arXiv subjects

Guanchong Cheng

Publications and source records attributed to Guanchong Cheng.

13 recordsLinked to original sources

arXiv2026

Radiative MHD Studies of Solar Spicules: Driving Mechanisms and the Role of Background Magnetic Field

We conduct 2.5D radiative magnetohydrodynamic (MHD) simulations to investigate the driving mechanisms of the solar spicules in coronal holes and how the different background magnetic fields affect their formation. The simulation model includes the upper convection zone, photosphere, chromosphere, and lower corona. We run several cases with different background magnetic fields to primarily explore the effects of magnetic field strength and inclination on the characteristics of the formed spicules, such as the maximum height, lifetime, maximum upward velocity, and deceleration. According to the results, we find that a weaker background magnetic field tends to cause solar spicules that exhibit higher heights, longer lifetimes and faster maximum upward velocities. Additionally, when the background magnetic field inclines, the generated spicules incline accordingly; compared with those in a vertical magnetic field, the spicules in the inclined magnetic field are relatively longer, with smaller decelerations, longer lifetimes and lower maximum upward velocities. Meanwhile, by tracking and analyzing the formation processes of the spicules in two cases with different magnetic field strengths, we find that most of the spicules are mainly driven by shock waves induced by convective and turbulent motions around the solar surface, while less than one third of the spicules are primarily driven by high-velocity reconnection outflows. These results provide a more in-depth basis for the theoretical understanding of the driving mechanisms and formation processes of solar spicules.

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arXiv2026

Energy and mass transport associated with impulsive spicular flows in solar coronal holes

How the solar atmosphere is heated from a temperature of about $5,000-6,000$\,K in the lower atmosphere to about $1-2$\,MK in the corona has challenged the astrophysical community for about 80 years. The same puzzle exists for the stellar coronae heating as well. In this study, we present a series of findings on solar spicules and their subsequent impact on the corona within a coronal hole environment, characterized by locally open magnetic field lines, combining insights from MHD simulations with observations. We find that the convective and turbulent motions around the solar surface cause plenty of shocks and small-scale magnetic reconnection in the lower atmosphere. The combined effects of shock compression and reconnection outflows then drive the formation of groups of spicules with a quasi-period of about $300$\,s and width of $\sim 200-500$\,km. The spicule upflows provide an averaged mass flux above $10^{-9}$\,kg\,m$^{-2}$\,s$^{-1}$ in the lower corona to sustain the solar wind in coronal holes, and they continuously trigger further new local slow-mode waves and shocks. These waves supply an energy flux of $10-100$\,W\,m$^{-2}$ in the lower corona, and they are dissipated by heat conduction and compression heating to sustain the corona temperature of about $1$\,MK. The results also indicate that the upward propagating disturbances (PDs) observed in extreme ultraviolet (EUV) passbands are caused by both spicule upflows and slow-mode waves and shocks. Our findings help to understand the long standing problem of coronal heating and the origin of solar winds in coronal hole regions.

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arXiv2025

Coronal Mass Ejections Deflected by Newly Emerging Flux: A Combined Analytic and Numerical Study

Newly emerging flux (NEF) has been widely studied as a trigger of solar filament eruptions, but its influence on the subsequent dynamics remains poorly explored. Because NEF typically emerges adjacent to filaments, it imposes magnetic asymmetry that can drive non-radial eruptions and complicate space-weather forecasting. We bridge analytic catastrophe theory with 2D resistive MHD simulations: analytic solutions provide magnetic configurations containing a flux rope at the loss-of-equilibrium point, which are then used as initial conditions for simulations to examine the following dynamics. We find that NEF governs the kinematics of filament eruptions in two ways. First, by reshaping coronal stability, NEF can create or eliminate a higher equilibrium in corona, thereby producing failed eruptions or CMEs. In the transitional situation where a metastable equilibrium appears, the rising filament decelerates and stalls before re-accelerating into a CME, consistent with observed two-step eruptions. Second, by breaking symmetry, NEF deflects eruptions away from the radial direction: depending on its polarity, it acts as a repulsor or an attractor on eruptive filaments, and the deflection magnitude increases with the degree of asymmetry. Our theory yields two characteristic angles that predict the deflection directions of CMEs and failed eruptions, and simulations closely aligns with these predictors. These results highlight the NEF not only as a trigger but also as a key factor that governs both the acceleration and deflection of eruptions during their propagation in the low corona.

