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Ann. Geophys., 20, 2003-2025, 2002
www.ann-geophys.net/20/2003/2002/
© European Geosciences Union 2002
New insights from a nonlocal generalization of the Farley-Buneman instability problem at high latitudes
J. Drexler, J.-P. St.-Maurice, D. Chen, and D. R. Moorcroft
Department of Physics and Astronomy, University of Western Ontario, London, Canada
Correspondence to: J. Drexler (jdrexler@uwo.ca)
Abstract. When their growth rate
becomes too small, the E-region Farley-Buneman and gradient-drift instabilities
switch from absolute to convective. The neutral density gradient is what gives
the instabilities their convective character. At high latitudes, the
orientation of the neutral density gradient is close to the geomagnetic field
direction. We show that this causes the wave-vector component along the
geomagnetic field to increase with time. This in turn leads to wave
stabilization, since the increase goes hand-in-hand with an increase in
parallel electric fields that ultimately short-circuits the irregularities. We
show that from an equivalent point of view, the increase in the parallel wave
vector is accompanied by a large upward group velocity that limits the time
during which the perturbations are allowed to grow before escaping the unstable
region. The goal of the present work is to develop a systematic formalism to
account for the propagation and the growth/decay of high-latitude Farley-Buneman
and gradient-drift waves through vertical convective effects. We note that our
new formalism shies away from a plane wave decomposition along the magnetic
field direction. A study of the solution to the resulting nonlinear aspect
angle equation shows that, for a host of initial conditions, jump conditions
are often triggered in the parallel wave-vector (defined here as the vertical
derivative of the phase). When these jump conditions occur, the waves turn into
strongly damped ion-acoustic modes, and their evolution is quickly terminated.
We have limited this first study to Farley-Buneman modes and to a flow
direction parallel to the electron E × B drift. Our initial
findings indicate that, irrespective of whether or not a jump in aspect angle
is triggered by initial conditions, the largest amplitude modes are usually
near the ion-acoustic speed of the medium (although Doppler shifted by the ion
motion), unless the growth rates are small, in which case the waves tend to
move at the same drift as the ambient electrons.
Key words. Ionosphere (auroral
ionosphere; ionospheric irregularities; plasma waves and instabilities)
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