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The dynamics of magnetically trapped fluids. I - Implications for umbral dots and penumbral grains

Choudhuri, Arnab Rai (1986) The dynamics of magnetically trapped fluids. I - Implications for umbral dots and penumbral grains. In: Astrophysical Journal, 302 . pp. 809-825.

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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.

Item Type: Journal Article
Publication: Astrophysical Journal
Publisher: Institute of physics
Additional Information: Copyright for this article belongs to Institute of physics.
Department/Centre: Division of Physical & Mathematical Sciences > Physics
Date Deposited: 19 Feb 2010 11:19
Last Modified: 19 Sep 2010 05:55
URI: http://eprints.iisc.ac.in/id/eprint/25652

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