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Magnetohydrodynamics of Sunspots

Published online by Cambridge University Press:  19 July 2016

N.O. Weiss
N.O. Weiss*
Affiliation:
Department of Applied Mathematics and Theoretical Physics University of Cambridge, Cambridge CB3 9EW, UK

Abstract

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Recent numerical investigations of fully compressible nonlinear magnetoconvection have clarified the nature of convection in sunspot umbrae. In a shallow layer with a strong vertical magnetic field oscillations give way to travelling waves as the Rayleigh number is increased but in a deep stratified layer oscillatory behaviour only appears after secondary bifurcations. This behaviour leads to a model that explains the formation of umbral dots. Penumbral structure is more difficult to explain owing to the apparent presence of adjacent horizontal and inclined fields in dark and bright filaments. The inner penumbra lies above a transition zone where volume currents are needed to maintain an overall static equilibrium; instabilities in this region may be responsible for filamentary structure in the penumbra as well as for fine structure at the umbral-penumbral boundary.

Type
V. Photospheric Flux Tubes
Information
Symposium - International Astronomical Union , Volume 142: Basic Plasma Processes on the Sun , 1990 , pp. 139 - 148
Copyright
Copyright © Kluwer 1990 

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