Columnar vortices aligned with the rotation axis frequently dominate the large scales in rapidly-rotating turbulence, both in laboratory experiments and in numerical simulations. We argue that, often, these columnar vortices are simply low-frequency inertial wave packets propagating away from a localised disturbance (a turbulent eddy or buoyant blob). An important feature of these low-frequency wave packets is that they transport negative helicity upward and positive helicity downward (relative to the rotation vector). This generation and subsequent spatial segregation of positive and negative helicity is potentially important for planetary dynamos.
Recent numerical simulations suggest that magnetic field generation in planets occurs predominantly within columnar vortices located outside the tangent cylinder, and these dynamos may be classified (at least approximately) as α2. The simulations, which operate in a regime very far from that of the planets, also suggest that the helicity outside the tangent cylinder is predominantly negative in the north and positive in the south. In the more viscous and weakly forced simulations this helicity is often attributed to Ekman pumping at the mantle. However, Ekman pumping is less evident in the less viscous and more strongly driven simulations, and almost certainly plays no significant dynamical role in planetary interiors. We argue that, in the absence of Ekman pumping, the observed helicity distribution is a consequence of low-frequency helical wave packets initiated near the equatorial plane by buoyant plumes floating out towards the mantle. This provides a mechanism for maintaining a dynamo in the Earth, as well as in the gas giants.
