The Dynamics of Magnetically Trapped Fluids. I. Implications for Umbral Dots and Penumbral Grains
Abstract
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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- March 1986
- DOI:
- 10.1086/164042
- Bibcode:
- 1986ApJ...302..809C
- Keywords:
-
- Magnetohydrodynamics;
- Solar Granulation;
- Solar Magnetic Field;
- Sunspots;
- Fluid Pressure;
- Incompressible Fluids;
- Magnetic Flux;
- Polytropic Processes;
- Steady Flow;
- Solar Physics;
- HYDROMAGNETICS;
- SUN: MAGNETIC FIELDS;
- SUN: SUNSPOTS