Robert L. Lysak

Satellite measurements and theories of low altitude auroral particle acceleration

Several previous and new S3-3 satellite results on DC electric fields, field-aligned currents, and waves are described, interpreted theoretically, and applied to the understanding of auroral particle acceleration at altitudes below 8000 km. These results include the existence of two spatial scale sizes (less than 0.1 degree and a few degrees invariant latitude) in both the perpendicular and parallel electric fields; the predominance of S-shaped rather than V-shaped equipotential contours on both spatial saales; the correlated presence of field-aligned currents, low frequency wave turbulence, coherent ion cyclotron wave emissions and accelerated upmoving ions and downgoing electrons; intense waves inside electrostatic shocks and important wave-particle interactions therein; correlations of field-aligned currents with magnetospheric boundaries that are determined by convection electric field measurements; electron acceleration producing discrete auroral arcs in the smaller scale fields and producing inverted-V events in the larger scale fields; ion and electron acceleration due to both wave-particle interactions and the parallel electric fields. Further analyses of acceleration mechanisms and energetics are presented.

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.

Polar Spacecraft Based Comparisons of Intense Electric Fields and Poynting Flux Near and Within the Plasma Sheet-Tail Lobe Boundary to UVI Images: An Energy Source for the Aurora

In this paper, we present measurements from two passes of the Polar spacecraft of intense electric and magnetic field structures associated with Alfven waves at and within the outer boundary of the plasma sheet at geocentric distances of 4-6 R(sub E), near local midnight. The electric field variations have maximum values exceeding 100 mV/m and are typically polarized approximately normal to the plasma sheet boundary. The electric field structures investigated vary over timescales (in the spacecraft frame.) ranging front 1 to 30 s. They are associated with strong magnetic field fluctuations with amplitudes of 10-40 nT which lie predominantly ill the plane of the plasma sheet and are perpendicular to the local magnetic field. The Poynting flux associated with the perturbation fields measured at these altitudes is about 1-2 ergs per square centimeters per second and is directed along the average magnetic field direction toward the ionosphere. If the measured Poynting flux is mapped to ionospheric altitudes along converging magnetic field lines. the resulting energy flux ranges up to 100 ergs per centimeter squared per second. These strongly enhanced Poynting fluxes appear to occur in layers which are observed when the spacecraft is magnetically conjugate (to within a 1 degree mapping accuracy) to intense auroral structures as detected by the Polar UV Imager (UVI). The electron energy flux (averaged over a spatial resolution of 0.5 degrees) deposited in the ionosphere due to auroral electron beams as estimated from the intensity in the UVI Lyman-Birge-Hopfield-long filters is 15-30 ergs per centimeter squared per second. Thus there is evidence that these electric field structures provide sufficient Poynting flux to power the acceleration of auroral electrons (as well as the energization of upflowing ions and Joule heating of the ionosphere). During some events the phasing and ratio of the transverse electric and magnetic field variations are consistent with earthward propagation of Alfven surface waves with phase velocities of 4000-10000 kilometers per second. During other events the phase shifts between electric and magnetic fields suggest interference between upward and downward propagating Alfven waves. The E/B ratios are about an order of magnitude larger than typical values of C/SIGMA(sub p), where SIGMA(sub p), is the height integrated Pedersen conductivity. The contribution to the total energy flux at these altitudes from Poynting flux associated with Alfven waves is comparable to or larger than the contribution from the particle energy flux and 1-2 orders of magnitude larger than that estimated from the large-scale steady state convection electric field and field-aligned current system.

Electrodynamic coupling of the magnetosphere and ionosphere

The auroral zone ionosphere is coupled to the outer magnetosphere by means of field-aligned currents. Parallel electric fields associated with these currents are now widely accepted to be responsible for the acceleration of auroral particles. This paper will review the theoretical concepts and models describing this coupling. The dynamics of auroral zone particles will be described, beginning with the adiabatic motions of particles in the converging geomagnetic field in the presence of parallel potential drops and then considering the modifications to these adiabatic trajectories due to wave-particle interactions. The formation of parallel electric fields can be viewed both from microscopic and macroscopic viewpoints. The presence of a current carrying plasma can give rise to plasma instabilities which in a weakly turbulent situation can affect the particle motions, giving rise to an effective resistivity in the plasma. Recent satellite observations, however, indicate that the parallel electric field is organized into discrete potential jumps, known as double layers. From a macroscopic viewpoint, the response of the particles to a parallel potential drop leads to an approximately linear relationship between the current density and the potential drop.The currents flowing in the auroral circuit must close in the ionosphere. To a first approximation, the ionospheric conductivity can be considered to be constant, and in this case combining the ionospheric Ohms Law with the linear current-voltage relation for parallel currents leads to an outer scale length, above which electric fields can map down to the ionosphere and below which parallel electric fields become important. The effects of particle precipitation make the picture more complex, leading to enhanced ionization in upward current regions and to the possibility of feedback interactions with the magnetosphere.Determining adiabatic particle orbits in steady-state electric and magnetic fields can be used to determine the self-consistent particle and field distributions on auroral field lines. However, it is difficult to pursue this approach when the fields are varying with time. Magnetohydrodynamic (MHD) models deal with these time-dependent situations by treating the particles as a fluid. This class of model, however, cannot treat kinetic effects in detail. Such effects can in some cases be modeled by effective transport coefficients inserted into the MHD equations. Intrinsically time-dependent processes such as the development of magnetic micropulsations and the response of the magnetosphere to ionospheric fluctuations can be readily treated in this framework.The response of the lower altitude auroral zone depends in part on how the system is driven. Currents are generated in the outer parts of the magnetosphere as a result of the plasma convection. The dynamics of this region is in turn affected by the coupling to the ionosphere. Since dissipation rates are very low in the outer magnetosphere, the convection may become turbulent, implying that nonlinear effects such as spectral transfer of energy to different scales become important. MHD turbulence theory, modified by the ionospheric coupling, can describe the dynamics of the boundary-layer region. Turbulent MHD fluids can give rise to the generation of field-aligned currents through the so-called α-effect, which is utilized in the theory of the generation of the Earths magnetic field. It is suggested that similar processes acting in the boundary-layer plasma may be ultimately responsible for the generation of auroral currents.

