Instabilities of the Earth’s magnetic field over multi-millennial timescales

Earth’s magnetic field varies over a wide range of time scales. The very long multi-millennial scales are associated with variations of the internal core field. The short timescales, annual and shorter, are related to field contributions generated by current systems in the ionosphere and magnetosphere. The most dramatic changes occur at the longest periods, namely geomagnetic reversals and excursions. They represent a significant decrease in the field intensity, with reversals having a complete polarity flip and longer duration in contrast to excursions. While the reversals are global events, the excursions can be of global and regional nature. For the latter, only part of the Earth is affected with very low intensity and transitional directions. The last geomagnetic reversal, Matuyama-Brunhes, happened 780,000 years ago, and the last global excursion, the Laschamps excursion, occurred 41,000 years ago. These events are studied with indirect measurements, paleomagnetic data from sedimentary cores and volcanic rocks. Global data compilations enable us to reconstruct the field’s spatial and temporal variations and to better understand the core processes responsible for these variations.

Magnetic field lines during a period of high dipole moment, for example 1900 AD (top) and the Laschamps geomagnetic excursion, 41,000 years ago (bottom). The figures show the dipole-dominated structure at stable periods and very complex, multipolar structure of the magnetic field in transitional epochs. The color maps represent the radial field component at the Earth’s surface. The blue lines are pointing inward and red lines in an outward direction.

Credit: Maximilian Schanner, GFZ Potsdam, 2022. https://sec23.git-pages.gfz-potsdam.de/korte/pymagglobal-3d-fieldlines/

doi: 10.5880/GFZ.2.3.2020.005

While the current configuration of the geomagnetic field can be approximated with a dipole (having two poles, magnetic north and south pole), during geomagnetic excursions, the dipole is not a dominant component. The figures present the magnetic field lines of a dipole vs. non-dipole-dominated field, where the former is characteristic of the present-day and the latter of the Laschamps excursion. Over very long time scales, tens of thousands to millions of years, the field closely approximates a geocentric axial dipole. This hypothesis allows using the paleomagnetic data for reconstructing plate tectonics.

Direct observations indicate that the magnitude of the Earth's magnetic axial dipole has decreased since 1840, and predictions of geomagnetic secular variation show that it will likely continue to decrease over the following decades. A weakened geomagnetic field may lead to numerous hazards like satellite outages, disturbances in communications and navigation, and induced currents in pipelines and transmission lines. The following questions come naturally:

Will the geomagnetic field go through an excursion or a reversal in the future? Most probably yes, considering the dynamic nature of the field and the frequency variations of these transitional events.

Is the geomagnetic field heading towards an excursion or reversal now? Probably not, based on analyses of available paleomagnetic field models covering the past 10,000 and 100,000 years and the fact that the field at present seems to be clearly stronger than the long-term average.

Can we predict such events? No, this remains an open question because it is hard to foresee the long-term changes. The geomagnetic field intensity needs to decay further for several centuries to recognize and distinguish a future transitional event from normal secular variation.

Sanja Panovska is a postdoctoral researcher at the Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences. Her primary research focus is reconstructing the long-term evolution of the geomagnetic field based on paleomagnetic data over different timescales from the Holocene, to 100,000 years and over the Earth’s geological history. She is also involved in studies on productions of cosmogenic isotopes, paleomagnetosphere and paleoaurora.