D. Bibcode: 2004IAUS..219..171F Altcode: 2003IAUS..219E..24F Particle beam bombardments on the solar chromosphere will produce non-thermal ionization and excitation. The effect on hydrogen lines has been extensively investigated by using non-LTE theory and semi-empirical flare models. It has been found that Halpha line is widely broadened and shows a strong central reversal. Significant enhancements at the wings of Lyalpha and Lybeta lines are also predicted. In the case of proton bombardment less strong broadening and no large central reversal are expected. We found that the coronal mass and the atmospheric condition give much influence on the line profiles. The line profiles are good tools for diagnosing the total flux of the particle beam but less sensitive to the power index. One can use these factors to diagnose the particle beam in the chromosphere. Title: Hα and Hard X-Ray Observations of a Two-Ribbon Flare Associated with a Filament Eruption Authors: Ding, M. D.; Chen, Q. R.; Li, J. P.; Chen, P. F. Bibcode: 2003ApJ...598..683D Altcode: 2003astro.ph..8085D We perform a multiwavelength study of a two-ribbon flare on 2002 September 29 and its associated filament eruption, observed simultaneously in the Hα line by a ground-based imaging spectrograph and in hard X-rays by RHESSI. The flare ribbons contain several Hα-bright kernels that show different evolutionary behaviors. In particular, we find two kernels that may be the footpoints of a loop. A single hard X-ray source appears to cover these two kernels and to move across the magnetic neutral line. We explain this as a result of the merging of two footpoint sources that show gradually asymmetric emission owing to an asymmetric magnetic topology of the newly reconnected loops. In one of the Hα kernels, we detect a continuum enhancement at the visible wavelength. By checking its spatial and temporal relationship with the hard X-ray emission, we ascribe it to electron-beam precipitation. In addition, we derive the line-of-sight velocity of the filament plasma based on the Doppler shift of the filament-caused absorption in the Hα blue wing. The filament shows rapid acceleration during the impulsive phase. These observational features are in principal consistent with the general scenario of the canonical two-ribbon flare model. Title: Interpretation of the infrared continuum in a solar white-light flare Authors: Ding, M. D.; Liu, Y.; Yeh, C. -T.; Li, J. P. Bibcode: 2003A&A...403.1151D Altcode: We explain the origin of the infrared continuum in the solar flare of 10 March 2001 which shows a positive contrast in the maximum phase but possibly a negative contrast in the early phase. The general feature is consistent with a flare model heated by an electron beam. By making non-LTE model calculations, we find that in the early phase, when the electron beam bombards an unheated atmosphere, the non-thermal ionization by the beam results in an increased H- opacity which then reduces the emergent intensity. With the flare development, the atmosphere is gradually heated. In particular, radiative backwarming plays a chief role in the heating of the temperature minimum region and upper photosphere. We estimate the temperature rise in these regions for a fully heated atmosphere in which energy balance is attained. In this case, the continuum emission rises above the quiescent value. Therefore, this corresponds to the flare maximum phase. We further find that the energy flux of the electron beam deduced from the hard X-ray emission is large enough to account for the continuum contrast. Title: Observations and Spectral Analyses of Solar Flares Authors: DING, M. D. Bibcode: 2003JKAS...36S..49D Altcode: No abstract at ADS Title: Spectral Diagnostics of Non-Thermal Particles in the Solar Chromosphere Authors: FANG, C.; XU, Z.; DING, M. D. Bibcode: 2003JKAS...36S..55F Altcode: No abstract at ADS Title: Magnetic Reconnection in the Solar Lower Atmosphere Authors: Fang, C.; Chen, P. F.; Ding, M. D. Bibcode: 2003ASPC..289..425F Altcode: 2003aprm.conf..425F Accumulating observational evidence indicates that magnetic reconnection is a fundamental process in the solar lower atmosphere, which is responsible for many localized activities and the global maintenance of the hot dynamical corona. Meanwhile, qualitative theoretical considerations and quantitative numerical simulations demonstrate the applicability of the reconnection to a thin layer in the lower atmosphere. This paper reviews the research progress in the related observations, theories and numerical simulations. Title: Non-LTE Calculation of the Ni I 676.8 Nanometer Line in a Flaring Atmosphere Authors: Ding, M. D.; Qiu, Jiong; Wang, Haimin Bibcode: 2002ApJ...576L..83D Altcode: The Ni I 676.8 nm line is used by the Solar and Heliospheric Observatory Michelson Doppler Imager to measure the magnetic field and velocity field in the solar atmosphere. We make non-LTE calculations of this line in an atmosphere that is bombarded by an energetic electron beam. This case is associated with the occurrence of solar flares. The electron beam produces nonthermal ionization and excitation of the hydrogen atoms and redistributes the level populations. This results in an enhanced opacity near the Ni I line and an upward shift of its formation height, as well as an increased line source function. We find that the Ni I line may appear in emission in the presence of a fairly strong electron beam and preferentially in a cool atmosphere (i.e., sunspot umbrae/penumbrae). On the other hand, if there is no bombarding electron beam, the profile can hardly turn to emission even though the atmosphere may be heated to higher temperatures through other ways. This result implies that the sign reversal of the longitudinal magnetic field observed in some flare events may not be a true reversal but just an artifact associated with the production of an emission profile. Title: A statistical study of soft X-ray flares during solar cycle 22 Authors: Yin, Suying; Chen, Pengfei; Ding, Mingde; Fang, Cheng Bibcode: 2002JNUNS..38..457Y Altcode: Solar flares are among the most energetic events on the Sun. The eruption of solar flares has a great influence on the space environment and the Earth. A statistical study of flares occurring in a whole solar cycle is important in understanding the main characteristics of flares and useful for further study on the flare mechanism. In this paper, we make a complete statistics of the soft X-ray (SXR) flares during solar cycle 22 according to the GOES satellite data. Among the 20930 flares, less than 1% belong to X-class, 10% to M-class, and about 60% to C-class. The flare activity in this cycle undergoes a similar behavior to the sunspot number and experiences two peaks in 1989 and 1991, in which the mean flare index is 426 and 468, respectively. The numbers of flares of C-, M-, and X-classes increase rapidly during the ascending phase of the solar cycle, and decrease slowly during the decay phase. The ascending phase takes only about 2 years while the descending phase more than 4 years. The occurrence ratio of flares has a power-law distribution in dependence on the peak soft X-ray flux; the average power-law index for dN/dF, where N is the number of flares and F is the peak flux, is about -2.135 and the correlation coefficient is -0.987. Smaller (B-class and some C-class) flares are affected by the background noise so that they are not included in the derivation of flare index. Moreover, the percentage of SXR flares that have optical counterparts increases with the soft X-ray flux which amounts to 94% for X-class, 83% for M-class, 63% for C-class, and only 30% for B-class, respectively. Title: Non-LTE Calculation of the Ni I 676.8 nm Line in a Flaring Atmosphere Authors: