publications | Kai E. Yang

2024

Spectropolarimetric Inversion in Four Dimensions with Deep Learning (SPIn4D). I. Overview, Magnetohydrodynamic Modeling, and Stokes Profile Synthesis

Kai E. Yang, Lucas A. Tarr, Matthias Rempel, and 8 more authors

The Astrophysical Journal, Nov 2024

Abs DOI

The National Science Foundation’s Daniel K. Inouye Solar Telescope (DKIST) will provide high-resolution, multiline spectropolarimetric observations that are poised to revolutionize our understanding of the Sun. Given the massive data volume, novel inference techniques are required to unlock its full potential. Here, we provide an overview of our “SPIn4D” project, which aims to develop deep convolutional neural networks (CNNs) for estimating the physical properties of the solar photosphere from DKIST spectropolarimetric observations. We describe the magnetohydrodynamic (MHD) modeling and the Stokes profile synthesis pipeline that produce the simulated output and input data, respectively. These data will be used to train a set of CNNs that can rapidly infer the four-dimensional MHD state vectors by exploiting the spatiotemporally coherent patterns in the Stokes profile time series. Specifically, our radiative MHD model simulates the small-scale dynamo actions that are prevalent in quiet-Sun and plage regions. Six cases with different mean magnetic fields have been explored; each case covers six solar-hours, totaling 109 TB in data volume. The simulation domain covers at least 25 × 25 × 8 Mm, with 16 × 16 × 12 km spatial resolution, extending from the upper convection zone up to the temperature minimum region. The outputs are stored at a 40 s cadence. We forward model the Stokes profile of two sets of Fe i lines at 630 and 1565 nm, which will be simultaneously observed by DKIST and can better constrain the parameter variations along the line of sight. The MHD model output and the synthetic Stokes profiles are publicly available, with 13.7 TB in the initial release.
Stellar flares are far-ultraviolet luminous

Vera L. Berger, Jason T. Hinkle, Michael A. Tucker, and 6 more authors

Monthly Notices of the Royal Astronomical Society, Aug 2024

Abs DOI

We identify 182 flares on 158 stars within 100 pc of the Sun in both the near-ultraviolet (NUV: 1750-2750 Å) and far-ultraviolet (FUV: 1350-1750 Å) using high-cadence light curves from the \textitGalaxy Evolution Explorer (GALEX). Ultraviolet (UV) emission from stellar flares plays a crucial role in determining the habitability of exoplanetary systems. However, whether such UV emission promotes or threatens such life depends strongly on the energetics of these flares. Most studies assessing the effect of flares on planetary habitability assume a 9 000 K blackbody spectral energy distribution that produces more NUV flux than FUV flux (\mathcalR ≡F_\rm FUV / F_\rm NUV ≈\frac16). Instead, we observe the opposite with the excess FUV reaching \mathcalR ≈\frac12 \– 2, roughly 3\–12 times the expectation of a 9 000 K blackbody. The ratio of FUV to NUV time-integrated flare energies is 3.0 times higher on average than would be predicted by a constant 9 000 K blackbody during the flare. Finally, we find that the FUV/NUV ratio at peak tentatively correlates (\sim2 σsignificance) both with total UV flare energy and with the G-RP color of the host star. On average, we observe higher FUV/NUV ratios at peak in E_\textUV>10^32 erg flares and in flares on fully convective stars.
Superflare rate variability on M dwarfs

James Crowley, Michael S. Wheatland, and Kai Yang

Monthly Notices of the Royal Astronomical Society, May 2024

Abs DOI

M dwarfs are often highly flare productive, and changes in the rate of flaring can potentially affect the radiation and energetic particles an exoplanet is exposed to. Rate variability has been found to be rare on G-type stars, and there has been limited study of the variability of flaring on M dwarfs. Using data from the TESS mission up to Observation Sector 66, we analyse 658 stars ranging from K7 to M5 to test for superflare rate variability by calculating an optimal piecewise constant-rate model for flare occurrence using a Bayesian algorithm and identify 66 stars with rate changes, a larger proportion than our previous study into G-type stars (7 out of 270 stars). We find that rate variability does not depend on the stellar parameters for our sample. We investigate how the flare-frequency distribution for these stars varies and whether this variation affects exoplanet habitability. No stars in our sample provide sufficient UV flux to support prebiotic chemistry. We investigate the phase distributions on the rate variable stars, finding 11 cases of potential phase dependence in flaring across seven stars. We also compare the Bayesian method to another measure of activity, the fraction of flux emitted through flares, and do not observe a consistent relationship between this flaring activity metric and the optimal Bayesian model. We discuss the possible reasons for the discrepancy and identify a problem with the flaring activity metric due to the power-law distribution of the flare events.

