Astrophysical Plasma Environments Research Articles

Towards synthetic magnetic turbulence with coherent structures

Synthetic turbulence is a relevant tool to study complex astrophysical and space plasma environments inaccessible by direct simulation. However, conventional models lack intermittent coherent structures, which are essential in realistic turbulence. We present a novel method featuring coherent structures, conditional structure function scaling and fieldline curvature statistics comparable to magnetohydrodynamic turbulence. Enhanced transport of charged particles is investigated as well. This method presents significant progress towards physically faithful synthetic turbulence.

Theoretical investigation of electron-impact ionization of W<sup>6+</sup> ion

<sec>Due to its unique characteristics, metal tungsten has been selected as the wall material for the tokamak magnetic confinement fusion device. The wall material directly interacts with the plasma for a long time, thus causing tungsten atoms and ions to be sputtered and ionized into different charge states, which then enter the tokamak device as plasma impurities. To ensure stable plasma combustion conditions, highly complex model is currently being used to evaluate the behavior of tungsten impurities and their influence on the tokamak plasma. This requires various high-precision atomic data for tungsten atoms and different ionized states of tungsten ions. Electron collision ionization, as a fundamental atomic physical process, is widely encountered in laboratory and astrophysical plasma environments. The parameters such as electron collision ionization cross-sections and rate coefficients are crucial for plasma radiation transport simulations and state diagnostics.</sec><sec>Electron-impact single-ionization (EISI) cross sections of the ground state and metastable state for W<sup>6+</sup> ions are calculated by using the level-to-level distorted-wave (LLDW) method. The contributions of direct ionization (DI) cross section and excited autoionization (EA) cross section to the total EISI cross section are primarily considered.</sec><sec>Comparison of our calculation results with the experimental data from Stenke et al. (Stenke M, Aichele K, Harthiramani D, Hofmann G, Steidl M, Volpel R, Salzborn E <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://iopscience.iop.org/article/10.1088/0953-4075/28/13/021">1995 <i>J. Phys. B: At. Mol. Opt. Phys.</i> <b>28</b> 2711</ext-link>) reveals that the EISI cross section considering only the ground state is significantly smaller than the experimental result. Therefore, it is imperative to take into account the contribution from the metastable state. To determine the fraction of ions in long-lived energy levels within the parent ion beam, three models are employed.</sec><sec>Our results, which include the contribution of metastable states, accord well with the experimental results of Stenke et al. Compared with the theoretical calculation result of Pindzola et al. our calculaiton provides a more comprehensive understanding of the electron-impact single-ionization process for W<sup>6+</sup> ions. The comparison is illustrated in the attached figure.</sec>

Effect of trapping and reflection on dust acoustic solitary waves in nonthermal opposite polarity dust plasmas

The study of resonant wave-particle interactions (WPIs) is crucial in plasma systems where charged plasma particles interact via long-range electromagnetic waves. Our research focuses on exploring the impact of trapping and reflection, along with the superthermality of Kappa resonance electrons and ions, on the characteristics of dust acoustic waves (DAWs) in opposite polarity dust plasma (OPDP). Both linear and non-linear analyses were conducted. Two distinct types of dust acoustic modes, namely fast and slow, have been observed in the linear regime of two different instances of WPIs. Moving on to the non-linear regime, the Schamel KdV (SKdV) equation has been derived using the reductive perturbation technique. In both cases, a stationary solution in the form of a dust acoustic double-layer wave (DADLW) has been successfully obtained. Our findings are highly relevant to astrophysical plasma environments with non-thermal trapped and reflected particles.

Higher-order dust kinetic Alfvén wave solitons and quasi-periodic waves in a polarized dusty plasma

This investigation presents the study of dust kinetic Alfvén wave solitons (DKAWSs) and quasi-periodic waves in a polarized dusty plasma comprising inertial dust, Cairns–Tsallis (CT) distributed ions and electrons. Using the reductive perturbation method, the KdV equation is derived and the Tanh-method is employed to derive its solution to study the dust kinetic Alfvén solitary structures. Furthermore, by considering the contribution of higher-order effects, the KdV-type inhomogeneous equation is derived and its solution results in the formation of dressed DKAWSs. The combined effects of polarization force and other plasma parameters on the characteristics of different kinds of DKAWSs are analyzed. Most importantly, from the conservation of KdV equations in the presence of external periodic perturbations, the study of quasi-periodic wave has also been illustrated. The analysis shows that the periodic solitonic behavior is transformed to quasi-periodic and also approaches chaotic oscillations. By changing different physical parameters, the numerical analysis of nonlinear DKAWs from periodic to quasi-periodic and to chaotic behavior has also been presented with external perturbation. The findings of this investigation are useful to understand the insight into physics of nonlinear phenomena for studying the dynamics of DKAWSs in space and astrophysical plasma environments.

