Ion and electron motions in the outer electron diffusion region of collisionless magnetic reconnection

Two-dimensional particle-in-cell simulations are performed to study the coupling between ion and electron motions in collisionless magnetic reconnection.The electron diffusion region(EDR),where the electron motions are demagnetized,is found to have a two-layer structure:an inner EDR near the reconnection site and an outer EDR that is elongated to nearly 10 ion inertial lengths in the outflow direction.In the inner EDR,the speed of the electron outflow increases when the electrons move away from the X line.In the outer EDR,the speed of the electron outflow first increases and then decreases until the electrons reach the boundary of the outer EDR.In the boundary of the outer EDR,the magnetic field piles up and forms a depolarization front.From the perspective of the fluid,a force analysis on the formation of electron and ion outflows has also been investigated.Around the X line,the electrons are accelerated by the reconnection electric field in the out-of-plane direction.When the electrons move away from the X line,we find that the Lorentz force converts the direction of the accelerated electrons to the x direction,forming an electron outflow.Both electric field forces and electron gradient forces tend to drag the electron outflow.Ion acceleration along the x direction is caused by the Lorentz force,whereas the pressure gradient force tends to decelerate the ion outflow.Although these two terms are important,their effects on ions are almost offset.The Hall electric field force does positive work on ions and is not negligible.The ions are continuously accelerated,and the ion and electron outflow velocities are almost the same near the depolarization front.

Ion dynamics in laser-produced collisionless perpendicular shock: one-dimensional particle-in-cell simulation

Recently, perpendicular shocks have been generated in laboratory experiments by the interaction between a laser-produced supersonic plasma flow and a magnetized ambient plasma. Here, we explore the ion dynamics and the formation of such kinds of shock with a one-dimensional(1D)particle-in-cell simulation model using achievable parameters for laser experiments. A small part of the ambient ions is first reflected by the laser-driven piston. These piston-reflected ions interact with the upstream plasma and form a shock then. By analyzing the contribution of the electric force and the Lorentz force during the reflection, shock-reflected ions are found to be accelerated by two different mechanisms: shock drift acceleration and shock surfing acceleration,where shock drift acceleration is the dominant one. Very few ions are reflected twice by the shock and accelerated to a large velocity, implying that a more energetic population of ions can be observed in future experiments.

The effect of the guide field on energy conversion during collisionless magnetic reconnection

Magnetic reconnection is well known as an efficient mechanism for transferring magnetic energy into plasma energy.However,how the energy conversion and partition between different species is influenced by the shear angle of the reconnecting magnetic component(i.e.,the guide field strength)is not well understood.Using 2.5-dimensional particle-in-cell simulations,we investigated the energy conversion in reconnection with different guide fields.We found that the overall energy conversion first decreases steeply and then increases slowly when the guide field increases fromB_(g)=0 toB_(g)=4.The increase in energy conversion in the large guide field regime is due to the electron energy gain through the perpendicular channelJ_(⊥)·E_(⊥).The overall energy conversion is predominantly contributed byJ_(⊥)·E_(⊥) rather thanJ||E||.We further find that energy conversion mainly occurs within the reconnection front and the flux pileup region.However,the contribution from the fore reconnection front becomes important in large guide field regimes(3<B_(g)≤4)because of the enhanced electron energy gain.

Collisionless magnetic reconnection in the magnetosphere

Magnetic reconnection underlies the physical mechanism of explosive phenomena in the solar atmosphere and planetary magnetospheres, where plasma is usually collisionless. In the standard model of collisionless magnetic reconnection,the diffusion region consists of two substructures: an electron diffusion region is embedded in an ion diffusion region,in which their scales are based on the electron and ion inertial lengths. In the ion diffusion region, ions are unfrozen in the magnetic fields while electrons are magnetized. The resulted Hall effect from the different motions between ions and electrons leads to the production of the in-plane currents, and then generates the quadrupolar structure of out-of-plane magnetic field. In the electron diffusion region, even electrons become unfrozen in the magnetic fields, and the reconnection electric field is contributed by the off-diagonal electron pressure terms in the generalized Ohm’s law. The reconnection rate is insensitive to the specific mechanism to break the frozen-in condition, and is on the order of 0.1. In recent years, the launching of Cluster, THEMIS, MMS, and other spacecraft has provided us opportunities to study collisionless magnetic reconnection in the Earth’s magnetosphere, and to verify and extend more insights on the standard model of collisionless magnetic reconnection. In this paper, we will review what we have learned beyond the standard model with the help of observations from these spacecraft as well as kinetic simulations.

