Magnetoconvection in a spherical shell: Equatorial symmetry during the transition from the weak- to the strong-field regime
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| Format: | Preprint |
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2026
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| _version_ | 1866915869558308864 |
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| author | Gostelow, Luke J. Teed, Robert J. |
| author_facet | Gostelow, Luke J. Teed, Robert J. |
| contents | At small but supercritical Rayleigh numbers, simulations of dynamos in spherical shells often separate into two broad regimes characterised either by their relative magnetic field strength (weak/strong) or by their dominant force balance (VAC/MAC). These regimes can tend smoothly from one to the other but can also be bistable, a phenomenon which occurs particularly at large $\Pm$. We show that in either case the transition correlates with a breaking of equatorial symmetry.
Nonlinear simulations of the geodynamo cannot be performed at accurate parameters and hence it is important to ensure that the correct (strong-field) branch is tracked as a distinguished limit is tracked towards a correct parameterisation from the simulations that we can perform. In order to understand the transition to strong-field dynamos, and better understand the mechanisms that occur in both branches, we report on a series of magnetoconvection simulations (that is, with the magnetic field fixed at the outer boundary) with which we bridge the gap between the strong- and weak-field regimes, and show that symmetry-breaking is triggered by the sudden growth of the magnetic field and in turn supports the dynamo in the strong-field regime. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2603_08261 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Magnetoconvection in a spherical shell: Equatorial symmetry during the transition from the weak- to the strong-field regime Gostelow, Luke J. Teed, Robert J. Fluid Dynamics Geophysics 76W05 (Primary) 85-11 (Secondary) At small but supercritical Rayleigh numbers, simulations of dynamos in spherical shells often separate into two broad regimes characterised either by their relative magnetic field strength (weak/strong) or by their dominant force balance (VAC/MAC). These regimes can tend smoothly from one to the other but can also be bistable, a phenomenon which occurs particularly at large $\Pm$. We show that in either case the transition correlates with a breaking of equatorial symmetry. Nonlinear simulations of the geodynamo cannot be performed at accurate parameters and hence it is important to ensure that the correct (strong-field) branch is tracked as a distinguished limit is tracked towards a correct parameterisation from the simulations that we can perform. In order to understand the transition to strong-field dynamos, and better understand the mechanisms that occur in both branches, we report on a series of magnetoconvection simulations (that is, with the magnetic field fixed at the outer boundary) with which we bridge the gap between the strong- and weak-field regimes, and show that symmetry-breaking is triggered by the sudden growth of the magnetic field and in turn supports the dynamo in the strong-field regime. |
| title | Magnetoconvection in a spherical shell: Equatorial symmetry during the transition from the weak- to the strong-field regime |
| topic | Fluid Dynamics Geophysics 76W05 (Primary) 85-11 (Secondary) |
| url | https://arxiv.org/abs/2603.08261 |