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Cylinders aligned with the planetary rotation axis have a special significance in the dynamics of planetary dynamo regions. The azimuthal Lorentz forces on these geostrophic cylinders is expected to cancel to a large degree, establishing the so-called Taylor state. Deviations from this state take the form of torsional oscillations (TOs) that are supposed to represent important fast flow variations. These oscillations have reportedly been identified in the secular variation signal from the top of Earth's core. We have performed several dynamo simulations at different parameters to check whether Taylor state and TOs can also be identified in a numerical model. Taylor states are approached when viscous effects are small at Ekman numbers of or below and Reynolds stresses are kept low by choosing moderate Rayleigh numbers. One-dimensional magnetic Alfvén waves that travel towards the boundaries then become prominent in the motion of the geostrophic cylinders. These waves obey the TO theory but are also damped and modified by other effects. For example, fast variations of likely convective origin remain important in all our simulations. Reynolds stresses may play a more sizable role for the dynamics in Earth's dynamo region than commonly assumed. They may also contribute to the motions of geostrophic cylinders and severely reduce the significance of TOs for the fast core dynamics. The amplitude of TOs amounts to not more than a few percent of the total flow amplitude in the simulations, which renders these motions insignificant for the long-term dynamo process. [ABSTRACT FROM AUTHOR]

We propose a spectral transfer model for the secular variation of the geomagnetic core field to explain the simultaneous decrease in dipole field intensity and the increase in non-dipole field intensity from 1840 to the present in terms of a dynamo cascade process. The main assumption of this model is that magnetic energy is transferred between adjacent spherical harmonic degrees in the Mauersberger–Lowes spectrum of the geomagnetic field. The key parameters are a set of coefficients that indicate the rate and direction of magnetic energy transfer through the spectrum. Applying the spectral transfer model to the historical period, we find that the quadrupole family of the core field can be characterized by a persistent inverse magnetic energy cascade from higher towards lower spherical harmonics. In the dipole family of the core field, we find cascade behaviour generally from lower to higher spherical harmonics, consistent with axial dipole decrease, but with a high level of time variability that correlates with variations in the dipole family intensity. During time intervals when the dipole family intensity rapidly decreases, energy appears to cascade towards higher spherical harmonics, beyond the limit of the observable part of the core field spectrum. During time intervals when the dipole family intensity is nearly constant, a more limited forward cascade appears to trap energy at intermediate spherical harmonics. Similar fluctuations in the rate and direction of spectral transfer are also seen in the Mauersberger–Lowes spectrum of a numerical dynamo model during a dipole decrease event that led to a polarity excursion. We discuss the possibility of this scenario for the current geomagnetic dipole decrease. [ABSTRACT FROM AUTHOR]

Investigates four subgrid-scale models in numerical calculations of magnetoconvection in the Earth's core. Large-eddy simulations; Strategy for modelling large-scale flow; Reliance of the approach on spatial filtering to eliminate scales smaller that the grid spacing.