Collisionless Space Plasmas Research Articles

Laboratory Study of Magnetic Reconnection in Lunar-relevant Mini-magnetospheres

Mini-magnetospheres are small ion-scale structures that are well suited to studying kinetic-scale physics of collisionless space plasmas. Such ion-scale magnetospheres can be found on local regions of the Moon, associated with the lunar crustal magnetic field. In this paper, we report on the laboratory experimental study of magnetic reconnection in laser-driven, lunar-like ion-scale magnetospheres on the Large Plasma Device at the University of California, Los Angeles. In the experiment, a high-repetition rate (1 Hz), nanosecond laser is used to drive a fast-moving, collisionless plasma that expands into the field generated by a pulsed magnetic dipole embedded into a background plasma and magnetic field. The high-repetition rate enables the acquisition of time-resolved volumetric data of the magnetic and electric fields to characterize magnetic reconnection and calculate the reconnection rate. We notably observe the formation of Hall fields associated with reconnection. Particle-in-cell simulations reproducing the experimental results were performed to study the microphysics of the interaction. By analyzing the generalized Ohm’s law terms, we find that the electron-only reconnection is driven by kinetic effects through the electron pressure anisotropy. These results are compared to recent satellite measurements that found evidence of magnetic reconnection near the lunar surface.

Exact ion acoustic N solitary waves with variable velocity in inhomogeneous plasma induced by charged space debris

We have considered the propagation of nonlinear ion acoustic waves in a weakly inhomogeneous, cold (T i =0), collisionless space plasma in presence of charged space debris objects. The (1+1) dimensional evolution equation for the nonlinear ion acoustic wave is shown to have the form of a forced KdV equation with variable coefficients. The forcing term comes from the orbiting charged space debris and the variable coefficients of the forced KdV equation arise due to the equilibrium plasma quantities that depend on the slowly varying spatial coordinate. We have derived the exact one solitary wave, two solitary wave and then exact N solitary wave solutions of the forced system for some given choices of pinned debris functions where the velocity of the solitary waves varies with the slowly varying spatial coordinate. To the best of our knowledge, these exact, debris induced N solitary wave solutions with variable velocity in inhomogeneous plasma are important and new; hence may generate new ideas on research in this field.

The muphyII code: Multiphysics plasma simulation on large HPC systems

Collisionless astrophysical and space plasmas cover regions that typically display a separation of scales that exceeds any code's capabilities. To help address this problem, the muphyII code utilizes a hierarchy of models with different inherent scales, unified in an adaptive framework that allows stand-alone use of models as well as a model-based dynamic and adaptive domain decomposition. This requires ensuring excellent conservation properties, careful treatment of inner-domain model boundaries for interface coupling, and robust time-stepping algorithms, especially with the use of electron subcycling. This multi-physics approach is implemented in the muphyII code, tested on different scenarios of space plasma magnetic reconnection, and evaluated against space probe data and higher-fidelity simulation results from literature. Adaptive model refinement is highlighted in particular, and a hybrid model with kinetic ions, pressure-tensor fluid electrons, and Maxwell fields is appraised.

Particle-in-cell simulations – ion beam instabilities and the generation of Alfvén and whistler waves in low β plasma

ABSTRACT Ion beam-driven instabilities in a collisionless space plasma with low β, i.e. low plasma and magnetic pressure ratio, are investigated using particle-in-cell (PIC) simulations. Specifically, the effects of different ion drift velocities on the development of Buneman and resonant electromagnetic (EM) right-handed (RH) ion beam instabilities are studied. Our simulations reveal that both instabilities can be driven when the ion beam drift exceeds the theoretical thresholds. The Buneman instability, which is weakly triggered initially, dissipates only a small fraction of the kinetic energy of the ion beam while causing significant electron heating, owing to the small electron-ion mass ratio. However, we find that the ion beam-driven Buneman instability is quenched effectively by the resonant EM RH ion beam instability. Instead, the resonant EM RH ion beam instability dominates when the ion drift velocity is larger than the Alfvén speed, leading to the generation of RH Alfvén waves and RH whistler waves. We find that the intensity of Alfvén waves decreases with decrease of ion beam drift velocity, while the intensity of whistler waves increases. Our results provide new insights into the complex interplay between ion beams and plasma instabilities in low β collisionless space plasmas.

