A flux tube is a bundle of magnetic field lines bounded by a surface everywhere tangent to the magnetic field. Gauss's law for magnetism makes the magnetic flux through any transverse cross-section constant along the tube. For a narrow tube with nearly uniform field magnitude and transverse area , this gives .
The cross-sectional area of a narrow flux tube, measured normal to its magnetic field. Constancy of the magnetic flux gives along the tube. This area controls the mass transport when flow follows the field.
