ESPCI web site
Reversals of a magnetic field generated by a dynamo process
Magnetic fields of many planets and stars are generated by a dynamo process, i.e. by electromagnetic induction due to the motion of an electrically conducting fluid. They often display reversals : the magnetic field flips between two states of opposite polarities. This occurs roughly periodically for the solar magnetic field and randomly in the case of the geodynamo. Both types of reversals have been observed in a laboratory experiment. After a short review of the observational and experimental results, the dynamics of the magnetic fields generated by these strongly different flows will be understood using low dimensional models for the large scale magnetic modes. The role of broken symmetries and mode coupling in the reversal mechanism will be emphasized. Other examples of reversals of a vector field on a fluctuating background that involve a wide range of scales from reversals of the magnetization of nanoparticles to reversals of a large scale circulation in laboratory experiments or in atmospheric flows, will be shortly discussed.