Laboratoire de Physique Théorique
Toulouse - UMR 5152

Many turbulent flows can generate large scale fields that spontaneously break the symmetry of their forcing. A large scale circulation is often observed in flows driven at a smaller scale by a periodic force or by a pattern forming instability. Large scale winds (resp. currents) exist in the atmosphere (resp. ocean). When the fluid is electrically conducting, a turbulent flow can generate a large scale magnetic field through the dynamo effect. In all these situations, extreme events during which the large scale field reverses, i.e. changes sign everywhere in space on a rather short time scale, are observed. The most famous example concerns the Earth magnetic field that is dominantly an axial dipole which randomly flips between two states of opposite polarities. Less known are the nearly periodic reversals of the magnetic field of the sun or of the zonal wind in the tropical stratosphere. After a short review of the observational and experimental results, the dynamics of the large scale field will be understood using low dimensional models. The role of broken symmetries and mode coupling in the reversal mechanism will be emphasized. We will next consider the statistical properties of random reversals that can display waiting times with a power-law decay of their probability density function. We will show that this is a general mechanism for 1/f type spectra in turbulent flows.