Magnetoconvection in sheared magnetic fields

Bian, N. H. and Garcia, O. E. (2008) Magnetoconvection in sheared magnetic fields. Physics of Plasmas, 15(10), 102901-1-102901-8. (doi: 10.1063/1.2953214)

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Abstract

The development of magnetoconvection in a sheared magnetic field is investigated. The equilibrium magnetic fieldB0 is horizontal and its orientation varies linearly along the vertical axis. Preliminary consideration of the transition from the inertial to the viscous regime of the gravitational resistive interchange instability, reveals that the latter is characterized by the existence of viscoresistive boundary layers of vertical width which scales as Q−1/6, where Q is the Chandrasekhar number. The situation is analogous to the one encountered in magnetically confined laboratory plasmas, where convective flows are constrained by the magnetic shear to develop in boundary layers located around resonant magnetic surfaces in order to fulfill the “interchange condition” k⋅B0=0, where k is the wave vector of the magnetic perturbation. It follows that when the effect of thermal diffusion is taken into account in the process, convection can only occur above a certain critical value of the Rayleigh number which scales as Q2/3 for large Q. At the onset, the convection pattern is a superposition of identically thin convective rolls everywhere aligned with the local magnetic field lines and which therefore adopt the magnetic field geometry, a situation also reminiscent of the penumbra of sunspots. Using this degeneracy, equations describing the weakly nonlinear state are obtained and discussed. A reduced magnetohydrodynamic description of magnetoconvection is introduced. Since it is valid for arbitrary magnetic field configurations, it allows a simple extension to the case where there exists an inclination between the direction of gravity and the plane spanned by the equilibrium magnetic field. These reduced magnetohydrodynamic equations are proposed as a powerful tool for further investigations of magnetoconvection in more complex field line geometries.

Item Type:Articles
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:UNSPECIFIED
Authors: Bian, N. H., and Garcia, O. E.
College/School:College of Science and Engineering > School of Physics and Astronomy
Journal Name:Physics of Plasmas
Publisher:American Institute of Physics
ISSN:1070-664X
ISSN (Online):1089-7674

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