Cartesian magnetic flux function
= Cartesian magnetic flux function
{title2=$\psi$}
For a $y$-independent solenoidal <magnetic field>, the transverse components can be written $B_x=-\psi_z$ and $B_z=\psi_x$. Contours of $\psi$ are transverse <magnetic field lines>. The independent axial component $B_y$ can twist the <flux tube>. Under the <MHD induction equation>, an additive time-dependent gauge can be chosen so that $D\psi/Dt=0$.