2026-04-24 核融合科学研究所
図1：地球磁場の模式図（左上）、海洋底の岩石の残存磁場から得られた地球磁場反転の履歴（Ogg, Geologic Time Scale 2020, Elsevier (2020)のデータより作成）（左下）と、地球磁場反転シミュレーションの結果（右）。
＜関連情報＞
四重対流の球殻ダイナモにおける２つの安定なダイポール極性 Bi-stable dipole polarity in spherical shell dynamo with quadruple convection
Hiroki Hasegawa,Hiroaki Ohtani & Tetsuya Sato
Scientific Reports Published:03 March 2026
DOI:https://doi.org/10.1038/s41598-026-42280-x
Abstract
To study nonlinear dynamo properties, we performed extensive MHD simulations in a spherical shell system, focusing on a quadruple convection pattern with equal amounts of positive and negative flow helicities in the southern and northern hemispheres. Simulations using 50 different magnetic noise patterns indicate that the system settles into a nonlinear dipole-dominated equilibrium, with north or south polarity occurring at similar rates. This finding is consistent with the results from 50 additional simulations in which the direction of the convection is reversed. Both stable polarity states exhibit significant robustness, showing resistance to reversal under minor magnetic disturbances. This indicates that a bi-stable polarity equilibrium exists in the dipole-dominated magnetic field of a spherical shell dynamo system. The stable polarity state can be disrupted by introducing anomalous magnetic diffusivity or viscosity, or plasma instability, which are beyond the scope of this report.
