arXiv subjects

Yi-Hsin Liu

Publications and source records attributed to Yi-Hsin Liu.

At least 37 records · Page 2Linked to original sources

arXiv2021

The Acceleration of Charged Particles and Formation of Power-law Energy Spectra in Nonrelativistic Magnetic Reconnection

Magnetic reconnection is a primary driver of particle acceleration processes in space and astrophysical plasmas. Understanding how particles are accelerated and the resulting particle energy spectra is among the central topics in reconnection studies. We review recent advances in addressing this problem in nonrelativistic reconnection that is relevant to space and solar plasmas and beyond. We focus on particle acceleration mechanisms, particle transport due to 3D reconnection physics, and their roles in forming power-law particle energy spectra. We conclude by pointing out the challenges in studying particle acceleration and transport in a large-scale reconnection layer and the relevant issues to be addressed in the future.

physics.plasm-ph↗

arXiv2021

The Effect of Thermal Pressure on Collisionless Magnetic Reconnection Rate

Modeling collisionless magnetic reconnection rate is an outstanding challenge in basic plasma physics research. While the seemingly universal rate of an order $\mathcal{O}(0.1)$ is often reported in the low-$β$ regime, it is not clear how reconnection rate scales with a higher plasma $β$. Due to the complexity of the pressure tensor, the available reconnection rate model is limited to the low plasma-$β$ regime, where the thermal pressure is arguably negligible. However, the thermal pressure effect becomes important when $β\gtrsim \mathcal{O}(1)$. Using first-principle kinetic simulations, we show that both the reconnection rate and outflow speed drop as $β$ gets larger. A simple analytical framework is derived to take account of the self-generated pressure anisotropy and pressure gradient in the force-balance around the diffusion region, explaining the varying trend of key quantities and reconnection rates in these simulations with different $β$. The predicted scaling of the normalized reconnection rate is $\simeq \mathcal{O}(0.1/\sqrt{β_{i0}})$ in the high $β$ limit, where $β_{i0}$ is the ion $β$ of the inflow plasma.

astro-ph.SR↗

arXiv2020

Radiation and Polarization Signatures from Magnetic Reconnection in Relativistic Jets--I. A Systematic Study

Blazars are relativistic magnetized plasma outflows from supermassive black holes that point very close to our line of sight. Their emission is nonthermal dominated and highly variable across the entire electromagnetic spectrum. Relativistic magnetic reconnection has been proposed as the driver of particle acceleration during blazar flares. While recent particle-in-cell simulations have self-consistently studied the evolution of magnetic reconnection and particle acceleration therein, the resulting radiation signatures have not been systematically explored. In particular, the polarization signatures, which directly reflect the characteristic strongly dynamical magnetic field evolution during reconnection, have not been carefully investigated. In this paper, we present a systematic study of radiation and polarization signatures arising from magnetic reconnection in blazars, based on combined PIC and polarized radiation transfer simulations with various physical parameters. We identify a harder-when-brighter trend in the spectral evolution. Moreover, higher-frequency bands tend to flare earlier than lower-frequency bands in the synchrotron spectral component. Most importantly, polarization signatures appear more variable with higher frequencies. We find that the temporal polarization variations strongly depends on the guide field strength. Specifically, reconnection with significant guide field component leads to very high polarization degree that contradict to typical blazar observations, while large polarization angle rotations are unique signatures of magnetic reconnection between nearly anti-parallel magnetic field lines. These rotations are at least $90^o$ and can extend to $>180^o$, and they may rotate in both directions. These results imply that blazars that have shown large polarization angle rotations intrinsically have more nearly anti-parallel magnetic field morphology.

astro-ph.HE↗

arXiv2020

A new Look at the Electron Diffusion Region in Asymmetric Magnetic Reconnection

A new look at the structure of the electron diffusion region in collisionless magnetic reconnection is presented. The research is based on a particle-in-cell simulation of asymmetric magnetic reconnection, which include a temperature gradient across the current layer in addition to density and magnetic field gradient. We find that none of X-point, flow stagnation point, and local current density peak coincide. Current and energy balance analyses around the flow stagnation point and current density peak show consistently that current dissipation is associated with the divergence of nongyrotropic electron pressure. Furthermore, the same pressure terms, when combined with shear-type gradients of the electron flow velocity, also serve to maintain local thermal energy against convective losses. These effects are similar to those found also in symmetric magnetic reconnection. In addition, we find here significant effects related to the convection of current, which we can relate to a generalized diamagnetic drift by the nongyrotropic pressure divergence. Therefore, only part of the pressure force serves to dissipate the current density. However, the prior conclusion that the role of the reconnection electric field is to maintain the current density, which was obtained for a symmetric system, applies here as well. Finally, we discuss related features of electron distribution function in the EDR.

