Flux tube
A flux tube is a generally tube-like (cylindrical) region of space containing a magnetic field, B, such that the field is perpendicular to the normal vector, . 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.
As used in astrophysics, a flux tube generally has a larger magnetic field and other properties that differ from the surrounding space. They are commonly found around stars, including the Sun, which has many flux tubes of around 300 km diameter.Sunspots are also associated with larger flux tubes of 2500 km diameter. Some planets also have flux tubes. A well-known example is the flux tube between Jupiter and its moon Io.
In 1861, James Clerk Maxwell gave rise to the concept of a flux tube inspired by Michael Faraday's work in electrical and magnetic behavior in his paper titled "On Physical Lines of Force". Maxwell described flux tubes as:
"If upon any surface which cuts the lines of fluid motion we draw a closed curve, and if from every point of this curve we draw lines of motion, these lines of motion will generate a tubular surface which we may call a tube of fluid motion."
...
Wikipedia