An analysis of two experimental observations of Langmuir wave collapse is performed. The corresponding experimental data are shown as evidence against the occurrence of collapses. The physical reason preventing the collapses is found to be the nonresonant electron diffusion in momentum. In this process, plasma thermal electrons are efficiently heated at the expense of wave energy, therefore intense collisionless wave dissipation takes place. The basic reason for the underestimation of nonresonant electron diffusion in the traditional theory is shown to be the substitution of a real plasma by a plasma probabilistic ensemble.

A study of nonresonant electron diffusion refraining from ensemble substitution is performed. It is shown that its intensity is sufficient for suppression of Zakharov's short-wavelength modulational plasma instability [Zakharov, V. E., Sov. Phys. JETP35, 908 (1972)]. This explains the nonoccurrence of Zakharov's Langmuir wave collapse in experiments.

We investigate particle acceleration by strong lower-hybrid turbulence produced by the relaxation of an energetic perpendicular ion ring distribution. Ion ring distributions are associated with counterstreaming plasma flows in a magnetic field, and are found at perpendicular shocks as a result of ion reflection from the shock surface. Using a 2½D particle-in-cell (PIC) code that is fully electromagnetic and relativistic, we show that the ion ring is unstable to the generation of strong plasma turbulence at the lower-hybrid resonant frequency. The lower-hybrid wave turbulence collapses in configuration space, producing density cavities. The collapse of the cavities is halted by particle acceleration, producing energetic electron and ion tails. For solar flare plasmas with temperatures of 1 keV and a ratio of the plasma frequency to the electron cyclotron frequency of ½, we demonstrate electron acceleration to energies up to MeV, while the ions are accelerated to energies in the region of several MeV.

Starting from a memory-function formalism coupled with the Green– Kubo formula and an approximate expression for the generalized Coulomb logarithm, the electric conductivity of a dense high-temperature hydrogen plasma is studied. A pseudopotential model, taking account of short-range quantum effects and long-range screening-field effects, is employed to include quantum mechanical and polarization effects. An analytical formula for the Coulomb logarithm is proposed when the thermal de Broglie wavelengths are rather smaller than the Debye radius. A minimum in the curve of electrical conductivity is found and some physical evidence for its appearance is produced.

The composition of a non-ideal plasma and the thermodynamic functions and conductivity of the system are calculated using the minimum Gibbs free energy method. Interactions of charged particles are described by a pseudopotential model that takes into account collective phenomena (high-order correlation effects) in the plasma.

An electrodeless plasma produced in an ion guide ion source by a high-energy heavy ion beam propagating through a noble gas has been studied. The plasma parameters in Ar and He buffer gases have been calculated and analysed for projectile beam intensities from 10$^{9}$ to 10$^{17}$ pps cm$^{-2}$. The plasma is characterized by very low electron and gas temperatures, but with an appreciable number of electrons with energy sufficient to ionize atoms of most chemical elements. The ion guide ion source plasma, combined with laser-enhanced ionization of the studied element, is used to improve the detection limit by enhancing the signal-to-noise ratio.

Magnetosonic–gravity waves in an isothermal non-dissipative atmosphere, with a uniform horizontal external magnetic field have been considered in the literature in two cases: (i) ‘one-dimensional’ magnetosonic–gravity waves, in the case of zero horizontal wavenumber and (ii) ‘two-dimensional’ magnetosonic–gravity waves, in which the horizontal wave vector lies in the plane of gravity and the external magnetic field. In the present paper, an extension of case (i) is considered that is distinct from case (ii). This case (iii) is that of magnetosonic–gravity waves with a horizontal wave vector orthogonal to the plane of gravity and the external magnetic field. Since the wave fields depend only on two spatial coordinates and time, the problem could be called ‘two-and-half’-dimensional. The three-dimensional magnetosonic–gravity wave propagates a magnetic field perturbation parallel to the external magnetic field, and velocity perturbations transverse to it. Elimination for the vertical velocity perturbation leads to a second-order wave equation, with four regular singularities. Three regular singularities specify (a) the wave fields at high altitude, where there are two cut-off frequencies involving the acoustic cut-off frequency; (b) the wave fields in the deep layers, where another two cut-off frequencies appear, involving both the acoustic and gravity cut-off frequencies; and (c) the transition between the two regimes, occurring across a critical layer, where one solution of the wave equation vanishes and the other has a logarithmic singularity in the amplitude and also a phase jump. The whole altitude range can be covered using the three pairs of solutions of the wave equation, obtained by expanding in Frobenius–Fuchs series about each regular singularity. The power series solutions are used to plot the wave fields, for several values of the three dimensionless parameters of the problem, namely the plasma $\beta$, frequency and wavenumber. It is shown that the presence of a horizontal wave vector transverse to the plane of gravity and the external magnetic field, can change the properties of the waves significantly: first, the two cut-off frequencies may cease to exist, in which case the full wave frequency spectrum can propagate; secondly, the critical layer occurs at different altitudes for different frequencies, allowing gradual absorption of the waves (e.g. in the solar transition region).

