arXiv subjects

Mingzhe Guo

Publications and source records attributed to Mingzhe Guo.

35 records · Page 2Linked to original sources

arXiv2023

Influence of the Lower Atmosphere on Wave Heating and Evaporation in Solar Coronal Loops

We model a coronal loop as a three-dimensional magnetic cylinder in a realistic solar atmosphere that extends from the chromosphere to the corona. Kink oscillations, believed ubiquitous in the solar corona, are launched in the loop. Heating is expected due to the dissipation of wave energy at small structures that develop from the Kelvin-Helmholtz instability induced by kink oscillations. Increases in temperature and internal energy can be observed in the coronal counterpart of the driven loop. With the presence of thermal conduction, chromospheric evaporation can also be seen. Although the volume averaged temperature and density changes seem slight ($\sim4\%$ relative to a non-driven loop), the enthalpy flow from the lower atmosphere redistributes the density and temperature in the vertical direction, thus enhancing the dissipation of wave energy in the corona. The efficient heating in the coronal counterpart of the loop can complement the thermal conductive losses shown in the current model and thus maintain the internal energy in the corona.

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arXiv2023

Excitation of Multi-periodic Kink Motions in Solar Flare Loops: Possible Application to Quasi-periodic Pulsations

Magnetohydrodynamic (MHD) waves are often invoked to interpret quasi-periodic pulsations (QPPs) in solar flares. We study the response of a straight flare loop to a kink-like velocity perturbation using three-dimensional MHD simulations and forward model the microwave emissions using the fast gyrosynchrotron code. Kink motions with two periodicities are simultaneously generated,with the long-period component P_L = 57s being attributed to the radial fundamental kink mode and the short-period component P_S=5.8s to the first leaky kink mode. Forward modeling results show that the two-periodic oscillations are detectable in the microwave intensities for some lines of sight. Increasing the beam size to (1")^2 does not wipe out the microwave oscillations. We propose that the first leaky kink mode is a promising candidate mechanism to account for short-period QPPs. Radio telescopes with high spatial resolutions can help distinguish between this new mechanism with such customary interpretations as sausage modes.

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arXiv2022

Three-Dimensional Propagation of Kink Wave Trains in Solar Coronal Slabs

Impulsively excited wave trains are of considerable interest in solar coronal seismology. To our knowledge, however, it remains to examine the three-dimensional (3D) dispersive propagation of impulsive kink waves in straight, field-aligned, symmetric, low-beta, slab equilibria that are structured only in one transverse direction. We offer a study here, starting with an analysis of linear oblique kink modes from an eigenvalue problem perspective. Two features are numerically found for continuous and step structuring alike, one being that the group and phase velocities may lie on opposite sides of the equilibrium magnetic field ($\vec{B}_0$), and the other being that the group trajectories extend only to a limited angle from $\vec{B}_0$. We justify these features by making analytical progress for the step structuring. More importantly, we demonstrate by a 3D time-dependent simulation that these features show up in the intricate interference patterns of kink wave trains that arise from a localized initial perturbation. In a plane perpendicular to the direction of inhomogeneity, the large-time slab-guided patterns are confined to a narrow sector about $\vec{B}_0$, with some wavefronts propagating toward $\vec{B}_0$. We conclude that the phase and group diagrams lay the necessary framework for understanding the complicated time-dependent behavior of impulsive waves.

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arXiv2022

Oblique Quasi-Kink Modes in Solar Coronal Slabs Embedded in an Asymmetric Magnetic Environment: Resonant Damping, Phase and Group Diagrams