2023

A Possible Mechanism for the “Late Phase” in Stellar White-light Flares

Kai E. Yang, Xudong Sun, Graham S. Kerr, and 1 more author

The Astrophysical Journal, Dec 2023

Abs DOI

M dwarf flares observed by the Transiting Exoplanet Survey Satellite (TESS) sometimes exhibit a peak-bump light-curve morphology, characterized by a secondary, gradual peak well after the main, impulsive peak. A similar late phase is frequently detected in solar flares observed in the extreme ultraviolet from longer hot coronal loops distinct from the impulsive flare structures. White-light emission has also been observed in off-limb solar flare loops. Here, we perform a suite of one-dimensional hydrodynamic loop simulations for M dwarf flares inspired by these solar examples. Our results suggest that coronal plasma condensation following impulsive flare heating can yield high electron number density in the loop, allowing it to contribute significantly to the optical light curves via free-bound and free–free emission mechanisms. Our simulation results qualitatively agree with TESS observations: the longer evolutionary timescale of coronal loops produces a distinct, secondary emission peak; its intensity increases with the injected flare energy. We argue that coronal plasma condensation is a possible mechanism for the TESS late-phase flares.
Data-constrained Magnetohydrodynamic Simulation of an Intermediate Solar Filament Eruption

Yang Guo, Jinhan Guo, Yiwei Ni, and 5 more authors

The Astrophysical Journal, Nov 2023

Abs DOI

Solar eruptive activities could occur in weak magnetic field environments and over large spatial scales, which are especially relevant to eruptions involving intermediate or quiescent solar filaments. To handle the large scales, we implement and apply a flux rope embedding method using regularized Biot–Savart laws in the spherical coordinate system. Combined with a potential field source surface model and a magneto-frictional method, a nonlinear force-free field comprising a flux rope embedded in a potential field is constructed. Using the combined nonlinear force-free field as the initial condition, we then perform a zero-βdata-constrained magnetohydrodynamic (MHD) simulation for an M8.7 flare at 03:38 UT on 2012 January 23. The MHD model reproduces the eruption process, flare ribbon evolution (represented by the quasi-separatrix layer evolution), and kinematics of the flux rope. This approach could potentially model global-scale eruptions from weak field regions.
Investigating Pre-eruptive Magnetic Properties at the Footprints of Erupting Magnetic Flux Ropes

Wensi Wang, Jiong Qiu, Rui Liu, and 4 more authors

The Astrophysical Journal, Feb 2023

Abs DOI

It is well established that solar eruptions are powered by free magnetic energy stored in the current-carrying magnetic field in the corona. It has also been generally accepted that magnetic flux ropes (MFRs) are a critical component of many coronal mass ejections. What remains controversial is whether MFRs are present well before the eruption. Our aim is to identify progenitors of MFRs, and investigate pre-eruptive magnetic properties associated with these progenitors. Here we analyze 28 MFRs erupting within 45 degree from the disk center from 2010 to 2015. All MFRs’ feet are well identified by conjugate coronal dimmings. We then calculate the magnetic properties at the feet of the MFRs, prior to their eruptions, using Helioseismic and Magnetic Imager vector magnetograms. Our results show that only 8 erupting MFRs are associated with significant nonneutralized electric currents, 4 of which also exhibit pre-eruptive dimmings at the footprints. Twist and current distributions are asymmetric at the two feet of these MFRs. The presence of pre-eruption dimmings associated with nonneutralized currents suggests the preexisting MFRs. Furthermore, the evolution of conjugate dimmings and electric currents within the footprints can provide clues about the internal structure of MFRs and their formation mechanism.

2022

Observed Rate Variations in Superflaring G-type Stars

James Crowley, Michael S. Wheatland, and Kai Yang

The Astrophysical Journal, Dec 2022

Abs DOI

Flare occurrence on the Sun is highly variable, exhibiting both short-term variation due to the emergence and evolution of active regions, and long-term variation from the solar cycle. On solar-like stars, more energetic stellar flares (superflares) have been observed, and it is of interest to determine whether the observed rates of superflare occurrence exhibit similar variability to solar flares. We analyze 274 G-type stars using data from the Transiting Exoplanet Survey Satellite and identify seven stars which exhibit statistically significant changes in the rate of superflare occurrence by fitting a piecewise constant-rate model with the Bayesian blocks algorithm. We investigate the properties of these stars and their flaring rates, and discuss the possible reasons for the low number of stars with detectable rate variation.