Ding, M. D.; Qiu, J.; Wang, H. Bibcode: 2002AAS...200.9103D Altcode: 2002BAAS...34..953D The Ni I 676.8 nm line is used by SOHO/MDI to measure the magnetic field and velocity field in the solar atmosphere. We make non-LTE calculations of this line in a flaring atmosphere which is heated by an energetic electron beam. The electron beam produces nonthermal ionization and excitation of the hydrogen atoms and redistributes the level populations. This results in an enhanced opacity near the Ni I line and an upward shift of its formation height. We find that the line may appear emission in the presence of a fairly strong electron beam. On the other hand, if there is no bombarding electron beam, the profile can hardly turn to emission even though the atmosphere may be heated through other ways. This result implies that at least in some cases, the sign reversal of the longitudinal magnetic field observed in some flare events may not be a true reversal, but just an artifact associated with the production of an emission profile. Title: The Temperature and Density Structure of an Hα Flaring Loop Authors: Ding, M. D.; Liu, Y.; Chen, P. F. Bibcode: 2002SoPh..207..125D Altcode: We develop a simple method to deduce the temperature and density in the loop of a limb flare from the spectral observations of two lines, Hα and Ca ii 8542 Å. We first build a grid of homogeneous slab models with various temperature and density values and compute the emergent line intensities, and then find the relevant model that can match the observed intensities. This is an approximate method because there are several other factors which can influence the line intensities. We apply this method to the limb flare of 11 November 1998 and deduce the values of temperature and hydrogen number density at different spatial points in the flaring loop, as well as their temporal variations. The loop contains relatively high density and possibly the loop top is slightly hotter and more condensed than the legs of the loop at the flare maximum time. A favorable scenario to produce this result is that magnetic reconnection occurs above the loop, and the reconnection outflow may heat and condense the plasma near the loop top. Title: Non-LTE Inversion of an Hα Flaring Loop Authors: Ding, M. D.; Liu, Y.; Chen, P. F.; Fang, C. Bibcode: 2002stma.conf...89D Altcode: No abstract at ADS Title: Cyclic variations in the solar lower atmosphere Authors: Fang, C.; Zhang, Y. X.; Ding, M. D.; Livingston, W. C. Bibcode: 2002AdSpR..29.1947F Altcode: The Ca II K line has been measured regularly nearly every month since 1974 at Kitt Peak. It is well known that the K 1 component of the Ca II K line is formed in the temperature minimum region (TMR) of the solar atmosphere. Our study of the data of CaII K profiles over two solar cycles indicates that both in full disc integrated spectra and in center disc spectra, the distance between the red K 1 and the blue K 1 of the profiles and its average intensity show periodic variations. But the variation for the full disc integrated spectra fluctuates in the same way as the sunspot number does, while that for the center disc spectra has a time delay with respect to sunspot number. Non-LTE computations yield a cyclic temperature variation of about 17 K of the TMR in the quiet-Sun atmosphere and a cyclic variation of about 15-20 km in the height position of the TMR. Title: Muti-wavelength Analysis of an X1.6 Flare on 2001 October 19 Authors: Li, J. P.; Ding, M. D. Bibcode: 2002aprm.conf..441L Altcode: An X1.6 two-ribbon flare was observed on 2001 October 19, using the imaging spectrograph in the solar tower of Nanjing University. We obtained a time series of 2D spectra of the Hα line. Combined with observations made with the Nobeyama radioheliograph (NoRH), Yohkoh/SXT and HXT, SOHO/MDI, and the Huairou Solar Observing Station (HSOS), we have made a detailed analysis of this flare event. The images show a very complex flaring region. The X-ray and radio time profiles for this event have a double-peaked structure. We find two bright kernels, out of four in total, showing temporal variations of the line-of-sight velocity, the line width and the line center intensity of Hα that correspond respectively with the two peaks in the hard X-ray time profile. These two kernels are found to be the foot points of two loops of the flare. A discussion is made about this event based on multi-wavelength analysis. Title: Waiting Time Distribution of Coronal Mass Ejections Authors: Yeh, C. -T.; Ding, M. D.; Chen, P. F. Bibcode: 2002aprm.conf..471Y Altcode: Coronal mass ejections (CME) and flares are believed to be the result of rapid release of magnetic energy. The energy release process in solar flares can be interpreted as avalanches of many small reconnection events, that is, the statistical properties of the flaring events can be described in a simple sandpile model. In this paper, we use the avalanche model to investigate the frequency distribution of coronal mass ejections, and we find the frequency distribution of coronal mass ejections is similar to that of flares. Title: Magnetic Reconnection in the Solar Lower Atmosphere Authors: Fang, C.; Chen, P. F.; Ding, M. D. Bibcode: 2002stma.conf....3F Altcode: No abstract at ADS Title: Enhanced Emission at the Infrared Continuum in the Flare of 2001 March 10 Authors: Liu, Y.; Ding, M. D.; Fang, C. Bibcode: 2001ApJ...563L.169L Altcode: We have obtained a time series of two-dimensional spectra of Hα and Ca II λ8542 for a flare on 2001 March 10. This flare showed an enhanced emission at the continuum near the Ca II 8542 Å line. The continuum contrast is estimated to be 3%-5%. This emission lasted about half a minute, showing a good time correlation with the peak of the microwave radio flux at 7.58 GHz. The flare can be classified as a type I white-light flare. A preliminary analysis shows that a nonthermal electron beam cannot directly produce the continuum emission. Heating in the lower atmosphere resulting from the radiative backwarming may account for the continuum enhancement. Title: The role of non-thermal electrons in the hydrogen and calcium lines of stellar flares Authors: Ding, M. D.; Fang, C. Bibcode: 2001MNRAS.326..943D Altcode: 2001astro.ph..5097D There is observational evidence showing that stellar and solar flares occur with a similar circumstance, although the former are usually much more energetic. It is expected that the bombardment by high-energy electrons is one of the chief heating processes of the flaring atmosphere. In this paper we study how a precipitating electron beam can influence the line profiles of Lyα, Hα, Caii K and λ8542. We use a model atmosphere of a dMe star and make non-LTE computations taking into account the non-thermal collisional rates owing to the electron beam. The results show that the four lines can be enhanced to different extents. The relative enhancement increases with increasing formation height of the lines. Varying the energy flux of the electron beam has different effects on the four lines. The wings of Lyα and Hα become increasingly broad with the beam flux; change of the Caii K and λ8542 lines, however, is most significant in the line centre. Varying the electron energy (i.e. the low-energy cut-off for a power-law beam) has a great influence on the Lyα line, but little on the Hα and Caii lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Lyα line. The Lyα/Hα flux ratio is thus sensitive to the electron energy. Title: On the Fast Fluctuations in Solar Flare Hα Blue Wing Emission Authors: Ding, M. D.; Qiu, Jiong; Wang, Haimin; Goode, Philip R. Bibcode: 2001ApJ...552..340D Altcode: Fine temporal structures in hard X-ray and microwave emissions of solar flares have been known for many years. Recent observations with high time and spatial