Studies of collisionless shockwaves using high-power laser pulses in laboratories

The remarkable experimental progress in the studies of collisionless shockwave（CS） in laboratories employing highpower lasers is briefly reviewed. The results show that CS can be generated in laser-produced plasmas due to the microturbulence associated with instabilities. CS is one of the most important astronomical phenomena. It has been found in supernova remnants（SNRs）, Sun–Earth space, etc. This paper focuses on CS in ways relevant to SNRs. Laboratory astrophysics（LA）, a new interdisciplinary frontier of astrophysics, plasma and laser physics, has developed rapidly in recent years. As an accessory to the astronomical observation, LA experimenters can closely study some astronomical events scaled-down to controllable phenomena.

Analytical Determination of Collisionless Sheath Properties for Triple Frequency Capacitively Coupled Plasma

A triple frequency capacitively coupled plasma （TF-CCP） has been considered to investigate the behavior of the sheath parameters. A self-consistent time-independent collisionless model has been developed. The sheath width and potential are calculated using the present model and compared with those calculated using a single-frequency （SF）, a dual-frequency （DF） and a triple-frequency （TF） model for time independent collisionless cases. The sheath motion and sheath potential are found to be larger compared with those of SF and DF CCPs for an inhomogeneous sheath, and that of TF CCP for a homogeneous sheath. The effects of the source parameters, i.e., current magnitudes, frequencies and phase difference, on the sheath parameters are investigated. The sheath parameters show higher values at higher source currents whereas they decrease with the increase of excitation frequencies. It has also been found that, by the proper choice of source frequencies and phase differences, it is possible to adjust the energy of ions when they hit the electrode.

Solutions to mKdV Eequation of Electromagnetic Wave Propagation in Cold Collisionless Plasma

The equation of electromagnetic wave propagation through cold collisionless plasma can be reduced to the modified Kortweg-de Vries （mKdV） equation. Using a new technique, whose keys are the trial solution in terms of the exponential function and the ideas of the like-terms＇ balance, some groups of accurate analytical solutions for this mKdV equation, such as solitary wave solutions, can be obtained. It is successfully shown that this method may be still valid for solving other nonlinear plasma equations.

Deformations at Earth’s dayside magnetopause during quasi-radial IMF conditions:Global kinetic simulations and Soft X-ray Imaging

The Solar wind Magnetosphere Ionosphere Link Explorer(SMILE)is a joint mission of the European Space Agency(ESA)and the Chinese Academy of Sciences(CAS).Primary goals are investigating the dynamic response of the Earth's magnetosphere to the solar wind(SW)impact via simultaneous in situ magnetosheath plasma and magnetic field measurements,X-Ray images of the magnetosheath and magnetic cusps,and UV images of global auroral distributions.Magnetopause deformations associated with magnetosheath high speed jets(HSJs)under a quasi-parallel interplanetary magnetic field condition are studied using a threedimensional(3-D)global hybrid simulation.Soft X-ray intensity calculated based on both physical quantities of solar wind proton and oxygen ions is compared.We obtain key findings concerning deformations at the magnetopause:(1)Magnetopause deformations are highly coherent with the magnetosheath HSJs generated at the quasi-parallel region of the bow shock,(2)X-ray intensities estimated using solar wind h+and self-consistentO7+ions are consistent with each other,(3)Visual spacecraft are employed to check the discrimination ability for capturing magnetopause deformations on Lunar and polar orbits,respectively.The SMILE spacecraft on the polar orbit could be expected to provide opportunities for capturing the global geometry of the magnetopause in the equatorial plane.A striking point is that SMILE has the potential to capture small-scale magnetopause deformations and magnetosheath transients,such as HSJs,at medium altitudes on its orbit.Simulation results also demonstrate that a lunar based imager(e.g.,Lunar Environment heliospheric X-ray Imager,LEXI)is expected to observe a localized brightening of the magnetosheath during HSJ events in the meridian plane.These preliminary results might contribute to the pre-studies for the SMILE and LEXI missions by providing qualitative and quantitative soft X-ray estimates of dayside kinetic processes.

Clustering of the collisionless particles in a gravitational field and rotation curve

The problem of dark matter in present astrophysics has been disputed for a long time and has not been solved yet. More and more evidence shoves that mostly the dark matter is probably not baryonic, but the collisionless particles which only participate in the gravitational interaction. These collisionless particles can condense in any potential well and provide the 'missing mass'.

