The effects of lithium and heavier ions (i.e. having specific charge Zj e/mj smaller than the proton's), on the electromagnetic cyclotron instability in warm anisotropic (bi-maxwellian) proton plasmas, are analytically and numerically investigated. It is found that the occurrence of resonances for modes with ωr = Ωj (ωr, real wave frequency; Ωj, ion gyrofrequency), which produce a cut-off in the phase velocity, leads to the possible existence of two different situations. (i) If the proton thermal anisotropy is smaller than a critical value A pc (which depends on the ion specific charge), the resonance and therefore the ‘stop band’ occurs outside the unstable range. The maximum growth rate, as well as the instability range in k space, may increase with the relative ion concentration ion and warm proton particle densities, respectively. There is an optimum value of η for maximum enhancement of the instability. (ii) If Ap > Apc, the unstable spectrum in Wr space is divided by the ‘stop band’ into two distinct regions. In the higher-frequency region, the maximum growth rate, as well as the width of the unstable spectrum, decrease with increasing relative ion concentration. For η > ηc, the instability is completely suppressed. In the lower-frequency region, however, the maximum growth rate and the extension in k space may increase with η.

We show theoretically that the electron density and temperature of a plasma could be deduced from the measurements of the transfer impedance between two small dipole antennae, each much shorter than a Debye length, separated by a distance of ten or more Debye lengths. In contrast to the quadripole probe, this ‘double-dipole probe’ relies on not producing perturbations in the plasma, rather than on minimizing their effects. The plasma is assumed to be warm and isotropic, and the motion of the ions is neglected. First, it is shown that, in a Maxwellian plasma, the frequency response of a double-dipole probe is easier to interpret than that of a quadripole probe with the customary square layout. Then, in a second step, the transfer impedance of the former probe is calculated in a Cauchy plasma, and the results are compared with those previously obtained in a Maxwellian plasma. By so doing, we show that, for large distances between the dipoles, the real part of the transfer impedance is sensitive to the form of the tail of the distribution function.

In a high-density magnetized plasma composed of two ion species of different charge-to-mass ratios, electrostatic waves propagating across the magnetic field exhibit a resonance at the Buchsbaum or ion-ion hybrid frequency, in addition to the resonances at the upper and lower hybrid frequencies. In this paper, the possibility of stimulated scattering of electromagnetic waves incident normal to the magnetic field from electrostatic waves at the ion-ion hybrid frequency is investigated. Based on the cold-plasma equations, it is found that such a process is theoretically possible. Formulas for the threshold power and growth rate are obtained, which show that the threshold power is much greater, and the growth rate much less, than those of stimulated scattering from upper and lower hybrid waves.

Dispersion relations are obtained for surface waves propagating along plasma—vacuum and plasma—plasma interfaces in a direction perpendicular to a uniform magneto-static field (that lying parallel to the interface), by setting to zero reflexion coefficient for an incident plane wave. Complete solutions to the dispersion relations are presented for both cases and the results are displayed diagrammatically. Comparisons are made with the work of other authors.

Some comments are made about how to determine the speed with which magnetic flux is carried towards an X-type neutral point and reconnected. Conditions in the diffusion region near the neutral point are also investigated, with the conclusion that the streamlines and magnetic field lines cannot both be locally hyperbolic. Instead, two distinct modes may be possible. In the first, the magnetic field lines are straight, and the diffusion region does not differ greatly from that described by Parker (1963). In the second, the fluid velocity components increase cubically away from the neutral point with the result that, for a given reconnection rate, the diffusion region is typically five times greater than in the Parker model.

A family of exact solutions to the MHD equations is presented for steady incompressible two- and three-dimensional flow in the vicinity of the stagnation point, which forms in a current sheet separating two colliding plasma streams. The magnetic field in each plasma is strictly parallel to the current sheet, but can have different magnitudes and directions. Resistive and viscous effects are accounted for. These flows are of considerable interest in connexion with the magnetic field merging process. They represent the limit of resistive field annihilation with zero reconnexion.

