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The coupling between a rapidly expanding cloud of ionized debris and an ambient magnetized plasma is revisited with a hybrid (kinetic ion/fluid electron) simulation code that allows a study over a wide range of plasma parameters. Over a specified range of hypothetical conditions, simple scaling laws in terms of the total debris mass and explosion speed are derived and verified for the maximal size of the debris cloud and the fraction of debris that free-streams from the burst along the magnetic field. The amount of debris that escapes from the burst with minimal coupling to the background magnetic field increases with the debris gyroradius. Test cases with two different debris species--including a heavy minority species with a relatively large gyroradius--highlight how the collisionless coupling of the debris depends on the single-particle trajectories as well as the overall conservation of energy and momentum., Comment: To appear in JGR: Space Physics

Various systems can be modeled as a point-like explosion of ionized debris into a magnetized, collisionless background plasma -- including astrophysical examples, active experiments in space, and laser-driven laboratory experiments. Debris streaming from the explosion parallel to the magnetic field may drive multiple resonant and non-resonant ion-ion beam instabilities, some of which can efficiently couple the debris energy to the background and may even support the formation of shocks. We present a large-scale hybrid (kinetic ions + fluid electrons) particle-in-cell (PIC) simulation, extending hundreds of ion inertial lengths from a 3-D explosion, that resolves these instabilities. We show that the character of these instabilities differs notably from the 1-D equivalent by the presence of unique transverse structure. Additional 2-D simulations explore how the debris beam length, width, density, and speed affect debris-background coupling, with implications for the generation of quasi-parallel shocks., Comment: 21 pages, 10 figures, the article has been accepted by Physics of Plasmas (2021)

The Right-Hand Resonant Instability (RHI) is one of several electromagnetic ion/ion beam instabilities responsible for the formation of parallel magnetized collisionless shocks and the generation of ultra-low frequency (ULF) waves in their foreshocks. This instability has been observed for the first time under foreshock-relevant conditions in the laboratory through the repeatable interaction of a preformed magnetized background plasma and a super-Alfvenic laser-produced plasma. This platform has enabled unprecedented volumetric measurements of waves generated by the RHI, revealing filamentary current structures in the transverse plane. These measurements are made in the plasma rest frame with both high spatial and temporal resolution, providing a perspective that is complementary to spacecraft observations. Direct comparison of data from both the experiment and the Wind spacecraft to 2D hybrid simulations demonstrates that the waves produced are analogous to the ULF waves observed upstream of the terrestrial bow shock.

Larmor coupling is a collisionless momentum exchange mechanism believed to occur in various astrophysical and space-plasma environments. The phenomenon is now observed in a laboratory experiment. The AMPTE (Active Magnetospheric Particle Tracer Explorers) mission provided in situ measurements of collisionless momentum and energy exchange between an artificial, photo-ionized barium plasma cloud and the streaming, magnetized hydrogen plasma of the solar wind 1,2,3. One of its most significant findings was the unanticipated displacement of the barium ion ‘comet head’ (and an oppositely directed deflection of the streaming hydrogen ions) transverse to both the solar wind flow and the interplanetary magnetic field, defying the conventional expectation that the barium ions would simply move downwind4. While subsequent theoretical and computational efforts5,6,7 to understand the cause of the transverse motion reached differing conclusions, several authors5 attributed the observations to Larmor coupling8,9, a collisionless momentum exchange mechanism believed to occur in various astrophysical and space-plasma environments10,11 and to participate in cosmic magnetized collisionless shock formation12,13,14. Here we present the detection of Larmor coupling in a reproducible laboratory experiment that combines an explosive laser-produced plasma cloud with preformed, magnetized ambient plasma in a parameter regime relevant to the AMPTE barium releases. In our experiment, time-resolved Doppler spectroscopy reveals ambient ion acceleration transverse to both the laser-produced plasma flow and the background magnetic field. Utilizing a detailed numerical simulation, we demonstrate that the ambient ion velocity distribution corresponding to the measured Doppler-shifted spectrum is qualitatively and quantitatively consistent with Larmor coupling.

