A flux tube is a generally tube-like (cylindrical) region of space containing a magnetic field, such that the field at the side surfaces is parallel to those surfaces. Both the cross-sectional area of the tube and the field contained may vary along the length of the tube, but the magnetic flux is always constant. Flux tubes can be easily created w...
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http://en.wikipedia.org/wiki/Flux_tube
A tube-like space in which there is a strong magnetic field. Small flux tubes at the surface of the Sun, with a diameter of less than about 300 km, form filigrees and plages and may appear bright. Bigger flux tubes, up to about 2,500 km diameter, appear dark and are called pores or sunspots.
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http://www.daviddarling.info/encyclopedia/F/flux_tube.html
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