Abstracts from Journal Papers:


Electron conic acceleration by the ionospheric Alfven resonator

by B. J. Thompson and R. L. Lysak

Resonant Alfven waves trapped in the ionosphere can produce electron conic distributions and earthward electron fluxes with energies in the keV range. Waves reflected by the ionosphere and by the decrease in the Alfven speed beyond 2 earth radii form an oscillating parallel electric field structure when electron inertial effects are included. Auroral electrons accelerated by the resonator precipitate at the cavity's resonant frequency of approximately 1 Hz, and are a possible source of the auroral flickering which has been observed in the 1 Hz range. The electron conic distributions at 3 and 4 earth radii which are generated by the resonator are also common observances, and the particle fluxes from the simulation are consistent with observed fluxes.
Previous models have investigated the generation of field-aligned currents by oscillating parallel electric fields and parallel heating by diffusion. This model has advantages over the others in that it has the following properties. First, the primary electron acceleration occurs at altitudes from 5000 to 10,000 km, instead of at lower altitudes. In addition, the parallel electric field is calculated for 1 to 4 earth radii, which is a more realistic range than that of models which involve much smaller distances. Finally, the Alfven wave structure takes into account the decrease in Alfven speed above 2 earth radii which results in the generation of a physically probable electric field comparable to that of observed fields. Furthermore, the electric field is calculated from the model of the ionospheric Alfven resonator, as the theory accounts for both the accelerating mechanism and its effects.

The effect of static electric fields in the Alfven wave model of auroral acceleration

by B. J. Thompson and R. L. Lysak

Thompson and Lysak [1995] investigated the effect of inertial Alfven waves through a numerical model of which included a MHD wave simulation along with a test particle code. It was shown that a significant parallel electric field (on the order of 10 mV) can result from the inclusion of the inertial term in the generalized Ohm's law. This oscillating field can accelerate electrons to keV energies, and can produce electron conic distributions as well as bursts of field-aligned electrons. Lysak [1993] included a calculation of resonator eigenmodes for Alfven waves propagating between a geocentric distance of around 4 Earth radii and the ionosphere. The eigenfrequencies of the lowest modes are typically in the 1 Hz range; since a typical transit time for an electron in the region is on the order of one second, the eigenmodes are ideal for resonant electron acceleration.

Quasiperiodicity of electron trajectories trapped in the auroral zone

by B. J. Thompson, R. L. Lysak, and K. H. Knuth

Electron Acceleration by the ionospheric Alfvén resonator

by B. J. Thompson and R. L. Lysak

Alfvén waves reflected by the ionosphere and by inhomogeneities in the Alfvén speed can develop an oscillating parallel electric field when electron inertial effects are included. These waves, which have wavelengths of the order of an Earth radius, can develop a coherent structure spanning distances of several Earth radii along geomagnetic field lines. This system has characteristic frequencies in the range of 1 Hz and can exhibit electric fields capable of accelerating electrons to several keV. These electric fields have the potential to accelerate electrons in several senses: via Landau resonance, bounce or transit time resonance as discussed by André and Eliasson [1992] or through the effective potential drop which appears when the transit time of the electrons is much smaller than the wave period, so that the electric fields appear effectively static. A time-dependent model of wave propagation is developed which represents inertial Alfvén wave propagation along auroral field lines. The disturbance is modeled as it travels earthward, experiences partial reflections in regions of rapid variation, and finally reflects off a conducting ionosphere to continue propagating antiearthward. The wave experiences partial trapping by the ionospheric Alfvén resonator, which is the effective cavity formed between the ionosphere and the Alfvén speed peak discussed earlier by Polyakov and Rapoport [1981] and Trakhtengerts and Feldstein [1981, 1984, 1991] and later by Lysak [1991, 1993]. Results of the wave simulation and an accompanying test particle simulation are presented, which indicate that inertial Alfvén waves are a possible mechanism for generating electron conic distributions and field-aligned particle precipitation. The model incorporates conservation of energy by allowing electrons to affect the wave via Landau damping, which appears to enhance the effect of the interactions which heat electron populations.

Chaotic scattering and trapping of auroral electrons by ULF waves

by B. J. Thompson and R. L. Lysak

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