Energy transfer among flow and magnetic fields with different equatorial symmetry during the dipole reversal in a geodynamo simulation
Takumi Kera, Tohoku University
The geomagnetic field has reversed its polarity, and some numerical dynamos have suggested that anti-symmetric flow with respect to the equator plays a role in reversals. Olson et al., (2004) suggested that the equatorial antisymmetric flow is temporarily strengthened, and transports a locally generated reversal magnetic field. Nishikawa and Kusano (2008) explained this asymmetric velocity field enhancement by the increase of energy transfer from magnetic field to asymmetric velocity field in polarity reversal phase by the Lorentz force by simulation with the large magnetic Prandtl number (Pm = 15). In the present research, we investigate how anti-symmetric flow merges and is sustained in the dynamo in which reversals occur with Pm = 5 and Ekman number E = 6e-3.
We perform dynamo simulations using Calypso (Matsui et al., 2014) to represent dipole dominant dynamo with reversals. We investigate the buoyancy flux and work of the Lorentz force in terms of the equatorial symmetry in the stable polarity phase and in the polarity reversal phase, respectively.
The results show that strong upwelling plumes emerge during the reversal (see Movie). This temperature field suggests that this plumes cause the high amplitude of asymmetric buoyancy flux. However, looking at the kinetic energy spectra, the amplitude of the axisymmetric and antisymmetric poloidal flow has small changes, while a characteristic increase in the toroidal component of the axisymmetric and antisymmetric flow is observed. This result suggests that the toroidal component may play a more important role in reversals. Consequentially, more investigation is required if meridional circulation has an important role for a dipole reversal.
Looking at the energy transfers for the flows during the reversal, the buoyancy flux and work of the advection term to the antisymmetric velocity field increase from the time average in the stable phase. On the other hand, the work of the Lorentz force for the antisymmetric flow has smaller change from the time average than that for the buoyancy flux and work of the advection term.
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