An electromagnetic particle-in-cell simulation code is used to investigate particle density effects on the nonlinear amplification of inverse-bremsstrahlung electron acceleration, according to the schemes proposed by Kawata et al. and by Hussein and Pato. It is shown that although the space-charge self-consistent field does not modify the basic results obtained with the one-particle model, the electromagnetic fields produced by the transverse particle current play a substantial role for large beam densities.

Kinetic Alfvén solitons with hot electrons and finite electron inertia in a low-beta (β=8πn0 T/B2G, the ratio of the kinetic to the magnetic pressure) plasma is studied analytically, with the ion motion being considered dominant through the polarization drift. Both compressive and rarefactive kinetic Alfvén solitons are found to exist within a definite range of kz (the direction of propagation of the kinetic Alfvén solitary waves with respect to the direction of the magnetic field) for each pair of assigned values of β and M (Mach number). Unlike in previous theoretical investigations, β appears as an explicit parameter for the kinetic Alfvén solitons in this case. In addition, consideration of the electron pressure gradient is found to suppress the speed of both the Alfvén solitons considerably for A (=2QM2/βk2z, with Q the electron-to-ion mass ratio) less than unity.

The evolution of solitary waves is studied using the KP equations derived by employing the reductive perturbation technique. It is revealed that the relativistic effect plays a role in the formation of double layers in multi-temperature electron plasmas. Because of the singularity in soliton propagation introduced by the zeros in the nonlinear coefficients of the KP wave equations. Owing to the appearance of zeros in the nonlinearity, there might be a continuous modification of the KP equation. The relevant properties of compressive and rarefactive solitons as well as shock-like soliton are considered. It is possible that the layers of various solitary waves might be observed in plasmas.

The bulk flows and magnetic-field fluctuations of the plasma sheet are investigated using single-point measurements from the ISEE-2 Fast Plasma Experiment and fluxgate magnetometer. Ten several-hour-long intervals of continuous data (with 3 s and 12 s time resolution) are analysed. The plasma-sheet flow appears to be strongly ‘turbulent’ (i.e. the flow is dominated by fluctuations that are unpredictable, with rms velocities[Gt]mean velocities and with field fluctuations≈mean fields). The flow velocities are typically sub-Alfvénic. The flow-velocity probability distribution P(v) is constructed, and is found to be well fitted by exponential functions. Autocorrelation functions [Ascr](τ) are constructed, and the autocorrelation times τcorr for the flow velocities are found to be about 2 min. From the flow measurements, an estimate of the mixing length in the plasma sheet is produced, yielding Lmix≈2 Earth radii; correspondingly, the plasma-sheet material appears to be well mixed in density and temperature. An eddy viscosity for the plasma sheet is also estimated. Power spectra, which are constructed from the v(t) and B(t) time series, have portions that are power laws with spectral indices that are near the range of those expected for turbulence theories. The plasma sheet may provide a laboratory for the study of turbulence in parameter regimes different from that of solar-wind turbulence: the plasma sheet is a β[Gt]1, hot-ion plasma, and the turbulence may be strongly driven rather than well developed. The turbulent nature of the flow and the disordered nature of the magnetic field have implications for the transport of plasma-sheet material, for the penetration of the solar-wind electric field into the plasma sheet, and for the calculation of particle orbits in the magnetotail.

The properties of longitudinal and transverse oscillations in isotropic unmagnetized plasmas of arbitrary composition are investigated on the basis of the Maxwell equations and the relativistic Vlasov equation. We show that in such plasmas (i) superluminal longitudinal and transverse waves undergo no collisionless Landau damping; and (ii) all isotropic particle distributions functions are stable against the excitation of longitudinal and transverse waves. Specializing to the textbook example of longitudinal fluctuations in a pure-electron equilibrium plasma, we derive various analytical expressions for the longitudinal dispersion relation valid for any value of the electron plasma temperature characterized by the dimensionless parameter μ=mec2=kBTe, which allow simpler approximations for non-relativistic (μ[Gt]1) and ultrarelativistic (μ[Lt]1) temperatures. The resulting longitudinal dispersion relation for nonrelativistic temperatures μ[Gt]1 can be expressed in terms of the Fried and Conte plasma dispersion function, but does not agree with the earlier results of Landau, Jackson and others, which were calculated starting from the non-relativistic form of the Vlasov–Maxwell equations. We establish a significant difference in the argument of the plasma dispersion function. This difference only shows up because we have started from a relativistically correct formulation of the kinetic theory, and it profoundly influences the results for the real and imaginary part of the frequency also for non-relativistic plasma temperatures. We show that the non-relativistic limit of the longitudinal dispersion relation reduces to the Landau–Jackson form only if the (unphysical) limit of an infinitely large value of the speed of light c→∞ is taken.

