The observational signature of modelled torsional waves and comparison to geomagnetic jerks

Cox, Grace; Livermore, Philip W.; Mound, J. E.

doi:10.1016/j.pepi.2016.03.012

Cited by 22 publications

(21 citation statements)

References 75 publications

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“…7, 8). This confirms the conclusions previously obtained by Cox et al (2016) regarding the difficulty to offer an explanation for geomagnetic jerks solely resting on torsional waves, as was initially advocated for by Bloxham et al (2002). Future studies should therefore attempt to highlight other possible mechanisms for these sudden changes in the geomagnetic secular acceleration.…”

Section: Dynamic Variability Versus Kinematic Invariance Along the Pathsupporting

confidence: 88%

Geomagnetic acceleration and rapid hydromagnetic wave dynamics in advanced numerical simulations of the geodynamo

Aubert

2018

Geophysical Journal International

107

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Geomagnetic secular acceleration, the second temporal derivative of Earth's magnetic field, is a unique window on the dynamics taking place in Earth's core. In this study, the behaviours of the secular acceleration and underlying core dynamics are examined in new numerical simulations of the geodynamo that are dynamically closer to Earth's core conditions than earlier models. These new models reside on a theoretical path in parameter space connecting the region where most classical models are found to the natural conditions. The typical time scale for geomagnetic acceleration is found to be invariant along this path, at a value close to 10 years that matches Earth's core estimates. Despite this invariance, the spatio-temporal properties of secular acceleration show significant variability along the path, with an asymptotic regime of rapid rotation reached after 30% of this path (corresponding to a model Ekman number E = 3 10 −7 ). In this regime, the energy of secular acceleration is entirely found at periods longer than that of planetary rotation, and the underlying flow acceleration patterns acquire a two-dimensional columnar structure representative of the rapid rotation limit. The spatial pattern of the secular acceleration at the coremantle boundary shows significant localisation of energy within an equatorial belt. Rapid hydromagnetic wave dynamics is absent at the start of the path because of insufficient time scale separation with convective processes, weak forcing and excessive damping but can be clearly exhibited in the asymptotic regime. This study reports on ubiquitous axisymmetric geostrophic torsional waves of weak amplitude relatively to convective transport, and also stronger, laterally limited, quasi-geostrophic Alfvén waves propagating in the cylindrical radial direction from the tip of convective plumes towards the core-mantle boundary. In a system similar to Earth's core where the typical Alfvén velocity is significantly larger than the typical convective velocity, quasi-geostrophic Alfvén waves are shown to be an important carrier of flow acceleration to the core surface that links with the generation of strong, short-lived and intermittent equatorial pulses in the secular acceleration energy. The secular acceleration time scale is shown to be insensitive to magnetic signatures from torsional waves because of their weak amplitude, and from quasi-geostrophic Alfvén waves because of their intermittent character, and is therefore only indicative of convective transport phenomena that remain invariant along the parameter space path.

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Section: Dynamic Variability Versus Kinematic Invariance Along the Pathsupporting

confidence: 88%

Geomagnetic acceleration and rapid hydromagnetic wave dynamics in advanced numerical simulations of the geodynamo

Aubert

2018

Geophysical Journal International

107

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“…3c, d). This effect is similar to that observed by Cox et al (2016), who explore whether global torsional waves can couple with heterogeneous background magnetic fields to produce localized signals in SV resembling geomagnetic jerks.…”

Section: Generation Of Secular Variation and Accelerationsupporting

confidence: 80%

Extracting waves from noisy geomagnetic data – A synthetic study of equatorially trapped waves in Earth’s core

Knezek

Buffett

2019

Physics of the Earth and Planetary Interiors

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Geomagnetic Secular Variation (SV) and acceleration (SA) can provide a wealth of information about fluid motions in the core. However, recovering this information from noisy data is complicated by the structure of the magnetic field at the core-mantle boundary. We demonstrate the challenge using a numerical model for equatorial waves to produce synthetic records of SV and SA. We show that interpreting magnetic oscillations directly as waves in Earth's core can lead to spurious conclusions due to coupling between non-dipole components of the magnetic field and wave structures. Using the structure of our modeled waves, we derive a set of methods to detect waves using magnetic field observations. We show that these methods can recover the wave period, longitudinal wavenumber, equatorial symmetry, latitudinal extent, and amplitude for synthetic equatorial waves in Earth's core when the wave fluid velocity is ∼0.5 km/yr or greater.

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“…Recent studies by Gillet et al (2010Gillet et al ( , 2015 and Holme and de Viron (2013) point to a ∼ 6-year variation originating in the core that is also observable in length-ofday data. The signal was also discussed by Abarca del Rio et al (2000), Mound and Buffet (2006), Cox et al (2016), and Duan et al (2018). We note, however, that both periodicities are very close to the harmonics of the solar cycle, suggesting a possible external origin.…”

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confidence: 63%

Geomagnetic field declination: from decadal to centennial scales

2018

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Abstract. Declination annual mean time series longer than 1 century provided by 24 geomagnetic observatories worldwide, together with 5 Western European reconstructed declination series over the last 4 centuries, have been analyzed in terms of the frequency constituents of the secular variation at inter-decadal and sub-centennial timescales of 20-35 and 70-90 years. Observatory and reconstructed time series have been processed by several types of filtering, namely Hodrick-Prescott, running averages, and Butterworth. The Hodrick-Prescott filtering allows us to separate a quasioscillation at a decadal timescale, which is assumed to be related to external variations and called the "11-year constituent", from a long-term trend. The latter has been decomposed into two other oscillations called "inter-decadal" and "sub-centennial" constituents by applying a Butterworth filtering with cutoffs at 30 and 73 years, respectively. The analysis shows that the generally accepted geomagnetic jerks occur around extrema in the time derivative of the trend and coincide with extrema in the time derivative of the 11-year constituent. The sub-centennial constituent is traced back to 1600 in the five 400-year-long time series and seems to be a major constituent of the secular variation, geomagnetic jerks included.

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The observational signature of modelled torsional waves and comparison to geomagnetic jerks

Cited by 22 publications

References 75 publications

Geomagnetic acceleration and rapid hydromagnetic wave dynamics in advanced numerical simulations of the geodynamo

Geomagnetic acceleration and rapid hydromagnetic wave dynamics in advanced numerical simulations of the geodynamo

Extracting waves from noisy geomagnetic data – A synthetic study of equatorially trapped waves in Earth’s core

Geomagnetic field declination: from decadal to centennial scales

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