There has been considerable interest in magnetoacoustic waves in static, straight, field-aligned, one-dimensional equilibria where the exteriors of a magnetic slab are different between the two sides. We focus on trapped, transverse fundamental, oblique quasi-kink modes in pressureless setups where the density varies continuously from a uniform interior (with density $ρ_{\rm i}$) to a uniform exterior on either side (with density $ρ_{\rm L}$ or $ρ_{\rm R}$), assuming $ρ_{\rm L}\leρ_{\rm R}\leρ_{\rm i}$. The continuous structuring and oblique propagation make our study new relative to pertinent studies, and lead to wave damping via the Alfv$\acute{\rm e}$n resonance. We compute resonantly damped quasi-kink modes as resistive eigenmodes, and isolate the effects of system asymmetry by varying $ρ_{\rm i}/ρ_{\rm R}$ from the ``Fully Symmetric'' ($ρ_{\rm i}/ρ_{\rm R}=ρ_{\rm i}/ρ_{\rm L}$) to the ``Fully Asymmetric'' limit ($ρ_{\rm i}/ρ_{\rm R}=1$). We find that the damping rates possess a nonmonotonic $ρ_{\rm i}/ρ_{\rm R}$-dependence as a result of the difference between the two Alfv$\acute{\rm e}$n continua, and resonant absorption occurs only in one continuum when $ρ_{\rm i}/ρ_{\rm R}$ is below some threshold. We also find that the system asymmetry results in two qualitatively different regimes for the phase and group diagrams. The phase and group trajectories lie essentially on the same side (different sides) relative to the equilibrium magnetic field when the configuration is not far from a ``Fully Asymmetric'' (``Fully Symmetric'') one. Our numerical results are understood by making analytical progress in the thin-boundary limit, and discussed for imaging observations of axial standing modes and impulsively excited wavetrains.

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arXiv2022

Influence of fine structures on gyrosynchrotron emission of flare loops modulated by sausage modes

Sausage modes are one leading mechanism for interpreting short period quasi-periodic pulsations (QPPs) of solar flares. Forward modeling their radio emission is crucial for identifying sausage modes observationally and for understanding their connections with QPPs. Using the numerical output from three-dimensional magnetohydrodynamic (MHD) simulations, we forward model the gyrosynchrotron (GS) emission of flare loops modulated by sausage modes and examine the influence of loop fine structures. The temporal evolution of the emission intensity is analyzed for an oblique line of sight crossing the loop center. We find that the low- and high-frequency intensities oscillate in-phase at the period of sausage modes for models with or without fine structures. For low-frequency emissions where the optically thick regime arises, the modulation magnitude of the intensity is dramatically reduced by the fine structures at some viewing angles. On the contrary, for high-frequency emissions where the optically thin regime holds, the effect of fine structures or viewing angle is marginal. Our results show that the periodic intensity variations of sausage modes are not wiped out by the fine structures, and sausage modes remains a promising candidate mechanism for QPPs even when flare loops are fine-structured.

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arXiv2022

Impulsively Generated Kink Wave Trains in Solar Coronal Slabs

We numerically follow the response of density-enhanced slabs to impulsive, localized, transverse velocity perturbations by working in the framework of ideal magnetohydrodynamics (MHD). Both linear and nonlinear regimes are addressed. Kink wave trains are seen to develop along the examined slabs, sharing the characteristics that more oscillatory patterns emerge with time and that the apparent wavelength increases with distance at a given instant. Two features nonetheless arise due to nonlinearity, one being a density cavity close to the exciter and the other being the appearance of shocks both outside and inside the nominal slab. These features may be relevant for understanding the interaction between magnetic structures and such explosive events as coronal mass ejections. Our numerical findings on kink wave trains in solar coronal slabs are discussed in connection with typical measurements of streamer waves.

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arXiv2022

Divert More Attention to Vision-Language Tracking

Relying on Transformer for complex visual feature learning, object tracking has witnessed the new standard for state-of-the-arts (SOTAs). However, this advancement accompanies by larger training data and longer training period, making tracking increasingly expensive. In this paper, we demonstrate that the Transformer-reliance is not necessary and the pure ConvNets are still competitive and even better yet more economical and friendly in achieving SOTA tracking. Our solution is to unleash the power of multimodal vision-language (VL) tracking, simply using ConvNets. The essence lies in learning novel unified-adaptive VL representations with our modality mixer (ModaMixer) and asymmetrical ConvNet search. We show that our unified-adaptive VL representation, learned purely with the ConvNets, is a simple yet strong alternative to Transformer visual features, by unbelievably improving a CNN-based Siamese tracker by 14.5% in SUC on challenging LaSOT (50.7% > 65.2%), even outperforming several Transformer-based SOTA trackers. Besides empirical results, we theoretically analyze our approach to evidence its effectiveness. By revealing the potential of VL representation, we expect the community to divert more attention to VL tracking and hope to open more possibilities for future tracking beyond Transformer. Code and models will be released at https://github.com/JudasDie/SOTS.

