arXiv subjects

C. Fang

Publications and source records attributed to C. Fang.

85 records · Page 5Linked to original sources

arXiv2012

Understanding the white-light flare on 2012 March 9 : Evidence of a two-step magnetic reconnection

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). We analyzed WLF observations in radio, H-alpha, 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). Continuum emission enhancement clearly appeared at the 3600 angstrom and 4250 angstrom 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 H-alpha 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. 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.

astro-ph.SR↗

arXiv2012

A New MHD Code with Adaptive Mesh Refinement and Parallelization for Astrophysics

A new code, named MAP, is written in Fortran language for magnetohydrodynamics (MHD) calculation with the adaptive mesh refinement (AMR) and Message Passing Interface (MPI) parallelization. There are several optional numerical schemes for computing the MHD part, namely, modified Mac Cormack Scheme (MMC), Lax-Friedrichs scheme (LF) and weighted essentially non-oscillatory (WENO) scheme. All of them are second order, two-step, component-wise schemes for hyperbolic conservative equations. The total variation diminishing (TVD) limiters and approximate Riemann solvers are also equipped. A high resolution can be achieved by the hierarchical block-structured AMR mesh. We use the extended generalized Lagrange multiplier (EGLM) MHD equations to reduce the non-divergence free error produced by the scheme in the magnetic induction equation. The numerical algorithms for the non-ideal terms, e.g., the resistivity and the thermal conduction, are also equipped in the MAP code. The details of the AMR and MPI algorithms are described in the paper.

astro-ph.IM↗

arXiv2012

Numerical Simulation of Solar Microflares in a Canopy-Type Magnetic Configuration

Microflares are small activities in solar low atmosphere, some are in the low corona, and others in the chromosphere. Observations show that some of the microflares are triggered by magnetic reconnection between emerging flux and a pre-existing background magnetic field. We perform 2.5D compressible resistive MHD simulations of magnetic reconnection with gravity considered. The background magnetic field is a canopy-type configuration which is rooted at the boundary of the solar supergranule. By changing the bottom boundary conditions in the simulation, new magnetic flux emerges up at the center of the supergranule and reconnects with the canopy-type magnetic field. We successfully simulate the coronal and chromospheric microflares, whose current sheets are located at the corona and the chromosphere, respectively. The microflare of coronal origin has a bigger size and a higher temperature enhancement than that of chromospheric origin. In the microflares of coronal origin, we also found a hot jet ($\sim$$1.8 \times 10^6$ K), which is probably related to the observational EUV/SXR jets, and a cold jet ($\sim$$10^4$ K), which is similar to the observational H$α$/Ca surges, whereas there is only an H$α$/Ca bright point in the microflares of chromospheric origin. The study of parameter dependence shows that the size and strength of the emerging magnetic flux are the key parameters which determine the height of the reconnection location, and further determine the different observational features of the microflares.

astro-ph.SR↗

arXiv2011

"EIT Waves" and Coronal Mass Ejections

Coronal "EIT waves" appear as EUV bright fronts propagating across a significant part of the solar disk. The intriguing phenomenon provoked continuing debates on their nature and their relation with coronal mass ejections (CMEs). In this paper, we first summarize all the observational features of "EIT waves", which should be accounted for by any successful model. The theoretical models constructed during the past 10 years are then reviewed. Finally, the implication of the "EIT wave" research to the understanding of CMEs is discussed. The necessity is pointed out to revisit the nature of CME frontal loop.

astro-ph.SR↗

arXiv2011

Detailed Analysis of Fan-Shaped jets in Three Dimensional Numerical Simulation

We performed three dimensional resistive magnetohydrodynamic simulations to study the magnetic reconnection using an initially shearing magnetic field configuration (force free field with a current sheet in the middle of the computational box). It is shown that there are two types of reconnection jets: the ordinary reconnection jets and fan-shaped jets, which are formed along the guide magnetic field. The fan-shaped jets are much different from the ordinary reconnection jets which are ejected by magnetic tension force. There are two driving forces for accelerating the fan-shaped jets. The one is the Lorentz force which dominates the motion of fluid elements at first and then the gas pressure gradient force accelerates the fluid elements in the later stage. The dependence on magnetic reconnection angle and resistivity value has also been studied. The formation and evolution of these jets provide a new understanding of dynamic magnetohydrodynamic jets.

astro-ph.SR↗

arXiv2010

Numerical Simulations of Chromospheric Microflares

With gravity, ionization, and radiation being considered, we perform 2.5D compressible resistive MHD simulations of chromospheric magnetic reconnection using the CIP-MOCCT scheme. The temperature distribution of the quiet-Sun atmospheric model VALC and the helium abundance (10%) are adopted. Our 2.5D MHD simulation reproduces qualitatively the temperature enhancement observed in chromospheric microflares. The temperature enhancement $ΔT$ is demonstrated to be sensitive to the background magnetic field, whereas the total evolution time $Δt$ is sensitive to the magnitude of the anomalous resistivity. Moveover, we found a scaling law, which is described as $ΔT/Δt \sim {n_H}^{-1.5} B^{2.1} {η_0}^{0.88}$. Our results also indicate that the velocity of the upward jet is much greater than that of the downward jet and the X-point may move up or down.

astro-ph.SR↗

arXiv2010

The orbital characters of bands in iron-based superconductor BaFe1.85Co0.15As2

The unconventional superconductivity in the newly discovered iron-based superconductors is intimately related to its multi-band/multi-orbital nature. Here we report the comprehensive orbital characters of the low-energy three-dimensional electronic structure in BaFe$_{1.85}$Co$_{0.15}$As$_2$ by studying the polarization and photon energy dependence of angle-resolved photoemission data. While the distributions of the $d_{xz}$, $d_{yz}$, and $d_{3z^2-r^2}$ orbitals agree with the prediction of density functional theory, those of the $d_{xy}$ and $d_{x^2-y^2}$ orbitals show remarkable disagreement with theory. Our results point out the inadequacy of the existing band structure calculations, and more importantly, provide a foundation for constructing the correct microscopic model of iron pnictides.

cond-mat.supr-con↗

arXiv2010

Why Are Halo Coronal Mass Ejections Faster?

