If you have ever seen the northern lights (aurora borealis), you have probably wondered where they originate from. It turns out, they are a stunning visual demonstration of the Earth’s magnetic field interacting with cosmic rays. The magnetic field is estimated to a have originated around 3.5 billion years ago based on analysis of Australian soil. They protect the Earth from solar wind, which allows all life to exist. Moreover, it is what makes compasses work. In fact, the south pole is actually the north side of the “bar magnet” of the magnetic field! Let’s find out where it originates from and how it works.
The main scientific consensus about how the field is generated is called the dynamo theory. It was first proposed by Joseph Larmor in 1919. For an object to become magnetised, the domains in it need to be aligned. A magnetic field is generated through the movement of charge. The Earth has a liquid core made out of iron, so its movement causes a magnetic field. However, it is so hot, reaching temperatures greater than the Curie point temperature of iron at 1043K, that the domains become disorganised. So, how come the Earth has a magnetic field? Scientists propose that it is maintained by a self-sustaining dynamo. The Coriolis effect due to the rotation of the planet creates an initial magnetic field. In turn, this field induces a current in the liquid core, which generates another magnetic field that supports the existing one. Researchers are now refining their computer models to show this theory holds in practice.
Interestingly enough, the field is not even across the planet’s surface. It is measured at around 30 microteslas in the region of South America and South Africa, and at around 60 microteslas in northern Canada and Siberia. Furthermore, there are daily variations due to the field’s interaction with Sun rays on the scale of 25 nanoteslas. Using magnetometers, it is possible to detect variations in the magnetic field due to the composition of the ocean floor. The volcanic rock basalt, which contains large amounts of the magnetic mineral magnetite, can distort the direction of the compass needle. This phenomenon was already observed by Icelandic sailors in the 18th century. It now offers an interesting way to map out the composition of the oceanic floor. Moreover, since this rock cools rapidly, it can create a historical record of how the magnetic field varied over time.
You might have heard about a prediction of the apocalypse where the magnetic poles of the Earth flip. Based on basalt samples, scientists have noticed that the magnetic field of the Earth reversed polarity at supposedly random intervals, ranging from millions to thousands of years, with an average period of 300,000 years. The last time this occurred was 781,000 years ago, so you might imagine we are due for one. Nonetheless, these are probably not as abrupt as portrayed in popular culture. The duration of such flip ranges from several thousand years to a human lifetime. Nonetheless, this is still a point of contention of researchers, as examination of the lava flow on the Steens mountain in Oregon suggest that the rate of change of the field could have been as rapid as 6 degrees per day.
What causes the magnetic field to reverse? We don’t have a clear answer for that either. Some computer simulations produce a field that is not exactly a stable dynamo as we said before, and it spontaneously switches due to the messy nature of the currents or perhaps the cooling of the core. Another theory is that large comet impacts trigger the reversal, nevertheless the age of large craters does not exactly correlate with the dates of these flips.
Looking at the Earth’s magnetic field and comparing it to the fields of other extraterrestrial bodies, is a fascinating way of studying them. Weiss, a professor of Planetary Sciences at MIT, suggests that determining the presence of a magnetic field is evidence for the presence of a metallic core. Furthermore, the history of the field on a planet, suggests about the climate change processes that happened on it. For instance, Mars is thought to have had a magnetic field in the past, but lost it at a similar time to the vanishing of the thicker atmosphere and the warm climate, which supported the presence of liquid water.
Magnetic field analysis of planets is a fascinating research field to go into if you are interested in developing detailed computer simulations and combining geology with astrophysics. I recommend checking out Researchgate and some journals if you want to find out more, as this is a constantly changing field with a lot of complexities that I don’t have space for here.
References
Brunhes–Matuyama reversal. (2021). Wikipedia. https://en.wikipedia.org/wiki/Brunhes%E2%80%93Matuyama_reversal
dynamo theory | geophysics. (n.d.). Encyclopedia Britannica. https://www.britannica.com/science/dynamo-theory
Earth’s magnetic field. (n.d.). Earth’s Magnetic Field. https://web.ua.es/docivis/magnet/earths_magnetic_field2.html
Explained: Dynamo theory. (2010, March 25). MIT News | Massachusetts Institute of Technology. https://news.mit.edu/2010/explained-dynamo-0325
Vejayan, V. V. R., & V.V. (2017, May 1). What creates Earth’s magnetic field? Cosmos Magazine. https://cosmosmagazine.com/earth/earth-sciences/what-creates-earths-magnetic-field/