Non-zero Electron Temperature Research Articles

MAGNETIC HELICITY OF ION KINETIC TURBULENCE WITH A NONZERO ELECTRON TEMPERATURE

ABSTRACT Hybrid numerical simulations of strong turbulence with a nonzero electron temperature are carried out in the proton kinetic range. The turbulent cascade is initiated by a large-scale spectrum with a nonzero cross-helicity. The turbulence evolves freely and produces a magnetic helicity spectrum with a peak at smaller scales. Testing is performed to verify that the shape of the peak is not affected by numerical artifacts. The magnetic helicity spectrum is shown to be determined by both the electron and proton thermal pressures, rather than the proton pressure alone. Implications for the observed correlations between the magnetic helicity and the plasma parameters in the solar wind are discussed.

Nonzero electron temperature effects in nonlinear mirror modes

The nonlinear theory of the magnetic mirror instability (MI) accounting for nonzero electron temperature effects is developed. Based on our previous low-frequency approach to the analysis of this instability and including nonzero electron temperature effects a set of equations describing nonlinear dynamics of mirror modes is derived. In the linear limit a Fourier transform of these equations recovers the linear MI growth rate in which the finite ion Larmor radius and nonzero electron temperature effects are taken into account. When the electron temperature Te becomes of the same order as the parallel ion temperature T∥ the growth rate of the MI is reduced by the presence of a parallel electric field. The latter arises because the electrons are dragged by nonresonant ions which are mirror accelerated from regions of high to low parallel magnetic flux. The nonzero electron temperature effect also substantially modifies the mirror mode nonlinear dynamics. When Te≃T∥, the transition from the linear to nonlinear regime occurred for wave amplitudes that are only half that which was inherent to the cold electron temperature limit. Further nonlinear dynamics developed with the explosive formation of magnetic holes, ending with a saturated state in the form of solitary structures or cnoidal waves. This shows that the incorporation of nonzero temperature results in a weak decrease in their spatial dimensions of the holes and increase in their depth.

Parametric interaction of kinetic Alfvén waves with convective cells

The theory of parametric interaction of kinetic Alfvén waves (KAWs) with convective cells in finite β plasmas is reconsidered. We use nonlinear equations that account for nonzero electron temperature effects. With the help of a standard multiscale expansion, we then derive a general equation for the electrostatic and magnetostatic convective cells. It is found that the electrostatic convective cells represent vortex plasma motions (VPM) with very large impedance ratio. They are associated with the large‐scale part of the dispersion relation. On the other hand, the short‐scale convective cells correspond to filamentary field‐aligned current (FAC) structures or magnetostatic modes. A close inspection of our new dispersion relation shows that the electrostatic convective cells as well as the magnetostatic modes do not grow when the Alfvén waves are in the kinetic regime, i.e., in finite β plasmas. The relevance of our theory to satellite observations is stressed.

Slow drift mirror modes in finite electron‐temperature plasma: Hydrodynamic and kinetic drift mirror instabilities

We develop a fully kinetic linear theory of the drift mirror (DM) instability accounting for arbitrary particle velocity distribution functions including nonzero electron temperature effects and plasma pressure anisotropy. In the quasi‐hydrodynamic limiting case the theory reproduces the results obtained for the ion mirror instability. However, for the very low frequency electron DM modes which can develop in a nonuniform plasma of nonzero electron temperature, Te ≠ 0, such an equivalence does not exist. We refer to these modes as slow drift mirror (SDM) modes in order to distinguish them from the conventional ion‐DM mode. Two new instabilities, one hydrodynamic and one kinetic, that lead to the growth of SDM modes are found in the fully kinetic regime. The first instability develops for values of the ion anisotropy lower than required for the classical ion‐DM instability. The second instability occurs when the conditions for the ion‐DM instability are satisfied as well. The frequency of the SDM mode is less than the frequency of the ion‐DM mode, ω ≪ ωDM ∼ ωn, where ωn is the density gradient‐drift frequency of the ions. However, when the electron temperature is of the order of the parallel ion temperature, the SDM instability growth rate may become comparable or even higher than that of the ion‐DM instability. The free energy necessary for these new instabilities is taken from two sources. The main source is the energy stored in the ion pressure anisotropy. The second source is the electron pressure gradient which builds up in a plasma of nonzero electron temperature.

