C. E. Seyler

SMALL SCALE ALFVÉNIC STRUCTURE IN THE AURORA

This paper presents a comprehensive review of dispersive Alfvén waves in space and laboratory plasmas. We start with linear properties of Alfvén waves and show how the inclusion of ion gyroradius, parallel electron inertia, and finite frequency effects modify the Alfvén wave properties. Detailed discussions of inertial and kinetic Alfvén waves and their polarizations as well as their relations to drift Alfvén waves are presented. Up to date observations of waves and field parameters deduced from the measurements by Freja, Fast, and other spacecraft are summarized. We also present laboratory measurements of dispersive Alfvén waves, that are of most interest to auroral physics. Electron acceleration by Alfvén waves and possible connections of dispersive Alfvén waves with ionospheric-magnetospheric resonator and global field-line resonances are also reviewed. Theoretical efforts are directed on studies of Alfvén resonance cones, generation of dispersive Alfvén waves, as well their nonlinear interactions with the background plasma and self-interaction. Such topics as the dispersive Alfvén wave ponderomotive force, density cavitation, wave modulation/filamentation, and Alfvén wave self-focusing are reviewed. The nonlinear dispersive Alfvén wave studies also include the formation of vortices and their dynamics as well as chaos in Alfvén wave turbulence. Finally, we present a rigorous evaluation of theoretical and experimental investigations and point out applications and future perspectives of auroral Alfvén wave physics.

Broadband ELF plasma emission during auroral energization: 1. Slow ion acoustic waves

High-resolution measurements by the Freja spacecraft of broadband extremely low frequency (BB-ELF) emission from dc up to the lower hybrid frequency (a few kHz) are reported from regions of transverse ion acceleration (TAI) and broad-energy suprathermal electron bursts (STEB) occuring in the topside ionospheric auroral regions. A gradual transition of the broadband emission occurs near the local O+ cyclotron frequency (ƒO+ ≈ 25 Hz) from predominantly electromagnetic below this frequency to mostly electrostatic above this frequency. The emission below 200 Hz often reach amplitudes up to several hundred mV/m and density perturbations (δn/n) of tens of %. An improved analysis technique is presented, based on the quantity |δE/(δn/n)| versus frequency and applied to the Freja plasma wave measurements. The method can be used to infer the dispersion relation for the measured emission as well as give estimates of the thermal plasma temperatures. The BB-ELF emission is found to consist partly of plasma waves with an ion Boltzmann response, which is interpreted as originating from the so-called slow ion acoustic wave mode (SIA). This emission is associated with large bulk ion (O+) temperatures of up to 30 eV and low electron temperatures (1–2 eV) and therefore occurs during conditions when Te/Ti ≪ 1. The BB-ELF emissions also contain other wave mode components, which are not equally easy to identify, even though it is reasonably certain that ion acoustic/cyclotron waves are measured. The ion Boltzmann component is characterized by a dominantly perpendicular polarization with respect to the Earths magnetic field direction and a small magnetic component with amplitudes around 0.1–1 nT. The ion Boltzmann component dominates the lower-frequency part (30–400 Hz) of the BB-ELF emissions. The BB-ELF emission have often an enhanced spectral power when certain waveform signatures, interpreted as solitary kinetic Alfven waves (SKAW), or when large-amplitude electric fields, possibly related to black aurora, are encountered in regions often associated with large-scale auroral density depletions. A scenario where the SKAW provides the original free energy and via the BB-ELF emission causes intense transverse ion heating (TAI) is suggested.

Continuum modeling of electromechanical dynamics in large-scale power systems

In this paper, we propose a continuum model for real power systems that tend to be highly irregular in terms of their geographical topology and the power injections, loads, and shunt elements at the bus locations. The continuum model presented here therefore relaxes the isotropy and homogeneity constraints assumed in our prior work. The network, with its transmission lines, generators, and loads, are treated as a continuum in spatial coordinates. We are consequently able to model the system as a pair of nonlinear partial differential equations (PDEs). The first PDE is the continuum equivalent of the load flow equations of the power system and is a boundary value problem. The second equation is the continuum equivalent of the swing equations of the power system. The parameters of these equations are functions of spatial coordinates and the network topology is embedded in them. The computational effort needed to solve the PDEs depends on the uniformity in the parameter distributions. A systematic approach of smoothening the parameter distributions is also proposed. While a continuum system with these smooth parameter distributions looses some of its ability to accurately model the detailed behavior of the power system, the global behavior of the system remains preserved. Furthermore, the electromechanical wave propagation behavior observed in actual power systems is readily recognized from the PDE model. A theoretical analysis of the continuum model as well as test simulations show that disturbances in the systems phase angles propagate through the continuum system with velocities much slower than the speed of light and exhibit dispersion phenomena.

Electron acceleration by Alfven waves in the magnetosphere

The self-consistent electron kinetics of Alfven waves on the electron inertial scale is studied using a two-dimensional hybrid-kinetic description. The ions follow a fluid description for Alfven waves at frequencies below the ion cyclotron frequency. The parallel electron dynamics are treated kinetically using particle-in-cell techniques. In this model the electron plasma mode is eliminated and only the physics of the Alfven waves is retained. At sufficiently large amplitudes, it is found that oblique Alfven waves break due to finite electron inertia in a cold plasma. The consequence of wave breaking is the formation of an electron beam which can be unstable to the beam-plasma instability. The electrons supporting the parallel current thermalize into a non-Maxwellian distribution with an energetic tail up to several keV, assuming a reasonable magnetospheric Alfven speed. In hot plasma simulations, electron trapping is the principal mechanism of electron acceleration. It is proposed that wave breaking or electron trapping of oblique Alfven waves at 1 RE can result in electron acceleration and may explain some observed auroral phenomena.