The creation of a repeatable collisionless quasi-parallel shock in the laboratory would provide a valuable platform for experimental studies of space and astrophysical shocks. However, conducting such an experiment presents substantial challenges. Scaling the results of hybrid simulations of quasi-parallel shock formation to the laboratory highlights the experimentally demanding combination of dense, fast, and magnetized background and driver plasmas required. One possible driver for such experiments is high-energy laser-produced plasmas (LPPs). Preliminary experiments at the University of California, Los Angeles, have explored LPPs as drivers of quasi-parallel shocks by combining the Phoenix Laser Laboratory [Niemann et al., J. Instrum. 7, P03010 (2012)] with a large plasma device [Gekelman et al., Rev. Sci. Instrum. 87, 025105 (2016)]. Beam instabilities and waves characteristic of the early stages of shock formation are observed, but spatial dispersion of the laser-produced plasma prematurely terminates the process. This result is illustrated by experimental measurements and Monte Carlo calculations of LPP density dispersion. The experimentally validated Monte Carlo model is then applied to evaluate several possible approaches to mitigating LPP dispersion in future experiments.

We report the first measurements of the formation and structure of a magnetized collisionless shock by a laser-driven magnetic piston in a current-free laboratory plasma. This new class of experiments combines a high-energy laser system and a large magnetized plasma to transfer energy from a laser plasma plume to the ambient ions through collisionless coupling, until a self-sustained MA∼ 2 magnetosonic shock separates from the piston. The ambient plasma is highly magnetized, current free, and large enough (17 m × 0.6 m) to support Alfven waves. Magnetic field measurements of the structure and evolution of the shock are consistent with two-dimensional hybrid simulations, which show Larmor coupling between the debris and ambient ions and the presence of reflected ions, which provide the dissipation. The measured shock formation time confirms predictions from computational work.

Motivated by recent advances in laboratory experiments on parallel ion-beam instabilities, we present a theoretical framework for—and simulations of—their evolution toward shock formation and Fermi acceleration. After reviewing the linear theory of beam instabilities, with an emphasis on how magnetic helicity and polarization depend on properties of the beam, we compare the evolution and saturation of three distinct parameter regimes: (I) the left-handed "non-resonant" regime; (II) the right-handed beam-gyroresonant regime; (III) the balanced, mixed-turbulence regime.

As a result of cyclotron interaction with Alfven waves propagating from the sun, pitch angle diffusion of resonant particles takes place and a shell-like distribution function of resonant ions is formed at each distance from the sun. Stability of the solar wind ion shell-like distribution function with respect to excitation of waves at larger distances is addressed. It is shown in linear approximation, that in the case when the phase velocity of Alfven waves decreases with distance, ions with shell distribution excite outward propagating Alfven waves with smaller phase velocities when they advance to larger distances. The nonlinear dynamics of the wave spectrum as well as the evolution of the ion distribution function are studied. The characteristic spectrum at the high-frequency edge of the magnetohydrodynamic fluctuations is explained.

The ion drag force on dust grains in a dusty plasma contributes significantly to the formation of equilibrium dust layers and void regions in radio-frequency discharges. In addition to the drag due to the direct collection of ions and to momentum transfer of ions in the electric field near grains, dust grains can also be subject to a drag force due to the effect of coherent waves, such as dust acoustic waves that can grow because of the drift of plasma ions relative to charged grains induced by an externally imposed electric field. Here, we examine wave drag due to unstable dust acoustic waves in a collisional dusty plasma using numerical simulations. We study the drag as a function of grain size as well as neutral-ion and neutral-dust collisions in both one-dimensional periodic systems, in which it is easier to study the instability properties per se, and in aperiodic configurations in order to assess effects associated with dust drag and void formation. For the parameter range considered, we find that in the absence of background collisions, the instability tends to saturate by trapping the plasma ions in the electrostatic waves, which does not affect the dust grains very much. Including ion-neutral collisions tends to suppress ion trapping, which in turn leads to larger wave amplitudes and trapping of the dust, resulting in significant drag on the dust grains. Inclusion of neutral-dust collisions leads to a grain size-dependent result, with the persistence of trapping of, and thus drag on, larger grains only. For the parameters of this study, the wave drag force is much larger than the ion drag usually considered.