Impact of Heavy Ions on the Structure of Current Sheets in the Gravity Field of Exoplanets and Stars

Boundary current sheets (CSs) can be formed in collisionless space plasmas in the environment of exoplanets and cold stars. Usually they represent curved surfaces carrying the electric current analogous to the well-known planetary ionospheres, magnetopauses, or stellar coronas surrounding celestial bodies. At smaller local scales, some of them can be imagined as planar current layers of a finite scale located parallel to the surface of a celestial object and, correspondingly, perpendicular to the direction of the gravitational force. In some cases, this force crossed with magnetic field can influence the dynamics of charged particles in CSs and substantially change the structure of both the current layer and the magnetic field. We have generalized our prior model, taking into account a multi-ion plasma composition and a magnetic field configuration with a shear. It is shown that, due to the drift motion of plasma particles in the crossed gravitational and magnetic fields, the structure of CSs becomes more complex, accruing asymmetric and shifted profiles of the current and plasma densities that depend on dominating current carriers and the characteristics of the magnetic shear. We discuss possible applications of the results to the interpretation of observations of boundary layers in different space plasmas.

Importance of accurate consideration of the electron inertia in hybrid-kinetic simulations of collisionless plasma turbulence: The 2D limit

The dissipation mechanism of the magnetic energy in turbulent collisionless space and astrophysical plasmas is still not well understood. Its investigation requires efficient kinetic simulations of the energy transfer in collisionless plasma turbulence. In this respect, hybrid-kinetic simulations, in which ions are treated as particles and electrons as an inertial fluid, have begun to attract a significant interest recently. Hybrid-kinetic models describe both ion- and electron scale processes by ignoring electron kinetic effects so that they are computationally much less demanding compared to fully kinetic plasma models. Hybrid-kinetic codes solve either the Vlasov equation for the ions (Eulerian Vlasov-hybrid codes) or the equations of motion of the ions as macro-particles [Lagrangian particle-in-cell (PIC)-hybrid codes]. They consider the inertia of the electron fluid using different approximations. We check the validity of these approximations by employing our recently massively parallelized three-dimensional PIC-hybrid code Code Hybrid with Inertial Electron Fluid (CHIEF), which considers the electron inertia without any of the common approximations. In particular, we report the results of simulations of two-dimensional collisionless plasma turbulence. We conclude that the simulation results obtained using hybrid-kinetic codes, which use approximations to describe the electron inertia, need to be interpreted with caution. We also discuss the parallel scalability of CHIEF, to the best of our knowledge, the first PIC-hybrid code, which without approximations describes the inertial electron fluid.

Fast Inverse Transform Sampling of Non‐Gaussian Distribution Functions in Space Plasmas

AbstractNon‐Gaussian distributions are commonly observed in collisionless space plasmas. Generating samples from non‐Gaussian distributions is critical for the initialization of particle‐in‐cell simulations that investigate their driven and undriven dynamics. To this end, we report a computationally efficient, robust tool, Chebsampling, to sample general distribution functions in one and two dimensions. This tool is based on inverse transform sampling with function approximation by Chebyshev polynomials. We demonstrate practical uses of Chebsampling through sampling typical distribution functions in space plasmas.

Upper‐Hybrid Waves Driven by Meandering Electrons Around Magnetic Reconnection X Line

AbstractMagnetic reconnection is a fundamental process in collisionless space plasma environment, and plasma waves relevant to the kinetic interactions can have a significant impact on the multiscale behavior of reconnection. Here, we present Magnetospheric Multiscale (MMS) observations during an encounter of an X line of symmetric magnetic reconnection in the magnetotail. The X line is characterized by reversals of ion and electron jets and electromagnetic fields, agyrotropic electron velocity distribution functions (VDFs), and an electron‐scale current sheet. MMS observe large‐amplitude nonlinear upper‐hybrid (UH) waves on both sides of the neutral line, and the wave amplitudes have highly localized distribution along the normal direction. The inbound meandering electrons drive the UH waves, releasing the free energy stored from the reconnection electric field along the meandering trajectories. The interaction between the meandering electrons and the UH waves may modify the balance of the reconnection electric field around the X line.