Halo coronal mass ejections (CMEs) were found to be significantly faster than normal CMEs, which was a long-standing puzzle. In order to solve the puzzle, we first investigate the observed properties of 31 limb CMEs that display clearly loop-shaped frontal loops. The observational results show a strong tendency that slower CMEs are weaker in the white-light intensity. Then, we perform a Monte Carlo simulation of 20 000 artificial limb CMEs that have average velocity of $\sim$523 km s$^{-1}$. The Thomson scattering of these events is calculated when they are assumed to be observed as limb and halo events, respectively. It is found that the white-light intensity of many slow CMEs becomes remarkably reduced as they turn from being viewed as a limb event to as a halo event. When the intensity is below the background solar wind fluctuation, it is assumed that they would be missed by coronagraphs. The average velocity of "detectable" halo CMEs is $\sim$922 km s$^{-1}$, very close to the observed value. It also indicates that wider events are more likely to be recorded. The results soundly suggest that the higher average velocity of halo CMEs is due to that a majority of slow events and a part of narrow fast events carrying less material are so faint that they are blended with the solar wind fluctuations, and therefore are not observed.

astro-ph.SR↗

arXiv2009

Energy Spectrum of the Electrons Accelerated by a Reconnection Electric Field: Exponential or Power Law?

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.

astro-ph↗

arXiv2007

The acceleration characteristics of solar energetic particles in the 2000 July 14 event

In large gradual solar energetic particle (SEP) events, especially the ground-level enhancement (GLE) events, where and how energetic particles are accelerated is still a problem. By using imaging data from TRACE, Yohkoh/HXT, SOHO/MDI and SOHO/EIT, along with the data from the GOES, Apatity NM, and SOHO/LASCO CME catalog, the evolution of the X5.7 two-ribbon flare and the associated SEP event on 14 July 2000 are studied. It is found that the magnetic reconnection in this event consists of two parts, and the induced electric field Erec is temporally correlated with the evolution of hard X-ray and gamma-ray emission. In particular, the first hard X-ray and gamma-ray emission peak occurred at 10:22 UT, corresponding to the magnetic reconnection in the western part of the flare ribbons and the maximum Erec of 9.5 V/cm; the second emission peak at 10:27 UT, corresponding to the eastern part and the maximum Erec of 13.0 V/cm. We also analyze the SEP injection profiles as functions of time and CME-height, and find two-component injection which may result from different acceleration mechanisms. A reasonable conclusion is that reconnection electric field makes a crucial contribution to the acceleration of relativistic particles and to the impulsive component of the large gradual SEP event, while CME-driven shocks play a dominant role in the gradual component.

astro-ph↗

arXiv2007

Flare magnetic reconnection and relativistic particles in the 2003 October 28 event

An X17.2 solar flare occurred on 2003 October 28, accompanied by multi-wavelength emissions and a high flux of relativistic particles observed at 1AU. We present the analytic results of the TRACE, SOHO, RHESSI, ACE, GOES, hard X-ray (INTEGRAL satellite), radio (Onderejov radio telescope), and neutron monitor data. It is found that the inferred magnetic reconnection electric field correlates well with the hard X-ray, gamma-ray, and neutron emission at the Sun. Thus the flare's magnetic reconnection probably makes a crucial contribution to the prompt relativistic particles, which could be detected at 1 AU. Since the neutrons were emitted a few minutes before the injection of protons and electrons, we propose a magnetic-field evolution configuration to explain this delay. We do not exclude the effect of CME-driven shock, which probably plays an important role in the delayed gradual phase of solar energetic particles.

astro-ph↗

arXiv2001

The role of non-thermal electrons in the hydrogen and calcium lines of stellar flares

There is observational evidence showing that stellar and solar flares occur in 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 Ly_alpha, H_alpha, Ca II K and Ca II 8542. We use a model atmosphere of a dMe star and make non-LTE computations taking into account the non-thermal collisional rates due 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 Ly_alpha and H_alpha become increasingly broad with the beam flux; change of the Ca II K and Ca II 8542 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 Ly_alpha line, but little on the H_alpha and Ca II lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Ly_alpha line. The Ly_alpha/H_alpha flux ratio is thus sensitive to the electron energy.

astro-ph↗

arXiv2000

The role of nonthermal electrons in the optical continuum of stellar flares

We explore the possibility that the continuum emission in stellar flares is powered by nonthermal electrons accelerated during the flares. We compute the continuum spectra from an atmospheric model for a dMe star, AD Leo, at its quiescent state, when considering the nonthermal excitation and ionisation effects by precipitating electron beams. The results show that if the electron beam has an energy flux large enough, 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, 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 nonthermal effects may play the chief role in powering the continuum emission in some stellar flares.

astro-ph↗