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The dynamics of magnetically trapped fluids. I - Implications for umbral dots and penumbral grains
Choudhuri, A. R.
AA(Chicago, University, IL)
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 302, March 15, 1986, p. 809-825. (ApJ Homepage)
Publication Date:
Solar Physics
NASA/STI Keywords:
Magnetohydrodynamics, Solar Granulation, Solar Magnetic Field, Sunspots, Fluid Pressure, Incompressible Fluids, Magnetic Flux, Polytropic Processes, Steady Flow
Bibliographic Code:


A study of the magnetohydrodynamic system in which a nonmagnetized fluid in a gravitational field is surrounded by a fluid carrying a vertical magnetic field is presented. It is pointed out that this study can throw some light on the fine-structural features of a sunspot. The equilibrium configuration of the field-free fluid is a tapering column ending at an apex. The regions away form the apex can be studied by the slender flux tube approximation. A scheme developed to treat the apex indicates that, just below the apex, the radius of the tapering column opens up with a 3/2 power dependence on the depth below the apex. If the internal pressure of the field-free fluid is increased, the apex rises, and a static equilibrium may not be possible beyond a limit if the magnetic pressure drops quickly above a certain height. The nature of steady-flow solutions beyond this limit is investigated. Under conditions inside a sunspot, a column of field-free gas is found to rise with a velocity of about 100 km/hr. If umbral dots and penumbral grains are interpreted as regions where the field-free gas ultimately emerges, a very natural explanation of most of their observed properties is obtained.

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