arXiv subjects

Yao Chen

Publications and source records attributed to Yao Chen.

At least 217 records · Page 12Linked to original sources

arXiv2017

Interaction of two filaments in a long filament channel associated with twin coronal mass ejections

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, accompanying with post-eruption loops and flare ribbons. It likely indicated a large-scale eruption involving the long filament channel, 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 form a longer eruptive filament loop. Our results also provide a possible picture of the origin of twin CMEs, and show the large-scale magnetic topology of the coronal hole is important for the eventual propagation direction of CMEs.

astro-ph.SR↗

arXiv2016

An Eruptive Hot-Channel Structure Observed at Metric Wavelength as a Moving Type-IV Solar Radio Burst

Hot channel (HC) structure, observed in the high-temperature passbands of the AIA/SDO, is regarded as one candidate of coronal flux rope which is an essential element of solar eruptions. Here we present the first radio imaging study of an HC structure in the metric wavelength. The associated radio emission manifests as a moving type-IV (t-IVm) burst. We show that the radio sources co-move outwards with the HC, indicating that the t-IV emitting energetic electrons are efficiently trapped within the structure. The t-IV sources at different frequencies present no considerable spatial dispersion during the early stage of the event, while the sources spread gradually along the eruptive HC structure at later stage with significant spatial dispersion. The t-IV bursts are characterized by a relatively-high brightness temperature ($\sim$ 10$^{7}$ $-$ 10$^{9}$ K), a moderate polarization, and a spectral shape that evolves considerably with time. This study demonstrates the possibility of imaging the eruptive HC structure at the metric wavelength and provides strong constraints on the t-IV emision mechanism, which, if understood, can be used to diagnose the essential parameters of the eruptive structure.

astro-ph.SR↗

arXiv2016

Dynamics of a prominence-horn structure during its evaporation in the solar corona

The physical connection among and formation mechanisms of various components of the prominence-horn cavity system remain elusive. Here we present observations of such a system, focusing on a section of the prominence that rises and separates gradually from the main body. This forms a configuration sufficiently simple to yield clues to the above issues. It is characterized by embedding horns, oscillations, and a gradual disappearance of the separated material. The prominence-horn structure exhibits a large amplitude longitudinal oscillation with a period of ~150 minutes and an amplitude of ~30 Mm along the trajectory defined by the concave horn structure. The horns also experience a simultaneous transverse oscillation with a much smaller amplitude (~3 Mm) and shorter period (~10-15 minutes), likely representative of a global mode of the large-scale magnetic structure. The gradual disappearance of the structure indicates that the horn, an observational manifestation of the field-aligned transition region separating the cool and dense prominence from the hot and tenuous corona, is formed due to the heating and diluting process of the central prominence mass, while most previous studies suggest that it is the opposite process, i.e., the cooling and condensation of coronal plasmas, to form the horn. This study also demonstrates how the prominence transports magnetic flux to the upper corona, a process essential for the gradual build-up of pre-eruption magnetic energy.

astro-ph.SR↗

arXiv2016

Observation of a Metric Type N Solar Radio Burst

Type III and type-III-like radio bursts are produced by energetic electron beams guided along coronal magnetic fields. As a variant of type III bursts, Type N bursts appear as the letter "N" in the radio dynamic spectrum and reveal a magnetic mirror effect in coronal loops. Here, we report a well-observed N-shaped burst consisting of three successive branches at metric wavelength with both fundamental and harmonic components and a high brightness temperature ($>$10$^9$ K). We verify the burst as a true type N burst generated by the same electron beam from three aspects of the data. First, durations of the three branches at a given frequency increase gradually, may due to the dispersion of the beam along its path. Second, the flare site, as the only possible source of non-thermal electrons, is near the western feet of large-scale closed loops. Third, the first branch and the following two branches are localized at different legs of the loops with opposite sense of polarization. We also find that the sense of polarization of the radio burst is in contradiction to the O-mode and there exists a fairly large time delay ($\sim$3-5 s) between the fundamental and harmonic components. Possible explanations accounting for these observations are presented. Assuming the classical plasma emission mechanism, we can infer coronal parameters such as electron density and magnetic field near the radio source and make diagnostics on the magnetic mirror process.