Laboratory Verification of Electron‐Scale Reconnection Regions Modulated by a Three‐Dimensional Instability

AbstractDuring magnetic reconnection in collisionless space plasma, the electron fluid decouples from the magnetic field within narrow current layers, and theoretical models for this process can be distinguished in terms of their predicted current layer widths. From theory, the off‐diagonal stress in the electron pressure tensor is related to the thermal noncircular orbit motion of electrons around the magnetic field lines. This stress becomes significant when the width of the reconnecting current layer approaches the small characteristic length scale of the electron motion. To aid in situ spacecraft and numerical investigations of reconnection, the structure of the electron diffusion region is here investigated using the Terrestrial Reconnection Experiment. In agreement with the closely matched kinetic simulations, laboratory observations reveal the presence of electron‐scale current layer widths. Although the layers are modulated by a current‐driven instability, three‐dimensional simulations demonstrate that it is the off‐diagonal stress that is responsible for breaking the frozen‐in condition of the electron fluid.

The Atmospheric Wind of Hot Exoplanets and its Manifestations in Observations: from Energy Estimates to 3D MHD Models

The studies of close-orbit gaseous exoplanets—hot Jupiters and warm Neptunes—are reviewed. Transit spectral observations of the Hubble Space Telescope stimulated the numerical modeling of these objects. The supersonic outflow of the upper atmosphere of hot exoplanets and its interaction with the stellar plasma flow is a newly-discovered phenomenon in plasma physics and astrophysics that pools photo- and plasma-chemistry, collision-induced gas-dynamics, and collisionless space plasma. Starting from 2003, the modeling of physical processes in this field has considerably developed from simple quasi-empiric analytical formulas to global three-dimensional magnetic hydrodynamic (3D MHD) codes. The attained level makes it possible to interpret the in-transit measurements in absorption lines of different elements and estimate the parameters of both the planetary atmospheres and the stellar plasma wind.

Energy Conversion between Ions and Electrons through Ion Cyclotron Waves and Embedded Ion-scale Rotational Discontinuity in Collisionless Space Plasmas

Abstract Wave–particle interaction is a fundamental process in collisionless plasma, which results in the redistribution of energy between plasma waves and particle species. The analysis of high-resolution Magnetospheric Multiscale plasma and magnetic field data directly reveals the energy exchange between electromagnetic energy, particle bulk kinetic energy, and thermal kinetic energy in magnetosheath turbulence. This work focuses on the energy transfer associated with ion cyclotron waves (ICWs) and embedded rotational discontinuity (RD). We find that (1) the particle kinetic energy of ions is converted into electromagnetic energy; (2) the electrons are gaining energy from electromagnetic fields, having significant electron heating in the parallel direction around the RD; (3) the ICWs and RD connect and redistribute energy between ions and electrons in the postshock downstream sheath region; and (4) the interactions between pressure tensor and strain tensor redistribute the ion and electron bulk and thermal kinetic energies, but less significantly than direct field–particle interaction by one order of magnitude in the ICW turbulence with weak compressibility, in the sense that , .

Current Sheets with Multicomponent Plasma in Magnetospheres of Planets of the Solar System

A self-consistent hybrid model of a thin current sheet (TCS) with a thickness of the order of several ion gyroradii is proposed that takes into account the multicomponent nature of collisionless space plasma. Several plasma components can be present in the tails of the magnetospheres of terrestrial planets (for example, the Earth, Mercury, Mars, and Venus). Variations in the current sheet (CS) structure in magnetospheric plasma in the presence of heavy oxygen ions with different characteristics are analyzed. It is shown that high relative concentrations of oxygen ions, as well as their relatively high temperatures and drift velocities, lead to significant thickening of CS and the formation of an additional embedded scale. In this case, on the profiles of the main characteristics—current density and magnetic field—symmetric breaks appear, which correspond to a sharp change in the gradients of variation in values. A comparison is performed and a qualitative agreement is shown between the simulation results and observational data in the tail of the Martian magnetosphere.

