In the axisymmetric case, the poloidal and toroidal components of the magnetic field
are decoupled with each other.
That is, the poloidal () and toroidal () magnetic fields are made
by the toroidal () and poloidal () electric currents, respectively.
As for the Lorentz force
, the poloidal component comes from or
, while the toroidal component does from
Even if there is no toroidal magnetic field (thus no poloidal electric current),
there exists the poloidal component of the Lorentz force, which acts as a pressure to counter-balance
the self-gravity (4.2).
On the other hand, the toroidal component of the Lorentz force appears only the case with
the poloidal electric current and thus toroidal component of magnetic field .
This means that the angular momentum is transferred by the magnetic field only when exists.
Equation (B.14) explains how the angular momentum density is transferred.
The left-hand side of equation (B.14) represents
the advection of the angular momentum density, while the right-hand side
The induction equation of the magnetic field [eq.(B.17)] shows that is generated from poloidal magnetic field by the effect of rotational motion . This indicates that the angular momentum is transferred as follows: