Magnetic Flux Tubes and the Solar Dynamo

Abstract

The observed properties of sunspot groups on the surface of the Sun are consistent with the concept of magnetic flux tubes emerging from deep within the solar convection zone. in order to maintain coherence and orientation during their rise in the turbulent convective flows the magnetic field strength in these flux tubes should always exceed the equipartition value with respect to the kinetic energy density of the convective motions.

Through linear stability analysis and nonlinear numerical Simulations a consistent picture of the storage. instability, and rise of magnetic flux tubes has emerged in recent years. which is in accordance with the observed basic properties of Sunspot groups: magnetic flux tubs are generated and stored in mechanical equilibrium in a subadiabatically stratified overshoot layer below the convection zone: they become uustable with respect to an undulatory (Parker-type) instability once the field strength exceeds a critical value: flux loops form, move through the convection zone and give rise to bipolar sunspot regions when they emerge at the Surface. Both the stability criteria and the constraints set by the observed properties of sunspot groups (orientation tilt angle. asymmetry. Proper motions) require that the field strength at the bottom of the convection zone shou1d be of the order of 10⁵ G. an order of magnitude larger than the equipartition field strength with respect to the convective flows.

We present the line of arguments which leads to this view and discuss a number of open questions in connection with the implied super equipartition fields. These include the problem of how such strong fields are generated. the rôle played by the ‘explosion’ process. and the need for non-conventional dynamo models.