resolution revealed that emissions in the wings of Hα could also exhibit fast (subsecond) fluctuations. We argue that such fluctuations are physically related to the small-scale injection of high-energy electrons. We explore this through numerical calculations. The energy equation and the equations for energy-level populations in hydrogen, in particular including the nonthermal collisional excitation and ionization rates, are solved simultaneously for an atmosphere impacted by a short-lived electron beam. We determine the temporal evolution of the atmospheric temperature, the atomic level populations, and the Hα line intensity. We find that although the background Hα wing emission is mainly formed in the photosphere, the fast fluctuations are probably produced in the chromosphere, which is penetrated by ~20 keV electrons. To yield Hα wing fluctuations of amplitude comparable to the observations, a mean energy flux of ~(1-2)×1011 ergs cm-2 s-1 is required for the electron beam, if one adopts a Gaussian macrovelocity of 25 km s-1. Such a burst contains a total energy of 1025-1026 ergs. These parameters are compatible with elementary flare bursts. Title: Physical Parameters of a Flare on 22 December 1999 Derived from the Hα Line with a Modified Cloud Method Authors: Liu, Y.; Ding, M. D. Bibcode: 2001SoPh..200..127L Altcode: The flare of 22 December 1999 was observed in the Hα line using the imaging spectrograph in the solar tower of Nanjing University. We present a new technique different from the classical cloud model to fit the Hα line profile which avoids using the background profile. We obtain the four parameters of the flare chromosphere: the source function, the optical thickness at line center, the line-of-sight velocity and the Doppler width. The observed asymmetry profiles have been reproduced well by the theoretical ones based on our model. A discussion is made about the reliability of the results we have obtained using the present method. Title: Quiet-Sun Variability In a Temperature Minimum Region Authors: Zhang, Y. X.; Fang, C.; Ding, M. D.; Livingston, W. C. Bibcode: 2001ApJ...547L.179Z Altcode: The results of monthly monitoring Ca II K line profiles at a quiet-Sun region near solar disk center at Kitt Peak during about two solar cycles since 1974 are presented to investigate the variations of K1 emission, which is strongly dependent on the temperature minimum region (TMR). Cyclic variations are found in K1 emission that are similar to those in the sunspot numbers. However, there is an evident time lag of about 1.2 yr between the K1 emission and the sunspot numbers. Non-LTE computations show that the corresponding temperature variation and position fluctuation in the quiet-Sun TMR region are estimated to be about 17 K and 1.7×10-2 g cm-2, respectively. Title: Cyclic Variation in the Solar Lower Atmosphere Authors: Fang, C.; Zahng, Y. X.; Ding, M. D.; Livingston, W. C. Bibcode: 2001IAUS..203..387F Altcode: CaII K line has been measured regularly nearly every month since 1974 at Kitt Peak. It is known that the K1 component of the CaII K line is formed in the temperature minimum region (TMR) of the solar atmosphere. Our study on the data of CaII K line profiles over nearly two solar cycles indicates that both in full disc integrated spectra and in disc-center spectra, the distance between red K1 and blue K1 and its mean intensity show periodical variations, but the former fluctuates in the same way as the sunspot number does, while the later has a time delay with respect to the sunspot number. Non-LTE computation indicates a cyclic temperature variation of about 15 K of the TMR in the quiet-Sun atmosphere and a cyclic variation of about 15-20 km of the position of the TMR. Title: The role of non-thermal electrons in the optical continuum of stellar flares Authors: Ding, M. D.; Fang, C. Bibcode: 2000MNRAS.317..867D Altcode: 2000astro.ph..5388D The continuum emission of stellar flares in UV and visible bands can be enhanced by two or even three orders of magnitude relative to the quiescent level and is usually characterized by a blue colour. It is difficult for thermal atmospheric models to reproduce all these spectral features. If the flaring process involves the acceleration of energetic electrons which then precipitate downwards to heat the lower atmosphere, collisional excitation and ionization of ambient hydrogen atoms by these non-thermal electrons could be important in powering the continuum emission. To explore such a possibility, we compute the continuum spectra from an atmospheric model for a dMe star, AD Leo, at its quiescent state, when considering the non-thermal effects by precipitating electron beams. The results show that if the electron beam has an energy flux large enough (for example, F1~1012ergcm-2s-1), the U-band brightening and, in particular, the U-B colour are roughly comparable with observed values for a typical large flare. Moreover, for electron beams with a moderate energy flux F1<~1011ergcm-2s-1, a decrease of the emission at the Paschen continuum appears. This can explain at least partly the continuum dimming observed in some stellar flares. Adopting an atmospheric model for the flaring state can further raise the continuum flux, but it yields a spectral colour incomparable with observations. This implies that the non-thermal effects may play the chief role in powering the continuum emission in some stellar flares. Title: Diagnostics of non-thermal processes in chromospheric flares --- IV. Limb flare spectra for an atmosphere bombarded by an electron beam Authors: Fang, C.; Hénoux, J. -C.; Ding, M. D. Bibcode: 2000A&A...360..702F Altcode: Hα, Lyα, Lyβ, CaII K and λ8542 Å line profiles have been computed for limb flares with height distributions of temperatures as given by the semi- empirical models F1 and F2, by including the non-thermal collisional excitation and ionization of hydrogen and of ionized calcium that results from electron bombardment. In agreement with observations, the computed profiles of the hydrogen lines are very broad, especially at the height where the source function reaches its maximum. Non-thermal effects are less pronounced for CaII lines. Title: XPS studies on the electronic structure of bonding between solid and solutes: adsorption of arsenate, chromate, phosphate, Pb2+, and Zn2+ ions on amorphous black ferric oxyhydroxide Authors: Ding, M.; de Jong, B. H. W. S.; Roosendaal, S. J.; Vredenberg, A. Bibcode: 2000GeCoA..64.1209D Altcode: The adsorption of three anions (AsO43-, PO43-, and CrO42-) and two cations (Zn2+ and Pb2+) has been determined on a new form of amorphous black ferric oxyhydroxide that resembles in local structure β-FeOOH, akaganeite. The nature of the interaction between FeOOH substrate and the above mentioned adsorbates is characterised with X-ray photoelectron spectroscopy on the core and valence band levels of trivalent iron and oxygen using frontier molecular orbital theory as theoretical framework. Our findings indicate that substantial and variable charge transfer occurs in which the FeOOH substrate can function either as a Lewis acid or base in its interaction with different adsorbates. Our findings suggest that the valence band spectra can be used to estimate qualitatively the energy level separation between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the surface complexes and to assess the chemical affinity between substrate and adsorbate. Title: Cyclic variation in the temperature minimum region of solar atmosphere. Authors: Zhang, Yunxiang; Fang, Cheng; Livingston, W. C.; Ding, Mingde Bibcode: 2000AcASn..41...44Z Altcode: Calcium K line as well as some other solar spectral lines have been measured regularly nearly every month since 1974 at Kitt Peak. It is now widely