Using Dimensionality Reduction and Clustering Techniques to Classify Space Plasma Regimes

Collisionless space plasma environments are typically characterised by distinct particle populations. Although moments of their velocity distribution functions help in distinguishing different plasma regimes, the distribution functions themselves provide more comprehensive information about the plasma state, especially at times when the distribution function includes non-thermal effects. Unlike moments, however, distribution functions are not easily characterised by a small number of parameters, making their classification more difficult to achieve. In order to perform this classification, we propose to distinguish between the different plasma regions by applying dimensionality reduction and clustering methods to electron distributions in pitch angle and energy space. We utilise four separate algorithms to achieve our plasma classifications: autoencoders, principal component analysis, mean shift, and agglomerative clustering. We test our classification algorithms by applying our scheme to data from the Cluster-PEACE instrument measured in the Earth's magnetotail. Traditionally, it is thought that the Earth's magnetotail is split into three different regions (the plasma sheet, the plasma sheet boundary layer, and the lobes), that are primarily defined by their plasma characteristics. Starting with the ECLAT database with associated classifications based on the plasma parameters, we identify 8 distinct groups of distributions, that are dependent upon significantly more complex plasma and field dynamics. By comparing the average distributions as well as the plasma and magnetic field parameters for each region, we relate several of the groups to different plasma sheet populations, and the rest we attribute to the plasma sheet boundary layer and the lobes. We find clear distinctions between each of our classified regions and the ECLAT results.

Description of Large-Scale Processes in the Near-Earth Space Plasma

We suggest a solution of the problem of the description of magnetic and electric fields occurring during large-scale nonradiative processes in the collisionless space plasma. The key idea is that the quasi-neutrality condition and the field-aligned force equilibrium of electrons should be taken into account. Equations describing the plasma are divided into two parts, namely, a system of transport equations which describes the plasma motion, and a system of equations for fields. The fields are defined in the instantaneous action approximation via the current spatial distributions of hydrodynamic plasma parameters and boundary conditions obtained from the system of elliptic equations containing no partial time derivatives. Three forms of the generalized Ohm’s law corresponding to different levels of plasma magnetization are considered. It is shown that, depending on the form of a system of transport equations derived for each plasma component, five key variants of the equation system describing the plasma can be obtained from the three forms of the Ohm’s law. The first form of the generalized Ohm’s law refers to the general case in which all plasma components unmagnetized and the system of transport equations represents the Vlasov equations for each plasma component. The second form of the Ohm’s law corresponds to the case of unmagnetized ionic plasma components, while electrons are magnetized and their pressure tensor is expressed through their longitudinal and transverse pressures as well as through the magnetic field. In the latter case two variants of the system of transport equations are possible, and the ions are described by Vlasov equations in both of them. In the first variant, the electrons are described by the Vlasov equation in the drift approximation. In the second variant, the electrons are described by the system of Chew–Goldberger–Low equations of magnetogasdynamics. The third variant of Ohm’s law corresponds to the case in which all plasma components are magnetized, and the pressure tensor of each component is replaced by its expression through the longitudinal and transverse pressure, as well as through the magnetic field. In this case, two variants of the transport equation system are also possible. In the first variant, each component is described by the Vlasov equation in the drift approximation. In the second variant, each component is described by the system of the Chew–Goldberger–Low equations of magnetogasdynamics.

Energy Cascade Rate Measured in a Collisionless Space Plasma with MMS Data and Compressible Hall Magnetohydrodynamic Turbulence Theory.