2020

Reconstructing Highly-twisted Magnetic Fields

Victor M. Demcsak, Michael S. Wheatland, Alpha Mastrano, and 1 more author

Solar Physics, Aug 2020

Abs DOI

We investigate the ability of a nonlinear force-free code to calculate highly-twisted magnetic field configurations using the Titov and Démoulin (Astron. Astrophys. 351:707, 1999) equilibrium field as a test case. The code calculates a force-free field using boundary conditions on the normal component of the field in the lower boundary, and the normal component of the current density over one polarity of the field in the lower boundary. The code can also use the current density over both polarities of the field in the lower boundary as a boundary condition. We investigate the accuracy of the reconstructions with increasing flux-rope surface twist number N_t, achieved by decreasing the sub-surface line current in the model. We find that the code can approximately reconstruct the Titov–Démoulin field for surface twist numbers up to N_t≈8.8. This includes configurations with bald patches. We investigate the ability to recover bald patches, and more generally identify the limitations of our method for highly-twisted fields. The results have implications for our ability to reconstruct coronal magnetic fields from observational data.
Self-consistent Nonlinear Force-Free Field Reconstruction from Weighted Boundary Conditions

A. Mastrano, K. E. Yang, and M. S. Wheatland

Solar Physics, Jul 2020

Abs DOI

Photospheric vector magnetogram data are often used as boundary conditions for force-free coronal magnetic field extrapolations. In general, however, vector magnetogram data are not consistent with the force-free assumption. In this article, we demonstrate a way to deal with inconsistent boundary data, by generalizing the “self-consistency procedure” of Wheatland and Régnier (Astrophys. J. Lett.700, L88, 2009). In that procedure, the inconsistency is resolved by an iterative process of constructing two solutions based on the values of the force-free parameter αon the two polarities of the field in the boundary (the P and N polarities), and taking uncertainty-weighted averages of the boundary values in the P and N solutions. When the αvalues in the P and N regions are very different, the self-consistent solution may lose high αvalues from the boundary conditions. We show how, by altering the weighting of the uncertainties in the P or N boundary conditions, we can preserve high αvalues in the self-consistent solution. The weighted self-consistent extrapolation method is demonstrated on an analytic bipole field and applied to vector magnetogram data taken by the Helioseismic and Magnetic Imager (HMI) instrument on board the Solar Dynamics Observatory (SDO) spacecraft for NOAA active region AR 12017 on 2014 March 29
Relative Magnetic Helicity Based on a Periodic Potential Field

Kai E. Yang, Michael S. Wheatland, and Stuart A. Gilchrist

The Astrophysical Journal, May 2020

Abs DOI

Magnetic helicity is conserved under ideal magnetohydrodynamics and quasi-conserved even under a resistive process. The standard definition for magnetic helicity cannot be applied directly to an open magnetic field in a volume, because it is gauge-dependent. Instead, the relative magnetic helicity is widely used. We find that the energy of a potential magnetic field in a rectangular domain with periodic lateral boundary conditions is less than that of the field with a fixed normal component on all six boundaries. To make use of this lower energy potential field in the analysis of relative magnetic helicity, we introduce a new definition for magnetic helicity for the magnetic field, which involves the periodic potential field. We apply this definition to a sequence of analytic solutions and a numerical simulation. The results show that our new gauge-invariant helicity is very close to the current-carrying part of the relative magnetic helicity of the original magnetic field. We find also that the ratio between the current-carrying helicity and the relative magnetic helicity for the original and our defined relative helicity show different behavior. It seems that the new helicity is more sensitive to the component of the field due to the electric current in the volume, which is the source for instabilities and solar eruptive phenomena.