accepted that K1 is formed in the temperature minimum region (TMR) and that the distance of the component K1 from the line center and the intensity of K1 is well related to the position and temperature of TMR, respectively. The authors' study on the data of K line profiles and on the sunspot numbers over decades shows: in full disc integrated spectra the distance of K1 from the line center fluctuates in the same way as the sunspot number does, indicating a solar cyclic variation. Title: Numerical Study on the Effect of Heat Conduction on Magnetic Reconnection Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D. Bibcode: 2000AdSpR..26..525C Altcode: The effect of heat conduction on 2.5D magnetic reconnection, similar to that in Kopp-Pneuman model, is numerically studied. It is shown that the heat conduction accelerates the reconnection, increases the amount of shrinkage of the closed field lines, and increases the average rise speed of the SXR loop. MHD slow shocks contribute to the SXR loop heating. When the timescale of heat conduction is shorter than the Alfvén timescale, an adiabatic slow shock is dissociated into an isothermal slow shock and a heat conduction front Title: An unusually intense CA II K line wing: possible role of particle acceleration in the lower atmosphere Authors: Ding, M. D. Bibcode: 1999A&A...351..368D Altcode: The white-light flare on 1974 October 11 showed an unusual behavior: its Ca ii K line reached at flare maximum an intensity of K1 as high as half of the continuum. This spectral feature cannot be explained by ordinary flare models. We present an abnormal model with an extremely hot temperature minimum region (TMR) that can reproduce the unusually high K1 intensity. However, this poses a very severe constraint on the energy requirement in the lower atmosphere. Since canonical heating models are insufficient to provide enough heating energy in lower layers, an in situ energy source may be required. We further investigate the possible role of a non-thermal particle beam injected from a lower layer. A beam of hecta-keV electrons (or MeV protons) can sufficiently heat the TMR and lead to the formation of a very hot TMR provided that the energy flux of the beam is large enough. Whether such kinds of particle beams exist needs to be checked by more theoretical and observational studies. Title: Numerical simulation of the magnetic reconnection in solar flares. Authors: Fang, Cheng; Chen, Pengfei; di, Xiaohua; Ding, Mingde; Tang, Yuhua Bibcode: 1999PPMtO..18..330F Altcode: 2D magnetic reconnection in solar flares is numerically simulated. The results show that when the reconnection occurs at a high altitude, the evolution can reproduce the main characteristics (such as flaring loop motions and footpoints separation, etc.) of two-ribbon flares; while that with a low reconnection X-point shows a large similarity to compact flares. The results also indicate that there is no direct relation between the rises of soft X-ray loops and the X-point. Title: Multi-line two-dimensional spectroscopy of a limb flare Authors: Ding, M. D.; Fang, C.; Yin, S. Y.; Chen, P. F. Bibcode: 1999A&A...348L..29D Altcode: We present the result of a preliminary analysis of the 2D spectra of Hα and Ca ii lambda 8542 for a limb flare on 11 November, 1998. Near the top of the flaring loop, the Hα line is extraordinarily broadened. The effect of line opacity (or the saturation of line core) cannot fully account for the observed line width since it requires an extremely high loop density (n_H>~ 10(13) cm(-3) when T=10(4) K). The remaining possibility is the broadening by micro-turbulence or inhomogeneous mass motions. Since the two lines demonstrate different broadening effects, it is quite possible that they are formed in different fine structures which cannot be spatially resolved by observations. Title: Flaring Loop Motion and a Unified Model for Solar Flares Authors: Chen, P. F.; Fang, C.; Ding, M. D.; Tang, Y. H. Bibcode: 1999ApJ...520..853C Altcode: We performed 2.5-dimensional numerical simulations of magnetic reconnection for several models, some with the reconnection point at a high altitude (the X-type point in magnetic reconnection), and one with the reconnection point at a low altitude. In the high-altitude cases, the bright loop appears to rise for a long time, with its two footpoints separating and the field lines below the bright loop shrinking, which are all typical features of two-ribbon flares. The rise speed of the loop and the separation speed of its footpoints depend strongly on the magnetic field B0, to a medium extent on the density ρ0, and weakly on the temperature T0, the resistivity η, and the length scale L0, by which the size of current sheet and the height of the X-point are both scaled. The strong B0 dependence means that the Lorentz force is the dominant factor; the inertia of the plasma may account for the moderate ρ0 dependence; and the weak η dependence may imply that ``fast reconnection'' occurs; the weak L0 dependence implies that the flaring loop motion has geometrical self-similarity. In the low-altitude case, the bright loops cease rising only a short time after the impulsive phase of the reconnection and then become rather stable, which shows a distinct similarity to the compact flares. The results imply that the two types of solar flares, i.e., the two-ribbon flares and the compact ones, might be unified into the same magnetic reconnection model, where the height of the reconnection point leads to the bifurcation. Title: Numerical study on the flaring loop dynamics. Authors: Fang, Cheng; Chen, Pengfei; Tang, Yuhua; Ding, Mingde Bibcode: 1999PPMtO..18...93F Altcode: Numerical simulations are performed to study the flaring loop dynamics during magnetic reconnection. Two models are considered with different reconnection point heights. In the model with high reconnection point, the results can reproduce the loop motions in the two-ribbon flares, while in the model with low reconnection point, the evolution shows a large similarity to the compact flares. It implies that the two types of solar flares could be unified into the same model, where the height of the reconnection point leads to the bifurcation. Title: Simulation of Magnetic Reconnection with Heat Conduction Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D. Bibcode: 1999ApJ...513..516C Altcode: Magnetohydrodynamic (MHD) equations are numerically solved to study 2.5-dimensional magnetic reconnection with field-aligned heat conduction, which is also compared with the adiabatic case. The dynamical evolution starts after anomalous resistivity is introduced into a hydrostatic solar atmosphere with a force-free current sheet, which might be similar to the configuration before some solar flares. The results show that two jets (i.e., the outflows of the reconnection region) appear. The downward jet collides with the closed line-tied field lines, and a bright loop is formed with a termination shock at the loop top. As the reconnection goes on, the loop rises almost uniformly with a speed of tens of km s-1, and the two footpoints of the loop separate with a speed comparable to the loop rise speed. Besides the apparent loop motion, the magnetic loops below the loop top shrink weakly. Such a picture is consistent with that given by observations of two-ribbon solar flares. Moreover, the results indicate that the slow MHD shock contributes to the bright loop heating. Some detailed structures of the reconnection process are also discussed. Title: Heating in the Lower Atmosphere and the Continuum Emission of Solar White-Light Flares Authors: Ding, M. D.; Fang, C.; Yun, H. S. Bibcode: 1999ApJ...512..454D Altcode: Observationally, there is a small fraction of solar white-light flares (WLFs), the so-called