Drift mirror instability in space plasmas, 2, Nonzero electron temperature effects

A linear theory of drift mirror instability accounting for the nonzero electron temperature effects is developed. Generalizing our previous approach to the analysis of this instability by accounting for a nonvanishing parallel electric field, we have derived the expressions for the mode frequency and instability growth rate. The origin of the electric field is due to the electron pressure gradient which builds up in a plasma with nonzero electron temperature, because the electrons are dragged by mirror‐accelerated protons as they pass from regions of high magnetic flux into those of lower magnetic flux. The electrostatic force drag associated with the parallel electric field provides a substantial reduction of the wave phase velocity and increases the drift mirror instability threshold. It is shown that in a plasma with nonzero electron temperature the drift mirror mode is accompanied by the field‐aligned current which varies in phase with the compressional changes in the magnetic field. The transition to the cold electron temperature limit is discussed.

Electron correlations in an electron bilayer at finite temperature: Landau damping of the acoustic plasmon

We report angle-resolved Raman scattering observations of the temperature-dependent Landau damping of the acoustic plasmon in an electron bilayer system realized in a GaAs double-quantum-well structure. Corresponding calculations of the charge-density excitation spectrum of the electron bilayer using forms of the random-phase approximation (RPA), and the static local field formalism of Singwi, Tosi, Land and Sjölander (STLS) extended to incorporate non-zero electron temperature Te and phenomenological damping, are also presented. The STLS calculations include details of the temperature dependence of the intra- and inter-layer local field factors and pair correlation functions. Good agreement between experiment and the various theories is obtained for the acoustic plasmon energy and damping for Te TF/2, where TF is the Fermi temperature. However, contrary to current expectations, all of the calculations show significant departures from our experimental data for Te TF/2. From this, we go on to demonstrate unambiguously that real local field factors fail to provide a physically accurate description of exchange correlation behaviour in low-dimensional electron gases. Our results suggest instead that one must resort to a dynamical local field theory, characterized by a complex field factor to provide a more accurate description.

Relaxation of equilibrium density profiles in nonneutral electron plasma due to electron-neutral collisions

Under the assumption of constant frequency of electron-neutral collisions and constant electron temperature, analytical expressions are obtained for equilibrium space-time distributions of the electron density of a cylindrically symmetric, strongly magnetized, nonneutral electron plasma. It is found that the effect of nonzero electron temperature accelerates or slows down the relaxation and flattening of equilibrium density profiles depending on the initial density gradient sign.

The Alfvén resonance in a magnetized dusty plasma

Wave propagation in a dusty magnetized plasma at frequencies below and of the order of the ion-cyclotron frequency, and with a non-zero electron temperature is considered. The general dispersion relation for waves propagating at an arbitrary angle with respect to the external magnetic field and for finite ion-cyclotron frequency is derived and discussed. Modification of the Alfvén resonance absorption mechanism due to the presence of a finite proportion of the negative charge of the plasma on stationary dust grains is investigated.

Modified models of electron distribution in the magnetosphere at L = 2.3

It is shown that mathematical model results for the electron distribution in the magnetosphere can be reasonably well approximated by a conventional diffusive equilibrium model with decreased electron density at the reference level, the other parameters being the same as in the mathematical model. The magnetospheric conditions are appropriate to equinox and L = 2.3 for solar minimum and solar maximum. This conclusion reflects the fact that the electron density in the mathematical model decreases more rapidly when compared with the diffusive equilibrium model. The approximations to the model electron distribution results are used to compute the group delay of the signals from the NAA (24.0 kHz) and NSS (21.4 kHz) transmitters, with the perturbations due to the effects of finite electron density, ion effects and non-zero electron temperature effects taken into account.

