Xueyi Wang

Gyrokinetic particle simulation of beta-induced Alfvén eigenmode

The beta-induced Alfven eigenmode (BAE) in toroidal plasmas is studied using global gyrokinetic particle simulations. The BAE real frequency and damping rate measured in the initial perturbation simulation and in the antenna excitation simulation agree well with each other. The real frequency is slightly higher than the ideal magnetohydrodynamic (MHD) accumulation point frequency due to the kinetic effects of thermal ions. Simulations with energetic particle density gradient show exponential growth of BAE with a growth rate sensitive to the energetic particle temperature and density. The nonperturbative contributions by energetic particles modify the mode structure and reduce the frequency relative to the MHD theory. The finite Larmor radius effects of energetic particles reduce the BAE growth rate. Benchmarks between gyrokinetic particle simulation and hybrid MHD-gyrokinetic simulation show good agreement in BAE real frequency and mode structure.

Dipolarization fronts as earthward propagating flux ropes: A three‐dimensional global hybrid simulation

Dipolarization fronts (DFs) as earthward propagating flux ropes (FRs) in the Earths magnetotail are presented and investigated with a three-dimensional (3-D) global hybrid simulation for the first time. In the simulation, several small-scale earthward propagating FRs are found to be formed by multiple X line reconnection in the near tail. During their earthward propagation, the magnetic field Bz of the FRs becomes highly asymmetric due to the imbalance of the reconnection rates between the multiple X lines. At the later stage, when the FRs approach the near-Earth dipole-like region, the antireconnection between the southward/negative Bz of the FRs and the northward geomagnetic field leads to the erosion of the southward magnetic flux of the FRs, which further aggravates the Bz asymmetry. Eventually, the FRs merge into the near-Earth region through the antireconnection. These earthward propagating FRs can fully reproduce the observational features of the DFs, e.g., a sharp enhancement of Bz preceded by a smaller amplitude Bz dip, an earthward flow enhancement, the presence of the electric field components in the normal and dawn-dusk directions, and ion energization. Our results show that the earthward propagating FRs can be used to explain the DFs observed in the magnetotail. The thickness of the DFs is on the order of several ion inertial lengths, and the electric field normal to the front is found to be dominated by the Hall physics. During the earthward propagation from the near-tail to the near-Earth region, the speed of the FR/DFs increases from ~150 km/s to ~1000 km/s. The FR/DFs can be tilted in the GSM (x, y) plane with respect to the y (dawn-dusk) axis and only extend several Earth radii in this direction. Moreover, the structure and evolution of the FRs/DFs are nonuniform in the dawn-dusk direction, which indicates that the DFs are essentially 3-D.

Investigation of storm time magnetotail and ion injection using three‐dimensional global hybrid simulation

Dynamics of the near-Earth magnetotail associated with substorms during a period of extended southward interplanetary magnetic field is studied using a three-dimensional (3-D) global hybrid simulation model that includes both the dayside and nightside magnetosphere, for the first time, with physics from the ion kinetic to the global Alfvenic convection scales. It is found that the dayside reconnection leads to the penetration of the dawn-dusk electric field through the magnetopause and thus a thinning of the plasma sheet, followed by the magnetotail reconnection with 3-D, multiple flux ropes. Ion kinetic physics is found to play important roles in the magnetotail dynamics, which leads to the following results: (1) Hall electric fields in the thin current layer cause a systematic dawnward ion drift motion and thus a dawn-dusk asymmetry of the plasma sheet with a higher (lower) density on the dawnside (duskside). Correspondingly, more reconnection occurs on the duskside. Bidirectional fast ions are generated due to acceleration in reconnection, and more high-speed earthward flow injections are found on the duskside than the dawnside. Such finding of the dawn-dusk asymmetry is consistent with recent satellite observations. (2) The injected ions undergo the magnetic gradient and curvature drift in the dipole-like field, forming a ring current. (3) Ion particle distributions reveal multiple populations/beams at various distances in the tail. (4) Dipolarization of the tail magnetic field takes place due to the pileup of the injected magnetic fluxes and thermal pressure of injected ions, where the fast earthward flow is stopped. Oscillation of the dipolarization front is developed at the fast-flow braking, predominantly on the dawnside. (5) Kinetic compressional wave turbulence is present around the dipolarization front. The cross-tail currents break into small-scale structures with k⟂ρi∼1, where k⟂ is the perpendicular wave number. A sharp dip of magnetic field strength is seen just in front of the sharp rise of the magnetic field at the dipolarization front, mainly on the duskside. (6) A shear flow-type instability is found on the duskside flank of the ring current plasma, whereas a kinetic ballooning instability appears on the dawnside. (7) Shear Alfven waves and compressional waves are generated from the tail reconnection, and they evolve into kinetic Alfven waves in the dipole-like field region. Correspondingly, multiple field-aligned current filaments are generated above the auroral ionosphere.

Generation of nonlinear Alfvén and magnetosonic waves by beam–plasma interaction

One-dimensional (1-D) and two-dimensional (2-D) hybrid simulations are carried out to study the interaction between a background plasma and an ion beam, whose velocity is parallel to the ambient magnetic field B0. It is found that the beam–plasma interaction and the associated wave evolution can be divided into four phases. The simulation results in phase 1 in the early stage of wave evolution are consistent with the linear theory. Right-hand nonresonant instabilities are present and dominant in cases with a relatively strong ion beam (e.g., the ratio of beam ion density to background ion density >0.06 for beam velocity =10VA, where VA is the Alfven speed), while right-hand resonant instabilities are present in the weak beam cases. During phases 2 and 3, the waves grow to form nonlinear structure, and are then saturated. A detailed analysis shows that the wave evolution in these phases is through secondary instabilities associated with parametric decay or the wave modulation. In addition, it is shown for ...