type II WLFs, showing an increased visible continuum but no significant Balmer jump and less strong chromospheric line emission in comparison with type I WLFs. The classical point of view, that the flare energy is initially released in the corona and then transported downward, can hardly explain WLFs of this kind. In this paper we explore the possibility that type II WLFs originate from a deeper layer. Assuming an in situ energy release, in particular in the form of high-energy particles, in a region around the temperature minimum, the continuum emission is computed in different time stages during the flare evolution. At first, nonthermal excitation and ionization of hydrogen atoms caused by bombarding particles result in a decline of the visible continuum. Later on, the lower atmosphere is gradually heated through radiative transfer, mitigating the continuum decline. In the final stage, when the particle bombardment stops while the atmosphere still keeps a heated state, we obtain a positive continuum contrast without an obvious Balmer jump. This meets the condition required for type II WLFs. The presence or absence of a continuum decline in the early stage of the flare provides a diagnostic tool for nonthermal processes in the lower atmosphere. Title: Magnetic Reconnection and Flare Loop Motion Authors: Chen, P. F.; Fang, C.; Tang, Y. H.; Ding, M. D. Bibcode: 1999ASSL..240..337C Altcode: 1999numa.conf..337C No abstract at ADS Title: 2D spectra analysis of a SN/C5.6 flare on 19 March 1993. Authors: Yin, Suying; Fang, Cheng; Ding, Mingde; Huang, Youran; Gao, Xiufa Bibcode: 1999JNanU..35...21Y Altcode: On 19 March 1993, a SN/C5.6 flare occurred near the center of the solar disk at ≡02:19 UT. Twodimensional Hα spectra were obtained by using an imaging spectrograph installed at the Solar Tower Telescope of Nanjing University. The 2D line center intensity contours show that the flare has three main kernels. Their morphology, structure and evolution are different, while the red asymmetry at the Hα wings is common for the three regions. Based on the asymmetries of the Hα lines, the authors derive a time series of 2D velocity fields for the flare. The results indicate that the inferred downward velocities are various in different regions. Title: Multi-Wavelength Analysis of the Flare on 2 October 1993 Authors: Fang, C.; Tang, Y. H.; Hénoux, J. -C.; Huang, Y. R.; Ding, M. D.; Sakurai, T. Bibcode: 1998SoPh..182..163F Altcode: By use of Yohkoh hard X-ray flux and soft X-ray images, and of vector magnetograms and 2D spectral observations, a 1N/C6.5 flare observed on 2 October 1993 is analysed in detail. Evidence is provided not only morphologically but also quantitatively that the dynamics at kernels A and C of the flare in the impulsive phase were controlled mainly by electron beam bombardment, while the heating of kernel B is mainly due to heat conduction. By plotting the energy gradient of the electron energy flux as a function of energy for the various spectral indexes observed during the flare, the acceleration mechanism is found to be such that there is a constant energy E0, close to 20 keV, for which the electron flux d F1/dE is constant. It is shown that such a conclusion can be reached more directly by using the photon flux, which in that case must be constant for E=E0, whatever the value of the power index. This result implies also that the electron spectrum is represented by a power law and that the X-ray photons are produced in a thick target. Instantaneous momentum balance is shown to exist between the upflowing soft X-ray-emitting and the downflowing Hα- emitting plasma at the kernels of the flare. The observed Hα red asymmetry is well reproduced by the non-LTE computation, with the down-moving condensation included. The observation of the magnetic field suggests that the flare was triggered probably by magnetic flux emergence. Title: A possible mechanism for the Hα broad wings emission of Ellerman bombs Authors: Henoux, J. -C.; Fang, C.; Ding, M. D. Bibcode: 1998A&A...337..294H Altcode: In this paper, the non-thermal emission of Lyalpha , Lybeta and Hα lines generated through charge exchange by protons accelerated in the low chromosphere and moving with a small pitch angle around an horizontal magnetic field has been computed. Computations have been done for observations made at the center of the solar disk, i.e. in a situation where the solar magnetic field is perpendicular to the line of sight, for non zero pitch angles. In such conditions, the photons emitted through charge exchange are Doppler shifted symmetrically in both wings of the lines. The Hα far line wing intensities increase significantly, making possible to reproduce the characteristics of the spectra of Ellerman bombs. Thus it is proposed that a proton beam accelerated in the low chromosphere, and losing its energy there, is a viable candidate for explaining the observed EB spectra. Title: Two-dimensional spectral observations of sunspots in chromospheric lines. I. Asymmetries of CA II lines Authors: Ding, M. D.; Schleicher, H. Bibcode: 1998A&A...332..767D Altcode: We present two-dimensional spectral observations of two closely located small sunspots in chromospheric lines. Line asymmetries and shifts are derived for the Caii K line in particular. It is found that the majority of Caii K profiles show a blue asymmetry. Red asymmetry may also appear, but only in some tiny regions. Some ephemeral blue asymmetry patches are found in the umbrae, which seem to be related to umbral flashes. On the limb side of the penumbra, the Caii K profiles show a distinguishably blue asymmetry of K1. In contrast, the profiles on the center side are more likely to possess a blue asymmetry of K2, together with a slight red shift of K3. We discuss possible causes for such a peculiar spatial distribution of line asymmetries. Further line profile computations are needed to get a consistent and satisfactory explanation. Title: Line profiles in moustaches produced by an impacting energetic particle beam Authors: Ding, M. D.; Henoux, J. -C.; Fang, C. Bibcode: 1998A&A...332..761D Altcode: The Hα line profile in moustaches is characterized by enhanced wings and a deep central absorption. We explore the possibility that such a profile may be due to the effect of energetic particles bombarding the atmosphere. Computations show that the characteristics of moustache line profiles can be qualitatively reproduced in two extreme cases, either injection from the corona of high energy particles (ga 60 keV electrons or ga 3 MeV protons) or injection in a low-lying site, in middle chromosphere or deeper, of less energetic particles ( ~ 20 keV electrons or ~ 400 keV protons). The requirements on the energy and on the depth of the injection site of energetic particles are reduced in the case of observations close to the solar limb. The role of protons of energies below 1 MeV is slightly less significant than that of deka-keV electrons in the case of a high particle injection site, but such protons remain to be viable candidates in the case of lower particle injection sites and of observations at larger heliocentric angles. Observations at various wavelengths are needed to find which of these hypotheses is convenient for explaining a given event. Title: Particle Beam Diagnostics Based on UV and Optical Spectra of Solar Flares (invited) Authors: Fang, C.; Ding, M. D.; Hénoux, J. C. Bibcode: 1998asct.conf..497F Altcode: No abstract at ADS Title: Coronal Loops Above a Sunspot Region Authors: Fang, C.; Tang, Y. H.; Ding, M. D.; Zhao, J.; Sakurai, T.; Hiei, E. Bibcode: 1997SoPh..176..267F Altcode: By analysing the data of Yohkoh soft X-ray images, vector magnetograms and 2D spectral observations, coronal loops above a large sunspot on 16-19 May 