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arXiv2025

QFP Waves Driven by the Tuning-Fork Effect during Magnetic Reconnecion

Through three-dimensional MHD simulations, we have uncovered a kind of fast coronal wave originating from both ends of a current sheet (CS) during a solar eruption. These waves are observed to appear near the top and bottom ends of the reconnection-related CS. The simulations demonstrate the presence of termination shock regions above the two ends of the CS. As the reconnection outflows escape from the vertical CS and encounter these termination shocks, they undergo partial reflection, redirecting towards the CS terminal fork walls. The identified waves propagate rapidly at a speed of approximately 1400 km/s with a period of just 2 s. Concurrently, the time-evolution of intensity within a small region of the CS terminal fork structures, exhibits a similar oscillation period of 2 s. All these evidence supports the notion that these QFP (Quasi-periodic Fast-Propagating) waves were excited by tuning fork effects within the CS system. Essentially, the rapid reconnection outflows are reflected by the terminal shocks, striking the fork walls at the CS ends. Moreover, parts of the oscillations along the tuning fork handle are transformed into thermal energy, accumulating in the CS center and elevating the temperature. This is the first time to report such QFP waves resulting from tuning fork effects within the CS during a solar eruption. These waves are anticipated to manifest closely following the propagation of CMEs and adjacent to the related post-flare loops in observations, with partial confirmation in current observations.

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arXiv2025

Three dimensional magnetic reconnection mediated with plasmoids and the resulted multi-thermal emissions in the cool atmosphere of the Sun

Flux emergence is ubiquitous in the Sun's lower atmosphere, where the emerging magnetic flux can reconnect with the pre-existing magnetic field. We investigate plasmoid formation and the resulting multi-thermal emissions during three-dimensional magnetic reconnection in the lower solar atmosphere. We performed 3D radiation magnetohydrodynamic simulations using the MURaM code, which incorporates solar convection and radiative transfer. A flat magnetic flux sheet was introduced into the convection zone to trigger flux emergence. For comparison with previous observations, we used the RH1.5D code to synthesize H{\alpha} and Si IV spectral line profiles, and generated ultraviolet images using the optically thin approximation. The simulations show that flux emergence occurs as the imposed flux tube crosses the photosphere. In the lower solar atmosphere, magnetic reconnection forms thin, elongated current sheets, and plasmoid-like structures develop, producing numerous small twisted magnetic flux ropes that are expelled toward both ends of the reconnection region. This process results in the coexistence of hot plasma exceeding 20,000 K and cooler plasma below 10,000 K. Synthetic images and spectral line profiles through the reconnection region exhibit features characteristic of Ellerman bombs (EBs) and UV bursts. Cooler plasma associated with EBs can be found above hot plasma at altitudes exceeding 2 Mm above the solar surface, while hot plasma associated with UV bursts can extend downward into the lower chromosphere, reaching approximately 0.7 Mm above the surface. These results indicate that turbulent reconnection mediated by plasmoid instability can occur in small-scale events such as EBs and UV bursts, and that the coexistence of hot and cool plasma in such reconnection processes can account for UV bursts that are temporally and spatially connected to EBs.

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arXiv2025

The effect of plasma-$\beta$ on the heating mechanisms during magnetic reconnection in partially ionized low solar atmosphere

We performed numerical simulations of magnetic reconnection with different strength of magnetic fields from the solar photosphere to the upper chromosphere. The main emphasis is to identify dominant mechanisms for heating plasmas in the reconnection region under different plasma-$\beta$ conditions in the partially ionized low solar atmosphere. The numerical results show that more plasmoids are generated in a lower $\beta$ reconnection event. The frequent coalescence of these plasmoids leads to a significant enhancement of turbulence and compression heating, which becomes the dominant mechanism for heating plasma in a lower plasma-$\beta$ reconnection process. The average power density of the compression heating (Q$_{comp}$) decreases with increasing initial plasma-$\beta$ as a power function: Q$_{comp} \sim \beta_{0}^{-a}$, where the value $a$ is $1.9$ in the photosphere and decreases to about 1.29 in the upper chromosphere. In the photosphere and lower chromosphere, the joule heating contributed by electron-neutral collisions Q$_{en}=\eta_{en} J^2$ eventually dominates over the compression heating when the initial plasma-$\beta$ is larger than the critical value $\beta_{0-critical} = 8$. In the upper chromosphere, the ambipolar diffusion heating and the viscous heating will become equally important as the compression heating when the initial plasma-$\beta$ is larger than the critical value $\beta_{0-critical} = 0.5$. These results indicate that the compression heating caused by turbulent reconnection mediated with plasmoids is likely the major heating mechanism for the small-scale reconnection events with stronger magnetic fields such as active region EBs and UV bursts. However, the heating caused by the partial ionization effects can not be ignored for those reconnection events with weaker magnetic fields such as quiet Sun EBs and cold surges.