MHD waves in a three-dimensional dipolar magnetic field : A search for Pi2 pulsations

ULF pulsations have been numerically studied in a new three-dimensional dipole model, which allows a realistic Alfven speed profile for the plasmasphere and outer magnetosphere in the tailward region. This model includes more realistic boundary conditions at the outer boundary, allowing for partial reflection at the magnetopause and escape of wave energy down the tail. We investigate how Pi2 modes develop in time when an impulse associated with the substorm onset is assumed. It is shown that discrete compressional modes, which are initially excited by the plasmasphere, persistently arise in the nightside region. Our numerical results suggest that Pi2 pulsations are strongly associated with these virtual resonances in the plasmasphere. Wave spectra are examined both at the equator and the ionosphere. We discuss and compare them with current theoretical and observational characteristics.

Propagation of Alfvén waves through the ionosphere: Dependence on ionospheric parameters

Waves in the 1-Hz frequency band are often seen by both ground observations of magnetic fields and satellite observations of electric and magnetic fields. Comparison between the ground and satellite observations of these waves is complicated by the fact that such waves must pass through the strongly inhomogeneous and collisional ionosphere. While this is true for ULF waves at lower frequencies as well, waves near 1 Hz are more strongly affected since their wavelength is comparable with the scale size of the ionospheric minimum in the Alfven speed; therefore they can be trapped and, in the case of compressional waves, ducted in this region of low Alfven speed. A model is developed to describe the propagation of waves in this frequency range through the ionosphere. A variety of ionospheric models for this propagation have been used to assess the ground signatures of these waves under various conditions. This model is used to study the transient response of the ionosphere to an increase in the field-aligned current. The strength of the ground signal depends strongly on both the Pedersen and Hall conductivities of the ionosphere. Ground signatures are strongest when the Hall conductivity is greater than the Pedersen conductivity. An underdamped signature is seen when the conductivities are high, while an overdamped waveform results for low conductivities. The fundamental mode of the shear mode Alfven resonator is found not to couple to a ducted compressional wave, while higher harmonics of the wave are readily ducted through the ionospheric waveguide.

Towards a new paradigm: from a quasi-steady description to a dynamical description of the magnetosphere

A great deal of the research done on the dynamical process of the solar wind- magnetosphere interaction is based on large-scale, quasi-steady theoretical models, such as the classical reconnection model. However, it can be argued that the theoretical and observational foundations of these commonly believed paradigms are not always strong, and support for these models is sometimes weak, controversial or inconsistent. This paper discusses the need for a transition from an oversimplified quasi-steady paradigm towards a more realistic one including the dynamics of MHD waves and wave packets. The effects of localized wave packets may be most important in active plasma regions, where ideal MHD breaks down and localized, time-dependent processes become dominant. New insights into the theories of field-aligned current generation, auroral particle acceleration and the concept of reconnection may be found by including MHD wave propagation and wave packet dynamics.

The relationship between electrostatic shocks and kinetic Alfvén waves

Auroral satellites and sounding rockets frequently observe large electric fields perpendicular to the magnetic field that have a narrow scale length perpendicular to the magnetic field if they are interpreted as spatial structures. These fields have been variously attributed to electrostatic shock structures or to kinetic Alfven waves. These two models can be distinguished by considering the ratio of the magnetic field perturbation to the electric field. This ratio is calculated within the context of the electrostatic approximation, the fully kinetic Alfven wave dispersion relation considered by Lysak and Lotko [1996], and the cold fluid model including ionospheric reflection presented by Lysak [1991, 1993]. Results for this model show that the ratio of the electric to magnetic field is not always equal to the Alfven speed, especially for structures that are very narrow in the direction perpendicular to the magnetic field. These narrow structures have electric fields that are enhanced with respect to the Alfvenic value, and thus may appear as electrostatic.

Response of the dipole magnetosphere to pressure pulses

The response of the magnetosphere to pressure pulses at the magnetopause has been studied using a three-dimensional model of ULF waves in a dipole geometry. Pressure pulses at the magnetosphere directly excite congressional waves, which then convert to shear mode Alfven waves due to inhomogeneity. The behavior of the system depends on the frequency of the source at the magnetopause, with vortex structures tending to form on field lines resonant with the source frequency. The perturbations between the vortices are skewed toward noon, in agreement with observations.

Propagation of Alfvén waves through the ionosphere

Abstract Waves in the 1 Hz frequency band are commonly observed at all local times by both ground observations of magnetic fields as well as satellite observations of electric and magnetic fields. Such waves may be externally generated in the outer magnetosphere, such as Pc1 oscillations that are generally thought to be generated by ion cyclotron instabilities in or near the cusp region of the magnetopause, or may be self-consistently generated in the ionospheric resonant cavity. Modeling the ground signatures of such waves is complicated by the fact that at these frequencies, the electromagnetic skin depth is comparable to the ionospheric thickness and so the vertical structure of the ionosphere must be resolved. In addition, the Hall conductivity in the ionosphere couples shear Alfven waves to compressional (fast) mode waves that can propagate across field lines in the ionosphere. A model has been developed which can describe these interactions, and which can determine the ground signatures of Pc1 oscillations not only on the field line on which they are generated but also as they propagate horizontally through the ionospheric waveguide. First results from this model are presented.