2019

Evolution of a Magnetic Flux Rope toward Eruption

Wensi Wang, Chunming Zhu, Jiong Qiu, and 3 more authors

The Astrophysical Journal, Jan 2019

Abs DOI

It is well accepted that a magnetic flux rope (MFR) is a critical component of many coronal mass ejections (CMEs), yet how it evolves toward eruption remains unclear. Here we investigate the continuous evolution of a pre-existing MFR, which is rooted in strong photospheric magnetic fields and electric currents. The evolution of the MFR is observed by the Solar Terrestrial Relations Observatory (STEREO) and the Solar Dynamics Observatory (SDO) from multiple viewpoints. From STEREO’s perspective, the MFR starts to rise slowly above the limb five hours before it erupts as a halo CME on 2012 June 14. In SDO observations, conjugate dimmings develop on the disk, simultaneously with the gradual expansion of the MFR, suggesting that the dimmings map the MFR’s feet. The evolution comprises a two-stage gradual expansion followed by another stage of rapid acceleration/eruption. Quantitative measurements indicate that magnetic twist of the MFR increases from 1.0 \pm 0.5 to 2.0 \pm 0.5 turns during the five-hour expansion, and further increases to about 4.0 turns per astronomical unit (au) when detected as a magnetic cloud at 1 au two days later. In addition, each stage is preceded by flare(s), implying reconnection is actively involved in the evolution and eruption of the MFR. The implications of these measurements on the CME initiation mechanisms are discussed.

2018

Extremely Large Extreme-ultraviolet Late Phase Powered by Intense Early Heating in a Non-eruptive Solar Flare

Yu Dai, Mingde Ding, Weiguo Zong, and 1 more author

The Astrophysical Journal, Aug 2018

Abs DOI

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 (\approx3 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.
Observationally quantified reconnection providing a viable mechanism for active region coronal heating

Kai E. Yang, Dana W. Longcope, M. D. Ding, and 1 more author

Nature Communications, Feb 2018

Abs DOI

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.

2017

A circular white-light flare with impulsive and gradual white-light kernels

Q. Hao, K. Yang, X. Cheng, and 5 more authors

Nature Communications, Dec 2017

Abs DOI

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.
Homologous Circular-ribbon Flares Driven by Twisted Flux Emergence

Z. Xu, K. Yang, Y. Guo, and 3 more authors

The Astrophysical Journal, Dec 2017

Abs DOI

In this paper, we report two homologous circular-ribbon flares associated with two filament eruptions. They were well observed by the New Vacuum Solar Telescope and the Solar Dynamics Observatory on 2014 March 5. Prior to the flare, two small-scale filaments enclosed by a circular pre-flare brightening lie along the circular polarity inversion line around the parasitic polarity, which has shown a continuous rotation since its first appearance. Two filaments eventually erupt in sequence associated with two homologous circular-ribbon flares and display an apparent writhing signature. Supplemented by the nonlinear force-free field extrapolation and the magnetic field squashing factor investigation, the following are revealed. (1) This event involves the emergence of magnetic flux ropes into a pre-existing polarity area, which yields the formation of a 3D null-point topology in the corona. (2) Continuous input of the free energy in the form of a flux rope from beneath the photosphere may drive a breakout-type reconnection occurring high in the corona, supported by the pre-flare brightening. (3) This initiation reconnection could release the constraint on the flux rope and trigger the MHD instability to first make filament F1 lose equilibrium. The subsequent more violent magnetic reconnection with the overlying flux is driven during the filament rising. In return, the eruption of filament F2 is further facilitated by the reduction of the magnetic tension force above. These two processes form a positive feedback to each other to cause the energetic mass eruption and flare.
A reexamination of a filament oscillation event on 2013 March 15

Jialin Chen, Wenbin Xie, Yuhao Zhou, and 3 more authors

Astrophysics and Space Science, Sep 2017

Abs DOI

The key element in the research of solar activities is the coronal magnetic field, which is however difficult to measure directly. Filament (or prominence) oscillations offer a new approach to derive important information of the coronal magnetic field, which is called prominence seismology. However, it is vital to determine the oscillation mode before applying the prominence seismology since for a given magnetic structure of a filament, the two different modes of oscillation, namely, the longitudinal and transverse, have different eigen frequencies. In low-resolution observations, it is hard to distinguish the oscillation mode since both modes of oscillations are associated with lateral displacements, and the subtle difference between the two modes becomes unresolvable. On 2013 March 15, there is a filament oscillation event with a period of \approx63 minutes and a decay timescale of \approx105 minutes, which was explained in the literature to be a transverse oscillation or a mixture of both transversal and longitudinal components with the same period. With the analysis of the high-resolution SDO/AIA data, we reexamine the filament oscillation event, and argue that this event is a longitudinal oscillation. We tentatively propose a new method on how to identify the oscillation mode when the observational resolution is not so high. A numerical simulation is also provided in order to match the observations, which leads to the ratio between the depth and the width of the magnetic dip being 0.1.
Interaction of Two Filaments in a Long Filament Channel Associated with Twin Coronal Mass Ejections

