It is possible to show that certain magnetic field configurations in a star must be unstable. For instance, it is possible to show that both purely toroidal and purely poloidal fields are unstable. The figure below shows the meaning of toroidal and poloidal.
A toroidal field is unstable near the magnetic axis (the Tayler instability). One can imagine the field in terms of magnetic `rings', which are exerting pressure on each other. This is a little bit similar to a backbone under stress. As the diagram below illustrates, this pressure causes the rings to move.
It is also easy to understand why a poloidal field is unstable. One can think of a star with a poloidal field as being composed of two bar magnets which are free to rotate with respect to one another. The magnets will not stay aligned for long - they rotate until the north pole of one is next to the south pole of the other. Exactly this happens in a star. In energy terms, the instability is driven by the reduction of magnetic energy in the atmosphere, the energy in the stellar interior (or inside the magnets) remaining constant.
So our stable field configuration needs to have both a toroidal component and a poloidal component. One can imagine wrapping an elastic band around the two bar magnets to stop them from rotating, this is rather like adding a toroidal component to our poloidal field.
Or in a star, we would expect something like in the figure below (cross-section, with the shaded areas representing the toroidal field). The toroidal field threads through the closed lines of poloidal field. The toroidal field must be confined only to the closed poloidal lines, because toroidal field in other areas would result in a situation where you have field lines entering the star at one end, twisting around each other inside the star and then exiting at the other end. Since we have a potential field (no electric current) outside the star, these field lines would tend to untwist themselves (reconnection would then occur outside the star) and the toroidal field would disappear.
