2026.06.23

[日時]　2026 年 7 月 13 日 (月) 14 時 〜 16時

[開催形式] ハイブリッド開催
場所：　核融合科学研究所 研究 I 期棟 4 階 402 会議室 / オンライン　
（接続情報は別途メールにて、参加申込・招待された方へお知らせします）

[講師１]

Dr. Robert J. Teed (School of Mathematics and Statistics, University of Glasgow, UK；本年度客員准教授)

[講演題目]

The strong-field regime of spherical dynamos and its relevance to magnetic field generation in Earth’s core

[講演概要]
　　Planetary magnetic fields are produced by dynamo action through turbulent motions of an electrically conducting fluid within the interior of the planet. Numerical experiments of dynamo action relevant to Earth’s magnetic field have produced different regime branches identified within bifurcation diagrams [1,2]. Notable are distinct branches in which the resultant magnetic field is either weak or strong (when compared with the fluid flow). Weak-field solutions can be identified by the prominent role of viscosity and/or inertia on the motion whereas the magnetic field has a leading order effect on the flow in strong-field solutions. Bistability between branches can be found within a small window of parameter space, as long theorised [3].
　　One measure of the success of numerical models of the geodynamo is the ability to replicate the expected balance between forces operating within Earth’s core; Coriolis (rotational) and Lorentz (magnetic) forces are predicted to be most important. Recent work has demonstrated the value in considering lengthscale dependent force balances [4] and ‘gradient-free’ solenoidal forces [5].
　　I will review the approach in numerically modelling spherical dynamos and the challenges in doing so. I will discuss the branches and bifurcations of dynamo action previously explored in numerical simulations. Furthermore, invoking recent results, I shall highlight that the expected force balance of Earth’s core can be preserved in the strong-field regime as input parameters of numerical simulations are moved towards more realistic values [2].

[1] E. Dormy et al, Fluid Dynamics Res. 50, 011415 (2018). DOI: 10.1088/1873-7005/aa769c
[2] R. J. Teed & E. Dormy, Geophys. Res. Lett. 52 (20) (2025). DOI: 10.1029/2025GL118078
[3] P. Roberts, In: Cupal, I. (ed.), Proc. First Int. Workshop on Dynamo Theory and the Generation of the Earth’s Magnetic Field pp. 7–12. Czech. Geophys. Inst. Rep (1979)
[4] T. Schwaiger et al, Geophys. J. Inter. 219, S101–S114 (2019). DOI: 10.1093/gji/ggz192
[5] R. J. Teed & E. Dormy, J. Fluid Mech. 964, A26 (2023). DOI: 10.1017/jfm.2023.332　　

[講師２]

Mr. Takumi Kera (Department of Geophysics, Tohoku University), Dr. Hiroaki Matsui (Department of Earth and Planetary Sciences, Institute of Science Tokyo)

[講演題目]

Kinetic energy transfer during polarity reversals in a numerical dynamo simulation

[講演概要]

　　Earth’s magnetic field is generated by the motion of electrically conducting liquid iron in the outer core. Although the magnetic field is usually stable and dominated by an axial dipole, geological records show that it has reversed its polarity many times in the past.
　　MHD simulations of the rotating spherical shell have reproduced these characteristics of the geomagnetic field and play a major role in understanding the dynamo process and the fluid dynamics in the outer core. For dipole reversals, numerical geodynamo simulations suggest that they are associated with a temporary strengthening of fluid motions that are asymmetric with respect to the equatorial plane. However, under the constraints imposed by Earth’s rapid rotation, flow structures tend to favour equatorial symmetry.
　　In this study, we perform geodynamo simulations using the open-source code Calypso to investigate how energy is transferred between symmetric and antisymmetric flows during magnetic field reversals. We found that, before a reversal, less energy is transferred to maintain the magnetic field. In comparison, more energy is transferred from symmetric to antisymmetric flows, and more buoyancy energy is supplied directly to the antisymmetric flow. These results suggest that the advective inertial force plays a significant role in driving reversals.

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