The Earth’s magnetic field, generated 3,000 km below our feet in the liquid iron core, is vitally important to life on our planet. It extends into space, wrapping us in an electromagnetic blanket that protects the atmosphere and satellites from solar radiation.
However, the magnetic field is constantly changing in both strength and direction and has undergone some dramatic changes in the past. This includes enigmatic reversals of the magnetic poles, with the south pole becoming the north pole and vice versa.
A longstanding question has been how fast the field can change. Our new study, published in Nature Communications, has uncovered some answers.
Rapid changes in the magnetic field are of great interest because they represent the most extreme behavior of the molten iron ocean in the liquid core. By linking observed changes to core processes, we can learn important information about a region of our planet that would otherwise be inaccessible.
Read more: Why might Earth’s magnetic poles be about to swap places and how would it affect us?
Historically, the fastest changes in Earth’s magnetic field have been associated with inversions, occurring at irregular intervals several times every million years. But we discovered field changes that are much faster and more recent than any of the data associated with actual reversals.
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Today satellites help monitor changes in the field in both space and time, complemented by navigation records and ground observatories. This information reveals that the changes in the modern field are quite heavy, around a tenth of a degree per year. But, while we know that the field has been around for at least 3.5 billion years, we don’t know much about its behavior before 400 years ago.
To trace the ancient field, scientists analyze the magnetism recorded by sediments, lava flows, and man-made artifacts. This is because these materials contain microscopic magnetic grains that record the signature of the Earth’s field at the time they cooled (for the lavas) or added to the earth’s mass (for the sediments). Sediment records from central Italy at the time of the last polarity reversal nearly 800,000 years ago suggest relatively rapid field changes reaching one degree per year.
Such measures, however, are extremely challenging, and the results are still debated. For example, there are uncertainties in the process by which sediments acquire their magnetism.
Improved measurements
Our research takes a different approach by using physics-based computer models of the field generation process. This is combined with a recently published reconstruction of global variations in Earth’s magnetic field spanning the past 100,000 years, based on a compilation of measurements from sediments, lavas, and artifacts.
This shows that changes in the direction of the Earth’s magnetic field reached rates of up to ten degrees per year, ten times greater than the fastest variations currently reported.
The fastest observed changes in the direction of the geomagnetic field occurred about 39,000 years ago. This change was associated with a locally weak field in a confined region near the west coast of Central America. The event followed the global “Laschamp excursion”, a “failed reversal” of Earth’s magnetic field some 41,000 years ago in which the magnetic poles briefly moved away from the geographic poles before returning.
The faster changes appear to be associated with local weakening of the magnetic field. Our model suggests that this is caused by the movement of intense magnetic field patches across the surface of the liquid core. These patches are more prevalent at lower latitudes, suggesting that future searches for rapid direction changes should focus on these areas.
The impact of society.
Changes in the magnetic field, like inversions, are probably not life threatening. Humans managed to experience Laschamp’s dramatic excursion. Today, the threat is primarily due to our dependence on electronic infrastructure. Space weather phenomena, such as geomagnetic storms, arising from the interaction between the magnetic field and incoming solar radiation, could disrupt satellite communications, GPS, and power grids.
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This is troubling: The economic cost of a US power grid crash due to a space weather event has been estimated to be around $ 1 trillion. The threat is severe enough that space weather appears as a high priority on the UK National Risk Register.
Space weather events tend to be more frequent in regions where the magnetic field is weak, something we know can happen when the field is changing rapidly. Unfortunately, computer simulations suggest that directional changes emerge after the field strength begins to weaken, meaning that we cannot predict falls in field strength simply by monitoring the direction of the field. Future work with more advanced simulations may shed more light on this topic.
Is there another rapid change in the magnetic field on the way? This is very difficult to answer. The fastest changes are also the rarest events: for example, the changes identified around the Laschamp excursion are twice as fast as any other change that has occurred in the last 100,000 years.
This makes it difficult for scientists to predict rapid changes: they are “black swan events” that surprise and have great impact. One possible route forward is to use physics-based models of how the field behaves as part of the forecast.
We still have a lot to learn about the “speed limit” of Earth’s magnetic field. Rapid changes have not yet been observed directly during a polarity reversal, but should be expected as the field is believed to be weakening globally at this time.
