With the magnetohydrostatic equilibrium condition met, a cylindrical flux tube's plasma pressure of p ( R ) is given by the following relation written in cylindrical coordinates with R as the distance from the axis radially: 0 = d p d R + d d R ( B ϕ ϕ 2 + B z 2 2 μ μ ) + B ϕ ϕ 2 μ μ R The second term in the above equation gives the magnetic pressure force while the third term represents the magnetic tension force. The field line's twist around the axis from one end of the tube of length L to the other end is given by: Φ Φ ( R ) = L B ϕ ϕ ( R ) R B z ( R )