8 - Magnetic confinement and the sharp tachopause

Summary

The discovery by Spruit of a new small-scale turbulent dynamo has significantly changed the tachocline model proposed by Gough & McIntyre (1998). The small-scale dynamo is shear driven, is characteristic of stably stratified flows, and is mediated by the kink or ‘tipping’ instability elucidated for such flows by R. J. Tayler. The dynamo works best in high latitudes and supports turbulent Maxwell stresses large enough to dominate the angular momentum transport, taking over from the pure mean meridional circulation (MMC) proposed by Gough & McIntyre (1998). What survives from the Gough & McIntyre proposal is the laminar thermomagnetic boundary layer at the tachopause, essential for the confinement of the interior field B_i by high-latitude downwelling. That downwelling is, however, itself confined within a double boundary layer at the tachopause. The thermomagnetic boundary layer sits just underneath a modified Ekman layer, in which the turbulent Maxwell stress of the small-scale dynamo diverges.

The effects of compositional stratification in the helium settling layer under the tachopause are considered. It is concluded that Gough & McIntyre's (1998) ‘polar pits’ to burn lithium are dynamically impossible and that the tachopause is not only sharp but globally horizontal. That is, the tachopause, as marked by the top of the helium settling layer, follows a single heliopotential to within a very tiny fraction of a megametre from equator to pole.