Secular Variation

Slow changes in the Earth's magnetic field, known as geomagnetic secular variation, are driven by the restless motions of liquid metal in the Earth's core that comprise the geodynamo. 

  • Together with Nils Olsen and Lars Tøffner-Clausen (DTU Space) I have developed high resolution models of the rate of change of the geomagnetic field at the outer edge of its source region, the core-mantle boundary (CMB). We make use of gradients (differences) of satellite magnetic measurements along-track and between Swarm's approximately east-west satellite pair to better isolate the core field signature.

Radial component of secular variation at the CMB from the CHAOS-6x-7 field model.

Finlay, C.C., Olsen, N., Kotsiaros, S., Gillet, N. and Tøffner-Clausen, L. (2016) Recent geomagnetic secular variation from Swarm and ground observatories as estimated in the CHAOS-6 geomagnetic field model, Earth Planets Space, Vol 68, 112. doi: 10.1186/s40623-016-0486-1

  • Together with PhD student Clemens Kloss I am constructing models of the time-dependent core flow responsible for secular variation at mid and low latitudes. Flow fluctuations at low latitudes give rise to pulses of acceleration and jerks in the field.

An example of core surface flow at mid latitudes, derived from ground and satellite observations:

Kloss, C., Finlay, C.C. (2019) Time-dependent low latitude core flow and geomagnetic field acceleration pulses, Geophys. J. Int., Vol 217, 140-168.

  • Working with with Phil Livermore (Leeds)  and also Nicolas Gillet and Oliver Barrois (IsTerre, Grenoble) I have studied an accelerating jet of intense core flow at high latitudes under Alaska and Siberia that accounts for the distinctive pattern of secular variation in the northern polar region, and and may be linked to a more general planetary-scale eccentric gyre structure in the outer core.  The impact of these structures on the north magnetic pole is presently under investigation.

Visualization of the intense core jet at high northern latitudes (left), and streamlines from a full core flow inversion (right, produced using flow visualization tool developed by Nicolas Gillet and colleagues at IsTerre, Grenoble).

Barrois, O., Hammer, M.D., Finlay, C.C., Martin, Y., and Gillet, N. (2018) Assimilation of ground and satellite magnetic measurements: inference of core surface magnetic and velocity field changes, Geophys. J. Int, Vol 215, 695-712, doi:10.1093/gji/ggy297

Livermore, P.W., Hollerbach, R. and Finlay, C.C. (2017) An accelerating high-latitude jet in Earth’s core, Nature Geoscience, 10, 62-68, doi:10.1038/ngeo2859

Gillet, N., Jault, D., and Finlay, C.C. (2015) Planetary gyre, time-dependent eddies, torsional waves, and equatorial jets at the Earth's core surface, J. Geophys. Res., doi: DOI: 10.1002/2014JB011786.
7 MARCH 2021