The Sun's Internal Magnetic Field
From helioseismology it is known that the Sun's radiation zone rotates uniformly, whereas the convection zone rotates slowly near the poles and fast near the equator. The shear layer in between is known as the 'tachocline'.
The uniform rotation of the radiative region can be explained by the presence of a global-scale magnetic field, in accordance with Ferraro's law of isorotation. This field must be prevented from diffusing out into the convection zone.
The differential rotation of the convective region is due to angular momentum redistribution by the convective turbulence. The retrograde drag exerted by the convection zone on the high-latitude tachocline drives meridional flows that try to burrow into and thereby spin down the interior. To prevent this burrowing, the magnetic field must transport angular momentum to the poles from lower latitudes.
In middle and low latitudes the internal magnetic field can be confined by "magnetic pumping" by overshooting convective plumes. In high latitudes the field must be confined by the tachocline's meridional flow, which is expected to be downwelling near the pole.
In mid latitudes the field residing in the convective overshoot layer is wound up by the latitudinal shear, producing a prograde magnetic torque that is transmitted to low latitudes along the field lines. This prograde torque prevents the meridional tachocline flows from burrowing into the interior.
Above: Magnetic field lines that emerge into the overshoot layer are wound up by latitudinal shear (artist's impression).
In high latitudes the tachocline's downwelling meridional flow holds the internal field in advective–diffusive balance across a thin "magnetic confinement layer". The strong stable thermal stratification holds the confinement layer nearly flat.
Over the Sun's 5 billion year history, helium and other heavy elements gradually settle out of the convection zone, producing a vertical gradient in the mean molecular weight. This compositional stratification holds the bottom of the confinement layer (the 'tachopause') almost perfectly flat.
Above: A vertical cross-section through the magnetic confinement
layer. The magnetic field lines are
drawn in red and the velocity
streamlines in blue. Compositional stratification is indicated in green.
Below: Perspective view of streamlines and field lines.
