flux tube
Diagram of a flux tube showing the magnetic field lines in the tube walls. The same amount of magnetic flux enters the tube through surface as leaves the tube through surface.
A flux tube is 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 kilometers, form filigrees and plages and may appear bright. Bigger flux tubes, up to about 2,500 kilometers diameter, appear dark and are called pores or sunspots.
