Centrifugally driven instability of a rotationally dominated magnetodisc

Details Centrifugally driven instability of a rotationally dominated magnetodisc Centrifugally driven instability of a rotationally dominated magnetodisc

Mar 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jgr...103.4707l&link_type=abstract

Journal of Geophysical Research, vol. 103, p. 4707

Physics

Planetary Magnetospheres, Jupiter Atmosphere, Space Plasmas, Plasmas (Physics), Io, Boundary Conditions, Heuristic Methods, Turbulent Diffusion

Scientific paper

The dynamics of a rotationally dominated magnetodisk is considered on the basis of linearized MHD. The treatment is prompted by the concern that the dominant theoretical view on this subject is based on the reduced framework of interchange motion, in which changes in magnetic field are suppressed. A realistic perturbation of a magnetodisk generally does not satisfy this constraint, and the changing magnetic field is expected to stabilize any small-amplitude perturbation. We reexamine the problem in an idealized slab magnetodisk geometry but with dynamical terms fully installed in the MHD equations. We find that the traditional interchange instability, essentially a centrifugal buoyancy regulated by the ionosphere, is not a proper solution for a thin magnetodisk. For short-wavelength modes the actual situation of instability comes to resemble what is known in the literature as magnetic buoyancy, i.e., a centrifugally driven motion moderated by the strong local magnetic field. Physically, the instability results as the rotational effects couple the Alfven and slow modes; density perturbations formed by the latter are in turn driven radially by centrifugal buoyancy. The instability is manifest even for a perfectly uniform density distribution and for the characteristic parameters near the Io torus, dominates over the contribution from a radial density gradient. The theory of centrifugally driven radial diffusion is considered with this instability as the mechanism of eddy formation.

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