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Ann. Geophys., 24, 2043-2055, 2006
www.ann-geophys.net/24/2043/2006/
© European Geosciences Union 2006
Interchange instability of the plasma disk in Jupiter's middle magnetosphere and its relation to the radial plasma density distribution
P. A. Bespalov1, S. S. Davydenko1, S. W. H. Cowley2, and J. D. Nichols2
1Institute of Applied Physics, Russian Academy of Sciences, 46 Ulyanov St, 603950 Nizhny Novgorod, Russia
2Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK
Abstract. We analyse the interchange or flute instability of the
equatorial plasma disk in Jupiter's middle magnetosphere. Particular
attention is paid to wave coupling between the dense plasma in the
equatorial disk and the more rarefied plasma at higher latitudes, and
between the latter plasma and the conducting ionosphere at the feet of the
field lines. It is assumed that the flute perturbations are of small spatial
scale in the azimuthal direction, such that a local Cartesian approximation
may be employed, in which the effect of the centrifugal acceleration
associated with plasma rotation is represented by an "external" force in the
"radial" direction, perpendicular to the plasma flow. For such small-scale
perturbations the ionosphere can also be treated as a perfect electrical
conductor, and the condition is determined under which this approximation
holds. We then examine the condition under which flute perturbations are at
the threshold of instability, and use this to determine the corresponding
limiting radial density gradient within the plasma disk. We find that when
the density of the high-latitude plasma is sufficiently low compared with
that of the disk, such that coupling to the ionosphere is not important, the
limiting radial density profile within the disk follows that of the
equatorial magnetic field strength as expected. However, as the density of
the high-latitude plasma increases toward that of the equatorial disk, the
limiting density profile in the disk falls increasingly steeply compared
with that of the magnetic field, due to the increased stabilising effect of
the ionospheric interaction. An initial examination of Galileo plasma
density and magnetic field profiles, specifically for orbit G08, indicates
that the latter effect is indeed operative inside radial distances of ~20 RJ. At larger distances, however, additional density smoothing
effects appear to be important.
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