Oblique Bernstein wave propagation in electron‐ion plasma with electron quantization effects

AbstractWe have studied the role of electron quantization effects on the dispersion characteristics of the oblique electron Bernstein wave (EBW) in an electron‐ion plasma using Vlasov‐Poisson model. Our analysis shows that large thermal effects (as compared to quantization effects) give rise to the usual classical results while in the limit of strong magnetic field, the dispersion characteristics changes. The existence of a quantum Bernstein mode has been proposed which appears purely due to the electron quantization effects. The damping of the oblique EBW has also been analyzed in detail and we found that the magnitude of the damping of the wave decreases as we decrease the obliqueness parameter (parallel to perpendicular wavenumber ratio) which agrees with the earlier theoretical and experimental findings. Moreover, the strong quantization effects () also tend to suppress the damping of EBW which contributes to the stabilization of the waves. The present findings are relevant to the laboratory laser‐produced plasmas where strong external magnetic fields exist, and also to the dense astrophysical plasma environments.

Target and Velocity Dependence of Charge Exchange X-Ray Emission at Solar Wind Velocities

Laboratory measurements of X-ray emissions following charge exchange (CX) between highly charged ions and neutrals are important to assess their diagnostic utility for the nonequilibrium astrophysical plasma environments, where hot flows meet cold gases. With a high-resolution X-ray quantum microcalorimeter detector, we report the CX-induced X-ray spectra and line ratios in Ne8+ on He and Kr collisions at solar wind velocities of 392, 554, 678, and 876 km s−1, respectively. The experimentally determined line ratios quantify the differences in CX state selectivity and the following X-ray emission between He and Kr at different collision velocities. This suggests that target and velocity dependence should be considered for accurately modeling astrophysical CX plasmas.

Ion acoustic shocks with contribution of higher order effects in a superthermal beam-plasma

This investigation presents the study of ion acoustic (IA) shocks in a plasma comprising warm inertial ions, and superthermal hot electrons penetrated by an electron beam. Using the reductive perturbation method, the Burgers equation is derived. Tanh-method is employed to derive its solution to study the ion acoustic shock structures. Further, by considering the contribution of higher order effects, the inhomogeneous Burgers-type equation is derived and its solution results in the formation of humped-type IA shocks. The combined effects of electron beam and other plasma parameters on the characteristics of different kinds of ion acoustic shock structures are analysed. The findings of this investigation may be useful to understand insight of physics of nonlinear phenomena for studying dynamics of ion acoustic shocks in space as well as astrophysical plasma environments where superthermal electrons and electron beam are present.

Non-thermal electron velocity distribution functions due to 3D kinetic magnetic reconnection for solar coronal plasma conditions

Magnetic reconnection can convert magnetic energy into kinetic energy of non-thermal electron beams. Those accelerated electrons can, in turn, cause radio emission in astrophysical plasma environments, such as solar flares via micro-instabilities. The properties of the electron velocity distribution functions (EVDFs) of those non-thermal beams generated by reconnection are, however, still not well understood, in particular, properties that are necessary conditions for some relevant micro-instabilities. We aim at characterizing the EVDFs generated in 3D magnetic reconnection by means of fully kinetic particle-in-cell code simulations. In particular, our goal is to identify the possible sources of free energy offered by the generated EVDFs and their dependence on the strength of the guide field. By applying a machine learning algorithm on the EVDFs, we find that (1) electron beams with positive gradients in their 1D parallel (to the local magnetic field direction) velocity distribution functions are generated in both diffusion region and separatrices. (2) Electron beams with positive gradients in their perpendicular (to the local magnetic field direction) velocity distribution functions are observed in the diffusion region and outflow region near the reconnection midplane. In particular, perpendicular crescent-shaped EVDFs (in the perpendicular velocity space) are mainly observed in the diffusion region. (3) As the guide field strength increases, the number of locations with EVDFs featuring a perpendicular source of free energy significantly decreases. The formation of non-thermal electron beams in the field-aligned direction is mainly due to magnetized and adiabatic electrons, while in the direction perpendicular to the local magnetic field, it is attributed to unmagnetized electrons.