1994 have been studied. It is shown that the loops follow generally the alignment of concentrated magnetic flux. The results indicate that the soft X-ray emission is low just above the sunspot, while some loops connecting regions with opposite magnetic polarities show strong soft X-ray emission. Especially, the part of the loops near the weaker magnetic field region tends to be brighter than the one near the stronger magnetic field. The temperature around the top of the loops is typically ∼3 × 106 K, which is higher than that at the legs of the loops by a factor of 1.5-2.0. The density near the top of the loops is about 5 x 109 cm-3, which is higher than that of the leg parts of the loops. These loops represent probably the sites where strong magnetic flux and/or current are concentrated. Title: Lyman continuum as a diagnostic for nonthermal processes in solar flares. Authors: Ding, M. D.; Schleicher, H. Bibcode: 1997A&A...322..674D Altcode: We have computed the Lyman continuum intensities for different flare circumstances, namely, the temperature rise in the chromosphere, the downward shift of the transition region, and the nonthermal effect of a precipitating beam of electrons. All the three factors can enhance the intensity by various amounts. The third effect additionally lowers the color temperature of the Lyman continuum. It thus provides a diagnostic tool to discriminate whether there exists an obvious nonthermal effect of electron beam bombardment during solar flares. It is also suggested that the relative magnitudes of Lyman and Balmer continuum enhancements can be used to infer the energy distribution type of the beam electrons. Title: Influence of non-thermal processes on line asymmetries in solar flares. Authors: Ding, M. D.; Fang, C. Bibcode: 1997A&A...318L..17D Altcode: We make line profile calculations for a flare atmosphere considering simultaneously the effect of non-thermal excitation and ionization of the hydrogen atoms caused by precipitating high-energy electrons, and the effect of possible chromospheric downflows. The results confirm the earlier finding that a downflow, if confined to the upper chromosphere, can sometimes produce a blue asymmetry of the Hα line. In addition, we find that the existence of such non-thermal effects tends to enhance the blue-asymmetry magnitude. In particular, the larger the energy flux, or the harder the energy spectrum for the beam electrons, the easier it will be to produce a blue-asymmetric Hα profile, which is simultaneously intensified and broadened. Possible reasons which make the blue asymmetry less popular than the red asymmetry in observations are discussed. Title: Spectral Characteristics and Mechanisms of Two Types of Solar White-Light Flares Authors: Fang, C.; Ding, M. D. Bibcode: 1997tcca.conf..340F Altcode: No abstract at ADS Title: Spectral Diagnostics of the Energetic Particles in Solar Flares Authors: Fang, C.; Ding, M. D.; Henoux, J. C.; Gan, W. Q. Bibcode: 1996JKASS..29..295F Altcode: No abstract at ADS Title: Parametric study of the continuum emission of white-light flares. Authors: Ding, M. D.; Fang, C. Bibcode: 1996A&A...314..643D Altcode: In this paper, we study the effect of different flare-associated disturbances on the white-light continuum emission, using a non-LTE computation code. A systematic study is made to show the variational behavior of the continuum contrast in dependence on different input parameters and different locations on the Sun. The results show that (1) a purely chromospheric temperature rise is not sufficient, under the assumed physical conditions, to produce a significant continuum enhancement, especially at visible wavelengths, except for some extreme cases when the chromospheric temperature is rather high and the event occurs near the disk limb, (2) a downward shift of the transition region would have some effect, if the top column mass density becomes sufficiently large, and (3) the effect of nonthermal electron beam bombardment is obvious only when the transition region has not suffered a great shift. For these three cases, the continuum contrast in the Balmer continuum is much larger than that beyond the Balmer continuum, implying there is a Balmer jump in the modeled spectra. In some special cases, the chromospheric temperature rise enhances the absorption of the photospheric radiation, thus producing a spurious negative continuum contrast. A case for possible temperature rises around the temperature minimum region is also considered. The continuum contrast in the case of enhanced temperature minimum is found to be much flatter than for the above cases, implying a significant emission process in the lower atmosphere. Title: The Lyman continuum as a diagnostic for nonthermal processes in solar flares. Authors: Ding, M. D.; Schleicher, H. Bibcode: 1996AGAb...12..163D Altcode: No abstract at ADS Title: On a Possible Explanation of Chromospheric Line Asymmetries of Solar Flares Authors: Ding, M. D.; Fang, C. Bibcode: 1996SoPh..166..437D Altcode: We discuss the relationship between the chromospheric downward motions and the line asymmetries in solar flares by using a simple model. It is found that both the blue asymmetry and red asymmetry of the Hα line can be caused by downward motions, as long as the moving material is confined to different heights in the chromosphere. The Ca II K line, however, mainly shows a red asymmetry. The results can qualitatively explain some observations. Title: Chromospheric Evaporation in Four Solar Flares Observed by YOHKOH Authors: Ding, M. D.; Watanabe, T.; Shibata, K.; Sakurai, T.; Kosugi, T.; Fang, C. Bibcode: 1996ApJ...458..391D Altcode: We present an analysis of the Ca XIX soft X-ray spectra of four flares observed by the Yohkoh spacecraft. These spectra show evident blueshifted signatures. Using a two-component fitting technique, we obtain the time variations of the fitting parameters. The peak line-of-sight upflow velocities range from 180 to 320 km s-1. The durations of upflows with observable velocities (≥ 100 km s-1) are found to be 4-10 minutes. Particularly, for two of the four flares, the emission measure of the blueshifted component is larger than that of the stationary component at early times. This fact agrees qualitatively with the predictions of numerical simulations of chromospheric evaporation. There is a rough correlation between the nonthermal turbulent velocity and the upflow velocity, consistent with the results of Fludra et al. We also find that most Ca xix resonance profiles show no significant shift of the profile peak, regardless of the blue asymmetry which sometimes appears. The possible causes of this phenomenon are discussed. Title: Are White-Light Flares Related to High Energy Particles? Authors: Fang, C.; Ding, M. D.; Hu, J.; Yin, S. Y. Bibcode: 1996mpsa.conf..553F Altcode: 1996IAUCo.153..553F No abstract at ADS Title: The Improved Solar Tower of Nanjing University and a CCD Imaging Spectrograph Authors: Huang, Y. R.; Fang, C.; Ding, M. D.; Ji, G. P.; Gao, X. F.; Ying, S. Y.; Hu, J.; Xue, Y. Z. Bibcode: 1996gbaa.conf..303H Altcode: No abstract at ADS Title: Spectral Analysis of the White Light Flare on 1974OCT11 Authors: Yin, S. Y.; Fang, C.; Ding, M. D.; Hiei, E.; Fu, W. J. Bibcode: 1995AcASn..36..279Y Altcode: No abstract at ADS Title: A CCD Imaging Spectrograph in the Improved Solar Tower of Nanjing-University Authors: Huang, Y. R.; Fang, C.; Ding, M. D.; Gao, X. F.; Zhu, Z. G.; Ying, S. Y.; Hu, J.; Xue, Y. Z. Bibcode: 1995SoPh..159..127H Altcode: 1995SoPh..159..127Y Since 1992 the solar tower telescope of Nanjing University (118°51' E, 32°03' N) as well as its multichannel solar spectrograph, originally established in 1982, have been reconstructed and a two-channel imaging spectrograph has been operated