cs.CV↗

arXiv2022

Learning Target-aware Representation for Visual Tracking via Informative Interactions

We introduce a novel backbone architecture to improve target-perception ability of feature representation for tracking. Specifically, having observed that de facto frameworks perform feature matching simply using the outputs from backbone for target localization, there is no direct feedback from the matching module to the backbone network, especially the shallow layers. More concretely, only the matching module can directly access the target information (in the reference frame), while the representation learning of candidate frame is blind to the reference target. As a consequence, the accumulation effect of target-irrelevant interference in the shallow stages may degrade the feature quality of deeper layers. In this paper, we approach the problem from a different angle by conducting multiple branch-wise interactions inside the Siamese-like backbone networks (InBN). At the core of InBN is a general interaction modeler (GIM) that injects the prior knowledge of reference image to different stages of the backbone network, leading to better target-perception and robust distractor-resistance of candidate feature representation with negligible computation cost. The proposed GIM module and InBN mechanism are general and applicable to different backbone types including CNN and Transformer for improvements, as evidenced by our extensive experiments on multiple benchmarks. In particular, the CNN version (based on SiamCAR) improves the baseline with 3.2/6.9 absolute gains of SUC on LaSOT/TNL2K, respectively. The Transformer version obtains SUC scores of 65.7/52.0 on LaSOT/TNL2K, which are on par with recent state of the arts. Code and models will be released.

cs.CV↗

arXiv2021

Possible Signature of Sausage Waves in Photospheric Bright Points

Sausage waves have been frequently reported in solar magnetic structures such as sunspots, pores, and coronal loops. However, they have not been unambiguously identified in photospheric bright points (BPs). Using high-resolution TiO image sequences obtained with the Goode Solar Telescope at the Big Bear Solar Observatory, we analyzed four isolated BPs. It was found that their area and average intensity oscillate for several cycles in an in-phase fashion. The oscillation periods range from 100 to 200 seconds. We interpreted the phase relation as a signature of sausage waves, particularly slow waves, after discussing sausage-wave theory and the opacity effect.

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arXiv2021

Statistical properties of Hα jets in the polar coronal hole and their implications in coronal activities

Dynamic features, such as chromospheric jets, transition region network jets, coronal plumes and coronal jets, are abundant in the network regions of the solar polar coronal holes. We investigate the relationship between chromospheric jets and coronal activities (e.g., coronal plumes and jets).We analyze observations of a polar coronal hole including the filtergrams that were taken by the New Vacuum Solar Telescope (NVST) at the Hα-0.6 Åto study the Hα jets,and the Atmospheric Imaging Assembly (AIA) 171 Å images to follow the evolution of coronal activities. Hα jets are persistent in the network regions, only some regions (denoted as R1-R5) are rooted with discernible coronal plumes.With an automated method, we identify and track 1 320 Hα jets in the network regions. We find that the average lifetime, height and ascending speed of the Hα jets are 75.38 s, 2.67 Mm, 65.60 km s$^{-1}$, respectively. The Hα jets rooted in R1-R5 are higher and faster than those in the others. We also find that propagating disturbances (PDs) in coronal plumes have a close connection with the Hα jets. The speeds of 28 out of 29 Hα jets associated with PDs are about 50 km s$^{-1}$ . In a case of coronal jet, we find that the speeds of both the coronal jet and the Hα jet are over 150 km s$^{-1}$, suggesting that both cool and hot jets can be coupled together. Based on our analyses, it is evident that more dynamic Hα jets could release the energies to the corona, which might be the results of the development of Kelvin-Helmholtz instability (KHi) or small-scaled magnetic activities. We suggest that chromospheric jets, transition region network jets and ray-like features in the corona are coherent phenomena, and they are important tunnels for cycling energy and mass in the solar atmosphere.

