ON THE GENERATION OF THE SOLAR MAGNETIC-FIELD IN A REGION OF WEAK BUOYANCY

If the cyclic magnetic field of the Sun is generated by turbulent motions in a thin (109 cm) layer at the lower boundary of the solar convection zone, convincing arguments based on surface observations of the magnetic flux suggest that in the generating layer the magnetic energy must exceed the kinetic energy of the motions. An EDQNM closure model of the turbulent magnetohydrodynamic equations was integrated numerically to study whether this can be achieved in times shorter than the cycle period. The injection of helicity was chosen as the maximum compatible with realizability constraints but multiplied by exp (-a EML/EVS), where EML and EVS are the energies of the large-scale magnetic field and turbulent motions, respectively, and a is a factor between zero and unity. With the course of time the magnetic energy becomes increasingly concentrated in the largest scale, and EML eventually exceeds EVS. However, for a = 1, perhaps closest to the solar reality, EML can only exceed EVS in times that are large compared with the cycle period. Only models of the following type appear viable: the kinetic energy of the turbulent motions must be larger than, say, EVS ≈ 106 g cm-1 s-2 (for smaller values of EVS the a-effect can generate only weak, large-scale poloidal fields in times shorter than the cycle period). The action of differential rotation on this poloidal field generates then a toroidal field exceeding the equipartition value or of sufficient strength to interfere with the transport of heat and be amplified further in the solar convection zone.