successfully. The apertures of the coelostat and the secondary mirror are both 60 cm. The spherical objective mirror, having an aperture of 43 cm and a focal length of 2170 cm, produces a solar image of 20 cm diameter. Two auxiliary telescopes using a small fraction of the coelostat's aperture were set up for guiding and Hα monochromatic monitoring. A multichannel spectrograph can be operated in six wavebands simultaneously. A CCD imaging spectrograph can be used for data acquisition at Hα and CaII K line wavebands automatically and simultaneously. The instrument consists of two CCD cameras, an image processor (SR-151), a personal computer, and a mechanical scanning device. The principal characteristics of the instruments are described. Some observational results are presented as examples. Title: An Unusual Behaviour of the White-Light Flare on 11 October, 1974 Authors: Fang, C.; Yin, S. Y.; Hiei, E.; Ding, M. D.; Fu, Q. J. Bibcode: 1995SoPh..158..387F Altcode: It was found that in the spectrum of the white-light flare on 11 October, 1974, at the time of continuum maximum, the intensity of K1 in the CaII K line increased very significantly and reached nearly half of the continuum intensity. The duration of this unusual increase is less than 4 min. It seems that existing semi-empirical models can reproduce neither this characteristic nor the chromospheric condensation. Title: Analysis of 2-d flare spectra: Velocity fields derived from Hα line asymmetries Authors: Ding, M. D.; Fang, C.; Huang, Y. R. Bibcode: 1995SoPh..158...81D Altcode: We derive a time series of two-dimensional velocity fields for a flare region on 1992 December 16, based on the asymmetries of the Hα line. The Hα spectra were obtained by an imaging spectrograph at the Solar Tower Telescope of Nanjing University. Four sites with evident chromospheric downflows are found to appear and decay consecutively in the studied region. The value of maximum velocities is 30-40 km s−1 and the lifetime of downflows is 2-3 min at these sites. It is also shown that the asymmetries only exist at the line wing, while the line center has nearly no shifts for this flare. Finally, we make a discussion on the characteristics of the velocity distribution and its correlations with the intensity distribution, as well as with the hard X-ray emission. Title: Asymmetry and Doppler shift of the metallic lines of the solar flare on 11 October 1974. Authors: Yin, Suying; Fang, Cheng; Ding, Mingde Bibcode: 1995JNanU..31..221Y Altcode: On 11 October 1974 an 1N flare occurred near the solar disk center. The asymmetries and the Doppler shifts of the 41 metallic lines of the flare are analysed in detail. The results are helpful to understand the dynamic process in the lower atmosphere of solar flares. It is proposed that the movements around the temperature minimum region caused by the flare could explain these observational results. Title: On the spectral characteristics and atmospheric models of two types of white-light flares. Authors: Fang, C.; Ding, M. D. Bibcode: 1995A&AS..110...99F Altcode: Based on detailed analyses of the solar white-light flares (WLFs) of 1974 September 10, 1979 September 19 and 1991 October 24, the characteristics of two types of WLFs have been distinguished. For the Type I WLF, there is a good time correlation between the maximum of continuum emission and the peaks of hard X-rays and microwave radiations; there is also a strong Balmer jump in the spectra; the Balmer lines are strong and very broad. For the Type II WLF, the situations are quite different. For these two types of WLFs, the atmospheric models and the contributions to the continuum from the hydrogen atoms and negative hydrogen ions, as well as the net radiative cooling rates, have been compared in detail. The possible mechanisms for the two types of WLFs have also been discussed. Title: VizieR Online Data Catalog: White-light flares models (Fang+, 1995) Authors: Fang, C.; Ding, M. D. Bibcode: 1994yCat..41100099F Altcode: Based on detailed analyses of the solar white-light flares (WLFs) of 1974 September 10, 1979 September 19 and 1991 October 24, the characteristics of two types of WLFs have been distinguished. (3 data files). Title: Optical Spectra and Semi-Empirical Model of a White-Light Flare Authors: Ding, M. D.; Fang, C.; Gan, W. Q.; Okamoto, T. Bibcode: 1994ApJ...429..890D Altcode: We derive a semi-empirical atmospheric model for the 1979 September 19 white-light flare (WLF), based on the observed continuum at 3590 to 3990 A and the Ca II K line profile. The model has a strongly heated photosphere, but has a chromospheric structure within the range of ordinary flare models. According to this model, the white-light continuum is mainly produced by the H- emission in the photosphere. A negative Balmer jump appears in the continuum spectrum (without considering the line blanketing effect), due to the Balmer continuum absorption in the chromosphere. From the spectral features and model results, the traditional heating mechanisms that are proposed to transport the energy from the corona down to deep layers seem insufficient to fully explain the photospheric heating for this WLF. The heating energy may originate within the photosphere itself. The possible energy release processes are also discussed, but a satisfactory explanation needs further quantitative studies. Title: Recent progress in the study of white-light flares. Authors: Fang, Cheng; Ding, Mingde Bibcode: 1994PABei..12..100F Altcode: This article reviews the recent progress in the study of solar and stellar white-light flares (WLFs). The following topics are mainly discussed: (1) spectral features of two different types of WLFs; (2) semi-empirical models of the WLF atmosphere; and (3) heating mechanisms of the WLF atmosphere. A brief comparison of stellar WLFs and solar WLFs is also presented. Title: On the Propagation of Chromospheric Condensations in Solar Flares (II). Evolution of the Mass Motion Velocity Authors: Ding, M. D.; Fang, C. Bibcode: 1994Ap&SS.213..247D Altcode: We study the evolution of the mass motion velocity in the chromospheric condensation, when it propagates into the deeper atmosphere. The condensation is represented by a shock-like structure. Its momentum equation can be solved after some approximations. The computations are carried out for two cases, i.e., the case that the gas pressure just behind the condensation front is constant and the case that the pressure increase at the top of the condensation is constant. The results show that the duration of the condensation in the second case is considerably longer than that in the first case. The most evident difference of the velocity evolution between the two cases appears in their later phase. A comparison of the results in this paper with the dynamic simulations indicates that the second case may be closer to the real situation. Title: On the Propagation of Chromospheric Condensations in Solar Flares (I). Dynamic Simulations Authors: Ding, M. D.; Fang, C. Bibcode: 1994Ap&SS.213..233D Altcode: We simulate dynamically the downward propagation of the chromospheric condensation, which originates following the chromospheric evaporation during solar flares. Our attention is concentrated on the lower part of the atmosphere. The top of the chromosphere (base of the transition region) is regarded as the top boundary. The condensation is mimicked by assuming an impulsive pressure increase at the top boundary. Using such a method, we compute in detail the evolution process of a condensation. The results show that the condensation can penetrate into the deeper atmosphere, though it becomes very weak at the later phase. Moreover, we also discuss the possibility that the mass motions in the condensation may cause the asymmetries of