Collisionless reconnection is studied using two-dimensional (2-D) hybrid (particle ions, massless fluid electrons) and Hall-magnetohydrodynamics (Hall-MHD) simulations. Both use the full electron pressure tensor instead of a localized resistivity in Ohm’s law to initiate reconnection; an initial perturbation or boundary driving to the equilibrium is used. The initial configurations include one-dimensional (1-D) and 2-D current sheets both with and without a guide field. Electron dynamics from the two calculations are compared, and overall agreement is found between the calculations in both reconnection rate and global configuration [L. Yin et al., J. Geophys. Res. 106, 10761 (2001)]. It is shown that the electron drifts in the small-transverse-scale fields near the X point cause the electron motion to decouple from the ion motion, and that reconnection occurs due to electron viscous effects contained in the off-diagonal terms of the electron pressure tensor. Comparing the hybrid and Hall-MHD simulations s...

Quiet direct simulation Monte Carlo (QDSMC) is a new particle simulation technique that is applicable to a broad range of applications where the underlying system dynamics obey Fokker Planck equations. These include hydrodynamics, radiation transport, magnetohydrodynamics, diffusion, and collisional kinetic plasmas. At the beginning of each time step in QDSMC, the weights and abscissas of Gaussian-Hermite quadrature are used to deterministically create particles to sample the random process. At the end of the time step, particles are gathered to the computational mesh to obtain updated distributions of conserved quantities on the mesh and then the particles are destroyed. The creation and destruction of particles allows arbitrary dynamical range to be accessed quiescently with only a small number of particles per computational cell. The application of QDSMC to the simulation of Coulomb collisions is considered in this report, and the method is demonstrated on problems involving the collisional relaxation of non-Maxwellian distributions.

Collisionless coupling between super Alfv\'{e}nic ions and an ambient plasma parallel to a background magnetic field is mediated by a set of electromagnetic ion/ion-beam instabilities including the resonant right hand instability (RHI). To study this coupling and its role in parallel shock formation, a new experimental configuration at the University of California, Los Angeles utilizes high-energy and high-repetition-rate lasers to create a super-Alfv\'{e}nic field-aligned debris plasma within an ambient plasma in the Large Plasma Device (LAPD). We used a time-resolved fluorescence monochromator and an array of Langmuir probes to characterize the laser plasma velocity distribution and density. The debris ions were observed to be sufficiently super-Alfv\'{e}nic and dense to excite the RHI. Measurements with magnetic flux probes exhibited a right-hand circularly polarized frequency chirp consistent with the excitation of the RHI near the laser target. We compared measurements to 2D hybrid simulations of the experiment.

For two-dimensional reconnection in a thin Harris current sheet, fluid–ion dynamics from a Hall-magnetohydrodynamics (Hall-MHD) calculation using a scalar ion pressure are compared to the particle–ion dynamics obtained from a hybrid simulation and from test ions that are advanced in Hall-MHD fields. Skewed ion velocity distributions from the particle calculations are shown to produce off-diagonal elements of the ion pressure tensor. These comparisons demonstrate that the inclusion of off-diagonal terms in the ion pressure tensor is important to correctly model the ion out-of-plane momentum transport from the X point. It is shown that these effects can be modeled efficiently in Hall-MHD simulations in a predictor–corrector manner that uses particle ions to implement the ion gyro-radius corrections. By advancing test ions in the Hall-MHD fields at every time step, accumulating the ion pressure tensor onto the spatial grid, and adding its divergence to the ion momentum equation, this particle Hall-MHD simulation models well the ion out-of-plane momentum transport from the X point.

A new approach to particle simulation, called “quiet direct simulation Monte Carlo” (QDSMC), is described that can be applied to many problems of interest, including hydrodynamics, magnetohydrodynamics (MHD), and the modeling of collision plasmas. The essence of QDSMC is the use of carefully chosen weights for the particles (e.g., Gauss–Hermite, for Maxwellian distributions), which are destroyed each time step after the particle information is deposited onto the grid and reconstructed at the beginning of the next time step. The method overcomes the limited dynamical range and statistical noise typically found in particle simulations. In this article QDSMC is applied to hydrodynamics and MHD test problems, and its suitability for modeling semi-collisional plasma dynamics is considered.

