An analysis of the fluctuations of the geomagnetic dipole

TL;DR

This paper models the Earth's geomagnetic dipole fluctuations using a Fokker-Planck approach, revealing insights into the growth, quenching, and diffusion processes of the geomagnetic field based on Sint-2000 data.

Contribution

It introduces a method to extract drift and diffusion coefficients from geomagnetic data, showing that the diffusion depends weakly on dipole strength, contrary to theoretical expectations.

Findings

The geomagnetic dipole has a linear growth time of 13-33 kyr.

Nonlinear quenching follows a quadratic form.

Diffusion depends weakly on dipole strength.

Abstract

The time evolution of the strength of the Earth's virtual axial dipole moment (VADM) is analyzed by relating it to the Fokker-Planck equation, which describes a random walk with VADM-dependent drift and diffusion coefficients. We demonstrate first that our method is able to retrieve the correct shape of the drift and diffusion coefficients from a time series generated by a test model. Analysis of the Sint-2000 data shows that the geomagnetic dipole mode has a linear growth time of 13 to 33 kyr, and that the nonlinear quenching of the growth rate follows a quadratic function of the type [1-(x/x0)^2]. On theoretical grounds, the diffusive motion of the VADM is expected to be driven by multiplicative noise, and the corresponding diffusion coefficient to scale quadratically with dipole strength. However, analysis of the Sint-2000 VADM data reveals a diffusion which depends only very weakly on the dipole strength. This may indicate that the magnetic field quenches the amplitude of the turbulent velocity in the Earth's outer core.