Relativistic Self-Focusing of Hermite-cosine-Gaussian Laser Beam in Collisionless Plasma with Exponential Density Transition

This work reveals an exploration of self-focusing of Hermite-cosine-Gaussian laser beam in a collisionless plasma under relativistic nonlinearity. Self-focusing along with self-trapping of Hermite-cosine-Gaussian laser beam are analyzed for different values of laser intensity, plasma density, and decentered parameters. Mathematical analysis displays that these parameters play a major role in achieving the stronger and earlier self-focusing. Further, a comparative study between self-focusing of Hermite-cosine-Gaussian laser beam with and without exponential density ramp profile is introduced. Plasma density transition with exponential profile is found to be more effective in order to have stronger self-focusing. The present analysis may lead to very useful applications in the field of efficient harmonic generation, laser driven fusion etc.

Chaotic motion and heating of proton in perpendicular collisionless shocks

OBSERVATIONS and theoretical calculations show that a part of protons are reflected by theshock after the interaction between the solar wind and the quasi-perpendicular collisionlessshock with high Much number.These protons have large velocities in the direction per-pendicular to the magnetic field and move again towards the downstream of shock due to theeffect of gyration.Upon their second encounter with the shock,most of them are

Second harmonic generation of high power Cosh-Gaussian beam in cold collisionless plasma

The purpose of this study is to explore the second harmonic generation(SHG)of a high power Cosh-Gaussian beam in cold collisionless plasma.The ponderomotive force causes carrier redistribution from high field to low field region in presence of a Cosh-Gaussian beam thereby producing density gradients in the transverse direction.The density gradients so produced the results in electron plasma wave(EPW)generation at the frequency of the input beam.The EPW interacts with the input beam resulting in the production of 2nd harmonics.WKB and paraxial approximations are employed for obtaining the 2nd order differential equation describing the behavior of the beam’s spot size against normalized distance.The impact of well-established laser-plasma parameters on the behavior of the beam’s spot size and SHG yield are also analyzed.The focusing behavior of the beam and SHG yield is enhanced with an increase in the density of plasma,the radius of the beam and the decentred parameter,and with a decrease in the intensity of the beam.The results of the current problem are really helpful for complete information of laser-plasma interaction physics.

Formation of Non-Maxwellian Distribution and Its Role in Collisionless Driven Reconnection

The dynamical evolution of collisionless driven reconnection is investigated by using an electromagnetic particle simulation code in a microscopic open system.Strong in-plane electrostatic field is excited in the central region of current sheet under the influence of an external driving field.As a result of the amplification of unmagnetized meandering motion by the electrostatic field particle distribution function is modified from the shifted Maxwellian to an anisotropic one in the current sheet.An ion hole appears at the center of current sheet in the phase space,where distribution becomes two-peaked and no ions exist in low velocity region between two peaks.The strongmodification of distribution function leads to the generation of off-diagonal components of pressure tensor term,which is one of major causes to violate frozen-in constraint and trigger collisionless reconnection.

Electron density hole and quadruple structure of B_y during collisionless magnetic reconnection

In collisionless reconnection,the magnetic field near the separatrix is stronger than that around the X-line,so an electron-beam can be formed and flows toward the X-line,which leads to a decrease of the electron density near the separatrix.Having been accelerated around the X-line,the electrons flow out along the magnetic field lines in the inner side of the separatrix.A quadruple structure of the Hall magnetic field By is formed by such a current system.A 2D particle-in-cell (PIC) simulation code is used in this paper to study the collisionless magnetic reconnection without an initial guide field.The current system described above is proved by the simulations.Furthermore,the position of the peak of the Hall magnetic field By is found to be between the separatrix and the center of the current sheet,which is verified by Cluster observations.

Laboratory study of astrophysical collisionless shock at SG-Ⅱ laser facility

Astrophysical collisionless shocks are amazing phenomena in space and astrophysical plasmas, where supersonic flows generate electromagnetic fields through instabilities and particles can be accelerated to high energy cosmic rays. Until now, understanding these micro-processes is still a challenge despite rich astrophysical observation data have been obtained. Laboratory astrophysics, a new route to study the astrophysics, allows us to investigate them at similar extreme physical conditions in laboratory. Here we will review the recent progress of the collisionless shock experiments performed at SG-Ⅱ laser facility in China. The evolution of the electrostatic shocks and Weibel-type/filamentation instabilities are observed. Inspired by the configurations of the counter-streaming plasma flows, we also carry out a novel plasma collider to generate energetic neutrons relevant to the astrophysical nuclear reactions.