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arXiv2021

Fast Sausage Oscillations in Coronal Loops with Fine Structures

Fast sausage modes (FSMs) in flare loops have long been invoked to account for rapid quasi-periodic pulsations (QPPs) with periods of order seconds in flare lightcurves. However, most theories of FSMs in solar coronal cylinders assume a perfectly axisymmetric equilibrium, an idealized configuration apparently far from reality. In particular, it remains to examine whether FSMs exist in coronal cylinders with fine structures. Working in the framework of ideal magnetohydrodynamics (MHD), we numerically follow the response to an axisymmetric perturbation of a coronal cylinder for which a considerable number of randomly distributed fine structures are superposed on an axisymmetric background. The parameters for the background component are largely motivated by the recent IRIS identification of a candidate FSM in Fe XXI 1354 Å~observations. We find that the composite cylinder rapidly settles to an oscillatory behavior largely compatible with a canonical trapped FSM. This happens despite that kink-like motions develop in the fine structures. We further synthesize the Fe XXI 1354 Å~emissions, finding that the transverse Alfvén time characterizes the periodicities in the intensity, Doppler shift, and Doppler width signals. Distinct from the case without fine structuring, a non-vanishing Doppler shift is seen even at the apex. We conclude that density-enhanced equilibria need not be strictly axisymmetric to host FSM-like motions in general, and FSMs remain a candidate interpretation for rapid QPPs in solar flares.

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arXiv2021

Resonant Damping of Kink Modes in Solar Coronal Slabs

We examine resonantly damped kink modes in straight coronal slabs, paying special attention to the effects of the formulation for the transverse density distribution ("profile"). We work in the framework of pressure-less, gravity-free, resistive magnetohydrodynamics, and we adopt the dissipative-eigenmode perspective. The density profile is restricted to be one-dimensional, but nonetheless allowed to take a generic form characterized by a continuous transition layer connecting a uniform interior to a uniform exterior. A dispersion relation (DR) is derived in the thin-boundary limit, yielding analytical expressions for the eigenfrequencies that generalize known results in various aspects. We find that the analytical rather than the numerical solutions to the thin-boundary DR serve better the purpose for validating our self-consistent resistive solutions. More importantly, the eigenfrequencies are found to be sensitive to profile specifications, the ratio of the imaginary to the real part readily varying by a factor of two when one profile is used in place of another. Our eigenmode computations are also examined in the context of impulsively excited kink waves, suggesting the importance of resonant absorption for sufficiently oblique components when the spatial scale of the exciter is comparable to the slab half-width.

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arXiv2021

The First 3D Coronal Loop Model Heated by MHD Waves against Radiative Losses

In the quest to solve the long-standing coronal heating problem, it has been suggested half a century ago that coronal loops could be heated by waves. Despite the accumulating observational evidence of the possible importance of coronal waves, still no 3D MHD simulations exist that show significant heating by MHD waves. Here we report on the first 3D coronal loop model heating the plasma against radiative cooling. The coronal loop is driven at the footpoint by transverse oscillations and subsequently the induced Kelvin-Helmholtz instability deforms the loop cross-section and generates small-scale structures. Wave energy is transfered to smaller scales where it is dissipated, overcoming the internal energy losses by radiation. These results open up a new avenue to address the coronal heating problem.

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arXiv2020

Coronal heating by MHD waves

The heating of the solar chromosphere and corona to the observed high temperatures, imply the presence of ongoing heating that balances the strong radiative and thermal conduction losses expected in the solar atmosphere. It has been theorized for decades that the required heating mechanisms of the chromospheric and coronal parts of the active regions, quiet-Sun, and coronal holes are associated with the solar magnetic fields. However, the exact physical process that transport and dissipate the magnetic energy which ultimately leads to the solar plasma heating are not yet fully understood. The current understanding of coronal heating relies on two main mechanism: reconnection and MHD waves that may have various degrees of importance in different coronal regions. In this review we focus on recent advances in our understanding of MHD wave heating mechanisms. First, we focus on giving an overview of observational results, where we show that different wave modes have been discovered in the corona in the last decade, many of which are associated with a significant energy flux, either generated in situ or pumped from the lower solar atmosphere. Afterwards, we summarise the recent findings of numerical modelling of waves, motivated by the observational results. Despite the advances, only 3D MHD models with Alfvén wave heating in an unstructured corona can explain the observed coronal temperatures compatible with the quiet Sun, while 3D MHD wave heating models including cross-field density structuring are not yet able to account for the heating of coronal loops in active regions to their observed temperature.