Using a high-energy laser to produce a super-Alfvenic carbon-ion beam in a strongly magnetized helium plasma, we expect to be able to observe the formation of a collisionless parallel shock inside the Large Plasma Device. We compare early magnetic-field measurements of the resonant right-hand instability with analytical predictions and find excellent agreement. Hybrid simulations show that the carbon ions couple to the background plasma and compress it, although so far the background ions are mainly accelerated perpendicular to the mean-field direction.

The explosive expansion of a localized plasma cloud into a relatively tenuous, magnetized, ambient plasma characterizes a variety of astrophysical and space phenomena. In these rarified environments, collisionless electromagnetic processes rather than Coulomb collisions typically mediate the transfer of momentum and energy from the expanding “debris” plasma to the surrounding ambient plasma. In an effort to better understand the detailed physics of collisionless coupling mechanisms, compliment in situ measurements of space phenomena, and provide validation of previous computational and theoretical work, the present research jointly utilizes the Large Plasma Device and the Raptor laser facility at the University of California, Los Angeles to study the super-Alfvenic, quasi-perpendicular expansion of laser-produced carbon (C) and hydrogen (H) debris plasma through preformed, magnetized helium (He) ambient plasma via a variety of diagnostics, including emission spectroscopy, wavelength-filtered imaging, and ...

Much of the theoretical understanding concerning the structure and essential properties of the slow mode shock has been obtained from hybrid calculations in which a full kinetic description is used for the ions while the electrons are approximated as a massless adiabatic fluid. Owing to the relatively broad spatial and relatively slow temporal scales of the slow shock, one would expect this approximation to be well justified. In this work we reexamine the importance of electron dynamics using one-dimensional fully kinetic simulations which fully resolve all relevant spatial and temporal electron scales. The resulting shock structure and ion heating are in excellent agreement with hybrid simulations, indicating that the dissipation arising from kinetic electrons is relatively minor. However, electron heating is somewhat larger, and clear non-Maxwellian features are observed. In the upstream region, back-streaming electrons give rise to double-peaked distributions, while in the downstream region, bi-Maxwellian distributions are observed with Tell > T e ⊥. Although the acceleration mechanism for the back-streaming electrons is not fully understood, we present evidence that resonant wave-particle interactions may play an important role.

In this study we compare the results of two-dimensional hybrid (particle ions, massless fluid electrons) and Hall-MHD simulations of collisionless reconnection in a thin current sheet. Both calculations include the full electron pressure tensor (instead of a localized resistivity) in the generalized Ohm's law to initiate reconnection, and in both an initial perturbation to the Harris equilibrium is applied. As in the recent Geospace Environment Modeling (GEM) reconnection challenge studies, we find overall agreement between the two calculations in both the reconnection rate and the global configuration. Results of this study show that in addition to providing the reconnection electric field at the X point the divergence of the electron pressure tensor leads to in-plane electric fields that exert drag forces on the ions as they enter and exit the near-X-point region. The in-plane electric fields are enhanced in regions of small transverse scale along the edge of the sheet, and the resulting narrow electron current layers are demonstrated clearly. The possibility of improving magnetotail reconnection models by embedding a Hall-MHD calculation using the electron pressure tensor model inside a large-scale MHD simulation is suggested.

Fluid-ion dynamics from a Hall-MHD calculation and test ions that are advanced in Hall-MHD fields are compared to particle-ion dynamics from a hybrid simulation of two-dimensional reconnection in a thin current sheet. These comparisons demonstrate that off-diagonal terms of the ion pressure tensor are important to correctly model the ion out-of-plane momentum transport from the X point. These effects can be efficiently modeled in Hall-MHD simulations by advancing test ions in the Hall-MHD fields, accumulating the pressure tensor onto the spatial grid, and modifying the ion momentum equation. With ion gyro-radius corrections, the “particle Hall-MHD” simulation is shown to model accurately the ion out-of-plane momentum transport from the X point and the reconnection rate comparison with the hybrid simulation is improved.