physics.space-ph↗

arXiv2020

Recent Progress on Particle Acceleration and Reconnection Physics during Magnetic Reconnectionin the Magnetically-dominated Relativistic Regime

Magnetic reconnection in strongly magnetized astrophysical plasma environments is believed to be the primary process for fast energy release and particle energization. Currently there is strong interest in relativistic magnetic reconnection, in that it may provide a new explanation for high-energy particle acceleration and radiation in strongly magnetized astrophysical systems. We review recent advances in particle acceleration and reconnection physics in the magnetically-dominated regime. More discussion is focused on the physics of particle acceleration, power-law formation as well as the reconnection rate problem. In addition, we provide an outlook for studying reconnection acceleration mechanisms and kinetic physics in the next step.

astro-ph.HE↗

arXiv2019

Scaling of magnetic reconnection with a limited x-line extent

Contrary to all the 2D models, where the reconnection x-line extent is infinitely long, we study magnetic reconnection in the opposite limit. The scaling of the average reconnection rate and outflow speed are modeled as a function of the x-line extent. An internal x-line asymmetry along the current direction develops because of the flux transport by electrons beneath the ion kinetic scale, and it plays an important role in suppressing reconnection in the short x-line limit; the average reconnection rate drops because of the limited active region, and the outflow speed reduction is associated with the reduction of the $J \times B$ force, that is caused by the phase shift between the J and B profiles, also as a consequence of this flux transport.

physics.space-ph↗

arXiv2019

The Critical Role of Collisionless Plasma Energization on the Structure of Relativistic Magnetic Reconnection

During magnetically dominated relativistic reconnection, inflowing plasma depletes the initial relativistic pressure at the x-line and collisionless plasma heating inside the diffusion region is insufficient to overcome this loss. The resulting pressure drop causes a collapse at the x-line, essentially a localization mechanism of the diffusion region necessary for fast reconnection. The extension of this low-pressure region further explains the bursty nature of anti-parallel reconnection because a once opened outflow exhaust can also collapse, which repeatedly triggers secondary tearing islands. However, a stable single x-line reconnection can be achieved when an external guide field exists, since the reconnecting magnetic field component rotates out of the reconnection plane at outflows, providing additional magnetic pressure to sustain the opened exhausts.

astro-ph.HE↗

arXiv2019

Collisionless energy transfer in kinetic turbulence: field-particle correlations in Fourier space

Turbulence is ubiquitously observed in nearly collisionless heliospheric plasmas, including the solar wind and corona and the Earth's magnetosphere. Understanding the collisionless mechanisms responsible for the energy transfer from the turbulent fluctuations to the particles is a frontier in kinetic turbulence research. Collisionless energy transfer from the turbulence to the particles can take place reversibly, resulting in non-thermal energy in the particle velocity distribution functions (VDFs) before eventual collisional thermalization is realized. Exploiting the information contained in the fluctuations in the VDFs is valuable. Here we apply a recently developed method based on VDFs, the field-particle correlation technique, to a $β=1$, solar-wind-like, low-frequency Alfvénic turbulence simulation with well resolved phase space to identify the field-particle energy transfer in velocity space. The field-particle correlations reveal that the energy transfer, mediated by the parallel electric field, results in significant structuring of the ion and electron VDFs in the direction parallel to the magnetic field. Fourier modes representing the length scales between the ion and electron gyroradii show that energy transfer is resonant in nature, localized in velocity space to the Landau resonances for each Fourier mode. The energy transfer closely follows the Landau resonant velocities with varying perpendicular wavenumber $k_\perp$ and plasma $β$. This resonant signature, consistent with Landau damping, is observed in all diagnosed Fourier modes that cover the dissipation range of the simulation.

physics.plasm-ph↗

arXiv2019

Three-Dimensional X-line Spreading in Asymmetric Magnetic Reconnection

The spreading of the X-line out of the reconnection plane under a strong guide field is investigated using large-scale three-dimensional (3D) particle-in-cell (PIC) simulations in asymmetric magnetic reconnection. A simulation with a thick, ion-scale equilibrium current sheet (CS) reveals that the X-line spreads at the ambient ion/electron drift speeds, significantly slower than the Alfvén speed based on the guide field $V_{Ag}$. Additional simulations with a thinner, sub-ion-scale CS show that the X-line spreads at $V_{Ag}$ (Alfvénic spreading), much higher than the ambient species drifts. An Alfvénic signal consistent with kinetic Alfvén waves develops and propagates, leading to CS thinning and extending, which then results in reconnection onset. The continuous onset of reconnection in the signal propagation direction manifests as Alfvénic X-line spreading. The strong dependence on the CS thickness of the spreading speeds, and the X-line orientation are consistent with the collisionless tearing instability. Our simulations indicate that when the collisionless tearing growth is sufficiently strong in a thinner CS such that $γ/Ω_{ci}\gtrsim\mathcal{O}(1)$, Alfvénic X-line spreading can take place. Our results compare favorably with a number of numerical simulations and recent magnetopause observations. A key implications is that the magnetopause CS is typically too thick for Alfvénic X-line spreading to effectively take place.