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arXiv2020

Kink Oscillations in Solar Coronal Loops with Elliptical Cross-Sections. I. the linear regime

The cross sections of solar coronal loops are suggested to be rarely circular. We examine linear kink oscillations in straight, density-enhanced, magnetic cylinders with elliptical cross-sections by solving the three-dimensional magnetohydrodynamic equations from an initial-value-problem perspective. Motivated by relevant eigen-mode analyses, we distinguish between two independent polarizations, one along the major axis (the M-modes) and the other along the minor one (the m-modes). We find that, as happens for coronal loops with circular cross-sections, the apparent damping of the transverse displacement of the loop axis is accompanied by the accumulation of transverse Alfvénic motions and the consequent development of small-scales therein, suggesting the robustness of the concepts of resonant absorption and phase-mixing. In addition, two stages can in general be told apart in the temporal evolution of the loop displacement; a Gaussian time dependence precedes an exponential one. For the two examined density ratios between loops and their surroundings, the periods of the M-modes (m-modes) tend to increase (decrease) with the major-to-minor-half-axis ratio, and the damping times in the exponential stage for the M-modes tend to exceed their m-mode counterparts. This is true for the two transverse profiles we examine. However, the relative magnitudes of the damping times in the exponential stage for different polarizations depend on the specification of the transverse profile and/or the density contrast. The applications of our numerical findings are discussed in the context of coronal seismology.

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arXiv2019

Wave heating in simulated multi-stranded coronal loops

It has been found that the Kelvin-Helmholtz instability (KHI) induced by both transverse and torsional oscillations in coronal loops can reinforce the effects of wave heating. In this study, we model a coronal loop as a system of individual strands, and we study wave heating effects by considering a combined transverse and torsional driver at the loop footpoint. We deposit the same energy into the multi-stranded loop and an equivalent monolithic loop, and then observe a faster increase in the internal energy and temperature in the multi-stranded model. Therefore, the multi-stranded model is more efficient in starting the heating process. Moreover, higher temperature is observed near the footpoint in the multi-stranded loop and near the apex in the monolithic loop. The apparent heating location in the multi-stranded loop agrees with the previous predictions and observations. Given the differences in the results from our multi-stranded loop and monolithic loop simulations, and given that coronal loops are suggested to be multi-stranded on both theoretical and observational grounds, our results suggest that the multi-strandedness of coronal loops needs to be incorporated in future wave-based heating mechanisms.

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arXiv2018

Heating effects from driven transverse and Alfvén waves in coronal loops

Recent numerical studies revealed that transverse motions of coronal loops can induce the Kelvin-Helmholtz Instability (KHI). This process could be important in coronal heating because it leads to dissipation of energy at small spatial-scale plasma interactions. Meanwhile, small amplitude decayless oscillations in coronal loops have been discovered recently in observations of SDO/AIA. We model such oscillations in coronal loops and study wave heating effects, considering a kink and Alfv{é}n driver separately and a mixed driver at the bottom of flux tubes. Both the transverse and Alfv{é}n oscillations can lead to the KHI. Meanwhile, the Alfv{é}n oscillations established in loops will experience phase mixing. Both processes will generate small spatial-scale structures, which can help the dissipation of wave energy. Indeed, we observe the increase of internal energy and temperature in loop regions. The heating is more pronounced for the simulation containing the mixed kink and Alfv{é}n driver. This means that the mixed wave modes can lead to a more efficient energy dissipation in the turbulent state of the plasma and that the KHI eddies act as an agent to dissipate energy in other wave modes. Furthermore, we also obtained forward modelling results using the FoMo code. We obtained forward models which are very similar to the observations of decayless oscillations. Due to the limited resolution of instruments, neither Alfv{é}n modes nor the fine structures are observable. Therefore, this numerical study shows that Alfv{é}n modes probably can co-exist with kink modes, leading to enhanced heating.

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