astro-ph.SR↗

arXiv2016

Decameter Stationary Type IV Burst in Imaging Observations on the 6th of September 2014

First-of-its-kind radio imaging of decameter solar stationary type IV radio burst has been presented in this paper. On 6 September 2014 the observations of type IV burst radio emission have been carried out with the two-dimensional heliograph based on the Ukrainian T-shaped radio telescope (UTR-2) together with other telescope arrays. Starting at 09:55 UT and throughout 3 hours, the radio emission was kept within the observational session of UTR-2. The interesting observation covered the full evolution of this burst, "from birth to death". During the event lifetime, two C-class solar X-ray flares with peak times 11:29 UT and 12:24 UT took place. The time profile of this burst in radio has a double-humped shape that can be explained by injection of energetic electrons, accelerated by the two flares, into the burst source. According to the heliographic observations we suggest the burst source was confined within a high coronal loop, which was a part of a relatively slow coronal mass ejection. The latter has been developed for several hours before the onset of the event. Through analyzing about 1.5 million of heliograms (3700 temporal frames with 4096 images in each frame that correspond to the number of frequency channels) the radio burst source imaging shows a fascinating dynamical evolution. Both space-based (GOES, SDO, SOHO, STEREO) data and various ground-based instrumentation (ORFEES, NDA, RSTO, NRH) records have been used for this study.

astro-ph.SR↗

arXiv2016

Local Region Sparse Learning for Image-on-Scalar Regression

Identification of regions of interest (ROI) associated with certain disease has a great impact on public health. Imposing sparsity of pixel values and extracting active regions simultaneously greatly complicate the image analysis. We address these challenges by introducing a novel region-selection penalty in the framework of image-on-scalar regression. Our penalty combines the Smoothly Clipped Absolute Deviation (SCAD) regularization, enforcing sparsity, and the SCAD of total variation (TV) regularization, enforcing spatial contiguity, into one group, which segments contiguous spatial regions against zero-valued background. Efficient algorithm is based on the alternative direction method of multipliers (ADMM) which decomposes the non-convex problem into two iterative optimization problems with explicit solutions. Another virtue of the proposed method is that a divide and conquer learning algorithm is developed, thereby allowing scaling to large images. Several examples are presented and the experimental results are compared with other state-of-the-art approaches.

stat.ML↗

arXiv2016

Spin exchange collision mixing of the K and Rb ac Stark shifts

In a hybrid pumping alkali vapor cell that both K and Rb are filled, K atom spins are optically pumped by laser and Rb atom spins are polarized by the K spins through spin exchange. We find that the AC Stark shift of the Rb atoms is composed of not only the AC Stark shift of the Rb atoms caused by the far off resonant pumping laser which is tuned to the K absorption lines, but also the AC Stark shift of the K atom spins. The mixing of the light shifts through fast spin exchange between K and Rb atoms are studied in this paper and we demonstrate a K-Rb-21Ne co-magnetometer in which the AC Stark shift of the Rb atoms are reduced by the collision mixing.

physics.atom-ph↗

arXiv2016

Slipping magnetic reconnections with multiple flare ribbons during an X-class solar flare

With the observations of the Solar Dynamics Observatory, we present the slipping magnetic reconnections with multiple flare ribbons (FRs) during an X1.2 eruptive flare on 2014 January 7. A center negative polarity was surrounded by several positive ones, and there appeared three FRs. The three FRs showed apparent slipping motions, and hook structures formed at their ends. Due to the moving footpoints of the erupting structures, one tight semi-circular hook disappeared after the slippage along its inner and outer edge, and coronal dimmings formed within the hook. The east hook also faded as a result of the magnetic reconnection between the arcades of a remote filament and a hot loop that was impulsively heated by the under flare loops. Our results are accordant with the slipping magnetic reconnection regime in 3D standard model for eruptive flares. We suggest that complex structures of the flare is likely a consequence of the more complex flux distribution in the photosphere, and the eruption involves at least two magnetic reconnections.

