The generation mechanism of the equatorially antisymmetric flow associated with dipole reversal in geodynamo simulation

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) carried out a dynamo simulation with a large magnetic Prandtl number (Pm = 15), and explained that enhancement of asymmetric velocity field was caused by the increase of energy transfer from magnetic field to asymmetric velocity field due to the Lorentz force in the polarity reversal phase. In the present research, we investigate how anti-symmetric flow merges and is sustained in the dynamo in which reversals occur with lower Pm. 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. We decompose the buoyancy fluxes for the symmetric and antisymmetric flow as RaTsus r ro and RaTaua r ro, where subscripts s and a indicate symmetric and anti-symmetric components, respectively. We also decompose the work of Lorentz force into the four components, us (Js Ba),us (Ja Bs),ua (Js Bs)and ua (Ja Ba). In the reversal phase, the buoyancy flux to the asymmetric velocity field RaTaua r roincreases 1.05 times of that in the stable phase, while RaTsus r rodecreases 0.98 times of that in the stable phase. The energy transfer from the symmetric velocity to the symmetric magnetic field us (Ja Bs)keeps 0.92 times of the symmetric velocity to the antisymmetric magnetic field us (Js Ba)in the stably polarity phase. However, us (Ja Bs) is 1.12 times larger than us (Js Ba) during the reversal. In addition, the conversion of the asymmetric velocity to the asymmetric magnetic field ua (Js Bs) remains up to 90% of that in the stable phase, while the other terms decrease to nearly 60% from the stable phase. We discuss the processes how the work of Lorentz force decreases during the reversal.