In support of this theory it has been noted that the variations in magnetic field are inversely correlated with the rotation velocity.
The origin of such high magnetic fields in Ap stars is problematic and two theories have been proposed in order to explain them.
Another problem with this theory is to explain why only a small proportion of A-type stars exhibit these high field strengths.
It is also unclear whether it is possible to generate such large dipole fields using this explanation, due to the slow rotation of the star.
For studying these stars high-resolution spectroscopy is used, together with Doppler imaging which uses the rotation to deduce a map of the stellar surface.