Novel features of electromagnetic waves in an isotropic degenerate electron-ion plasma

Within the framework of kinetic theory, the nonlinear interaction of electromagnetic waves (EMWs) with a degenerate electron-ion plasma is studied to account for the electron quantum mechanical effects. For this purpose, a specific quantum regime is considered, for which the degenerate electron Fermi velocity is assumed to be of the order of the group velocity of EMWs. This eventually leads to the existence of a nonlinear Landau damping rate for the EMWs in the presence of electron ponderomotive force. The electron–ion density oscillations may have arisen from the nonlinear interaction of EMWs, leading to a new type of nonlinear Schrödinger equation in terms of a complex amplitude for electromagnetic pump waves. The profiles of nonlinear damping rates reveal that EMWs become less damped for increasing the quantum tunneling effects. The electrostatic response of the linear electrostatic waves is also investigated and derived from a linear dispersion for the ion-acoustic damping rate. The latter is a direct function of the electron Fermi speed and does not rely on the Bohm tunneling effect. The obtained results are numerically analyzed for two microwaves of different harmonics in the context of nonrelativistic astrophysical dense plasma environments, e.g. white dwarfs, where the electron quantum corrections cannot be ignored.

Dissipative dust acoustic solitons in magnetized nonextensive warm dusty plasma

A systematic theoretical investigation has been carried out in order to explore the propagation of both linear and nonlinear dust acoustic waves (DAWs) in a collisional magnetized warm dusty plasma in the presence of nonextensive electrons and ions. A linear dispersion relation is analyzed to show that the wave frequencies of both parallel and perpendicular modes are modified by the relevant plasma parameters. The standard reductive perturbative technique leads to a Korteweg–de Vries equation with a linear damping term for the propagation dynamics of the finite amplitude waves in warm dusty plasma. It is found that the dust-neutral collision induced dissipation is responsible for the linear damping. The analysis reveals that DAW propagates in the form of a weakly dissipative compressive/rarefactive solitons and the nonlinear excitations are significantly influenced by different plasma parameters, such as dust concentration, nonextensivity, external magnetic field strength, and dust-neutral collisionality. The present results can be relevant to the DAWs in some astrophysical plasma environments, such as Saturn’s rings, interstellar dusty clouds, etc.

Nonlinear dust ion acoustic shock wave structures in solar F corona region

Shock wave formation in dusty plasma consisting of mobile positive and negative ions, nonthermal electrons, and negatively charged static dust particles is theoretically studied in the presence of a magnetic field. Using the reductive perturbation technique, the basic set of fluid equations is reduced to the three-dimensional Zakarov–Kuznetsov Burgers nonlinear wave equation. The dissipation generated by ion kinematic viscosity in the system is taken into consideration. Distinct features of the dispersive (oscillatory) shock-like structures are discussed for different plasma parameters such as ion kinematic viscosities, ion density ratio, dust density ratio, nonthermal electrons, and magnetic field effect. It is observed that at large value of ion viscosities, nonthermal electrons, and ion density ratio, the formation of compressive shock strength increases. However, at certain critical values of nonthermal electrons, ion density ratio, the transition in the polarity of shock wave occurs within this range of plasma parameters. The considered parameters are reliable in the Solar F Corona (Dust corona) region. The findings of this work may be helpful in understanding shock formation in the Solar F Corona (Dust corona) region and astrophysical related plasma environments.

