Transient evolution regimes in a multiscale dynamo model: Timescales of the reversal mechanism

International audienceIn order to better understand the origin and nature of reversals of the Earth’s magnetic field, we examine the reversal mechanism of a multiscale dynamo model. This model can be described as a cellular automaton with long-range interaction. Different states correspond to different types of helical motions, and a hierarchical structure of length scales is used to mimic helicity transfers in fully developed turbulence. These multiscalehelical motions and a differential rotation are the ingredients of a schematic aw dynamo. The model exhibits rich behavior, including long periods of stable magnetic polarity (i.e., chrons), sudden reversals, excursions, and secular variation. Three transient regimes of evolution emerge: (1) Chrons are initiated by an amplification mechanism, which involves spontaneous reorientation of large length scale circulation and an a-quenchingmechanism reinforcing the asymmetry of flow during runaway growth in magnetic field intensity. (2) During the chrons the injection of turbulence at small length scale slowly restores the symmetry of the system and leads to reversal. (3) During reversals the magnetic field does not remember its previous polarity, its intensity collapses, and spontaneous reorientation of large length scale circulation is more likely to occur (see regime 1). Reversal duration therefore corresponds to an upper value of the time constants for underlying turbulence in the absence of a magnetic field. We observe a constant excursion rate during chrons and a power law relationship between the reversal rate and the magnitude of helical forcing until a limit for dynamo action is reached. Extrapolated to the Earth’s dipole field, this model predicts the duration of both chrons and reversals and sheds light on physical processes that may be responsible for their systematic occurrenc