Energy transfers during dynamo reversals

Pankaj Mishra¹, Christophe Gissinger¹, Emmanuel Dormy² and Stephan Fauve¹

¹ Laboratoire de Physique Statistique, Ecole Normale Supéieure, CNRS, Université P. et M. Curie, Université Paris Diderot - Paris, France
² MAG (CNRS/ENS/IPGP), LRA, Ecole Normale Supéieure - Paris, France

EPL, 104 (2013) 69002

Abstract. Using direct numerical simulations of the equations of magnetohydrodynamics, we study reversals of the magnetic field generated by the flow of an electrically conducting fluid in a sphere. We show that at low magnetic Prandtl numbers, Pm=0.5, the decrease of magnetic energy, ohmic dissipation and power of the Lorentz force during a reversal is followed by an increase of the power injected by the force driving the flow and an increase of viscous dissipation. Cross correlations show that the power of the Lorentz force is in advance with respect to the other energy flows. We also observe that during a reversal, the maximum of the magnetic energy density migrates from one hemisphere to the other and comes back to its initial position, in agreement with recent experimental observations. For larger magnetic Prandtl numbers (Pm=1,2), the magnetic field reversals do not display these trends and strongly differ one from another.