Ruisheng Zheng, Qingmin Zhang, Yao Chen, and 4 more authors

The Astrophysical Journal, Feb 2017

Abs DOI

Using the high-quality observations of the Solar Dynamics Observatory, we present the interaction of two filaments (F1 and F2) in a long filament channel associated with twin coronal mass ejections (CMEs) on 2016 January 26. Before the eruption, a sequence of rapid cancellation and emergence of the magnetic flux has been observed, which likely triggered the ascending of the west filament (F1). The east footpoints of rising F1 moved toward the east far end of the filament channel, accompanied by post-eruption loops and flare ribbons. This likely indicated a large-scale eruption involving the long filament channel, which resulted from the interaction between F1 and the east filament (F2). Some bright plasma flew over F2, and F2 stayed at rest during the eruption, likely due to the confinement of its overlying lower magnetic field. Interestingly, the impulsive F1 pushed its overlying magnetic arcades to form the first CME, and F1 finally evolved into the second CME after the collision with the nearby coronal hole. We suggest that the interaction of F1 and the overlying magnetic field of F2 led to the merging reconnection that forms a longer eruptive filament loop. Our results also provide a possible picture of the origin of twin CMEs and show that the large-scale magnetic topology of the coronal hole is important for the eventual propagation direction of CMEs.
Magnetic Separatrix as the Source Region of the Plasma Supply for an Active-region Filament

P. Zou, C. Fang, P. F. Chen, and 2 more authors

The Astrophysical Journal, Feb 2017

Abs DOI

Solar filaments can be formed via chromospheric evaporation followed by condensation in the corona or by the direct injection of cool plasma from the chromosphere to the corona. We here confirm with high-resolution Hαdata observed by the 1.6 m New Solar Telescope of the Big Bear Solar Observatory on 2015 August 21 that an active-region filament is maintained by the continuous injection of cold chromospheric plasma. We find that the filament is rooted along a bright ridge in Hα, which corresponds to the intersection of a magnetic quasi-separatrix layer with the solar surface. This bright ridge consists of many small patches whose sizes are comparable to the width of the filament threads. It is found that upflows originate from the brighter patches of the ridge, whereas the downflows move toward the weaker patches of the ridge. The whole filament is composed of two opposite-direction streams, implying that longitudinal oscillations are not the only cause of the counterstreamings, and unidirectional siphon flows with alternative directions are another possibility.

2016

Material Supply and Magnetic Configuration of an Active Region Filament

P. Zou, C. Fang, P. F. Chen, and 3 more authors

The Astrophysical Journal, Nov 2016

Abs DOI

It is important to study the fine structures of solar filaments with high-resolution observations, since it can help us understand the magnetic and thermal structures of the filaments and their dynamics. In this paper, we study a newly formed filament located inside the active region NOAA 11762, which was observed by the 1.6 m New Solar Telescope at Big Bear Solar Observatory from 16:40:19 UT to 17:07:58 UT on 2013 June 5. As revealed by the Hαfiltergrams, cool material is seen to be injected into the filament spine with a speed of 5–10 km s^−1. At the source of the injection, brightenings are identified in the chromosphere, which are accompanied by magnetic cancellation in the photosphere, implying the importance of magnetic reconnection in replenishing the filament with plasmas from the lower atmosphere. Counter-streamings are detected near one endpoint of the filament, with the plane-of-the-sky speed being 7–9 km s^−1 in the Hαred-wing filtergrams and 9–25 km s^−1 in the blue-wing filtergrams. The observations are indicative that this active region filament is supported by a sheared arcade without magnetic dips, and the counter-streamings are due to unidirectional flows with alternative directions, rather than due to the longitudinal oscillations of filament threads as in many other filaments.
Slipping Magnetic Reconnection of Flux-rope Structures as a Precursor to an Eruptive X-class Solar Flare