We consider the electromagnetic field produced by an azimuthal current of arbitrary shape, external to a plasma. In particular, we study absorption of the field energy by a radially inhomogeneous plasma cylinder with double ioncyclotron resonance. We calculate the power absorbed by plasma ions for an arbitrary ion distribution function. We show that, in some regions of the plasma it is possible for the work done by the field on the currents to be negative. We also show that, if the plasma has regions with a large density gradient (e.g. a narrow transitional plasma-vacuum layer), power absorption increases sharply. We consider, too, the exciting of the natural oscillations of the cylinder by external currents. In this case, power absorption is independant of the dissipative properties of the plasma.

The second-order elliptic partial differential equation, which describes a class of static ideally conducting magnetohydrodynamic equilibria with helical symmetry, is solved analytically. When the equilibrium is contained within an infinitely long conducting cylinder, the appropriate Dirichiet boundary-value problem may be solved in general in terms of hypergeometric functions. For a countably infinite set of particular cases, these functions are polynomials in the radial co-ordinate; and a solution may be obtained in a closed form. Necessary conditions are given for the existence of the equilibrium, which is described by the simplest of these functions. It is found that the Dirichlet boundary-value problem is not well-posed for these equiilbria; and additional information (equivalent to locating a stationary value of the hydrodynamic pressure) must be provided, in order that the solution be unique.

The paper compares the methods of re-normalization introduced by the Dupree—Weinstock theory of turbulence with those in the theories of Kadomtsev, Rudakov and Tsytovich. It is shown to what extent they are equivalent.

We examine stimulated Raman scattering in an inhomogeneous plasma, and demonstrate that there may be an exponentially growing decay, which owes its existence to reflexion of the electromagnetic decay wave. Unlike absolute instabilities, which depend on simultaneous vanishing of the wavenumber mismatch and its derivative, it can occur over a wide spectrum of decay wave frequencies, and may, therefore, be of greater practical importance than those instabilities.

The effect of the interaction of plasma and neutral gas on the structure of switchtype shock waves propagating in a partly-ionized gas is studied. These shocks, in which the magnetic field is perpendicular to the shock front either upstream or downstream, exhibit a spiralling behaviour of the magnetic field in the shock transition region, if the Hall term is important in the Ohm's law. Observation of this behaviour for shocks propagating into a plasma with a residual neutral content of ~ 15 % has implied an anomalously high resistivity of the plasma. We show that this can be partly explained by considering the collisions of ions with the neutral atoms in a magnetic field. We show that the extra dissipation due to the increase in resistivity goes primarily to the ions and neutrals. Thus even in the absence of viscous dissipation within each species, the heavy particles can be appreciably heated in a shock propagating into a partly-ionized gas in a magnetic field.

This paper studies the nonlinear coupling between an ion—acoustic wave and two Alfvén waves. On the basis of the MHD equations, this three-wave interaction is described within the coupled normal-mode theory. It is shown that a largeamplitude acoustic wave can decay into a pair of Alfvén waves in a high-β plasma. Calculations of the threshold condition and the maximum growth rate suggest that this decay process may occur both in the laboratory and in the ionosphere.

Ion-acoustic solitons are observed to form when a large-amplitude positive step voltage is applied to a single mesh grid. For the formation of the soliton, the sheath about the grid plays an important rôle. The number and the amplitude of the solitons are examined experimentally as a function of the nonlinear index, and are compared with theory. The observed width and the velocity of the soliton are well explained by the theory of a hot-ion fluid and Boltzmann electrons. The phase delay of two solitons by a head-on collision is also observed.

Charge transfer and diffusion cross-sections of low-lying excited states are calculated using ab initio quantum-mechanical potential-energy curves. These data are used to calculate the reactive thermal conductivity λR of an atmospheric nitrogen plasma. Differences of up to 20 % are found between the present values of λR and the corresponding values obtained by increasing the cross-sections of excited states by a factor of 2. These differences propagate, to a minor extent (10 %), in the total thermal conductivity.