*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.

*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.

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

*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 *