Longhui Yuan

Last Name

Yuan

First Name

Longhui

ORCID

0000-0002-3237-8409

Organisational unit

03734 - Jackson, Andrew / Jackson, Andrew

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Publications1 - 2 of 2

Waves in the Earth's core. IV. The structure of inviscid torsional oscillations in a spherical shell
Yuan, Longhui; Marti, Philippe David; Luo, Jiawen; et al. (2025)
Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences
We study the properties of cylindrical oscillations of electrically conducting fluid in the presence of a magnetic field, a phenomenon known in geomagnetism as torsional oscillations (TOs) since their discovery by J. B. Taylor and S. I. Braginsky. The chosen geometry is a spherical shell, consistent with Earth's present-day geometry. We concentrate on the scenario where fluid viscosity is absent in our calculations, but magnetic diffusivity is retained, appropriate to the geophysical conditions in Earth's fluid outer core. Two axisymmetric background magnetic fields that provide the restoring torques to the motions are considered, one of dipole parity and the other of quadrupole parity. The anticipated class of equatorially symmetric (ES) azimuthal motions is joined by an antisymmetric class that exists in the shell geometry but is absent in the full sphere. Compared to previous studies in a full sphere, our results reveal that computing the eigenmodes of TOs in a spherical shell under the inviscid limit is considerably more computationally challenging. Only one large-scale eigenmode exists (filling the whole shell), while many modes with higher frequencies tend to be concentrated inside the tangent cylinder. We complement our inviscid calculations with calculations in which viscosity is retained, and find convergence (with decreasing viscous diffusion) towards the inviscid results.
A joint investigation of Saturn’s deep zonal flow via its gravitational field and Ohmic dissipation
Yuan, Longhui; Kong, Dali (2023)
Monthly Notices of the Royal Astronomical Society
The Cassini Grand Finale provided a unique opportunity to study Saturn’s deep zonal flow. In this paper, we present a comprehensive deep zonal flow model for Saturn using a joint inversion of observed gravity and zonal flow-induced Ohmic dissipation in the semi-conducting region, under the assumption that the planet’s cloud-level wind is limited to a shallow weather layer. Our model unveils a strong equatorially symmetric zonal flow (O(100) m s⁻¹) and a weaker antisymmetric zonal flow (O(1) m s⁻¹ beneath the cloud-level winds. Furthermore, we show that the maximum depth of the deep zonal flow is around 7800 km, surpassing previous results derived from gravity alone and with the assumption that the rapid cloud-level winds extend deep into the planet’s interior. The meridional profile of the deep zonal flow differs significantly from the cloud-level zonal winds and predicts a strong westward zonal flow in the region with latitude around ±23°, where the observed cloud-top winds remain eastward. We also demonstrate that the zonal flow inside and outside the tangent cylinder exhibits significant differences in speed and scale. Moreover, our findings suggest that the coupling between the deep zonal flow and cloud-level winds varies across latitudes, with the shallow-wind model applicable to polar regions within the tangent cylinder and the deep-wind model more relevant to equatorial regions outside the tangent cylinder. Our findings highlight the importance of accounting for the planet’s deep zonal flow in future studies of Saturn’s atmospheric dynamics.

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Longhui Yuan

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