Molecular dynamics (MD) simulations are used to study dusty plasma crystal formation in three dimensions. The grain interaction model includes a spherically symmetric Debye-Huckel potential, an asymmetric wake potential, and a unidirectional external potential representing gravity and the sheath potential. We use a new form for the wake with ion-neutral collisions that reduce the interaction length of the wake. For the parameters considered, we obtain quasi-ordered structures in which the grains align into well-formed strings in the vertical direction and a more amorphous alignment of the strings themselves. Changes in the vertical alignment as a function of the wake parameters are analyzed.

The linear response of a dust grain to a flowing plasma has been well studied, both through analytic theory as well as by plasma simulation. The principal feature of the plasma response is a quasi-periodic wavetrain downstream of the grain, whose characteristics have been investigated as a function of the plasma conditions and grain properties. In linear theory, the magnitude of the wake potential scales as the grain charge Q. For large enough charge, one expects that nonlinear behavior will occur. Using one- and two-dimensional simulations with particle ions flowing past a fixed charged grain, we show that for sufficiently large Q, the wake potential becomes essentially independent of the grain charge. The potential is large enough to reflect some of the incident ions, which become trapped in the vicinity of the grain. Compared to our previous calculations, we also assume a larger value for the ion-neutral collision frequency, and show that the resulting wake is significantly shorter in length and steadier in time.

This paper presents the detailed comparative analysis of full particle and hybrid simulations of collisionless magnetic reconnection. The comprehensive hybrid simulation code employed in this study incorporates essential electron kinetics in terms of the evolution of the full electron pressure tensor in addition to the full ion kinetics and electron bulk flow inertia effects. As was demonstrated in our previous publications, the electron nongyrotropic pressure effects play the dominant role in supporting the reconnection electric field in the immediate vicinity of the neutral X point. The simulation parameters are chosen to match those of the Geospace Environmental Modeling (GEM) “Reconnection Challenge.” It is that these comprehensive hybrid simulations perfectly reproduce the results of full particle simulations in many details. Specifically, the time evolutions of the reconnected magnetic flux and the reconnection electric field, as well as spatial distributions of current density and magnetic field at all stages of the reconnection process, are found to be nearly identical for both simulations. Comparisons of variations of characteristic quantities along the x and z axes centered around the dominating X points also revealed a remarkable agreement. Noticeable differences are found only in electron temperature profiles, i.e., in the diagonal electron pressure tensor components. The deviation in the electron heating pattern in hybrid simulations from that observed in particle simulations, however, does not affect parameters essential for the reconnection process. In particular, the profiles of the off-diagonal components of the electron pressure tensor are found to be very similar for both runs and appear unaffected by heat flux effects. Both simulations also demonstrate that the Ey component of the electric field is nearly constant inside the diffusion region where ions are nonmagnetized. We demonstrate that the simple analytical estimate for the reconnection electric field as a convection electric field at the edge of the diffusion region very well reproduces the reconnection electric field observed in the simulations.

The average relative flow velocity between protons and alpha particles in collisionless plasmas can excite several distinct alpha/proton instabilities. Here linear theory and two-dimensional hybrid simulations in a homogeneous plasma model are used to study one such mode, the alpha/proton magnetosonic instability, using plasma parameters characteristic of the high-speed solar wind observed by the Ulysses spacecraft at high heliospheric latitudes. Wave-particle scattering by enhanced fluctuations from this mode reduces the alpha/proton relative speed and heats the alphas more strongly than the protons. Post-saturation results for the alpha/proton flow in the simulations are approximately bounded by the linear theory threshold condition for onset of the magnetosonic instability, which is also a statistical upper bound on the alpha/proton relative speed at high βp for some of the Ulysses observations. Thus it is likely that this instability is a constraining agent for the alpha/proton relative speed in some domains of the solar wind.

