JpGU-AGU Joint Meeting 2020

Presentation information

[J] Poster

S (Solid Earth Sciences ) » S-EM Earth's Electromagnetism

[S-EM22] Geomagnetism, paleomagnetism and rock magnetism

convener:Masahiko Sato(Department of Earth and Planetary Science, The University of Tokyo), Chie Kato(Faculty of Social and Cultural Studies, Kyushu University)

[SEM22-P04] Investigation of equatorial symmetry of the flow and magnetic fields in numerical dynamos with dipolar reversal

*takumi kera1, Yuto Katoh1, Yuki Nishida1, Hiroaki Matsui2 (1.tohoku university, 2.Dept. of Earth and Planetary Sciences, University of California, Davis Dept. of Earth and Planetary Sciences, University of California, Davis)

Keywords:Geomagnetic field, Geodynamo, Polarity reversals, Equatorial symmetry

Paleomagnetic observations reveal that the geomagnetic field has reversed its polarity every hundred of thousands to millions of years and each reversal typically takes 2000-12000 years to occur. Numerical dynamo simulations have been performed to gain insight into physical processes that give rise to polarity reversal temporal characteristics. Olson et al. (2011) found a transitional regime between dipolar and multipolar regimes by simulating with different Rayleigh number (Ra) and Ekman number (E). When dominated by rotation (i.e. models with low Ra and low E), magnetic field was dipole dominant without reversal. And when dominated by convection (i.e. high Ra and high E), magnetic field had multipolar structure with frequently reversing dipole component. In the transitional regime, simulations have long stable dipole-dominated chrons separated by relatively short reversals. A polarity reversal in their most Earth-like dynamo has precursory reversal with intensity low for 35 kyr followed by a rapid polarity change for about 400yr, with no postreversal rebound. The overall duration is approximately 50 kyr. However, mechanism for each phase or time scale remain not clear. Olson et al., (2004) analyzed the mechanism of magnetic polarity reversals in a dynamo model. In their model, the reversed flux is produced locally in the convective plumes and the transported from south to north by the meridional circulation. This result suggests that asymmetric flow with respect to equator play a role in reversals.
In the present study, we perform dynamo simulations in order to discuss dynamical processes of the reversal, using a geodynamo code Calypso [Matsui et al., 2014]. In particular, we investigate how equatorial flow asymmetry contributes reversal processes. In the present study, we fix aspect ratio, Ekman, Prandtl and magnetic Prandtl numbers of ri/ro = 0.15,E = 10-3,Pr = 1 and Pm = 5, respectively. The simulations are performed to 4 times of the magnetic diffusion time (td) with various Rayleigh number of Ra = 980, 1100, 1200 and 1300. Then we examine the dipole tilt angle. Generally, with increasing Ra, the tilt angle became more unstable. In Ra = 980 case, during simulation time, dipole field was stable without reversal. In Ra = 1100 case, dipole field tilts to vicinity of the opposite polarity at near the end of simulation (t = 4.0td). In Ra = 1200 case, dipole field keeps the normal polarity at 2.0 td < t < 3.5 td, and flips to the reverse polarity to the end of the simulation. In Ra = 1300 case, the dipole field has reverse polarity at 2.0 td < t < 3.5td, and has normal polarity at 1.0 td < t < 2.0td and 3.5 td < t < 4.0td. And we are performing with aspect ratio of the current Earth's outer and inner cores (i.e. ri/ro = 0.35), and we analyze the amplitude of symmetric and asymmetric components of flow and magnetic field respectively. For further research, we will also discuss how the equatorial antisymmetric flow component with respect to the equator is generated and maintained, focusing on the energy transfer between the flow and the magnetic field.