astro-ph.SR↗

arXiv2016

A Statistical Study of the Average Iron Charge Distributions inside Magnetic Clouds for Solar Cycle 23

Magnetic clouds (MCs) are the interplanetary counterpart of coronal magnetic flux ropes. They can provide valuable information to reveal the flux rope characteristics at their eruption stage in the corona, which are unable to be explored in situ at present. In this paper, we make a comprehensive survey of the average iron charge state ( Fe) distributions inside 96 MCs for solar cycle 23 using ACE (Advanced Composition Explorer) data. As the Fe in the solar wind are typically around 9+ to 11+, the Fe charge state is defined as high when the Fe is larger than 12+, which implies the existence of a considerable amount of Fe ions with high charge states (e.g., \geq 16+). The statistical results show that the Fe distributions of 92 (~ 96%) MCs can be classified into four groups with different characteristics. In group A (11 MCs), the Fe shows a bimodal distribution with both peaks higher than 12+. Group B (4 MCs) presents a unimodal distribution of Fe with its peak higher than 12+. In groups C (29 MCs) and D (48 MCs), the Fe remains higher and lower than 12+ throughout ACE passage through the MC, respectively. Possible explanations to these distributions are discussed.

astro-ph.SR↗

arXiv2016

Numerical simulations of Kelvin-Helmholtz instability: a two-dimensional parametric study

Using two-dimensional simulations, we numerically explore the dependences of Kelvin-Helmholtz instability upon various physical parameters, including viscosity, width of sheared layer, flow speed, and magnetic field strength. In most cases, a multi-vortex phase exists between the initial growth phase and final single-vortex phase. The parametric study shows that the evolutionary properties, such as phase duration and vortex dynamics, are generally sensitive to these parameters except in certain regimes. An interesting result is that for supersonic flows, the phase durations and saturation of velocity growth approach constant values asymptotically as the sonic Mach number increases. We confirm that the linear coupling between magnetic field and Kelvin-Helmholtz modes is negligible if the magnetic field is weak enough. The morphological behaviour suggests that the multi-vortex coalescence might be driven by the underlying wave-wave interaction. Based on these results, we make a preliminary discussion about several events observed in the solar corona. The numerical models need to be further improved to make a practical diagnostic of the coronal plasma properties.

astro-ph.SR↗

arXiv2016

Electron Acceleration at a Coronal Shock Propagating Through a Large-scale Streamer-like Magnetic Field

With a test-particle simulation, we investigate the effect of large-scale coronal magnetic fields on electron acceleration at an outward-propagating coronal shock with a circular front. The coronal field is approximated by an analytical solution with a streamer-like magnetic field featured by partially open magnetic field and a current sheet at the equator atop the closed region. We show that the large-scale shock-field configuration, especially the relative curvature of the shock and the magnetic field line across which the shock is sweeping, plays an important role in the efficiency of electron acceleration. At low shock altitudes, when the shock curvature is larger than that of magnetic field lines, the electrons are mainly accelerated at the shock flanks; at higher altitudes, when the shock curvature is smaller, the electrons are mainly accelerated at the shock nose around the top of closed field lines. The above process reveals the shift of efficient electron acceleration region along the shock front during its propagation. It is also found that in general the electron acceleration at the shock flank is not so efficient as that at the top of closed field since at the top a collapsing magnetic trap can be formed. In addition, we find that the energy spectra of electrons is power-law like, first hardening then softening with the spectral index varying in a range of -3 to -6. Physical interpretations of the results and implications on the study of solar radio bursts are discussed.