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arXiv2025

Numerical simulation of oscillatory magnetic reconnection modulated by solar convective motions

Oscillatory magnetic reconnection is a periodic magnetic reconnection process, during which the current sheet's orientation and the magnetic connections change periodically. This periodic variation is generally considered to originate from the magnetic reconnection itself rather than from external driving processes. We conduct 2.5-dimensional radiative magnetohydrodynamic simulations to investigate the emergence of a magnetic flux tube from the convection zone into the lower corona, where the emerging magnetic fields reconnect with background ones. During the reconnection process within 5771 s, the current sheet's orientation has been reversed 41 times, corresponding to 40 oscillation periods. Notably, the longest period is 30 minutes, which is consistent with the previous observational results. We find that the main factor leading to the reversal of the current sheet's orientation is the quasi-periodic external force provided by the emergence of plasma and magnetic fields from the convection zone. We also find the shifting of the upward outflows from the reconnection region along the horizontal direction due to the alternating changes of the reconnection inflow and outflow regions. In addition to the quasi-periodic change of the current sheet orientation, the reconnection rate at the main X-point also oscillates with a period between 100-400 s, which corresponds to the period of p-mode oscillations.

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arXiv2024

Theoretical Studies on the Evolution of Solar Filaments in Response to New Emerging Flux

New emerging flux (NEF) has long been considered a mechanism for solar eruptions, but detailed process remains an open question. In this work, we explore how NEF drives a coronal magnetic configuration to erupt. This configuration is created by two magnetic sources of strengths $M$ and $S$ embedded in the photosphere, one electric-current-carrying flux rope (FR) floating in the corona, and an electric current induced on the photospheric surface by the FR. The source $M$ is fixed accounting for the initial background field, and $S$ changes playing the role of NEF. We introduce the channel function $C$ to forecast the overall evolutionary behavior of the configuration. Location, polarity, and strength of NEF governs the evolutionary behavior of FR before eruption. In the case of $|S/M|<1$ with reconnection occur between new and old fields, the configuration in equilibrium evolves to the critical state, invoking the catastrophe. In this case, if polarities of the new and old fields are opposite, reconnection occurs as NEF is close to FR; and if polarities are the same, reconnection happens as NEF appears far from FR. With different combinations of the relative polarity and the location, the evolutionary behavior of the system gets complex, and the catastrophe may not occur. If $|S/M|>1$ and the two fields have opposite polarity, the catastrophe always takes place; but if the polarities are the same, catastrophe occurs only as NEF is located far from FR; otherwise, the evolution ends up either with failed eruption or without catastrophe at all.

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arXiv2024

Evidence for Plasmoid-mediated Magnetic Reconnection during a Small-scale Flare in the Partially Ionized Low Solar Atmosphere

Magnetic reconnection plays a crucial role in the energy release process for different kinds of solar eruptions and activities. The rapid solar eruption requires a fast reconnection model. Plasmoid instability in the reconnecting current sheets is one of the most acceptable fast reconnection mechanisms for explaining the explosive events in the magnetohydrodynamics (MHD) scale, which is also a potential bridge between the macroscopic MHD reconnection process and microscale dissipations. Plenty of high resolution observations indicate that the plasmoid-like structures exist in the high temperature solar corona, but such evidences are very rare in the lower solar atmosphere with partially ionized plasmas. Utilizing joint observations from the Goode Solar Telescope (GST) and the Solar Dynamics Observatory (SDO), we discovered a small-scale eruptive phenomenon in NOAA AR 13085, characterized by clear reconnection cusp structures, supported by Nonlinear Force-Free Field (NLFFF) extrapolation results. The plasmoid-like structures with a size about 150 km were observed to be ejected downward from the current sheet at a maximum velocity of 24 km$\cdot$s$^{-1}$ in the H$\alpha$ line wing images, followed by enhanced emissions at around the post flare loop region in multiple wave lengths. Our 2.5D high-resolution MHD simulations further reproduced such a phenomenon and revealed reconnection fine structures. These results provide comprehensive evidences for the plasmoid mediated reconnection in partially ionized plasmas, and suggest an unified reconnection model for solar flares with different length scales from the lower chromosphere to corona.

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arXiv2024

A magnetic reconnection model for the hot explosion with both ultraviolet and H{\alpha} wing emissions

Ellerman bombs (EBs) with significant H$\alpha$ wing emissions and ultraviolet bursts (UV bursts) with strong Si IV emissions are two kinds of small transient brightening events that occur in the low solar atmosphere.We numerically investigated the magnetic reconnection process between the emerging arch magnetic field and the lower atmospheric background magnetic field. We aim to find out if the hot UV emissions and much colder H$\alpha$ wing emissions can both appear in the same reconnection process and how they are located in the reconnection region. The open-source code NIRVANA was applied to perform the 2.5D magnetohydrodynamic (MHD) simulation. We developed the related sub-codes to include the more realistic radiative cooling process for the photosphere and chromosphere and the time-dependent ionization degree of hydrogen. The initial background magnetic field is 600 G, and the emerged magnetic field in the solar atmosphere is of the same magnitude, meaning that it results in a low- $\beta$ magnetic reconnection environment. We also used the radiative transfer code RH1.5D to synthesize the Si IV and H$\alpha$ spectral line profiles based on the MHD simulation results. Magnetic reconnection between emerged and background magnetic fields creates a thin, curved current sheet, which then leads to the formation of plasmoid instability and the nonuniform density distributions. The mix of hot tenuous and much cooler dense plasmas in the turbulent reconnection region can appear at about the same height, or even in the same plasmoid. The turbulent current sheet is always in a dense plasma environment with an optical depth larger than 6.5$\times$10$^{-5}$ due to the emerged magnetic field pushing high-density plasmas upward.