Recent progress on particle acceleration and reconnection physics during magnetic reconnection in the magnetically-dominated relativistic regime

Magnetic reconnection in strongly magnetized astrophysical plasma environments is believed to be the primary process for fast energy release and particle energization. Currently, there is strong interest in relativistic magnetic reconnection in that it may provide a new explanation for high-energy particle acceleration and radiation in strongly magnetized astrophysical systems. We review recent advances in particle acceleration and reconnection physics in the magnetically dominated regime. Much discussion is focused on the physics of particle acceleration and power-law formation as well as the reconnection rate problem. In addition, we provide an outlook for studying reconnection acceleration mechanisms and kinetic physics in the next step.

Electrostatic and electromagnetic waves driven by current free electron beam instability: Effects of positron, background composition, and ion-to-electron mass ratio

Counterstreaming electron beams drifting along the magnetic field in electron–proton plasmas may lead to the generation of electrostatic (ES) waves and structures widely observed in planetary magnetospheres. In electron–positron plasmas, streaming electron/positron beams along the background magnetic field may first result in ES waves followed by the electromagnetic (EM) waves with substantial magnetic field perturbations, as shown by Jao and Hau [Phys. Rev. E 98, 013203 (2018)]. In this study, we examine the formation of ES and EM waves driven by current free counterstreaming electron beams in electron–ion background plasmas and current free electron–positron beams in electron–proton–positron background plasmas based on one-dimensional EM particle-in-cell simulations with the drift velocity being along the background magnetic field. For the former cases, the effects of ion-to-electron mass ratios are examined, which have implications for two- and three-dimensional particle in cell simulations with reduced mass ratios. For the latter cases, the effects of positrons are examined, which have applications to astrophysical plasma environments and laboratory experiments. It is shown that the presence of positrons or light ions may shorten the occurrence times between the ES and EM waves driven by current free electron beam instability. The EM waves have relatively longer wavelengths as compared to the ES waves and the magnetic field evolution resembles parallel proton firehose instability. The effects of positrons, background compositions, and ion-to-electron mass ratios on the occurrence of ES and EM waves are consistent with the linear fluid theory.

Theory of ion holes in space and astrophysical plasmas

ABSTRACT Coherent bipolar electric field structures, ubiquitously found in various space and astrophysical plasma environments, play an important role in plasma transport and particle acceleration. Most of the studies found in the literature about them pertain to bipolar structures with positive potentials interpreted in terms of electron holes. Magnetospheric Multiscale spacecraft have recently observed a series of coherent electric field structures with negative potential in the Earth’s bow shock region, which are interpreted as ion holes. The existing theoretical models of ion holes are inadequate because they entail stringent conditions on the ratio of ion to electron temperature. This letter presents a new theory that provides a satisfactory explanation to these observations. A salient point is that this letter incorporates the electron dynamics in the theoretical formalism, which removes ambiguities associated with existing theories, thus showing that the new theory for ion holes may be widely applicable for space and astrophysical plasmas.

Quantum inertial Alfvén solitary waves: the effect of exchange-correlation and spin magnetization

ABSTRACT The effect of electron exchange-correlation and spin magnetization on nonlinear inertial Alfvén waves in electron-ion quantum plasma are succinctly discussed for 1, with β and Q are being the ratio of Fermi to magnetic pressures and the ratio of electron to ion masses, respectively. Using the quantum hydrodynamic fluid model, both the Sagdeev's potential and Korteweg de Vries equation are derived in low-β quantum plasma. It is found that in the presence of spin magnetization and exchange-correlation effects only dip soliton structures of inertial Alfvén waves are formed. The inertial Alfvén soliton dip structures move with super Alfvénic wave speed. The graphical analysis shows that the depth of Sagdeev's potential profiles and width of solitary wave structures significantly modified by changing the plasma parameters such as plasma density (exchange-correlation) and magnetic field intensity (spin magnetization). The importance of present work in the context of astrophysical plasma environments existing in compact stars is also discussed.

Two stream ion acoustic wave instability in warm dense plasmas

In this study we investigate the two-ion stream instability of plasma with a generalized Fermi-Dirac equation of state (EoS) for electron fluid. We use a hydrodynamic set of evolution equations for electron-ion plasma in order to obtain the linearized set of equations from which the dispersion relation of the plasma is obtained in the presence of ion acoustic wave instability. It is remarked that changes in the electron plasma fluid parameters such as the chemical potential and electron fluid temperature have fundamental effects on the growth rate of ion acoustic waves instability. It is however found that the plasma degeneracy has rather more significant effect on the two stream instability phenomenon. The generalized equation of state of electrons used in this research is valid for a wide density-temperature regimes of plasmas relevant to both laboratory as well as astrophysical environments including high-energy density fusion plasmas and fully degenerate compact stellar plasmas. We believe that using such a wide scope of plasma EoS provides a broader view of underlying physical mechanisms in plasma dynamics and instabilities and better understanding of astrophysical harsh plasma environments.