astro-ph.SR↗

arXiv2016

Solar jet-coronal hole collision and a related coronal mass ejection

Jets are defined as impulsive, well-collimated upflows, occurring in different layers of the solar atmosphere with different scales. Their relationship with coronal mass ejections (CMEs), another type of solar impulsive events, remains elusive. Using the high-quality imaging data of AIA/SDO, here we show a well-observed coronal jet event, in which part of the jets, with the embedding coronal loops, runs into a nearby coronal hole (CH) and gets bounced towards the opposite direction. This is evidenced by the flat-shape of the jet front during its interaction with the CH and the V-shaped feature in the time-slice plot of the interaction region. About a half-hour later, a CME initially with a narrow and jet-like front is observed by the LASCO C2 coronagraph, propagating along the direction of the post-collision jet. We also observe some 304 A dark material flowing from the jet-CH interaction region towards the CME. We thus suggest that the jet and the CME are physically connected, with the jet-CH collision and the large- scale magnetic topology of the CH being important to define the eventual propagating direction of this particular jet-CME eruption.

astro-ph.SR↗

arXiv2015

Gradual Magnetic Evolution of Sunspot Structure and Filament-Corona Dynamics Associated with the X1.8 Flare in AR 11283

In this paper, we present a study on persistent and gradual penumbral decay and correlated decline of the photospheric transverse field component during 10-20 hours before a major flare (X1.8) eruption on 2011 September 7. This long-term pre-eruption behavior is corroborated with the well-imaged pre-flare filament rising, the consistent expansion of coronal arcades overlying the filament, as well as the NLFFF modelling results in the literature. We suggest that both the long-term pre-flare penumbral decay and the transverse field decline are the photospheric manifestation of the gradual rise of the coronal filament-flux rope system. We also suggest that a C3 flare and subsequent reconnection process preceding the X1.8 flare play an important role in triggering the later major eruption.

astro-ph.SR↗

arXiv2015

An observational revisit of band-split solar type-II radio bursts

Band split of solar type II radio bursts, discovered several decades ago, is a fascinating phenomenon with the type-II lanes exhibiting two almost-parallel sub-bands with similar morphology. The underlying split mechanism remains elusive. One popular interpretation is that the splitting bands are emitted from the shock upstream and downstream, respectively, with their frequency ratio (γ) determined by the shock compression ratio. This interpretation has been taken as the physical basis for many published references. Here we report an observational analysis of type II events with nice split selected from the ground-based RSTN data from 2001 to 2014, in the metric-decametric wavelength. We investigate the temporal variation and distribution of γ, and conduct correlation analyses on the deduced spectral values. It is found that γ varies in a very narrow range with >80% of γ (one-minute averaged data) being between 1.15 to 1.25. For some well-observed and long-lasting events, γ does not show a systematic variation trend within observational uncertainties, from the onset to the termination of the splits. In addition, the parameters representing the propagation speed of the radio source (presumably the coronal shock) show a very weak or basically no correlation with γ. We suggest that these results do not favor the upstreamdownstream scenario of band splits.

astro-ph.SR↗

arXiv2015

Evidence of the Solar EUV hot channel as a magnetic flux rope from remote-sensing and in-situ observations

Hot channels (HCs), high temperature erupting structures in the lower corona of the Sun, have been proposed as a proxy of magnetic flux ropes (MFRs) since their initial discovery. However, it is difficult to make definitive proof given the fact that there is no direct measurement of magnetic field in the corona. An alternative way is to use the magnetic field measurement in the solar wind from in-situ instruments. On 2012 July 12, an HC was observed prior to and during a coronal mass ejection (CME) by the AIA high-temperature images. The HC is invisible in the EUVI low-temperature images, which only show the cooler leading front (LF). However, both the LF and an ejecta can be observed in the coronagraphic images. These are consistent with the high temperature and high density of the HC and support that the ejecta is the erupted HC. In the meanwhile, the associated CME shock was identified ahead of the ejecta and the sheath through the COR2 images, and the corresponding ICME was detected by \textit{ACE}, showing the shock, sheath and magnetic cloud (MC) sequentially, which agrees with the coronagraphic observations. Further, the MC contained a low-ionization-state center and a high-ionization-state shell, consistent with the pre-existing HC observation and its growth through magnetic reconnection. All of these observations support that the MC detected near the Earth is the counterpart of the erupted HC in the corona for this event. Therefore, our study provides strong observational evidence of the HC as an MFR.