The structure of the wake potential downstream of a stationary dust grain in a flowing plasma is studied on ion time scales using particle-in-cell simulation methods. The scaling of the wake is investigated as a function of Mach number and other parameters as well as the dimensionality of the system. The results are compared and discussed in relation to various theoretical expressions for the wake. Consistent with theory, in one dimension the wake wavelength scales as MλDe(1−M2)−1/2 for M 1. In two dimensions, a wake is formed for both M 1, while the wake wavelength scales as MλDe in both regimes. The amplitude of the wake peaks at M≈1 in both the one- and two-dimensional simulations.

Hot electrons and sub- and supersonic flows of cold ions around a charged dust particle create steady state wake and Debye screening fields. These linear, electrostatic fields are studied in two-dimensional planar or cylindrical geometry. An asymptotic analysis in the limit of large (compared to Debye length) downstream coordinate z yields analytic wakefields that are in good agreement with numerical integrations of the linear, steady state response function.

An electron temperature anisotropy T⊥e/T‖e > 1 leads to excitation of three distinct modes, the whistler, the electrostatic, and the Z-mode instabilities, in collisionless plasmas at frequencies below the electron cyclotron frequency |Ωe|. (Here perpendicular and parallel subscripts denote directions relative to the background magnetic field.) Two-and-one-half-dimensional particle-in-cell simulations are used to study the nonlinear consequences of the growth of these modes in homogeneous plasmas with ωe ∼ |Ωe|, where ωe is the electron plasma frequency. The simulations show that wave-particle scattering by enhanced fluctuations from the whistler and electrostatic anisotropy instabilities imposes a β-dependent upper bound on the electron temperature anisotropy at β‖e ≤ 0.10. The simulations also demonstrate that the maximum value of the dimensionless fluctuating magnetic fields increases with β‖e and that at sufficiently low β the electrostatic instability leads to non-Maxwellian suprathermal enhancements on the reduced electron velocity distribution fe(v‖).

The average flow speed of alpha particles relative to protons in the solar wind is observed to be of the order of or less than the Alfven speed. This flow, if sufficiently large, can lead to two distinct types of electromagnetic alpha/proton instabilities: magnetosonic and Alfven. Spatially homogeneous two-dimensional hybrid simulations of collisionless plasmas are used to study these growing modes and the consequent wave-particle scattering of each ion component. This scattering reduces the alpha/proton average flow speed, heats the alphas more strongly than the protons, and increases the alpha temperature in the directions perpendicular to the background magnetic field, all of which are consistent with solar wind observations.

An absence of theoretical justification for the magnitude of resistivity is one of the major limitations of large-scale simulations of magnetic reconnection in collisionless magnetospheric plasma. We took advantage of the results of recent progress in kinetic modeling of collisionless dissipation in the vicinity of the magnetically neutral X point aiming to find ways to represent small-scale kinetic effects in large-scale models. The study was based on a combination of hybrid and particle methods and on analytical analysis. A comprehensive hybrid simulation code which incorporates the leading terms in electron dynamics responsible for breaking the frozen magnetic flux constraint (electron bulk flow inertia and nongyrotropic pressure effects) was utilized. The results of the comprehensive hybrid model were found to be in excellent quantitative agreement with the results of full particle simulations with similar setups. Both simulations demonstrated that the actual reconnection electric field is determined primarily by kinetic quasi-viscous effects and less by electron bulk flow inertia. An analytical expression for the quasi-viscous reconnection electric field averaged over the nongyrotropic region was obtained. Similar behavior of the evaluated quasi-viscous electric field and actual reconnection electric field taken from the simulations was demonstrated. Conventional hybrid simulations with simple nongyrotropic corrections to the electric field where also performed. The model was further reduced for utilization in MHD models. Analytical expressions for the time evolution of the reconnected flux evaluated from the MHD model modified by nongyrotropic corrections appeared to be in very good agreement with the results of comprehensive kinetic simulations. The evaluated averaged quasi-viscous electric field can be substituted into large-scale simulation models.