This paper reviews some of the conditions for electron MHD, and shows how this leads to anomalous resistivity, which plays an important role in the behaviour of plasma opening switches (POS are switches, based on the modification of the global resistance of a plasma flow, used to shorten the characteristic time of pulse power machines and increase their performances). Anomalous resistivity is build up from a transfer of momentum between the electrons and ion acoustic waves, which in turn modify the momentum of ions. One of the main consequences of EMHD is the diffusion of B in the plasma. A rough model envisions this diffusion as a propagation of nonlinear waves that may compete with convective transport.

The effect of a stochastic electron background on the stability of low-m tearing modes is studied. The coherent direct interaction approximation is applied to the drift-kinetic equation in order to include the effects of current density diffusion, electron density diffusion and magnetic field diffusion in the tearing mode equations. It is found that for the m=1 mode in the collisionless regime the magnetic field diffusion destabilizes the mode further, while in the collisional regime these diffusions may not significantly affect the mode; for the m[ges ]2 modes in the collisional and semicollisional regimes the electron density diffusion stabilizes the modes, while in the collisionless regime the effects of current density diffusion and electron density diffusion may balance each other without significantly affecting the modes.

Electric current sheets develop in the solar corona when different flux systems come into contact. At these sheets magnetic energy is transformed into heat and kinetic energy by means of reconnection. We have previously demonstrated how to accelerate neutral sheet energy conversion by means of a transition to turbulent reconnection via ideal, three-dimensional secondary instabilities, as conjectured by Montgomery. In this paper we describe how our previous results are modified by the presence of a finite mean sheetwise magnetic field. We find a stabilization from this field, due to a decrease in energy transfer from the basic magnetic field to the three-dimensional perturbed fields. An increase in perturbed dissipative energy losses is also observed.

A theory of current and energy modulations in an electron beam propagating through a grounded drift tube is developed. The theoretical analysis is based on the assumption that each beam segment has a prescribed current profile at the drift-tube entrance. It is shown that the current modulation amplitude decreases, reaches its minimum value and increases as the beam propagates downstream. The current modulation in the linear regime is described by a sum of the forward and backward density waves. On the other hand, the energy modulation in the linear regime is described by the difference between the forward and backward density waves.

The two-dimensional spatial and temporal evolution of the components of a plasma surrounding an electrode whose potential is suddenly decreased is investigated numerically. The electrode contains a localized convex or a localized concave voltage perturbation. The quasineutral plasma consists of positive ions and various proportions of negative ions and electrons. The results are compared and contrasted with those obtained with a uniform electrode under identical plasma conditions.

The second-order approximation in a quasiparticle description of magnetized plasmas is studied. The second-order adiabatic invariant of quasiparticle motion is found. Global adiabatic invariants for a magnetized plasma are revealed, and their possible role in energy exchange between particles and fields, nonlinear mode cascades and global plasma stability is discussed.

A general class of solutions is found for the resistive hydromagnetic equation. The results are applied to the case of a prescribed stagnation point flow. It is shown that general solutions with magnetic null points exist. Scaling laws for the length and the width of the current sheet of the solutions are given for the general case. It is shown that the length of the current sheet increases with magnetic Reynolds number unless the outflow boundary conditions prevent the sheet length from growing. The latter would result in the appearance of boundary layers in the outflow region.