Electron dynamics in collisionless magnetic reconnection with a PIC simulation

Two-dimensional particle-in-cell (PIC) simulation is used to investigate electron dynamics in colli- sionless magnetic reconnection, and the proton/electron mass ratio is taken to be mi /me = 256. The results show that the presence of a strong initial guide field will change the direction of the electron flow. The electron density cavities and the parallel electric field can be found in the electron inflow re- gion along the separatrix, and the electron inflow and density cavities only appear in the second and fourth quadrants. What is different from the results with a smaller mass ratio is that new structures appear in the diffusion region near the X line: (1) Narrow regions of density enhancement and density cavities can be found synchronously in this region; and (2) corresponding to the electron density changes near the X line, the strong parallel electric fields are found to occur in the first and third quadrants. These electric fields perhaps play a more important role in acceleration and heating electrons than those fields located in the density cavities.

强激光与近临界密度等离子体相互作用中的无碰撞静电冲击波产生

本文通过一维粒子模拟(particle-in-cell)方法研究了强激光与近临界密度等离子体相互作用中的弱冲击波和强冲击波产生,并讨论了非相对论和相对论光强以及等离子体密度分布区间对无碰撞冲击波形成的影响.非相对论的弱驱动光与等离子体相互作用产生的是弱冲击波.由于电子加热不充分,电子能谱呈现出双温分布.较低温度的电子对弱冲击波的形成以及质子反射加速有重要作用.弱冲击波加速质子的能谱呈连续分布.在等离子体密度上升沿区间较大时,可观察到后孤子结构向离子声波结构演化并进一步演化为弱冲击波结构的过程.在相对论的强驱动光强下,电子加热比较充分可达到相对论温度,且呈现出单温分布.进一步分析密度分布区间大小对冲击波形成的影响时发现:1)当等离子体密度上升沿区间较大时,离子声波的势垒易被热电子屏蔽且离子声波结构在传输的过程中容易被后续的激光破坏而无法演化为无碰撞冲击波;2)当等离子密度分布区间较小时,离子声波中加速电场的有效距离(即德拜长度)和持续时间更长,这导致其结构在传输过程中更加稳定.当离子声波中加速的质子与靶后鞘层场加速的质子之间的速度差满足无碰撞冲击波的离子反射条件时,离子声波进一步演化为强的无碰撞冲击波,同时产生了准单能的质子.

无碰撞电流片哨声波不稳定性

通过数值求解文献[4]中物理模型A得到的一般形式的色散关系,讨论了无碰撞电流片低频波不稳定性问题.结果表明,哨声波能被无碰撞电流片直接激发.在中性片上(z/di=0),在较宽的波数范围内,斜哨声波是可以传播的,但它基本上是稳定的.在离子惯性区内(z/di<1,电子惯性区外),斜传播的哨声波是不稳定的.在离子惯性区边缘(z/di=1),斜传播的哨声波仍然是不稳定的,增长率更大,不稳定的波频率范围更高.此外,朝向中性片方向传播(kzdi<0)的哨声波比离开中性片方向传播(kzdi>0)的哨声波有更大的增长率.

空间等离子体与垂直无碰撞激波相互作用的数值实验

应用混合模拟方法数值研究了高Ａｌｆｖｅｎ—Ｍａｃｈ数垂直无碰撞激波与等离子体间的相互作用。结果表明，激波上游的磁场非常稳定，粒子分布近似为Ｍａｘｗｅｌｌ分布。激波下游磁场存在不规则的湍动，质子分布有一个高能尾，且有些质子被激波反射。跟踪少量高速质子的计算结果表明，在ｔ＝４０Ω时约有４０％的质于被激波反射，而７％质子可被加速，最大速度值可达到２０ＶＡ激波区的电场分布对质子的减速或加速起了主要的作用。

近地磁尾重联中哨声波和Hall磁场的Cluster观测

在2001～2003年Cluster飞船通过近地磁尾期间，共探测到14次重联事件，在这些事件中同时还观测到等离子体波活动．本文把14次事件分为三大类，其中：第1类包含了8次事件，它们是在等离子体片内先于重联事件观测到波活动，并且还同时观测到Hall磁场．经过分析判断，这类事件中观测到的波是右旋偏振的哨声波．第Ⅱ类包含了2次事件，这类事件也观测到了Hall磁场和右旋偏振的哨声波．第Ⅲ类也包含了2次事件，这类事件只是普通的重联事件，没有观测到Hall磁场，但是波活动明显先于重联事件．在我们观测的14次事件中，比较强烈的哨声波和Hall磁场是一一对应的，因此哨声波可能主要是在Hall磁场的四极结构区激发的．