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arXiv2023

Can the Parker Solar Probe Detect a CME-flare Current Sheet?

A current sheet (CS) is the central structure in the disrupting magnetic configuration during solar eruptions. More than 90\% of the free magnetic energy (the difference between the energy in the non-potential magnetic field and that in the potential one) stored in the coronal magnetic field beforehand is converted into heating and kinetic energy of the plasma, as well as accelerating charged particles, by magnetic reconnection occurring in the CS. However, the detailed physical properties and fine structures of the CS are still unknown since there is no relevant information obtained via in situ detections. The Parker Solar Probe (PSP) may provide us such information should it traverse a CS in the eruption. The perihelion of PSP's final orbit is located at about 10 solar radii from the center of the Sun, so it can observe the CS at a very close distance, or even traverses the CS, which provides us a unique opportunity to look into fine properties and structures of the CS, helping reveal the detailed physics of large-scale reconnection that was impossible before. We evaluate the probability that PSP can traverse a CS, and examine the orbit of a PSP-like spacecraft that has the highest probability to traverse a CS.

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arXiv2022

Plausibility of ultraviolet burst generation in the low solar chromosphere

Ultraviolet (UV) bursts and Ellerman bombs (EBs) are small-scale magnetic reconnection events taking place in the highly stratified, low solar atmosphere. It is still not clear whether UV bursts have to be generated at a higher atmospheric layer than EBs or whether both UV bursts and EBs can occur in the low chromosphere. We numerically studied the low $β$ magnetic reconnection process around the solar temperature minimum region (TMR). The time-dependent ionization degrees of hydrogen and helium are included in the MHD code, which lead to a more realistic magnetic diffusion caused by electron-neutral collision and ambipolar diffusion. A more realistic radiative cooling model from Carlsson & Leenaarts 2012 is included in the simulations. Our results in high resolution indicate that the plasmas in the reconnection region are heated up to more than $20,000$ K if the reconnecting magnetic field is as strong as $500$ G, which suggests that UV bursts can be generated in the dense low chromosphere. The dominant mechanism for producing the UV burst in the low chromosphere is heating, as a result of the local compression in the reconnection process. The thermal energy occurring in the reconnection region rapidly increases after the turbulent reconnection mediated by plasmoids is invoked. The average power density of the generated thermal energy in the reconnection region can reach over $1000$ erg cm$^{-3}$ s$^{-1}$, which is comparable to the average power density accounting for a UV burst. With the strength of the reconnecting magnetic field exceeding $900$ G, the width of the synthesized Si IV 1394 A line profile with multiple peaks can reach up to $100$ km s$^{-1}$, which is consistent with observations.

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arXiv2021

The Ellerman bomb and Ultraviolet burst triggered successively by an emerging magnetic flux rope

Ellerman bombs (EBs) and Ultraviolet (UV) bursts are common brightening phenomena which are usually generated in the low solar atmosphere of emerging flux regions. In this paper, we have investigated the emergence of an initial un-twisted magnetic flux rope based on three-dimensional (3D) magneto-hydrodynamic (MHD) simulations. The EB-like and UV burst-like activities successively appear in the U-shaped part of the undulating magnetic fields triggered by Parker Instability. The EB-like activity starts to appear earlier and lasts for about 80 seconds. Six minutes later, a much hotter UV burst-like event starts to appear and lasts for about 60 seconds. Along the direction vertical to the solar surface, both the EB and UV burst start in the low chromosphere, but the UV burst extends to a higher altitude in the up chromosphere. The regions with apparent temperature increase in the EB and UV burst are both located inside the small twisted flux ropes generated in magnetic reconnection processes, which are consistent with the previous 2D simulations that most hot regions are usually located inside the magnetic islands. However, the twisted flux rope corresponding to the EB is only strongly heated after it floats up to an altitude much higher than the reconnection site during that period. Our analyses show that the EB is heated by the shocks driven by the strong horizontal flows at two sides of the U-shaped magnetic fields. The twisted flux rope corresponding to the UV burst is heated by the driven magnetic reconnection process.

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