Asymmetric growth and decay of the geomagnetic dipole field examined with numerical dynamo simulations
Abstract
Studying direct magnetic and paleomagnetic field variations at Earth's surface provide a means to deepen our understanding of the dynamo operating in the liquid outer core. We use a suite of numerical dynamo simulations (generated with the Boussinesq Leeds Dynamo Code) to investigate the core processes responsible for the asymmetric axial dipole moment (ADM) growth and decay rates observed in the paleomagnetic record (Ziegler and Constable, 2011; Avery et al., 2017). Numerical dynamo simulations can be used to complement paleomagnetic records as they do not suffer the same limitations of spatial and temporal resolution. The magnetic and velocity fields are completely known; however, the simulations cannot yet run with Earth-like diffusivities or rotational rates. Our simulations include a range of Rayleigh and Roberts numbers resulting in dipole-dominated dynamos that have been run for multiple magnetic diffusion times.
For each simulation surface dipole energy output is evaluated by using low-pass filters to determine what timescales (if any) exhibit asymmetry in rates of change. We also examine the coherence spectra between the various terms of the magnetic induction equation to assess changes in the force balance as a function of frequency. Some simulations exhibit similar asymmetry between dipole growth and decay to that observed in the paleomagnetic record, and this behavior is associated with changes in magnetic energy that are more coherent with ohmic heating than with the work done by the Lorentz force in that frequency band. We focus on two instructive dynamo cases, one with Earth-like ADM behavior and one that is not Earth-like. Visualization of their magnetic fields at the core mantle boundary (CMB) and their internal fields reveal a link between the number of convective upwellings and ADM variations. We also map the contributions to changes in ADM from advection and diffusion at the CMB. For the Earth-like case dipole decay is dominated by diffusion although both advection and diffusion are in play. During dipole growth advection at the CMB is stronger and acts to increase the axial dipole moment. In the future, with higher spatial resolution paleomagnetic field reconstructions of long-term behavior, we could test if such a link between convective upwelling lengthscales and ADM variations exists for Earth.- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMDI21B0007A
- Keywords:
-
- 1510 Dynamo: theories and simulations;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 1560 Time variations: secular and longer;
- GEOMAGNETISM AND PALEOMAGNETISMDE: 7207 Core;
- SEISMOLOGYDE: 8124 Earth's interior: composition and state;
- TECTONOPHYSICS