Parametric instability of the helical dynamo

Marine Peyrot (1, 2) , Franck Plunian (1, 2) , Christiane Normand (3)

1 LEGI - Laboratoire des Écoulements Géophysiques et Industriels [Grenoble]
2 LGIT - Laboratoire de Géophysique Interne et Tectonophysique
3 SPhT - Service de Physique Théorique

We study the dynamo threshold of a helical flow made of a mean (stationary) plus a fluctuating part. Two flow geometries are studied, either (i) solid body or (ii) smooth. Two well-known resonant dynamo conditions, elaborated for stationary helical flows in the limit of large magnetic Reynolds numbers, are tested against lower magnetic Reynolds numbers and for fluctuating flows (zero mean). For a flow made of a mean plus a fluctuating part the dynamo threshold depends on the frequency and the strength of the fluctuation. The resonant dynamo conditions applied on the fluctuating (resp. mean) part seems to be a good diagnostic to predict the existence of a dynamo threshold when the fluctuation level is high (resp. low).

Domaines: Dynamique des Fluides [physics.flu-dyn]

Marine Peyrot, Franck Plunian, Christiane Normand. Parametric instability of the helical dynamo. Physics of Fluids, 2007, 19, pp.054109. ⟨10.1063/1.2734118⟩. ⟨hal-00381972⟩