some spectral lines as observations have indicated. Title: The Propagation of Chromospheric Condensations and the Asymmetry of Spectral Lines in Solar Flares Authors: Ding, M. D.; Fang, C. Bibcode: 1994AcApS..14..172D Altcode: No abstract at ADS Title: Study of the White Light Flare of 1974SEP10 Authors: Hu, J.; Fang, C.; Ding, M. D.; Hiei, E. Bibcode: 1994AcASn..35..396H Altcode: No abstract at ADS Title: Doppler Shifts of Metallic Lines for a White-Light Flare Authors: Ding, M. D.; Fang, C.; Okamoto, T. Bibcode: 1994SoPh..149..143D Altcode: In this paper we investigate the Doppler shifts of the metallic lines for a 3B white-light flare on September 19, 1979. The results show that there is no systematic shift at the line center, while there may appear some asymmetry at the line wing. A possible heating process of this flare is proposed to interpret the observed spectral features. Title: Hα Emission Line Profiles of Solar Flares with Turbulent Velocity Fields Authors: Ding, M. D.; Fang, C. Bibcode: 1993SoPh..147..305D Altcode: The Hα line profile in a flare atmospheric model superposed by a spatially correlated velocity field is studied in detail in this paper. The computations are carried out with the assumption that the velocity field is represented by a Kubo-Anderson process. It is found that the shape and the intensity of the Hα line profile depend greatly on the parameters of the velocity field. The variation of the profile and its relative deviation with respect to different correlation lengths is more complex than in the case of absorption lines. It is also shown that such a profile cannot be matched by those produced in the usually-used micro- and macro-turbulent approaches, especially for the velocity field with an intermediate correlation length. The results imply that the flare atmosphere should be highly turbulent in order to explain the observed Hα line profile with only weak central reversal in the flare spectra. Particularly, the effects of meso-turbulent fields should be taken into account in order to improve the spectral diagnostics and modelling of the flare atmosphere. Title: Ca II K emission line profile in a flare atmosphere with turbulent velocity fields. Authors: Ding, Mingde; Fang, Cheng Bibcode: 1993AcASn..34..279D Altcode: 1993AcASn..34..287D The authors studied in detail the properties of the Ca II K emission line profile in a flare atmosphere with the existence of turbulent velocity fields. The results show that (1) the intensity and shape of the emergent line profile depend strongly on the parameters of the turbulent field, (2) when the correlation length and the turbulent velocity are large enough, the theoretical profile has no central reversal, which can match the observed result, and (3) the profile in meso-turbulent fields can not be fitted by that in micro- and macro-turbulent approximations. These results will be useful in semi-empirical model calculations and spectral analyses of solar flares. Title: Hα and Ca II K line profiles of solar flare in turbulent velocity fields Authors: Fang, C.; Ding, M. D. Bibcode: 1993AdSpR..13i.321F Altcode: 1993AdSpR..13..321F No abstract at ADS Title: Spectral analysis of the 3B flare on September 19, 1979: asymmetry and Doppler shift of the metal lines. Authors: Ding, Mingde; Fang, Cheng; Okamoto, T. Bibcode: 1993AcApS..13..245D Altcode: The asymmetry and Doppler shift of metal lines of the 3B flare on September 19, 1979 are investigated. The results show that: (1) red asymmetries dominate in strong metal lines, but blue asymmetries also exist in some weak lines; (2) the maximum of the line asymmetry always precedes the maximum of the line intensity; (3) the blue asymmetry occurs generally in the early phase and can turn to a red one in the later phase; and (4) the line center has no obvious shift, regardless of the line asymmetry. It is proposed that the mass motions around the temperature minimum region caused by the impulsive heating or the propagation of the chromospheric condensation can explain these observational results. The study on metal lines can help understanding the dynamic process in the lower atmosphere of solar flares. Title: Spectral Analysis of the 3b Flare on 1979SEP19 - Asymmetry and Doppler Shift of the Metal Lines Authors: Ding, M. D.; Fang, C.; Tomizo, O. Bibcode: 1993AcApS..13..252D Altcode: No abstract at ADS Title: Mean Optical Depth and Contribution Function of Spectral Lines in a Turbulent Atmosphere Authors: Ding, M. D.; Fang, C. Bibcode: 1993Ap&SS.204..283D Altcode: In this paper we present a new definition and its analytic expressions for the mean optical depth and the mean contribution function of spectral lines in a turbulent atmosphere. These mean values are based on the radiative transfer equation and thus satisfy the general properties of the radiation field. They can be used to study the line formation process in turbulent atmospheres. Title: Semi-empirical model of the white light flare on September 19, 1979. Authors: Fang, Cheng; Ding, Mingde; Okamoto, T.; Gan, Weiqun Bibcode: 1993ScChA..36..217F Altcode: The intensities of the continuum emission and the Ca II Ka line of the white light flare on Sep 19, 1979 are measured and analyzed. Their variations with time are given. It is indicated that the continuum emission of this flare appeared in the early impulsive phase and lasted for about 5 - 6 min, with the time of maximum intensity 2 - 3 min earlier than that for the microwave radio burst. Based on the non-LTE theory, a semi-empirical model at a time with the continuum emission being relatively intensive is presented. The results show that the temperature in the flare photosphere is increased by 150 - 250K, and that the continuum emission is produced mainly by the negative hydrogen ion. Title: Center-to-limb variation of spectral lines and quiet sun atmospheric model. Authors: Ding, Mingde; Fang, Cheng Bibcode: 1991ChJSS..11..241D Altcode: Spectral lines of hydrogen and ionized calcium, as well as their center-to-limb variations, observed at Kitt Peak National Soalr Observatory, are used to diagnose the VAL-C', quiet Sun atmospheric model. The PANDORA computer program is used to perform the detailed non-LTE model calculations, especially for hydrogen and calcium. The main discrepanicies between the computed spectral lines and the observed data are: (1) The residual intensity in Hβ and λ8498 line centers is evidently higher, while that in Ca II H and K lines are relatively lower than the observed values. (2) The center-to-limb enhancement coefficient of the residual intensities of Hβ, and all the Ca II lines (resonance and infrared lines) near Δλ = 0.5 Å is a little larger than indicated by the observation. However, all other spectral features observed can be reproduced fairly well. Finally, the possible causes which can make the computed values deviate from the oberved ones is discussed, and a comment on revising the VAL-C' model is given. Title: A semi-empirical model of sunspot penumbra Authors: Ding, M. D.; Fang, C. Bibcode: 1989A&A...225..204D Altcode: A new semiempirical model of a sunspot penumbra is presented in this paper. The model is based on the profiles of H-alpha, H-beta, Ca II H and K and infrared lines observed at Kitt Peak Observatory on July 3, 1985. Non-LTE calculations are made to obtain the atmospheric model. In the temperature minimum region there is a notable negative value of the net radiative loss rate. By analyzing the magnetohydrostatic equilibrium of the model atmosphere, it is shown that the magnetic force has a great impact on the distribution of particle densities at and above the upper chromosphere, whereas it does not change substantially the emergent intensities of the seven lines except those of the line cores of Ca II H and K.