Proton velocity distributions in the high-speed solar wind are sometimes observed as two components of approximately equal temperature with an average relative drift velocity parallel to the background magnetic field. This relative drift gives rise to several proton/proton instabilities; for representative parameters, linear Vlasov theory demonstrates that the electromagnetic modes most likely to grow are a magnetosonic instability and an Alfven instability. Two-dimensional initial value hybrid simulations of both instabilities are carried out in a homogeneous plasma. Enhanced fluctuating fields from both instabilities yield a reduction in the relative drift speed and a characteristic heating of the more tenuous component temperature perpendicular to the background magnetic field. Ensembles of simulations yield constraints on the fluctuating field energy density, the relative drift speed, and component anisotropies. Some of these constraints are cast as concise analytic expressions which may be indicators of proton/proton instability activity in the solar wind.

We use molecular dynamics (MD) and particle-in-cell (PIC) simulation methods, in which dust grains are treated as discrete particles and the background plasma is included in the potential shielding (MD) or as a Boltzmann fluid (PIC), to investigate dust-acoustic waves in a one-dimensional, strongly coupled (with the Coulomb coupling parameter {Gamma} equal to the ratio of the Coulomb energy to the thermal energy, which is greater than 1) dusty plasma. We study cases both where the dust is represented by a small number of simulation particles that form into a regular array structure at large {Gamma} (crystal limit) and where the dust is represented by a much larger number of particles (fluid limit). We show that the measured frequency for dust acoustic waves satisfies either a fluidlike dispersion relation or a lattice wavelike dispersion relation, depending on {Gamma} and the number of simulation particles. Other PIC simulations, either with plasma ions represented as particles rather than as a Boltzmann fluid or with collisions between the dust and the background gas, have also been carried out and shown to agree with theoretical predictions. Numerical issues associated with smoothing of the accumulated charge density in PIC simulations have also been addressed; smoothingmore » is shown to affect wave dispersion at high wave numbers in the fluid limit and low wave numbers in the crystal limit. {copyright} {ital 1999} {ital The American Physical Society}« less

A study of the electron dynamics in the dissipation region of collisionless magnetic reconnection is presented. The investigation is based on a new 2.5-dimensional electromagnetic particle-in-cell simulation code. This code is applied to the problem of reconnection in two differently sized current sheets: one with a thickness of the ion inertial length and the other with electron inertial length thickness. The complete set of contributions to the reconnection electric field is calculated directly from the particle information. The two cases lead to quite different results. In the ion scale, sheet reconnection is significantly slower, and the dissipation is provided virtually exclusively by electron quasi-viscous effects. The electron scale sheet reconnects much faster, involving a bifurcation of the reconnection region and the formation of a magnetic island. In this latter case, dissipation appears to be primarily provided by electron inertial effects and here foremost by bulk electron acceleration. Finally, an attempt to represent the effects of electron pressure-based dissipation in a transport model is presented also.

The electromagnetic proton firehose instability may grow in a plasma if the proton velocity distribution is approximately bi-Maxwellian and T‖p > T⊥p, where the directional subscripts denote directions relative to the background magnetic field. Linear Vlasov dispersion theory in a homogeneous electron-proton plasma implies an instability threshold condition at constant maximum growth rate 1 − T⊥p/T‖p = Sp/β‖pαp over 1 < β‖p ≤ 10 where and Bo is the background magnetic field. Here Sp and αp are fitting parameters and αp ≃ 0.7. One- and two-dimensional initial value hybrid simulations of this growing mode are carried out under proton cyclotron resonant conditions in a homogeneous plasma on the initial domain 2 ≲ β‖p ≤ 100. The two-dimensional simulations show that enhanced fluctuations from this instability impose a bound on the proton temperature anisotropy of the form of the above equation with the fluid theory result αp ≃ 1.0. On this domain both one- and two-dimensional simulations yield a new form for the upper bound on the fluctuating field energy density from the proton resonant firehose instability where SB and αB are empirical parameters which are functions of the initial growth rate. This logarithmic behavior is qualitatively different from a fluid theory prediction and, like the anisotropy bound, should be subject to observational verification in any sufficiently homogeneous plasma in which the proton velocity distribution is approximately bi-Maxwellian.

