4 - Solar Dynamos; Computational Background

Summary

INTRODUCTION

The picture of the solar dynamo has evolved continuously over the past twenty years. Both observations and theoretical considerations have introduced new aspects to the problem. Meanwhile, there has been a growing recognition of the importance of the overshoot layer beneath the solar convection zone (CZ) as the place where magnetic flux tubes can be ‘stored’ over time scales comparable with the solar cycle period (e.g., Spiegel & Weiss 1980). Less clear, however, is the question of whether the field is also generated there, or whether the magnetic field is actually generated in the convection zone, and only then transported into the overshoot layer where it accumulates (Brandenburg et al 1991, Nordlund et al 1992).

Numerical simulations of hydromagnetic convection show generation of magnetic fields in the entire CZ, but there is a strong turbulent downward pumping, that causes the magnetic field to build up at the interface between the CZ and the radiative interior. Magnetic buoyancy causes magnetic flux tubes to float upwards, but at the same time convective motions push them down again. In numerical simulations it is seen that under these conditions the magnetic field plays an active role and can still be amplified. It is questionable whether the interface can be considered in isolation. Consequently, throughout this chapter we consider the evolution of the magnetic field in the entire CZ and allow for the interaction between the CZ and the radiative interior in most of the cases.

The magnetic fields generated by turbulence are so intermittent that it is difficult to understand how the solar magnetic field has such a systematic orientation, as demonstrated by Hale's polarity law (cf. Schiissler 1987).