physics.space-ph↗

arXiv2019

Determining the Dominant Acceleration Mechanism during Relativistic Magnetic Reconnection in Large-scale Systems

While a growing body of research indicates that relativistic magnetic reconnection is a prodigious source of particle acceleration in high-energy astrophysical systems, the dominant acceleration mechanism remains controversial. Using a combination of fully kinetic simulations and theoretical analysis, we demonstrate that Fermi-type acceleration within the large-scale motional electric fields dominates over direct acceleration from non-ideal electric fields within small-scale diffusion regions. This result has profound implications for modeling particle acceleration in large-scale astrophysical problems, since it opens up the possiblity of modeling the energetic spectra without resolving microscopic diffusion regions.

astro-ph.HE↗

arXiv2019

3D Magnetic Reconnection with a spatially confined X-line extent -- Implications for Dipolarizing Flux Bundles and the Dawn-Dusk Asymmetry

Using 3D particle-in-cell (PIC) simulations, we study magnetic reconnection with the x-line being spatially confined in the current direction. We include thick current layers to prevent reconnection at two ends of a thin current sheet that has a thickness on an ion inertial (di) scale. The reconnection rate and outflow speed drop significantly when the extent of the thin current sheet in the current direction is < O(10 di). When the thin current sheet extent is long enough, we find it consists of two distinct regions; an inactive region (on the ion-drifting side) exists adjacent to the active region where reconnection proceeds normally as in a 2D case. The extent of this inactive region is ~ O(10 di), and it suppresses reconnection when the thin current sheet extent is comparable or shorter. The time-scale of current sheet thinning toward fast reconnection can be translated into the spatial-scale of this inactive region; because electron drifts inside the ion diffusion region transport the reconnected magnetic flux, that drives outflows and furthers the current sheet thinning, away from this region. This is a consequence of the Hall effect in 3D. While this inactive region may explain the shortest possible azimuthal extent of dipolarizing flux bundles at Earth, it may also explain the dawn-dusk asymmetry observed at the magnetotail of Mercury, that has a global dawn-dusk extent much shorter than that of Earth.

physics.plasm-ph↗

arXiv2018

Strongly localized magnetic reconnection by the super-Alfvenic shear flow

We demonstrate the dragging of the magnetic field by the super-Alfvenic shear flows out of the reconnection plane can strongly localize the reconnection x-line in collisionless plasmas, reversing the current direction at the x-line. Reconnection with this new morphology, which is impossible in resistive-magnetohydrodynamic (MHD), is enabled by electron inertia. Surprisingly, the quasi-steady reconnection rate remains of order 0.1 even though the aspect ratio of the local x-line geometry is larger than unity. We explain this by examining the transport of the reconnected magnetic flux and the opening angle made by the upstream magnetic field, concluding that the reconnection rate is still limited by the constraint imposed at the inflow region. This study further suggests the nearly universal fast rate value of order 0.1 cannot be explained by the physics of tearing modes, nor can it be explained by a universal localization mechanism.

physics.plasm-ph↗

arXiv2018

Orientation and stability of asymmetric magnetic reconnection x-line

The orientation and stability of the reconnection x-line in asymmetric geometry is studied using three-dimensional (3D) particle-in-cell simulations. We initiate reconnection at the center of a large simulation domain to minimize the boundary effect. The resulting x-line has sufficient freedom to develop along an optimal orientation, and it remains laminar. Companion 2D simulations indicate that this x-line orientation maximizes the reconnection rate. The divergence of the non-gyrotropic pressure tensor breaks the frozen-in condition, consistent with its 2D counterpart. We then design 3D simulations with one dimension being short to fix the x-line orientation, but long enough to allow the growth of the fastest growing oblique tearing modes. This numerical experiment suggests that reconnection tends to radiate secondary oblique tearing modes if it is externally (globally) forced to proceed along an orientation not favored by the local physics. The development of oblique structure easily leads to turbulence inside small periodic systems.

physics.space-ph↗

arXiv2018

Drift Turbulence, Particle Transport, and Anomalous Dissipation at the Reconnecting Magnetopause