The first complete estimation of the compressible energy cascade rate |ϵ_{C}| at magnetohydrodynamic (MHD) and subion scales is obtained in Earth's magnetosheath using Magnetospheric MultiScale spacecraft data and an exact law derived recently for compressible Hall MHD turbulence. A multispacecraft technique is used to compute the velocity and magnetic gradients, and then all the correlation functions involved in the exact relation. It is shown that when the density fluctuations are relatively small, |ϵ_{C}| identifies well with its incompressible analog |ϵ_{I}| at MHD scales but becomes much larger than |ϵ_{I}| at subion scales. For larger density fluctuations, |ϵ_{C}| is larger than |ϵ_{I}| at every scale with a value significantly higher than for smaller density fluctuations. Our study reveals also that for both small and large density fluctuations, the nonflux terms remain always negligible with respect to the flux terms and that the major contribution to |ϵ_{C}| at subion scales comes from the compressible Hall flux.

Long-term Correlations of Polytropic Indices with Kappa Distributions in Solar Wind Plasma near 1 au

Abstract The polytropic behavior of space plasmas defines a power law between the plasma moments during the transition of the plasma from one state to another under constant specific heat. Knowledge of the polytropic index—the power-law exponent—is essential for understanding the dynamics of plasma particles, while a full kinetic description can be established by the study of the velocity distribution of plasma particles. The particle velocities of collisionless space plasmas, such as the solar wind, follow the kappa distribution function. The kappa index, the parameter that labels and governs these distributions, is an independent variable that describes the state of plasmas and is required for a complete description of the plasma properties. Previous studies showed and demonstrated how the kappa and polytropic indices are related to each other in the presence of potential energy, and their relationship also depends on the potential degrees of freedom. This paper extends these analyses and derives the kappa and polytropic indices of the solar wind proton plasmas using Wind observations during the last two solar cycles. We examine and show the systematic long-term correlation between these indices, the magnetic field strength, and the solar activity.

Introduction to Basic Space Plasma Physics and Its Methods for Computer Simulation

No less than 99.9% of visible matter in our universe is in the collisionless plasm state. Hence, it is essential for understanding of our universe to study collisionless space plasma. This paper introduces various basic theories with different approximations for space plasma. Various numerical methods for simulating space plasma with these basic equations are also described briefly.

Energy Conversion at Kinetic Scales in the Turbulent Magnetosheath

The process of conversion or dissipation of energy in nearly collisionless turbulent space plasma, is yet to be fully understood. The existing models offer different energy dissipation mechanisms w ...

Validation of Anisotropic Electron Fluid Closure Through In Situ Spacecraft Observations of Magnetic Reconnection

AbstractA valid fluid model for electrons in collisionless space plasmas is desirable for understanding the structure and evolution of magnetic reconnection geometries. Additionally, such a fluid model would be useful for the simulation of systems too large to be tractable in a fully kinetic model. Using Magnetospheric Multiscale spacecraft observations, we provide direct confirmation of the Lê et al., 2009 (https://doi.org/10.1103/PhysRevLett.102.085001) equations of state for the electron pressure tensor during guide field reconnection and demonstrate how the closure can be applied in efficient numerical simulation, yielding new physical insight to the electron heating problem. Furthermore, we use the Lê et al., 2009 equations of state to predict a scaling of electron heating in the exhaust comparable to the available observational data.

System of Kinetic Equations for Collisionless Space Plasma in the Approximation of Field-Aligned Force Equilibrium for Electrons

A system of kinetic equations describing relatively slow large-scale processes in collisionless magnetoplasma structures with a spatial resolution on the order of the proton thermal gyroradius is derived. The system correctly takes into account the electrostatic effects in the approximation of field-aligned force equilibrium for electrons. The plasma is considered quasineutral, and the magnetic field is described by the Ampere equation. The longitudinal component of the electric field is found explicitly from the equality of the field-aligned component of the electric force acting on plasma electrons and the divergence of the electron pressure tensor. The electric field component orthogonal to the magnetic field is determined by the distributions of the number densities, current densities, and stress tensors of all plasma species in the instantaneous long-range approximation described by a system of time-independent elliptic equations. Versions of the system of equations adapted to the case of magnetized electrons described by the Vlasov equation in the drift approximation, as well as to the case in which all plasma species are magnetized, are derived. The resulting systems of equations allow creating numerical models capable of describing large-scale processes in nonuniform collisionless space plasma.