Mean electromotive force for a ring of helical vortices

A ring of helical vortices is a common feature of the onset of thermal convective instability in a rapidly rotating spherical shell as expected in the Earth's core. In fact, there are good reasons that this kind of fluid motion plays an important role in the generation of earth's magnetic field. On the other hand, this ring of rolls can also result from a spherical Taylor-Couette instability as expected to be reproduced in a sodium experiment in Grenoble, France. In this work, the mean electromotive force (e.m.f) for such kind of flow structure is derived analytically following the lines of the mean field concept. The main idea is to split the basic mechanisms that could generate the dynamo action at large scale. Some numerical examples are also calculated.