[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-P11] Contribution of equatorially antisymmetric buoyancy to polarity reversals in geodynamo models

*Takumi Kera¹, Hiroaki Matsui³, Masaki Matsushima², Yuki Nishida¹, Yuto Katoh¹ (1.Department of Geophysics, Graduate School of Science, Tohoku University, 2.Tokyo Institute of Technology, 3.Dept. of Earth and Planetary Sciences, University of California, Davis)

Keywords:dynamo, reversal, core, equatorial symmetry, geomagnetic field, buoyancy

Paleomagnetic observations revealed that the geomagnetic field has reversed its polarity. Numerical dynamo simulations have represented dipole reversals and provided an insight into physical processes that give rise to polarity reversals. Both paleomagnetic observations and numerical dynamos have inferred that the equatorial symmetry of the magnetic and velocity fields is related to polarity reversals. The paleomagnetically inferred degree of the symmetry of the Earth's magnetic field during the past 150 Ma shows that geomagnetic field reversed polarity more frequently when the geomagnetic field was more symmetrical with respect to the equator (McFadden et al., 1991). Dynamo simulations revealed a strong inverse correlation between the stability and the equatorial symmetry of the magnetic field (Coe and Glatzmaier, 2006). Olson et al., (2004) proposed a process of magnetic polarity reversals in a dynamo model. In their model, the reversed magnetic field flux is produced locally in the convective plumes and transported from south to north by the meridional circulation. This result suggests that anti-symmetric flow with respect to the equator plays a role in reversals. However, it is an open question how to generate anti-symmetric flow under the condition of the strong rotation, in which symmetric flows with respect to the equator is dominant. In the present research, we investigate how anti-symmetric flow is growing and maintained in the dynamo in which reversals occur.

We perform three dynamo simulations using Calypso (Matsui et al., 2014) to represent dipole dominant dynamo with and without reversals and multipole dynamo, and compare the equatorial symmetry among the cases. In the present study, we set the fixed heat flux, non-slip, and connecting the potential field at the inner and outer boundaries as the boundary conditions for temperature, velocity, and magnetic field, respectively. For the dimensionless numbers, we fixed the three dimensionless numbers to E = 6×10^-4, Pr = 1, and Pm = 5. We also set Ra = 1500, 2000, and 2700 for the non-reversal dipolar, dipolar with reversal, and multipolar cases, respectively. For each case, we investigate the structure of velocity and magnetic field in terms of the symmetricity with respect to the equator.

For dipolar dynamo with reversals, flow and magnetic field structure for axial components is similar to the structure of non-reversal dynamo, when the dipole magnetic field sustains stably. The symmetric component is dominant for the velocity field, while the anti-symmetric component is dominant for the magnetic field. On the other hand, during the reversal, the structure is very similar to the multipolar case. Outside the tangent cylinder, flow fields are nearly anti-symmetric and anti-symmetric component of the magnetic field is larger than the symmetric component in the entire domain. This dominance of flow energy of anti-symmetric component during reversals may be caused by buoyancy flux associated with anti-symmetric temperature field. We also give an investigation of buoyancy flux due to symmetric and anti-symmetric components with respect to the equator.