It is shown here that the marginally mirror-trapped fraction of newly born fusion alpha particles in deuterium–tritium (DT) reaction-dominated tokamak plasmas can induce a two-stream cyclotron radiative instability for fast Alfvén waves propagating near the harmonics of the alpha-particle cyclotron frequency ωcα. This can explain both the experimentally observed temporal behaviour and spatially localized origin of the fusion-product ion cyclotron emission (ICE) in TFTR at frequencies ω≈mωcα.

In this paper commemorating the 60th birthday of David Montgomery, issues tantalizing to me that evolved from my collaborations with him are highlighted: sustainment of the magnetohydrodynamic dynamo, relationships between discrete and continuous models of media, and closures for nonlinear theories of discrete and continuous media. In particular, it is speculated that a tractable statistical model of a magnetohydrodynamic dynamo may arise from a proper formulation of a solenoidal expansion basis for the magnetic and velocity fields in the context of an appropriate closure, such as the eddy-damped quasinormal Markovian model.

A collision integral for three-wave interactions in a collisionless plasma is derived from the full plasma description by means of the Klimontovich–Dupree and Maxwell equations. This collision integral differs from its traditional counterpart (calculated within the framework of Vlasov theory) by an additional functional factor. This means that the changes in the wave spectral density, which are induced by three-wave interactions, occur with a rate other than that calculated in Vlasov theory.

A new generation mechanism for electromagnetic waves near the fundamental plasma frequency is discussed. It is shown that a travelling Langmuir pump wave can nonlinearly convert into electromagnetic, Langmuir and ion-acoustic daughter waves via a hybrid stimulated modulational instability. The linear and nonlinear temporal dynamics of this modulational process involving coupling to a resonant ion-acoustic wave are studied. The wave energy conservation relations are derived. The roles of frequency mismatch, dissipation and wave dispersion in the temporal evolution of nonlinear coupling of two wave triplets are analysed.

The role of magnetic fields during the initial stages of protostellar cloud collapse is investigated, in particular with respect to the scenario where the magnetic force can be an effective compressor due to diamagnetic effects. A multifluid approach involving electrons, ions of different atomic masses and neutrals is adopted, where each species is treated separately. The electron fluid is compressed by the magnetic pressure force, and the other ion species are pulled by the collective electric field developed by the space charge separation. The neutrals are also dragged owing to collisions with the ions. The difference in charge-to-mass ratio ensures that each ion species is accelerated differently, resulting in a distribution following their atomic masses. This model explores the scenario where the electromagnetic forces can achieve a supercritical mass-to-flux ratio in a magnetized cloud before dynamical collapse due to gravity takes over.

Scaling analysis is used to derive approximations of magnetohydrodynamics with self-consistent leading-order dynamics under general conditions of anisotropy. Both incompressible and weakly compressible limits are considered. The horizontal magnetic and velocity fields obey dynamics given by a reduced closed set of equations, but the vertical components have different decoupled dynamics in the different regimes. Conservation laws are also discussed. It is shown that the reduced equations are not self-consistent unless either the ratio of vertical to horizontal length scales is large or the fluid is gravitationally stratified and the ratio of length scales is small. New equations are derived for a rotating stratified plasma, which are an extension of the quasigeostrophic equations of neutral fluids.

The existence and properties of nonlinear lower-hybrid-like waves in a dusty plasma are discussed. It is found that both rarefactive and compressive types of solitons can exist in the presence of isothermal hot electrons and negatively charged dust grains. Only compressive solitons are found to occur when the dust grains are positively charged or the hot electrons are non-isothermal. The amplitude, velocity, width and nature of the nonlinear structure are greatly affected by the hot electron concentration as well as by the charge-to-mass ratio of the dust grains.

We present a Rayleigh–Ritz variational model of the electron sheath in crossed-field devices. Using laminar layer orbits, this model predicts equilibria in agreement with the classical Brillouin model. Other predictions include the existence of existence of non-classical, stable equilibria as well as limit cycles. This variational model also predicts the existence of a stable propagating diocotron mode, which is absent from the Brillouin model but is present in the double-box model of Kaup and Thomas. Comparisons of these results with those of the cold-fluid theory are discussed.