A particle simulation of current sheet instabilities under finite guide field

The instability of a Harris current sheet under a broad range of finite guide field (BG) is investigated using a linearized (δf) gyrokinetic electron and fully kinetic ion particle simulation code. The simulation is carried out in the two-dimensional plane containing the guide field along y and the current sheet normal along z. In this particle model, the rapid electron cyclotron motion is removed, while the realistic mass ratio mi∕me, finite electron Larmor radii, and wave-particle interactions are kept. It is found that for a finite BG∕Bx0⩽1, where Bx0 is the asymptotic antiparallel component of magnetic field, three unstable modes, i.e., modes A, B, and C, can be excited in the current sheet. Modes A and C, appearing to be quasielectrostatic modified two-stream instability/whistler mode, are located mainly on the edge of the current sheet. Mode B, on the other hand, is confined in the current sheet center and carries a compressional magnetic field (δBy) perturbation along the direction of electron drif...

An improved gyrokinetic electron and fully kinetic ion particle simulation scheme: benchmark with a linear tearing mode

An improved gyrokinetic electron and fully kinetic ion (GeFi) particle simulation scheme is presented for the investigation of linear collisionless tearing mode instability in a two-dimensional Harris current sheet under a finite guide field BG and a realistic ion-to-electron mass ratio mi/me. Due to the removal of the rapid electron cyclotron motion while retaining the finite electron Larmor radii, wave–particle interaction, and off-diagonal components of the electron pressure tensor, the GeFi model can be used to investigate the physics of magnetic reconnection with a realistic mi/me in a large-scale current sheet, which in general possesses wave modes ranging from Alfven waves to lower hybrid/whistler waves, with wave frequency ω < Ωe, where Ωe is the electron gyrofrequency. As a necessary step of utilizing the code for magnetic reconnection, the linearized GeFi scheme is benchmarked by comparing the simulation results using a δf method against direct numerical solutions of the tearing-instability eigenmode equations, as well as those obtained analytically via asymptotic matching.

Theory and simulation of lower-hybrid drift instability for current sheet with guide field

The stability of a thin current sheet with a finite guide field is investigated in the weak guide-field limit by means of linear theory and simulation. The emphasis is placed on the lower-hybrid drift instability (LHDI) propagating along the current flow direction. Linear theory is compared against the two-dimensional linear simulation based on the gyrokinetic electron/fully kinetic ion code. LHDI is a flute mode characterized by k⋅Btotal=0; hence, it is stabilized by a finite guide field if one is confined to k vector strictly parallel to the cross-field current. Comparison of the theory and simulation shows qualitatively good agreement.

Formation of dayside low‐latitude boundary layer under northward interplanetary magnetic field

[1] A three-dimensional (3-D) global hybrid simulation is carried out to study the formation of the dayside low-latitude boundary layer (LLBL) under a purely northward interplanetary magnetic field (IMF). Magnetic reconnection in both northern and southern hemispheres leads to a continued formation of newly closed field lines on the dayside, and a subsequent formation of the LLBL by capture of the magnetosheath ions on the original magnetosheath field lines, as the newly closed field lines shorten. The formation of the LLBL is associated with the tailward spreading of the transmitted ions along the magnetopause as the newly closed flux tubes convect tailward. Meanwhile, compressional waves and transverse waves in magnetic field are generated in the magnetosphere. The ion distributions in the cusp and the boundary layer show the presence of multiple ion beams of the transmitted magnetosheath ions, as well as ion heating.

Evolution of flux ropes in the magnetotail: A three-dimensional global hybrid simulation

Flux ropes in the Earths magnetotail are widely believed to play a crucial role in energy transport during substorms and the generation of energetic particles. Previous kinetic simulations are limited to the local-scale regime, and thus cannot be used to study the structure associated with the geomagnetic field and the global-scale evolution of the flux ropes. Here, the evolution of flux ropes in the magnetotail under a steady southward interplanetary magnetic field are studied with a newly developed three-dimensional global hybrid simulation model for dynamics ranging from the ion Larmor radius to the global convection time scales. Magnetic reconnection with multiple X-lines is found to take place in the near-tail current sheet at geocentric solar magnetospheric distances x=−30RE∼−15RE around the equatorial plane ( z=0). The magnetotail reconnection layer is turbulent, with a nonuniform structure and unsteady evolution, and exhibits properties of typical collisionless fast reconnection with the Hall eff...

Simulation of linear and nonlinear Landau damping of lower hybrid waves

The linear physics of lower hybrid waves (LHWs) and their nonlinear interaction with particles through Landau damping are studied with the gyrokinetic electron and fully kinetic ion (GeFi) particle simulation model in the electrostatic limit. Unlike most other wave modes, the LHWs can resonantly interact with both electrons and ions, with the former being highly magnetized and latter nearly unmagnetized around the lower hybrid frequency. Direct interactions of LHWs with electrons and/or ions are investigated for cases with various k∥/k,Ti/Te, and wave amplitudes. In the linear electron Landau damping (ELD), the dispersion relation and the linear damping rate obtained from our simulation agree well with the analytical linear theory. As the wave amplitude increases, the nonlinear Landau effects are present, and a transition from strong decay at smaller amplitudes to weak decay at larger amplitudes is observed. In the nonlinear stage, the LHWs in the long time evolution finally exhibit a steady Bernstein-Gre...