We study the nonlinear behavior of the low-frequency dust acoustic instability in a collisional dusty plasma by means of particle simulations. The instability arises due to the streaming of plasma ions and neutrals relative to charged dust grains. According to linear theory, the presence of collisions between the plasma ions and a neutral gas background reduces the growth rate of the instability. Nonlinearly, however, the presence of drifting neutrals maintains the initial relative drift between plasma and dust ions until the unstable waves grow to large amplitude and collisions due to wave-particle interactions exceed the neutral collisions. As a result, stronger nonlinear effects, as manifested by enhanced fluctuations, larger amounts of plasma and dust heating, and a temporary reduction of the relative drift velocity, can occur in the presence of collisions.

A numerical and analytical investigation of the formation and structure of thin current sheets which form in a magnetotail-like plasma model is presented. Current sheet formation is driven by the application of a boundary electric field similar to what would be expected from solar wind driving. As a result, a thin current sheet forms which involves strong Hall-type electric fields, which, by means of electric field drifts, strongly increase the electron contribution, and reduce the ion contribution to the cross-tail current density. This result is also supported by analytical investigations of the total plasma momentum. The thin current sheet forms on strong gradients in the ion density and the magnetic field. The simulation leads to a well defined equilibrium state.

Both electron inertia and nongyrotropic effects were analysed using 2 1/2-dimensional hybrid simulations of collisionless magnetic reconnection. The traditional hybrid approach which treats ions as particles and electrons as an isotropic massless fluid was modified. In the new model the electron mass dependence was introduced both in the expression for the electric field and in the evolution equation of the full electron pressure tensor. This comprehensive hybrid model includes full ion kinetics, incorporates Hall effects, describes the leading terms in electron dynamics responsible for breaking the frozen magnetic flux constraint and allows to consider arbitrary ion/electron temperature and mass ratios. We demonstrate that the kinetic quasi-viscous electron inertia associated with nongyrotropic pressure effects dominates over the electron bulk flow inertia in controlling the structure of the dissipation region around the neutral X line. The reconnection electric field based on the nongyrotropic pressure tends to reduce the current density and to relax gradients in the vicinity of the X line (similar to the localized anomalous viscosity). On the other hand, the reconnection electric field based on electron bulk flow inertia tends to require an increased current density, with gradient scales comparable with the electron skin depth. The dependence on the ion/electron temperature ratio and on the current carrier also is discussed. An analytical analysis which supports the results of the numerical simulations is also presented.

Two-dimensional hybrid simulations of super-Alfv\'enic expanding debris plasma interacting with an inhomogeneous ambient plasma are presented. The simulations demonstrate improved collisionless coupling of energy to the ambient ions when encountering a density gradient. Simulations of an expanding cylinder running into a step function gradient are performed and compared to a simple analytical theory. Magnetic flux probe data from a laboratory shock experiment are compared to a simulation with a more realistic debris expansion and ambient ion density. The simulation confirms that a shock is formed and propagates within the high density region of ambient plasma.

Magnetospheric banded chorus is enhanced whistler waves with frequencies (r)

The electromagnetic proton cyclotron instability and the mirror instability are driven by the proton temperature anisotropy T⊥p/T‖p > 1, where ⊥ and ‖ denote directions relative to the background magnetic field. Linear theory and one-dimensional hybrid simulations imply that the former mode grows more rapidly over 0.05 ≤ β‖p ≤ 5 and that wave-particle scattering by its enhanced fluctuations imposes an upper bound on the temperature anisotropy of the form where and B0 is the background magnetic field. Here Sp and αp are fitting parameters, and . This paper describes results from more general two-dimensional hybrid simulations, which permit both instabilities to grow simultaneously. These simulations confirm the one-dimensional results on the initial domain ; enhanced fluctuations display the properties of the proton cyclotron instability and αp ≃ 0.4. On this domain the two-dimensional simulations also yield an upper bound for the fluctuating field energy density of the form with fitting parameter . The simulations on the initial domain 10 ≤ β‖p≤100 show spectral characteristics of both instabilities and exhibit a more stringent bound on the proton anisotropy, in agreement with observations in the terrestrial magnetosheath.