We explore the level of saturation of instabilities in a two-species plasma using a combination of matched asymptotic expansion and numerical computation. The plasma is assumed to be spatially periodic, and the domain size is chosen to allow a single mode to become unstable when a bump is added to the tail of the distribution of the lighter species. We consider two versions of the problem, arising when the mass ratio of the two species is either very small, or of the order of unity. For small mass ratios, the initial saturation level of the mode amplitude, as measured by the electric field disturbance, follows the ‘trapping scaling’. For mass ratios of order unity, nonlinear effects become important at the level predicted by Crawford and Jayaraman, but the instability does not saturate there and continues to grow. In both cases, the initial onset of nonlinearity does not reflect the longer- time evolution of the system. In fact, the system passes through multiple stages of evolution in which the electric field amplitude is not simply predicted; none of the previously published scalings are adequate. Eventually, for both cases, the distribution of the lighter ions becomes significantly rearranged, and much (though not all) of the destabilizing bump is flattened. A better predictor of the strength of the instability is given by the extent of these rearrangements.

We investigate the structure of nonlinear stationary waves propagating obliquely to the magnetic field in a cold bi-ion plasma. By using the constants of motion that follow from the multi fluid equations, the system may be described by four coupled first-order differential equations. A new constant of motion characterizing a bi-ion flow (called the ‘energy difference integral’) is found. The combination of relations between the flow speeds derived from the conservation laws, which we call the ‘momentum–energy hodographs’, reveal some important features of stationary waves and solitons. Soliton solutions representing both compressions and rarefactions in the ion fluids exist in specific windows in the ‘Alfvén Mach number–obliquity’ space. In other windows, solutions characterized by both oscillating and soliton properties (‘oscillitons’) exist. Critical Mach numbers and propagation angles narrow the size of the windows where smooth soliton solutions can be constructed.

We present two findings: (i) in the absence of compressibility of (cold) neutrals, the instability triggered by neutral drift due to gradients in plasma density and magnetic field and (ii) the equilibrium pressure-gradient- (PG-) driven instabilities with and without neutral dynamics. The results of detailed analysis suggest that on account of neutral dynamics the growth of these instabilities is suppressed by a neutral perturbational effect. Suppression of PG-driven instabilities is highlighted and the critical wavelength for sustainable growth is found. The physics involved in the mechanism of instabilities is discussed. The endeavour in these investigations is justifiable in its usefulness for understanding the phenomena pertinent to partially ionized plasma (PIP). The stability analysis presented here may be useful for understanding the plasma processes in ‘deep’ coma (very near to the nucleus of coma), at the edges of a plasma column, near a divertor region and in the case of sublimating gas from a cloud.

The Monte Carlo method is used to simulate ion transport in an Ar plasma electrode sheath with a non-uniform dust. Charge exchange and elastic collisions between ions and neutral atoms and also the collection and Coulomb scattering of ions on the dust particles are examined during the motion of ions in the sheath. In order to study the effect of the non-uniform dust density and size on ion transport, we choose an exponent dust density distribution with a uniform dust size and a normal dust radius distribution with a uniform density and compare the simulation results with those for a uniform dust. It is found that both a non-uniform and a uniform dust density affect the ion density arriving at the electrode significantly and to the same degree. At the same time, it is also found that a non-uniform and uniform dust size influence the ion density arriving at the electrode greatly, but with a slight difference. Therefore, although the dust content is very low in most processing plasmas, its influence becomes evident whether its content is uniform or non-uniform in content and size. So, we can come to the conclusion that the key factors affecting the influence of dust particles on plasma behaviour are the linear density and the average radius of dust particles rather than their distribution.