Group delay times of whistler-mode signals from VLF transmitters observed at Faraday, Antarctica

The group delay times ( t g ) of whistler-mode waves generated by the NAA ( f= 24.0 kHz) and NSS ( f = 21.4 kHz) U.S. Navy transmitters and recorded at Faraday, Antarctica ( L= 2.3), after following a ducted field-aligned path are analysed theoretically for different L-shells of propagation using models of electron density, temperature, and ion composition distribution for typical day and night-time conditions. t g is presented as the sum of (1) a group delay time calculated for the simplest model of wave propagation parallel to the magnetic field in a cold, dense plasma with the effects of ions neglected ( t go ) and (2) the corrections due to finite electron density, that is, finite ratio of electron plasma frequency to electron gyro frequency ( Δt gc ), contribution of ions ( Δt gr ), and non-zero electron temperature ( Δt gh ). It is pointed out that the correction Δ t gc is the dominant one, while the ratio Δt gh/ Δt gc is only about 1 % for L close to 2.3. The total correction Δt gs , = Δt gc + Δt gr + Δt gh at L = 2.3is about 10 ms and is to be taken into account when interpreting the measurements of t g . However, on the assumption of strictly longitudinal propagation, the parameter [ t gm (NSS) – t gm (NAA)] t gm (NSS) [index m indicates measured parameters] can be used for estimating L without taking into account the corrections Δt gs , if we do not require an accuracy better than ± 0.02.

Whistler-mode polarization in a rarefied plasma

Approximate expressions for whistler-mode polarization taking into account the effect of finite electron density, the contribution of ions and the effect of non-zero electron temperature are derived in the form convenient for practical applications. These expressions are particularly important for the study of whistler-mode polarization in a rarefied magnetospheric plasma where the electron plasma frequency is not well above the electron gyrofrequency.

Storey angle for whistler-mode waves

The concept of the Storey angle, i.e. the angle of maximal deviation of the direction of whistler-mode group velocity from the magnetic field, is generalized for the case of a finite ratio of wave frequency to electron gyrofrequency with the effects of finite electron density, the contribution of ions, non-zero electron temperature and anisotropic character of their distribution function being taken into account.

Energy absorption in cold inhomogeneous plasmas: the Herlofson paradox

The Herlofson paradox is exemplified by a capacitor containing cold, collisionless, inhomogeneous plasma as the dielectric: its response to a sinusoidal driving signal can exhibit continuous energy absorption, even though the system is lossless. The underlying mechanism has been explained generally by Barston in terms of the transient response of the system. In this paper, we confirm Barston 's conclusions by examining in detail several analytically tractable cases of delta-function and sinusoidal excitation, and consider the effects of collisions and non-zero electron temperature in determining the steady state fields and dissipation. Energy absorption without dissipation in plasmas is analogous to that occurring after application of a signal to a network of lossless resonant circuits. This analogy is pursued, and extended to cover Landau damping in a warm homogeneous plasma, in which the resonating elements are the electron streams making up the velocity distribution. Some of the practical consequences of resonant absorption are discussed, together with a number of paradoxical plasma phenomena which can also be elucidated by considering a superposition of normal modes rather than a single Fourier component.

Beam-Plasma Interaction in a Warm Plasma†

ABSTRACT Theoretical studies of electron beam-plasma interactions have predicted the occurrence of amplifying waves having growth rates maximizing near to the electron plasma frequency. For the special case of an infinite beam interacting with a cold infinite plasma, an infinite growth rate is expected at the plasma frequency. In practice, the theoretical gain becomes finite if factors such as collisions or non-zero electron temperature are taken into account, or if the beam and plasma ore not both infinite. In this paper, beam-plasma interaction is investigated in planar geometry for both finite and infinite beams in an infinite, uniform, warm plasma. In the finite beam case, the question of appropriate boundary conditions to use at the beam edge arises. For cold plasmas, the Hahn approximation involving postulatian of an equivalent surface charge at the beam edge is usually applied. In the case of a warm plasma, it is unlikely that this approximation would describe accurately the plasma behaviour, thoug...

Internal Resonances of a Discharge Column

Dipole resonance of a cold plasma column can be predicted quite simply and observed experimentally. It is normally accompanied, however, by a series of subsidiary resonances. These have recently been explained phenomenologically as depending on nonuniform electron density and nonzero electron temperature for their existence. Numerical results of an analysis of the resonances by Nickel, Parker, and Gould are compared to the recently published experimental work on which the phenomenological description was founded, and it is shown that there is very good agreement. These observations and the theory have considerable relevance to the pressure waves described by Bohm and Gross, and to the effectiveness of Landau damping under conditions of nonuniform electron density. It is also possible to use the computed data to estimate the error in dipole resonance measurements of electron density.