Abstract
Rather weak fossil magnetic fields in the radiative core can produce the solar tachocline if the field is almost horizontal in the tachocline region, i.e. if the field is confined within the core. This particular field geometry is shown to result from a shallow penetration of the meridional flow existing in the convection zone into the radiative core. Two conditions are thus crucial for a magnetic tachocline theory: (i) the presence of meridional flow of a few metres per second at the base of the convection zone, and (ii) a magnetic diffusivity inside the tachocline smaller than 108 cm2 s−1. Numerical solutions for the confined poloidal fields and the resulting tachocline structures are presented. We find that the tachocline thickness runs as Bp−1/2 with the poloidal field amplitude falling below 5% of the solar radius for Bp > 5 mG. The resulting toroidal field amplitude inside the tachocline of about 100 G does not depend on the Bp. The hydromagnetic stability of the tachocline is only briefly discussed. For the hydrodynamic stability of latitudinal differential rotation we found that the critical 29% of the 2D theory of Watson (1981 Geophys. Astrophys. Fluid Dyn.16 285) are reduced to only 21% in 3D for marginal modes of about 6 Mm radial scale.
Highlights
The tachocline is a thin shell inside the Sun where the rotation pattern changes strongly
The transition from differential to rigid rotation detected by helioseismology (Wilson et al 1997; Kosovichev et al 1997; Schou et al 1998) is shown in figure 1
For the toroidal field we impose the vacuum condition B = 0 at z = 0 motivated by the very large turbulent magnetic diffusivity inside the convection zone compared with microscopic diffusivity of the radiative interior
Summary
The tachocline is a thin shell inside the Sun where the rotation pattern changes strongly. For the toroidal field we impose the vacuum condition B = 0 at z = 0 motivated by the very large turbulent magnetic diffusivity inside the convection zone compared with microscopic diffusivity of the radiative interior. In case the poloidal field of the solar cycle diffuses into the core (Forgacs-Dajka & Petrovay 2002) ν and η must be replaced by the turbulent diffusivity values. As their magnetic Prandtl number is not much smaller than unity the resulting toroidal magnetic field after (9) becomes much stronger than 1000 G
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