The status of observations and theory of high latitude ionospheric and magnetospheric plasma turbulence

A review is given for the current status of both observations and theory of high latitude ionospheric plasma turbulence. The principal purpose of this review is to draw connections between the existing body of experimental evidence of fluid-like plasma turbulence in the ionosphere and the predictions of various fluid plasma models which have been proposed to describe the dynamics of the turbulent ionosphere. To place the ionospheric problem in perspective, a tutorial summary of 3-D and 2-D turbulent cascade theory is included along with citations of its applications in neutral fluid turbulence and the supportive experimental evidence found mainly in atmospheric flows. The high latitude observational evidence for low frequency (ω < Ωi) macroscale (L > ϱi) turbulence is summarized. The evidence includes observations of irregularities which occur over an altitude range of 400 km to 8000 km or possibly higher, and vary over scale sizes of 5 meters (ϱi) to about 2000 km. At the shorter wavelengths the irregularities are known to be nonpropagating hence convective nonlinear processes are important and this suggests that turbulence is the rule. The mathematical description of the turbulent dynamics of much of the ionosphere is suggested to be embodied in a low frequency fluid model, the essential features of which have been discussed in the literature. Application of standard cascade theory to this model leads to testable predictions of the power spectra for the density and electric field. A comparison of the spectral theory and the observations indicates significant agreement in some cases and ambiguity in others, but no apparent contradictions. Suggestions are made for future experimental diagnostics which could resolve ambiguities.

A symmetric regularized‐long‐wave equation

A symmetric version of the regularized‐long‐wave equation is shown to describe weakly nonlinear ion acoustic and space‐charge waves. The equation possesses hyperbolic secant squared solitary waves and has four known invariants. Numerical solutions are compared with previous results on the regularized‐long‐wave equation.

Kinetic tilting stability of field-reversed configurations

Stability of the internal tilting mode in an elongated prolate field‐reversed configuration (FRC) is investigated numerically. Eigenfrequencies are calculated from a Vlasov‐fluid dispersion functional that is separated into fluid and kinetic portions. The latter are evaluated by a Monte Carlo method following a sample of the equilibrium orbits. An innovative Fourier transform technique is developed to reduce the operation count of the algorithm. Kinetic growth rates obtained for an experimentally relevant equilibrium indicate essential stabilization of internal tilting in an FRC with s≲2, where s measures the number of thermal gyroradii in the configuration. For s→∞ the kinetic growth rates approach previous magnetohydrodynamic (MHD) values.

Lower hybrid wave phenomena associated with density depletions

A fluid description of lower hybrid, whistler and magnetosonic waves is applied to study wave phenomena near the lower hybrid resonance associated with plasma density depletions. The goal is to understand the nature of lower hybrid cavitons and spikelets often associated with transverse ion acceleration events in the auroral ionosphere. Three-dimensional simulations show the ponderomotive force leads to the formation of a density cavity (caviton) in which lower hybrid wave energy is concentrated (spikelet) resulting in a three-dimensional collapse of the configuration. Plasma density depletions of the order of a few percent are shown to greatly modify the homogeneous linear properties of lower hybrid waves and account for many of the observed features of lower hybrid spikelets.

Observation and analysis of lower hybrid solitary structures as rotating eigenmodes

Lower hybrid solitary structures (LHSS) are observed to be composed of wave modes rotating in the right-handed sense about the geomagnetic field. The data analyzed were measured at altitudes near 800 km in the auroral ionosphere by the plasma wave interferometer aboard the AMICIST rocket. Clear evidence of these modes is obtained by an estimation of the local frequency-wavenumber spectrum derived from wavelet analysis of the electric field. This evidence demonstrates that the phase velocity direction reverses as the payload traverses the structure, implying that the structure is composed of rotating electric fields. These observations are consistent with the result of three-dimensional numerical simulations investigating lower hybrid wave behavior in the presence of a density cavity. This suggests that the observed characteristics of LHSS may be explained by the excitation of localized lower hybrid eigenmodes and the scattering of background VLH hiss from plasma density depletions.

Magnetic bubbles and kinetic Alfvén waves in the high-latitude magnetopause boundary

We present a detailed analysis of magnetic bubbles observed by the Polar satellite in the high-latitude magnetopause boundary. The bubbles which represent holes or strong depressions (up to 98%) of the ambient magnetic field are filled with heated solar wind plasma elements and observed in the vicinity of strong magnetopause currents and possibly near the reconnection site. We analyze the wave modes in the frequency range 0-30 Hz at the magnetopause (bubble) layer and conclude that the broadband waves in this frequency range represent most likely spatial turbulence of kinetic Alfven waves (KAW), Doppler-shifted to higher frequencies (in the satellite frame) by convective plasma flows. We present also results of a numerical simulation which indicate that the bubbles are produced by a tearing mode reconnection process and the KAW fluctuations are related to the Hall instability created by macroscopic pressure and magnetic field gradients. The observed spatial spectrum of KAW extends from several ion gyroradii (∼ 500 km) down to the electron inertial length (∼ 5 km).