Ting Li, Kai Yang, Yijun Hou, and 1 more author

The Astrophysical Journal, Oct 2016

Abs DOI

We present the quasi-periodic slipping motion of flux-rope structures prior to the onset of an eruptive X-class flare on 2015 March 11, obtained by the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory. The slipping motion occurred at the north part of the flux rope and seemed to successively peel off the flux rope. The speed of the slippage was 30−40 km s^−1, with an average period of 130 \pm 30 s. The Si iv \lambda1402.77 line showed a redshift of 10−30 km s^−1 and a line width of 50−120 km s^−1 at the west legs of slipping structures, indicative of reconnection downflow. The slipping motion lasted about 40 minutes, and the flux rope started to rise up slowly at the late stage of the slippage. Then an X2.1 flare was initiated, and the flux rope was impulsively accelerated. One of the flare ribbons swept across a negative-polarity sunspot, and the penumbral segments of the sunspot decayed rapidly after the flare. We studied the magnetic topology at the flaring region, and the results showed the existence of a twisted flux rope, together with quasi-separatrix layer (QSL) structures binding the flux rope. Our observations imply that quasi-periodic slipping magnetic reconnection occurs along the flux-rope-related QSLs in the preflare stage, which drives the later eruption of the flux rope and the associated flare.
Observation of Magnetic Reconnection at a 3D Null Point Associated with a Solar Eruption

J. Q. Sun, J. Zhang, K. Yang, and 2 more authors

The Astrophysical Journal Letters, Oct 2016

Abs DOI

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 \approx02:23 UT and \approx02:40 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.
Quantifying the Topology and Evolution of a Magnetic Flux Rope Associated with Multi-flare Activities

Kai Yang, Yang Guo, and M. D. Ding

The Astrophysical Journal, Jun 2016

Abs DOI

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.
Observing the release of twist by magnetic reconnection in a solar filament eruption

Zhike Xue, Xiaoli Yan, Xin Cheng, and 9 more authors

Nature Communications, Jun 2016

Abs DOI

Magnetic reconnection is a fundamental process of topology change and energy release, taking place in plasmas on the Sun, in space, in astrophysical objects and in the laboratory. However, observational evidence has been relatively rare and typically only partial. Here we present evidence of fast reconnection in a solar filament eruption using high-resolution H-alpha images from the New Vacuum Solar Telescope, supplemented by extreme ultraviolet observations. The reconnection is seen to occur between a set of ambient chromospheric fibrils and the filament itself. This allows for the relaxation of magnetic tension in the filament by an untwisting motion, demonstrating a flux rope structure. The topology change and untwisting are also found through nonlinear force-free field modelling of the active region in combination with magnetohydrodynamic simulation. These results demonstrate a new role for reconnection in solar eruptions: the release of magnetic twist.
Observations of an X-shaped Ribbon Flare in the Sun and Its Three-dimensional Magnetic Reconnection

Y. Li, J. Qiu, D. W. Longcope, and 2 more authors

The Astrophysical Journal Letters, May 2016

Abs DOI

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.
Bidirectional Outflows as Evidence of Magnetic Reconnection Leading to a Solar Microflare

Jie Hong, M. D. Ding, Ying Li, and 5 more authors

The Astrophysical Journal Letters, Mar 2016

Abs DOI

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 \pm(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.

2015

Is Flux Rope a Necessary Condition for the Progenitor of Coronal Mass Ejections?

Y. Ouyang, K. Yang, and P. F. Chen

The Astrophysical Journal, Dec 2015

Abs DOI

A magnetic flux rope structure is believed to exist in most coronal mass ejections (CMEs). However, it has been long debated whether the flux rope exists before eruption or if it is formed during eruption via magnetic reconnection. The controversy has continued because of our lack of routine measurements of the magnetic field in the pre-eruption structure, such as solar filaments. However, recently an indirect method was proposed to infer the magnetic field configuration based on the sign of helicity and the bearing direction of the filament barbs. In this paper, we apply this method to two erupting filament events, one on 2014 September 2 and the other on 2011 March 7, and find that the first filament is supported by a magnetic flux rope and the second filament is supported by a sheared arcade, i.e., the first one is an inverse-polarity filament and the second one is a normal-polarity filament. With the identification of the magnetic configurations in these two filaments, we stress that a flux rope is not a necessary condition for the pre-CME structure.
On the 2012 October 23 Circular Ribbon Flare: Emission Features and Magnetic Topology

Kai Yang, Yang Guo, and M. D. Ding

The Astrophysical Journal, Jun 2015

Abs DOI

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.