Singular Spectral Analysis Decomposition of Geomagnetic and Stochastic Axial Dipole Moment Models Reveal Long-Period Oscillations
Abstract
Earth's magnetic field varies on a broad range of time scales driven by fluid motion in the outer core through a self-sustaining dynamo. We use singular spectral analysis (SSA), a technique used to decompose geophysical and climatic time series into statistically independent components that can be identified as trends, oscillations, and noise, to examine long period variations of Earth's axial dipole moment (ADM). Cluster analysis is used to objectively group SSA components of variation by their covariance. Monte Carlo - SSA (MC-SSA) is an extension of SSA analysis used to test the significance of the signal above that predicted for red noise (Allen and Smith, 1996). We apply MC-SSA here to explore the spectra of ADM variations described in the field models: PADM2M (Ziegler et al. 2010), GGF100k (Panovska et al. 2018), and a stochastic dipole model (Scullard and Buffett, 2018). The axial dipole component of GGF100k has a periodic oscillation with a period of ~30kyr, which can be explored further with the higher degree spherical harmonic components, up to degree 10, available for this field model. The noise component in PADM2M largely matches the signal of GGF100k; this is not surprising since PADM2M is smoothed and has a resolution of ~10kyr. PADM2M has a nearly linear trend, showing that the Brunhes chron has a higher average ADM than the Matuyama. PADM2M displays long-period oscillatory signal with periods of ~100 and 500 kyr. These are longer than the expected characteristic time scales of advection and diffusion which are approximately 200 years and 50 kyr respectively. A possible source of this long-period ADM periodicity is the interaction between poloidal and toroidal fields. A stochastic model with drift and diffusion terms for the toroidal and poloidal components can produce similar long-period variations with physically motivated choices for the model parameters. This has the potential to tell us about Earth's toroidal field, which cannot be observed outside of the outer core.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMGP33A..07A
- Keywords:
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- 1503 Archeomagnetism;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1517 Magnetic anomalies: modeling and interpretation;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1521 Paleointensity;
- GEOMAGNETISM AND PALEOMAGNETISM;
- 1595 Planetary magnetism: all frequencies and wavelengths;
- GEOMAGNETISM AND PALEOMAGNETISM