astro-ph.SR↗

arXiv2015

Black Phosphorus-Polymer Composites for Pulsed Lasers

Black phosphorus is a very promising material for telecommunication due to its direct bandgap and strong resonant absorption in near-infrared wavelength range. However, ultrafast nonlinear photonic applications relying on the ultrafast photo-carrier dynamics as well as optical nonlinearity in black phosphorus remain unexplored. In this work, we investigate nonlinear optical properties of solution exfoliated BP and demonstrate the usage of BP as a new saturable absorber for high energy pulse generation in fiber laser. In order to avoid the oxidization and degradation of BP, we encapsulated BP by polymer matrix which is optically transparent in the spectrum range of interest to form a composite. Two fabrication approaches were demonstrated to produce BP-polymer composite films which were further incorporated into fiber laser cavity as nonlinear media. BP shows very fast carrier dynamics and BP-polymer composite has a modulation depth of 10.6%. A highly stable Q-switched pulse generation was achieved and the single pulse energy of ~194 nJ was demonstrated. The ease of handling of such black phosphorus-polymer composite thin films affords new opportunities for wider applications such as optical sensing, signal processing and light modulation.

cond-mat.mtrl-sci↗

arXiv2015

Acceleration phases of a solar filament during its eruption

Filament eruptions often lead to coronal mass ejections (CMEs), which can affect critical technological systems in space and on the ground when they interact with the geo-magnetosphere in high speeds. Therefore, it is an important issue to investigate the acceleration mechanisms of CMEs in solar/space physics. Based on observations and simulations, the resistive magnetic reconnection and the ideal instability of magnetic flux rope have been proposed to accelerate CMEs. However, it remains elusive whether both of them play a comparable role during a particular eruption. It has been extremely difficult to separate their contributions as they often work in a close time sequence during one fast acceleration phase. Here we report an intriguing filament eruption event, which shows two apparently separated fast acceleration phases and provides us an excellent opportunity to address the issue. Through analyzing the correlations between velocity (acceleration) and soft (hard) X-ray profiles, we suggest that the instability and magnetic reconnection make a major contribution during the first and second fast acceleration phases, respectively. Further, we find that both processes have a comparable contribution to accelerate the filament in this event.

astro-ph.SR↗

arXiv2015

First Taste of Hot Channel in Interplanetary Space

Hot channel (HC) is a high temperature ($\sim$10 MK) structure in the inner corona revealed first by Atmospheric Imaging Assembly (AIA) on board \textit{Solar Dynamics Observatory}. Eruption of HC is often associated with flare and coronal mass ejection. Previous studies suggest that HC is a good proxy of magnetic flux rope (MFR) in the inner corona, in addition to another well-known MFR candidate, the prominence-cavity structure that is with a normal coronal temperature ($\sim$1-2 MK). In this paper, we report a high temperature structure (HTS, $\sim$1.5 MK) contained in an interplanetary coronal mass ejection induced by an HC eruption. According to the observations of bidirectional electrons, high temperature and density, strong magnetic field, and its association with the shock, sheath, and plasma pile-up region, we suggest that the HTS is the interplanetary counterpart of the HC. The scale of the measured HTS is around 14 R$_\odot$, and it maintained a much higher temperature than the background solar wind even at 1 AU. It is significantly different from the typical magnetic clouds (MCs), which usually have a much lower temperature. Our study suggests that the existence of a corotating interaction region ahead of the HC formed a magnetic container to inhibit the HC expansion and cooling down to a low temperature.

astro-ph.SR↗