Using linear kinetic plasma theory the relation between electron density and magnetic field fluctuations for low-frequency plasma waves for Maxwellian background distribution functions of arbitrary temperatures in a uniform magnetic field is derived. By taking the non-relativistic temperature limit this ratio is calculated for the diffuse intercloud medium in our Galaxy. The diffuse intercloud medium is the dominant phase of the interstellar medium with respect to radio wave propagation, dispersion and rotation measure studies. The differences between the relation of electron density and magnetic field fluctuations from the linear kinetic theory compared with the classical magnetohydrodynamics theory are established and discussed.

This paper discusses the excitation of Bernstein waves by pickup ions, which occur in the low corona, particularly the active regions. The present research is motivated by the study of solar plasma and radio-emission processes. However, emphasis is placed in this paper on the stability analysis rather than applications of the result. The analysis encompasses two limiting cases: one is when the pickup ions have small velocity dispersion; the other is when the ion velocity dispersion is significant.

Distribution of antenna-driven electromagnetic wave fields within the local Alfvén resonance is studied under resonant conditions such that the axial period of the fundamental harmonic of the magnetohydrodynamic oscillations in the axial direction is twice as large as the pitch length, and the poloidal number of the fundamental harmonic is twice as small as the polarity of the helical coils. It is shown that taking account of the helical inhomogeneity of the steady magnetic field can remove the infinite discontinuity of the wave fields that takes place in a straight magnetic field rather than with electron inertia or a finite ion Larmor radius.

Advection–diffusion equations occur in a wide variety of fields in many contexts of active and passive transport in fluids and plasmas. The effects of sheared advective flows in the presence of irreversible processes such as diffusion and viscosity are of considerable current interest in tokamak and astrophysical contexts, where they are thought to play a key role in both transport and the dynamical structures characteristic of electromagnetic plasma turbulence. In this paper we investigate the spectral and evolutionary properties of relatively simple, linear, advection–diffusion equations. We apply analytical approaches based on standard Green function methods to obtain insight into the nature of the spectra when the advective and diffusive effects occur separately and in combination. In particular, the physically interesting limit of small (but finite) diffusion is studied in detail. The analytical work is extended and supplemented by numerical techniques involving a direct solution of the eigenvalue problem as well as evolutionary studies of the initial-value problem using a parallel code, CADENCE. The three approaches are complementary and entirely consistent with each other when an appropriate comparison is made. They reveal different aspects of the properties of the advection–diffusion equation, such as the ability of sheared flows to generate a direct cascade to high wave numbers transverse to the advection and the consequent enhancement of even small amounts of diffusivity. The invariance properties of the spectra in the low diffusivity limit and the ability of highly sheared, jet-like flows to ‘confine’ transport to low shear regions are demonstrated. The implications of these properties in a wider context are discussed and set in perspective.

Magnetic reconnection is one of the most efficient ways of transforming magnetic into kinetic and thermal energies. We prove a general identity relating the energy transfer in a neighborhood of a current sheet, where reconnection is assumed to occur. With some reasonable hypotheses regarding the geometry of stream and field lines, we prove that for a constant rate of transformation of magnetic energy, the width of the current sheet must grow with the plasma conductivity. Hence an enhanced diffusivity seems necessary for certain classical models of fast reconnection to work.

Extra and very low-frequency (ELF/VLF) wave generation by modulated polar electrojet currents is studied numerically. Through Ohmic heating by the amplitude-modulated high-frequency heating wave, the conductivity and thus the current of the electrojet are modulated accordingly to set up the ionospheric antenna current. Stimulated thermal instability, which can further enhance the electrojet current modulation, is studied. It is first analysed analytically to determine the threshold heating power for its excitation. The nonlinear evolutions of the generated ELF/VLF waves enhanced by the instability are then studied numerically. Their spectra are also evaluated. The field intensity of the emission at the fundamental modulation frequency is found to increase with the modulation frequency in agreement with the Tromso observations. The efficiency enhancement by the stimulated thermal instability is hampered by inelastic collisions of electrons with neutral particles (mainly due to vibration excitation of N2), which cause this instability to saturate at low levels. However, the electron inelastic collision loss rate drops rapidly to a low value in the energy regime from 3.5 to 6 eV. As the heating power exceeds a threshold level, significant electron heating enhanced by the instability is shown, which indeed causes a steep drop in the electron inelastic collision loss rate. Consequently, this instability saturates at a much higher level, resulting to a near step increase (of about 10–13 dB depending on the modulation wave form) in the spectral intensity of ELF radiation. The dependence of the threshold power of the HF heating wave on the modulation frequency is determined.

