Quanming Lu

The process of electron acceleration during collisionless magnetic reconnection

Two-dimensional particle-in-cell simulations are performed to study electron acceleration in collisionless magnetic reconnection. The process of electron acceleration is investigated by tracing typical electron trajectories. When there is no initial guide field, the electrons can be accelerated in both the X-type and O-type regions. In the X-type region, the electrons can be reflected back and enter the acceleration region several times before they leave the diffusion region. In this way, the electrons can be accelerated by the inductive electric field to high energy. In the O-type region, the trapped electrons can be accelerated when they are trapped in the magnetic island. When there is an initial guide field, the electrons can only be accelerated in the X-type region, and no obvious acceleration is observed in the O-type region. In the X-type region, the electrons are not demagnetized and they gyrate with the force of the guide field. Although no electron reflection is observed in this region, the acceleration efficiency can be enhanced through staying longer time in the diffusion region due to their gyration motion.

Delayed phase explosion during high-power nanosecond laser ablation of silicon

An important parameter for high-irradiance laser ablation is the ablation crater depth, resulting from the interaction of individual laser pulses on a targeted surface. The crater depth for laser ablation of single-crystal silicon shows a dramatic increase at a laser intensity threshold of approximately 2×1010 W/cm2, above which, large (micron-sized) particulates were observed to eject from the target. We present an analysis of this threshold phenomenon and demonstrate that thermal diffusion and subsequent explosive boiling after the completion of the laser pulse is a possible mechanism for the observed dramatic increase of the ablation depth. Calculations based on this delayed phase explosion model provide a satisfactory estimate of the measurements. In addition, we find that the shielding of an expanding mass plasma during laser irradiation has a profound effect on this threshold phenomenon.

Magnetic Reconnection in the Near Venusian Magnetotail

Magnetic Reconnection Magnetic reconnection (MR) has been observed in the magnetospheres of planets with an intrinsic magnetic field, such as Earth, Mercury, Jupiter, and Saturn. MR is a universal plasma process that occurs in regions of strong magnetic shear and converts magnetic energy into kinetic energy. On Earth, MR is responsible for magnetic storms and auroral events. Using data from the European Space Agency Venus Express spacecraft, Zhang et al. (p. 567, published online 5 April; see the Perspective by Slavin) present surprising evidence for MR in the magnetosphere of Venus, which is a nonmagnetized body. Venus Express observations show that magnetic reconnection occurs in the magnetotail of an unmagnetized planet. Observations with the Venus Express magnetometer and low-energy particle detector revealed magnetic field and plasma behavior in the near-Venus wake that is symptomatic of magnetic reconnection, a process that occurs in Earth’s magnetotail but is not expected in the magnetotail of a nonmagnetized planet such as Venus. On 15 May 2006, the plasma flow in this region was toward the planet, and the magnetic field component transverse to the flow was reversed. Magnetic reconnection is a plasma process that changes the topology of the magnetic field and results in energy exchange between the magnetic field and the plasma. Thus, the energetics of the Venus magnetotail resembles that of the terrestrial tail, where energy is stored and later released from the magnetic field to the plasma.

The mechanisms of electron acceleration in antiparallel and guide field magnetic reconnection

Two-dimensional particle-in-cell simulations are performed to investigate electron dynamics in antiparallel and guide field (in the presence of a strong guide field) magnetic reconnection, and the mechanisms of electron acceleration are compared. In the antiparallel reconnection, the dominant acceleration occurs in the vicinity of the X line, where the magnetic field is weak. Most of these electrons come from the regions just outside of the separatrices, which move into the vicinity of the X line along the magnetic field lines. Electrons can also be nonadiabatically accelerated in the pileup region by the reconnection electric field, where the gyroradii of the electrons are comparable to the curvature radii of the magnetic field lines. Most of these electrons come from the regions inside of the separatrices, which move into the pileup region along the magnetic field lines. In the guide field reconnection, electrons are accelerated by the parallel electric field. They are firstly accelerated when moving to...

