The Mystery of Earth’s Magnetic Field

By Bradley Kelton

Imagine you wake up in your dorm conveniently located on 115th street, get ready, and head out for class. Of course, identifying which direction is north in Manhattan is quite difficult, so to solve this problem, you take out your trusty compass and follow it to your class on 120th street. After walking the requisite number of blocks you find yourself at 110th instead! What could be the cause of this? The answer, of course, is that you forgot to re-calibrate your compass after the overnight magnetic field reversal. After all, who knew that the magnetic field flips polarity from time to time? Well, it turns out that it does.​

The geomagnetic field exists thanks to a process called differentiation, the separation of a bulk mass into layers with distinct thermal and mechanical properties. Accretion kicks off this process, as materials begin to separate themselves based on density, meaning that the heavier elements–metals like iron and nickel–end up in this dense zone, called the core. The core is then intensely heated by the decay of radioactive isotopes which produces a solid inner core and a liquid outer core. The reduced viscosity of the liquid outer core permits the free travel of ferromagnetic elements, like iron and nickel, which align themselves in magnetic domains, creating the generator powering Earth’s magnetic field (Buis, 2022).

From making life above sea level a possibility by shielding us from harmful solar rays to illuminating the sky with the brilliant blues and greens of the aurora borealis, the geomagnetic field is a fascinating part of the Earth system. And yet, these complexities are not fully understood by geophysicists. The polarity flips, for instance, happen once every few hundred-thousand years and are incredibly difficult to model. The best tools for studying the magnetic field are the hydromagnetic equations, a combination of Maxwell’s equations for electrodynamics and the principles of fluid dynamics (Simkanin and Hejda, 2013). These equations have no exact solutions, owing to the chaotic nature of the magnetic field.

How then do we know anything about the magnetic field? Geologists have done a remarkable job of reconstructing the history of Earth’s magnetic field by examining metallic crystals embedded in igneous rocks dating back billions of years. As lava flows cool, small metallic grains align themselves with the magnetic field, allowing us to reconstruct the past magnetic field. And as for predicting the future magnetic field, we can model very short term changes with machine learning and data assimilation methods, but these techniques cannot accurately explain reversals on geologic timescales nor forecast future reversals (Li et al., 2023).

So, what happens if you wake up tomorrow and find that south is north and north is south? How would you get to work or school without the help of your trusty compass? Well, fear not, because it turns out that the magnetic field isn’t quite so mercurial that it changes on any given day. Over several thousand years, the magnetic field strength weakens until it reaches 10% of its original strength and then it eventually re-magnetizes, this time in the opposite direction. Thousands of years might sound like a long time on the human scale, but geologically, this is quite fast. According to our observations the magnetic field has weakened by about 10% over the last 200 years (European Space Agency, 2020) so, who knows, maybe tomorrow that minor inconvenience of having to re-calibrate your compass will become a reality.

References
1. Buis, A. (2021). Earth’s Magnetosphere: Protecting Our Planet from Harmful Space Energy. NASA Science Editorial Team. https://science.nasa.gov/science-research/earth-science/earths-magnetosphere-protecting-our-planet-from-harmful-space-energy/ 
2. Simkanin, J. and Hejda, P. (2013). Magnetic fields generated by hydromagnetic dynamos at the low Prandtl number in dependence on the Ekman and magnetic Prandtl numbers. Physics of the Earth and Planetary Interiors, Volume 217, 2013, Pages 22-30, ISSN 0031-9201, https://doi.org/10.1016/j.pepi.2012.11.002 
3. Li, J. F., Lin, Y. F., and Zhang, K. K. (2023). The effect of model errors in ensemble sequential assimilation of geomagnetic field. Earth Planet. Phys., 7(1), 22–31. https://doi.org/10.26464/epp2023006
4. European Space Agency. (2020). Swarm probes weakening of Earth’s magnetic field. https://www.esa.int/Applications/Observing_the_Earth/FutureEO/Swarm/Swarm_probes_weakening_of_Earth_s_magnetic_field