Inferring core processes using stochastic models of the geodynamo

2021 ◽  
Author(s):  
William Davis ◽  
Bruce Buffett
Keyword(s):  
Magnetic Field ◽  
Stochastic Models ◽  
Dipole Field ◽  
Core Mantle Boundary ◽  
The Core ◽  
Field Generation ◽  
Magnetic Dipole Field ◽  
Time Variations ◽  
Random Part ◽  
Magnetic Field Variations

Summary Recent studies have represented time variations in the Earth’s axial magnetic dipole field as a stochastic process, which comprise both deterministic and random elements. To explore how these elements are affected by the style and vigour of convection in the core, as well as the core-mantle boundary conditions, we construct stochastic models from a set of numerical dynamo simulations at low Ekman numbers. The deterministic part of the stochastic model, the drift term, characterises the slow relaxation of the dipole back to its time-average. We find that these variations are predominantly accommodated by the slowest decay mode, enhanced by turbulent diffusion to enable a faster relaxation. The random part—the noise term—is set by the amplitude and timescale of variations in dipole field generation, including contributions from both velocity and internal magnetic field variations. Applying these interpretations to the paleomagnetic field suggest that reversal rates are very sensitive to rms variations in the field generation. Less than a 50 per cent reduction in rms field generation variations is sufficient to prevent reversals for the recent magnetic field.

scholarly journals Can one use Earth’s magnetic axial dipole field intensity to predict reversals?

2020 ◽  
Author(s):  
K Gwirtz ◽  
M Morzfeld ◽  
A Fournier ◽  
G Hulot
Keyword(s):  
Magnetic Field ◽  
Numerical Models ◽  
Dipole Field ◽  
Axial Dipole ◽  
Intensity Threshold ◽  
Magnetic Dipole Field ◽  
Model Hierarchy ◽  
Definition Of ◽  
Virtual Axial Dipole Moment ◽  
Prediction Strategy

Summary We study predictions of reversals of Earth’s axial magnetic dipole field that are based solely on the dipole’s intensity. The prediction strategy is, roughly, that once the dipole intensity drops below a threshold, then the field will continue to decrease and a reversal (or a major excursion) will occur. We first present a rigorous definition of an intensity threshold-based prediction strategy and then describe a mathematical and numerical framework to investigate its validity and robustness in view of the data being limited. We apply threshold-based predictions to a hierarchy of numerical models, ranging from simple scalar models to 3D geodynamos. We find that the skill of threshold-based predictions varies across the model hierarchy. The differences in skill can be explained by differences in how reversals occur: if the field decreases towards a reversal slowly (in a sense made precise in this paper), the skill is high, and if the field decreases quickly, the skill is low. Such a property could be used as an additional criterion to identify which models qualify as Earth-like. Applying threshold-based predictions to Virtual Axial Dipole Moment (VADM) paleomagnetic reconstructions (PADM2M and Sint-2000) covering the last two million years, reveals a moderate skill of threshold-based predictions for Earth’s dynamo. Besides all of their limitations, threshold-based predictions suggests that no reversal is to be expected within the next 10 kyr. Most importantly, however, we show that considering an intensity threshold for identifying upcoming reversals is intrinsically limited by the dynamic behavior of Earth’s magnetic field.

GRACEFUL: Probing the deep Earth interior by synergistic use of observations of the magnetic and gravity fields, and of the rotation of the Earth

2020 ◽  
Author(s):  
Mioara Mandea ◽  
Veronique Dehant ◽  
Anny Cazenave
Keyword(s):  
Magnetic Field ◽  
Gravity Field ◽  
Time Scales ◽  
Earth Rotation ◽  
Outer Core ◽  
Time Dependent ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
The Earth

<div> <p>To understand the processes involved in the deep interior of the Earth and explaining its evolution, in particular the dynamics of the Earth’s fluid iron-rich outer core, only indirect satellite and ground observations are available. They each provide invaluable information about the core flow but are incomplete on their own:</p> <p>-        The time dependent magnetic field, originating mainly within the core, can be used to infer the motions of the fluid at the top of the core on decadal and subdecadal time scales.</p> <p>-        The time dependent gravity field variations that reflect changes in the mass distribution within the Earth and at its surface occur on a broad range of time scales. Decadal and interannual variations include the signature of the flow inside the core, though they are largely dominated by surface contributions related to the global water cycle and climate-driven land ice loss.</p> <p>-        Earth rotation changes (or variations in the length of the day) also occur on these time scales, and are largely related to the core fluid motions through exchange of angular momentum between the core and the mantle at the core-mantle boundary.</p> <p>Here, we present the main activities proposed in the frame of the GRACEFUL ERC project, which aims to combine information about the core deduced from the gravity field, from the magnetic field and from the Earth rotation in synergy, in order to examine in unprecedented depth the dynamical processes occurring inside the core and at the core-mantle boundary.</p> </div>

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