Multispacecraft observation of electron pitch angle distributions in magnetotail reconnection

[1] In this paper, we present Cluster observations of a magnetotail reconnection event without the presence of an obvious guide magnetic field and analyze electron pitch angle distributions in the vicinity of the X line and the outflow region, respectively. In the vicinity of the X line, at lower energies the distributions are highly anisotropic (field-aligned bidirectional anisotropic), while at higher energies, the electrons are observed to flow away from the X line along the magnetic field lines. The electron distributions change largely in the outflow region. At the edge of the outflow region, at lower energies, the electrons flow toward the X line, while the electrons at higher energies are directed away from the X line. When the satellites approach the center of the current sheet, at lower energies, the electrons have field-aligned bidirectional distributions, while at higher energies, the electron distributions are isotropic. The generation mechanisms of such distributions are explained by following typical electron trajectories in the electric and magnetic fields of magnetic reconnection, which are obtained in two-dimensional particle-in-cell simulations. It is shown that the observed high-energy electrons directed away from the X line both in the vicinity of the X line and in the outflow region are due to the acceleration by the reconnection electric field near the X line, and the field-aligned bidirectional distributions at lower energies are caused by the effects of the magnetic mirror in the reconnection site. The isotropic distributions at higher energies in the outflow region are the results of the electron stochastic motions when their gyroradii are comparable to the curvature radii of the magnetic field lines.

Plasmoid Ejection and Secondary Current Sheet Generation from Magnetic Reconnection in Laser-Plasma Interaction

Reconnection of the self-generated magnetic fields in laser-plasma interaction was first investigated experimentally by Nilson et al. [Phys. Rev. Lett. 97, 255001 (2006)] by shining two laser pulses a distance apart on a solid target layer. An elongated current sheet (CS) was observed in the plasma between the two laser spots. In order to more closely model magnetotail reconnection, here two side-by-side thin target layers, instead of a single one, are used. It is found that at one end of the elongated CS a fanlike electron outflow region including three well-collimated electron jets appears. The (>1 MeV) tail of the jet energy distribution exhibits a power-law scaling. The enhanced electron acceleration is attributed to the intense inductive electric field in the narrow electron dominated reconnection region, as well as additional acceleration as they are trapped inside the rapidly moving plasmoid formed in and ejected from the CS. The ejection also induces a secondary CS.

Electrostatic waves in an electron-beam plasma system

A one-dimensional (1D) electrostatic particle-in-cell simulation was performed to study wave excitation processes due to a tenuous electron beam in a plasma system, which is composed of hot and cold electron components. In this case, three types of electrostatic waves are excited, namely, Langmuir waves, electron-acoustic waves, and beam-driven waves. The beam-driven waves have a broad frequency spectrum, which extends from (0.1–0.2)ωpe (ωpe is the electron plasma frequency) to (1.5–2.5)ωpe, with phase speeds close to the speed of the electron beam. The interactions among these waves are investigated for different values of density, temperature, and speed of the electron beam, etc. One special case is that when the density of the electron beam is sufficiently high, compressive solitary waves with bipolar structure of the electric field are generated. The generation mechanism of these solitons is mainly due to the trapping of a fraction of the beam electrons by the potential well of the enhanced beam-drive...

New evidence for generation mechanisms of discrete and hiss‐like whistler mode waves

Linear theory suggests that whistler mode wave growth rates are proportional to the ratio of hot electron (~1 to 30 keV) density to total electron density (Nh/Nt), whereas nonlinear wave theory suggests that an optimum linear growth rate is required to generate rising tone chorus from hiss-like emissions. Using the Time History of Events and Macroscale Interactions during Substorms waveform data collected by three probes over the past ~5 years, we investigate the correlation between Nh/Nt and wave amplitude/wave occurrence rate for rising tone, falling tone, and hiss-like emissions separately. Statistical results show that the rising and falling tones preferentially occur in the region with a limited Nh/Nt range, whereas both the occurrence rate and wave amplitudes of hiss-like emissions become larger for higher values of Nh/Nt. Our statistical results not only provide an important clue on the generation mechanism of hiss-like emissions, but also provide supporting experimental evidence for the nonlinear theory of generating rising tone chorus.

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.