Using fully kinetic 3D simulations, the reconnection dynamics of asymmetric current sheets are examined at the Earth's magnetopause. The plasma parameters are selected to model MMS magnetopause diffusion region crossings with guide fields of 0.1, 0.4, and 1 of the reconnecting magnetosheath field. In each case, strong drift-wave fluctuations are observed in the lower-hybrid frequency range at the steep density gradient across the magnetospheric separatrix. These fluctuations give rise to cross-field electron particle transport. In addition, this turbulent mixing leads to significantly enhanced electron parallel heating in comparison to 2D simulations. We study three different methods of quantifying the anomalous dissipation produced by the drift fluctuations, based on spatial averaging, temporal averaging, and temporal averaging followed by integrating along magnetic field lines. Comparison of the different methods reveals complications in identifying and measuring the anomalous dissipation. Nevertheless, the anomalous dissipation from short wavelength drift fluctuations appears weak for each case, and the reconnection rates observed in 3D are nearly the same as in 2D models. The 3D simulations feature a number of interesting new features that are consistent with recent MMS observations, including cold beams of magnetosheath electrons that penetrate into the hotter magnetospheric inflow, the related observation of decreasing temperature in regions of increasing total density, and an effective turbulent diffusion coefficient that agrees with predictions from quasi-linear theory.

physics.plasm-ph↗

arXiv2017

On the collisionless asymmetric magnetic reconnection rate

A prediction of the steady-state reconnection electric field in asymmetric reconnection is obtained by maximizing the reconnection rate as a function of the opening angle made by the upstream magnetic field on the weak magnetic field (magnetosheath) side. The prediction is within a factor of two of the widely examined asymmetric reconnection model [Cassak and Shay, Phys. Plasmas 14, 102114, 2007] in the collisionless limit, and they scale the same over a wide parameter regime. The previous model had the effective aspect ratio of the diffusion region as a free parameter, which simulations and observations suggest is on the order of 0.1, but the present model has no free parameters. In conjunction with the symmetric case [Liu et al., Phys. Rev. Lett. 118, 085101, 2017], this work further suggests that this nearly universal number 0.1, essentially the normalized fast reconnection rate, is a geometrical factor arising from maximizing the reconnection rate within magnetohydrodynamic (MHD)-scale constraints.

physics.plasm-ph↗

arXiv2016

Why does steady-state magnetic reconnection have a maximum local rate of order 0.1?

Simulations suggest collisionless steady-state magnetic reconnection of Harris-type current sheets proceeds with a rate of order 0.1, independent of dissipation mechanism. We argue this long-standing puzzle is a result of constraints at the magnetohydrodynamic (MHD) scale. We perform a scaling analysis of the reconnection rate as a function of the opening angle made by the upstream magnetic fields, finding a maximum reconnection rate close to 0.2. The predictions compare favorably to particle-in-cell simulations of relativistic electron-positron and non-relativistic electron-proton reconnection. The fact that simulated reconnection rates are close to the predicted maximum suggests reconnection proceeds near the most efficient state allowed at the MHD-scale. The rate near the maximum is relatively insensitive to the opening angle, potentially explaining why reconnection has a similar fast rate in differing models.

physics.plasm-ph↗

arXiv2016

A model of global magnetic reconnection rate in relativistic collisionless plasmas

A model of global magnetic reconnection rate in relativistic collisionless plasmas is developed and validated by the fully kinetic simulation. Through considering the force balance at the upstream and downstream of the diffusion region, we show that the global rate is bounded by a value $\sim 0.3$ even when the local rate goes up to $\sim O(1)$ and the local inflow speed approaches the speed of light in strongly magnetized plasmas. The derived model is general and can be applied to magnetic reconnection under widely different circumstances.

astro-ph.HE↗

arXiv2016

Particle Acceleration during Magnetic Reconnection in a Low-beta Pair Plasma

Plasma energization through magnetic reconnection in the magnetically-dominated regime featured by low plasma beta ($β= 8 πnkT_0/B^2 \ll 1$) and/or high magnetization ($σ= B^2/(4 πnmc^2) \gg 1$) is important in a series of astrophysical systems such as solar flares, pulsar wind nebula, and relativistic jets from black holes, etc. In this paper, we review the recent progress on kinetic simulations of this process and further discuss plasma dynamics and particle acceleration in a low-$β$ reconnection layer that consists of electron-positron pairs. We also examine the effect of different initial thermal temperatures on the resulting particle energy spectra. While earlier papers have concluded that the spectral index is smaller for higher $σ$, our simulations show that the spectral index approaches $p=1$ for sufficiently low plasma $β$, even if $σ\sim 1$. Since this predicted spectral index in the idealized limit is harder than most observations, it is important to consider effects that can lead to a softer spectrum such as open boundary simulations. We also remark that the effects of 3D reconnection physics and turbulence on reconnection need to be addressed in the future.

astro-ph.HE↗