Abstract
Evidence for torsional oscillations (TOs) operating within the Earth's fluid outer core has been found in the secular variation of the geomagnetic field. These waves arise via disturbances to the predominant (magnetostrophic) force balance believed to exist in the core. The coupling of the core and mantle allow TOs to affect the length-of-day of the Earth via angular momentum conservation.Encouraged by previous work, where we were able to observe TOs in geodynamo simulations, we perform 3-D magnetoconvection simulations in a spherical shell in order to reach more Earth-like parameter regimes that proved hitherto elusive.At large Ekman numbers we find that TOs can be present but are typically only a small fraction of the overall dynamics and are often driven by Reynolds forcing at various locations throughout the domain. However, as the Ekman number is reduced to more Earth-like values, TOs become more apparent and can make up the dominant portion of the short timescale flow. This coincides with a transition to regimes where excitation is found only at the tangent cylinder, is delivered by the Lorentz force and gives rise to a periodic Earth-like wave pattern, approximately operating on a 4 to 5 year timescale. The core travel times of our waves also become independent of rotation at low Ekman number with many converging to Earth-like values of around 4 years.
Highlights
The geodynamo, which operates in the Earth’s iron-rich outer core and continuously replenishes its magnetic field, is believed to be one of a number of planetary dynamos that exists under a quasi-magnetostrophic regime
Across the set of runs we found the geostrophy parameter, U C, was able to give an excellent indication of the ability to observe torsional oscillations (TOs) in any given simulation
Perturbations of a quasi-Taylor state leading to TOs may, on some level, always be possible, but TOs must account for ∼40% of the short timescale flow, in order for the waves to be observed over convection
Summary
The geodynamo, which operates in the Earth’s iron-rich outer core and continuously replenishes its magnetic field, is believed to be one of a number of planetary dynamos that exists under a quasi-magnetostrophic regime (see, for example, Jones et al, 2011) This suggests that, owing to the rapidly rotating nature of the Earth, the predominant force balance within the core is between Lorentz, Archimedean and Coriolis forces; this is commonly known as the MAC balance. Violations of Taylor’s constraint manifest themselves as the acceleration of concentric cylinders with a restoring Lorentz force acting like a torsional spring that attempts to reimpose the Taylor state (Braginsky, 1970) This process leads to the excitation of torsional oscillations (TOs), which are the only Alfvén waves (Alfvén, 1942) in the core that act at large lengthscales, propagating in the cylindrical radial direction. TOs have long been presented as an explanation for certain changes in the Earth’s length-of-day via the coupling of solid and fluid regions at the core–mantle boundary (CMB) by angular momentum conservation (Jault et al, 1988; Jackson, 1997; Roberts and Aurnou, 2012), and may be connected with other features such as geomagnetic jerks (Bloxham et al, 2002; Brown et al, 2013)
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