Nonlinear propagation of ion plasma waves in dust-ion plasma including quantum-relativistic effect

In this paper we have theoretically investigated the quantum and relativistic effects on ion plasma wave in an unmagnetised dust-ion plasma. By using the method of normal mode analysis, we have obtained a linear dispersion relation. It has been analysed numerically for quantum and relativistic effects on the propagation of ion plasma wave. By using the standard reductive perturbation technique, we have derived a Korteweg–de Vries (KdV) equation which describes the nonlinear propagation of the wave. Numerically, it is shown that only compressive type of soliton can exist in the plasma under consideration. It is found that the solitary wave profile depends significantly on the quantum and relativistic parameters. The dust size, dust charge and the dust number density are also shown to have significant influences on these solitary waves. The results of this present investigation have some relevance to the nonlinear propagation of ion plasma wave in some astrophysical, space and laboratory plasma environments.

Dispersion characteristics of anisotropic unmagnetized ultra-relativistic transverse plasma wave with arbitrary electron degeneracy

Thermal momentum space anisotropy is ubiquitous in many astrophysical and laboratory plasma environments. Using Vlasov-Maxwell's model equations, a generalized polarization tensor for a collisionless ultra-relativistic unmagnetized electron plasma is derived. In particular, the tensor is obtained by considering anisotropy in the momentum space. The integral of moments of Fermi-Dirac distribution function in terms of Polylog functions is used for describing the border line plasma systems (TeTFe≈1) comprising arbitrary electron degeneracy, where Te and TFe, are thermal and Fermi temperatures, respectively. Furthermore, the effects of variation in thermal momentum space anisotropy on the electron equilibrium number density and the spectrum of electromagnetic waves are analyzed.

Electron-impact dissociation and ionization of CN+ ions

Absolute cross sections for various pathways of electron-impact ionization and dissociation of CN${}^{+}$ ions are measured, which provides strong support for more accurate modeling of relevant processes in astrophysical plasma environments.

The development of magnetic field line wander in gyrokinetic plasma turbulence: dependence on amplitude of turbulence

The dynamics of a turbulent plasma not only manifests the transport of energy from large to small scales, but also can lead to a tangling of the magnetic field that threads through the plasma. The resulting magnetic field line wander can have a large impact on a number of other important processes, such as the propagation of energetic particles through the turbulent plasma. Here we explore the saturation of the turbulent cascade, the development of stochasticity due to turbulent tangling of the magnetic field lines and the separation of field lines through the turbulent dynamics using nonlinear gyrokinetic simulations of weakly collisional plasma turbulence, relevant to many turbulent space and astrophysical plasma environments. We determine the characteristic time $t_{2}$ for the saturation of the turbulent perpendicular magnetic energy spectrum. We find that the turbulent magnetic field becomes completely stochastic at time $t\lesssim t_{2}$ for strong turbulence, and at $t\gtrsim t_{2}$ for weak turbulence. However, when the nonlinearity parameter of the turbulence, a dimensionless measure of the amplitude of the turbulence, reaches a threshold value (within the regime of weak turbulence) the magnetic field stochasticity does not fully develop, at least within the evolution time interval $t_{2}<t\leqslant 13t_{2}$. Finally, we quantify the mean square displacement of magnetic field lines in the turbulent magnetic field with a functional form $\langle (\unicode[STIX]{x1D6FF}r)^{2}\rangle =A(z/L_{\Vert })^{p}$ ($L_{\Vert }$ is the correlation length parallel to the magnetic background field $\boldsymbol{B}_{\mathbf{0}}$, $z$ is the distance along $\boldsymbol{B}_{\mathbf{0}}$ direction), providing functional forms of the amplitude coefficient $A$ and power-law exponent $p$ as a function of the nonlinearity parameter.