Magnetic Field Properties in Radiative Regions

Magnetic fields are also important in radiative regions, because they can efficiently contribute to the transport of angular momentum and affect the evolution (Fig. 13.3). The main effect of a magnetic field is to favor a strong internal coupling, which may lead to solid body rotation. This is for example expressed by the so-called Ferraro law of isorotation (Sect. 13.3.1).

Quite generally, there is a strong interaction between differential rotation and magnetic field. The coupling of different layers by the magnetic field only allows a very small degree of differential rotation, close to uniform rotation [401]. Even very weak poloidal fields are able to enforce solid body rotation, for the Sun the limit is around 0.03 G [402]. This is of interest for the internal solar rotation (Sect. 13.6). Also, the small shear of the poloidal field by differential rotation gives rise to an azimuthal field. Models in Sect. 13.6 confirm the nearly solid body rotation of stars with self-generated magnetic fields (Sect. 13.3.3).

Radiative equilibrium is broken on equipotentials of stars with a large-scale magnetic field (13.14) in the same way as it is broken in rotating stars. Thus, circulation currents are also driven in stars with a poloidal field, whether axisymmetric or not, with or without some toroidal field [430].

A critical aspect concerns the instabilities of the magnetic structures and their possible amplification by the winding-up of the lines by differential rotation. The magnetic instabilities may also directly transport the elements and the field they

Uniform rotation ?

l-'icld amplification ?

Field configuration 7

What time su a lus ?

Critical instabilities?

Transport of the elements ?

Tr;inspurl of angular momentum ?

Uniform rotation ?

l-'icld amplification ?

Field configuration 7

What time su a lus ?

Critical instabilities?

Transport of the elements ?

Tr;inspurl of angular momentum ?

Fig. 13.3 Questions regarding the magnetic field and evolution create may insure the internal coupling of the angular velocity in radiative regions (cf. Sect. 13.4).

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