Particle-in-Cell Simulation Study Of Reconnection Fronts And Particle Energy Gain In Collisionless Magnetic Reconnection

The solar-terrestrial space environment is an intricate and multifaceted system that encompasses the region between the Sun and the Earth,comprising of numerous physical processes,such as the interaction of the solar wind with the Earth’s magnetosphere,the structure and evolution of the Earth’s magnetosphere,and the acceleration and heating of plasma.Due to the interdependence of these processes,the system is highly nonlinear and time-varying,and operates on multiple spatial and temporal scales.Among these processes,magnetic reconnection is a significant explosive energy release mechanism that rapidly releases stored magnetic energy,accelerates and heats charged particles,and plays a pivotal role in the energy release and matter circulation of the solar-terrestrial space environment.Magnetopause and magnetotail are crucial regions within the Earth’s magnetosphere where magnetic reconnection occurs,and they are closely associated with storms and substorms.Previous studies have demonstrated that during the reconnection process,cold ions originating from the ionosphere or the inner magnetosphere can reach the outer magnetosphere and obtain energy.However,the mechanism by which these cold ions gain energy remains unclear.Moreover,the energy budget in the presence of cold ions is not well understood,and the kinetic process of cold ions during magnetic reconnection also requires further investigation.Additionally,reconnection fronts are generated during magnetic reconnection,and plasma in the fronts can undergo acceleration and heating.However,the dynamic evolution of reconnection fronts and their occurrence in asymmetric reconnection warrant further study.Satellites are unable to detect global data,making it impossible to track the evolution of reconnection,while the direct detection of cold ions remains a challenge.However,Particle-in-Cell simulation represents a powerful research tool that can study physical processes across different scales,distinguish between cold ions,hot ions,and electrons,and provide global simulation data to track the evolution of reconnection.This thesis employs Particle-in-Cell simulations to investigate reconnection fronts and particle energization.Specifically,2.5-dimensional Particle-in-Cell simulations are used,consisting of two-dimensional position space and three-dimensional velocity space.The simulation space is divided into grids,with the electromagnetic field defined on these grids,while charged particles can move throughout the simulation space.The electromagnetic field is updated based on Maxwell’s equations,while charged particles are governed by relativistic Newtonian equations.The ideal conducting boundary condition and periodic boundary condition are used,and the Harris current sheet model is used for initialization.This model provides the initial magnetic field and plasma temperature distribution,and the plasma density distribution is obtained based on pressure balance.The research in this thesis includes both symmetric and asymmetric reconnection,and the main findings are as follows:1.Reconnection fronts and particle energization in symmetric reconnection:The time evolution of force and energy balance at the reconnection front in symmetric reconnection from a Lagrangian frame is investigated.Despite hindrance from thermal pressure,the reconnection front accelerates towards the outflow direction under curvature force.During the motion of the reconnection front,the magnetic energy is significantly converted to plasma energy,as indicated by (?)>0.Nevertheless,the magnetic energy at the front increases gradually due to magnetic field compression at the front.The thermal energy of the plasma at the front increases due to the work done by pressure,resulting in (?)·(▽·(?))>0 at the front.Both electron and ion temperatures increase due to the pressure strain term-((?)·▽)·(?) and the pressure dilation term p▽·(?).The energy gain of cold ions in symmetric antiparallel reconnection is investigated.During the reconnection process,cold ions can be significantly accelerated.The study shows that the average energy gain per cold ion is smaller than the average energy gain per hot ion but larger than the average energy gain per electron.Cold ions with the highest energy are first accelerated around the X-line by the reconnection electric field E_y and further accelerated by the transverse electric field E_y in the reconnection front.They undergo meandering motions near the X-line while being accelerated by the reconnection electric field perpendicular to the reconnection plane.The velocity distribution function of cold ions near the X-line consists of a reverse incoming component and a component that is accelerated in the +y direction,forming a Dshaped velocity distribution function.The cold ions are picked up at the reconnection fronts and captured by the secondary magnetic islands,resulting in a bump in the cold ion energy spectrum.The energy at the location of the bump is close to the propagation velocity of the reconnection fronts and the secondary magnetic islands.Additionally,a crescent-shaped velocity distribution function is observed at the reconnection front,resulting from the finite Larmor radius effect of energetic cold ions in the flux pileup region.2.Reconnection fronts and particle energization in asymmetric reconnection:It has been predicted that robust asymmetric reconnection fronts can exist in asymmetric reconnection and is contrasted with symmetric reconnection fronts.The propagating speed and thickness of the asymmetric reconnection front are observed to be smaller than those of the symmetric reconnection front.While the symmetric reconnection front is both a current sheet and a boundary layer,the asymmetric reconnection front is solely a current sheet and not a boundary layer.Moreover,while there is a substantial energy conversion at the symmetric reconnection front,there is very little energy conversion at the asymmetric reconnection front.During asymmetric reconnection,a portion of the released magnetic energy is absorbed by cold ions,amounting to 10-25%of the total released magnetic energy.This absorbed energy is primarily converted into thermal energy of the cold ions.The released magnetic energy and the proportion of energy gained by cold ions is observed to increase with the initial density ratio of cold ions to hot ions in the magnetosphere.The Hall electric field E_z is found to be the primary accelerator of cold ions in the region of the magnetospheric separatrix,while the out-of-plane electric field E_y does negative work.The increase in thermal energy of cold ions is primarily due to stochastic heating,wherein the viscous heating associated with the non-gyrotropic pressure tensor-((?)·▽)·(?) is the most significant contribution,and it is most significant around the magnetosheath separatrix region.The velocity distribution function of cold ions in different reconnection regions consists of two types of particles that differ by their ability to reach the magnetosheath.In one type,the reconnection electric field E_y performs negative work,while in the other type,it performs positive work.The latter type particles are further accelerated in a stepwise manner by the Hall electric field E_z as they traverse the magneto spheric current sheet.As reconnection progresses,the velocity distribution functions of the cold ions diffuse in phase space,and the number density of low-energy particles rises.This process results in bulk heating,while the temperature of cold ions remains relatively stable.On the magnetosheath side,the temperature of cold ions experiences a significant increase,approximately 30 times higher than the initial cold ion temperature.This temperature enhancement is attributed to the kinetic effect of cold ions.This thesis centers on investigating the acceleration and heating mechanism of plasma during magnetic reconnection,specifically highlighting the existence of robust asymmetric reconnection fronts in asymmetric reconnection.The findings presented in this research can be further compared to satellite observations,enabling a deeper comprehension of energy transformation within the magnetic reconnection process.This understanding contributes to unraveling the mechanisms underlying energy circulation and release within the Earth’s magnetosphere,and holds significant reference value for studying the coupling between solar wind,magnetosphere,and ionosphere.