The eddy-damped quasi-normal Markovian (EDQNM) two-point statistical closure model is used to study the non-linear triadic energy transfer processes in three-dimensional, incompressible, isotropic, non-helical magnetohydrodynamic (MHD) turbulence in the primitive variable formulation. The triadic transfer functions, which arise from the closure of the three-point correlations in the kinetic energy and magnetic energy spectrum evolution equations, are calculated for non-helical MHD turbulence as a function of wavenumber $k$ for given values of $(p,q)$ with $q$ fixed and $p$ varied. These functions describe the magnitude of the energy transferred into or out of a given mode with wavenumber $k$ due to all allowed interactions of modes with wavenumbers $p$ and $q$ satisfying ${\bf k}={\bf p}+{\bf q}$, and their integrals over all $p$ and $q$ yield the kinetic energy and magnetic energy transfer spectra. Rather than solving the time-dependent, coupled EDQNM equations using initial, prescribed energy spectra, assumed forms of the kinetic energy spectrum $E_{v}(k)$ and the magnetic energy spectrum $E_{B}(k)$ having both a production subrange spectrum proportional to $k$ and a Kolmogorov inertial subrange spectrum proportional to $k^{-5/3}$ are used to evaluate instantaneous values of the triadic transfer functions. The two cases $r_{A}\,{=}\,1$ and $\frac12$ are considered, where $r_{A}$ is the Alfvén ratio. The individual contributions to the transfer functions are also computed in order to determine the dominant interactions that contribute to the total spectral energy transfers. The triadic transfers exhibit forms similar to those found in previous studies of incompressible, isotropic Navier–Stokes turbulence. The non-local-in-wavenumber triadic interactions dominate the local-in-wavenumber interactions, which indicates that the transfer process in non-helical MHD turbulence is primarily local in scale. As the Alfvén ratio decreases, it was found that the most non-local triadic interactions resulted in kinetic energy input and magnetic energy removal at most wavenumbers $k$. The decomposition of the triadic kinetic and magnetic energy transfer functions into their constituents showed that increasing non-locality of the wavenumber interaction involving modes $p$ and $q$ responsible for the net energy transfer into a given wavenumber $k$ corresponds to a qualitative change in the cascade dynamics; namely, there is less energy removal at smaller $k$ and primarily energy input at nearly all $k$. An increased degree of non-locality also results in more cancellation between constituent triadic transfer functions.

Experimental line profiles can be altered by so-called spectral interference that appears due to different causes. One of the most commonly appearing sources of interference is random noise due to the detector used for measuring the spectra in the laboratory. In this work, the noise is studied using a simulation procedure. This study helps to pinpoint the changes that the noise produces in the parameters of the experimental profiles and, consequently, in the information provided by them.

The lower-hybrid drift instability of a plasma driven by relative ion–electron motion is analyzed in the framework of the modified magnetohydrodynamic equations. The Hall contribution is expressed in terms that offer a simple physical interpretation of the process and allow a comprehensive study of various features and limits of instability. It is shown that in the chosen terms there are clear-cut ranges of magnetosonic drift, lower-hybrid drift, and kinetic versions of instability that have different properties. It is shown for the first time that the instability may have, besides a flute-like structure, a cell-like one as well. On the basis of the performed analysis, a new classification of the phenomenon is offered.

A theoretical investigation of the interaction between a charged-particle beam and ordinarily polarized surface waves at the first two harmonics of ion and electron cyclotron frequencies propagating across an external steady magnetic field is carried out. The case of weak plasma spatial dispersion when the Larmor radii of plasma particles are much less than the penetration depth of these waves into the plasma is considered. The indicated waves are eigenmodes of the planar plasma–vacuum–metal waveguide structure, which is exposed to a steady magnetic field oriented parallel to the plasma surface. It is supposed that a cold charged- particle beam propagates over the plasma surface in the vacuum region. The beam density is less than the plasma density. Excitation of these O modes at the first and second harmonics of electron and ion cyclotron frequencies caused by the both resonant beam and dissipative instabilities is studied analytically. The dependence of their growth rates on parameters of the considered waveguide structures is also examined.