Signatures of High‐Latitude Waves in Observations of Geomagnetic Acceleration

Chi-Durán, Rodrigo; Avery, Margaret S.; Buffett, B. A.

doi:10.1029/2021gl094692

Cited by 7 publications

(5 citation statements)

References 28 publications

(43 reference statements)

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“…The DMD method also identifies a nominal 20‐year wave, which has been previously reported in the Northern Hemisphere (Chi‐Durán et al., 2020, 2021). However, the origin of Modes 4 and 5 is less clear.…”

Section: Discussionmentioning

confidence: 83%

Extracting Spatial–Temporal Coherent Patterns in Geomagnetic Secular Variation Using Dynamic Mode Decomposition

Chi-Durán

Buffett

2023

Geophysical Research Letters

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Observations of the geomagnetic field offer a wealth of information about the dynamics of Earth's deep interior. Historical records from the past 400 years (Jackson et al., 2000) are commonly used to construct models of the geomagnetic field and its first-time derivative, often called secular variation. A large part of the secular variation is attributed to large-scale fluid motion near the surface of Earth's core (e.g., Holme et al., 2015). Other contributions include magnetic diffusion and the effects of unresolved small-scale flow. Recent efforts to account for these effects (Gillet et al., 2019) rely on geodynamo simulations to establish statistical correlations between the predicted flow and the magnetic field. While this approach represents the forefront of current research, it does mean that our ability to recover dynamics from magnetic-field observations is dependent on prior assumptions about the nature of the flow. A complementary approach relies on modern data-driven methods to identify and characterize patterns of change in the observations. One particular technique, known as dynamic mode decomposition (DMD) (Schmid, 2022), is particularly well-suited to the analysis of magnetic-field observations because it allows us to establish modes (waves) in the data before attaching a physical interpretation. There is no requirement for each mode to have a common physical basis or interpretation, although we do expect a common set of background conditions. A primary motivation for this study is to explore the feasibility of using new data-driven approaches to assess the geomagnetic field.Several factors prompt our interest in data-driven approaches. One is the availability of magnetic observations from satellite missions (e.g., Orsted, CHAMP, SWARM), which substantially improve the quality and quantity of information. Satellite-based observations give better spatial coverage and allow greater discrimination between the internal and external sources of the geomagnetic field compared to ground-based measurements (Friis-Christensen et al., 2006

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Section: Discussionmentioning

confidence: 83%

Extracting Spatial–Temporal Coherent Patterns in Geomagnetic Secular Variation Using Dynamic Mode Decomposition

Chi-Durán

Buffett

2023

Geophysical Research Letters

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“…As stated above also, the presence of density stratification near the core surface could also create new rapid geomagnetic signatures corresponding to the propagation of MAC waves within a stable layer (e.g. Buffett 2014 ;Chi-Dur án et al 2021 ). Including these ingredients and systematically studying their effects within the present assimilation framework should provide new ways to constrain the corresponding physical properties from the standpoint of geomagnetic variations.…”

Section: Strengthening the Geomagnetically Derived Geodynamical Const...mentioning

confidence: 93%

State and evolution of the geodynamo from numerical models reaching the physical conditions of Earth’s core

Aubert

2023

Geophysical Journal International

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Summary Our understanding of the geodynamo has recently progressed thanks to geomagnetic data of improved quality, and analyses resting on numerical simulations of increasing realism. Here these two advances are combined in order to diagnose the state and present dynamics of Earth’s core in physically realistic conditions. A sequential, ensemble-based framework assimilates the output of geomagnetic field models covering the past 180 years into a numerical geodynamo simulation, the physical realism of which is also advanced as data is assimilated. The internal dynamical structure estimated for the geodynamo at present reproduces previously widely documented features such as a planetary-scale, eccentric westwards gyre, and localisation of buoyancy release beneath the Eastern (0oE-180o) hemisphere. Relating the typical magnetic variation time scale of the assimilated states to the power at which they operate, the present convective power of the geodynamo is estimated at 2.95 ± 0.2 TW, corresponding to an adiabatic heat flow out of the core of 14.8 ± 1 TW if the top of the core is convectively neutrally stratified at present. For the first time, morphologically and dynamically relevant trajectories are obtained by integrating the estimated states forward for a few decades of physical time using a model reaching the physical conditions of Earth’s core. Such simulations accurately account for the spatio-temporal content of high-resolution satellite geomagnetic field models and confirm earlier interpretations in terms of rapid core dynamics. The enforcement of a realistic force balance approaching a Taylor state allows for propagation of weak (velocity perturbation of about 0.6km.yr−1) axisymmetric torsional waves with period about 5 years, supported by a magnetic field of root-mean-squared amplitude of 5.6 mT inside the core. Quasi-geostrophic magneto-Coriolis waves of interannual periods and significantly stronger velocity perturbation (about 7km.yr−1) are also reproduced, with properties that converge towards those recently retrieved from the analysis of geomagnetic variations before fully achieving Earth’s core conditions. The power spectral density of magnetic variations falls off rapidly at frequencies exceeding the inverse Alfvén time (about 0.6yr−1), which indicates that the excitation of hydromagnetic waves occurs preferentially at large spatial scales. The possibility to account for geomagnetic variations from years to centuries in physically realistic models opens the perspective of better constraining properties of the deep Earth through geomagnetic data assimilation.

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“…For example, a comprehensive investigation in a spherical, magnetised, shallow-water system was recently made by Márquez-Artavia et al (2017), where a simple background field was assumed; here recall an equivalent analysis in the hydrodynamic case was made by Longuet-Higgins (1968). Further works are finding peculiar eigenmodes such as equatorially or polarly trapped ones (e.g., Buffett and Matsui 2019;Nakashima 2020;Chi-Durán et al 2021).…”

Section: Waves: the Focus Of The Special Topicsmentioning

confidence: 99%

Waves in planetary dynamos

Hori

Nilsson

Tobias

2022

Rev. Mod. Plasma Phys.

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This Special Topic focuses on magnetohydrodynamic (MHD) processes in the deep interiors of planets, in which their fluid dynamos are in operation. The dynamo-generated, global, magnetic fields provide a background for our solar-terrestrial environment. Probing the processes within the dynamos is a significant theoretical and computational challenge and any window into interior dynamics greatly increases our understanding. Such a window is provided by exploring rapid dynamics, particularly MHD waves about the dynamo-defined basic state. This field is the subject of current attention as geophysical observations and numerical modellings advance. We here pay particular attention to torsional Alfvén waves/oscillations and magnetic Rossby waves, which may be regarded as typical axisymmetric and nonaxisymmetric modes, respectively, amongst a wide variety of wave classes of rapidly rotating MHD fluids. The excitation of those waves has been evidenced for the Earth — whilst their presence has also been suggested for Jupiter. We shall overview their dynamics, summarise our current understanding, and give open questions for future perspectives.

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Signatures of High‐Latitude Waves in Observations of Geomagnetic Acceleration

Cited by 7 publications

References 28 publications

Extracting Spatial–Temporal Coherent Patterns in Geomagnetic Secular Variation Using Dynamic Mode Decomposition

Extracting Spatial–Temporal Coherent Patterns in Geomagnetic Secular Variation Using Dynamic Mode Decomposition

State and evolution of the geodynamo from numerical models reaching the physical conditions of Earth’s core

Waves in planetary dynamos

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