Guest blog by Sharmila Kuthunur
We take for granted Earth’s reliable magnetic field, which unceasingly protects us from calamitous radiation. Without it, life as we know it would most certainly not have been possible. Lately, however, the intensity of our magnetic field has been decreasing at an alarming rate. And given its history, a pole reversal may well be brewing deep within our planet.
Magnetic pole reversals are a relatively common occurrence on Earth, where one occurs every few thousand years or so. Because of this, ancient remnants of volcanoes can provide researchers with interesting insights on how our magnetic field has evolved over time.
By looking inside volcanic rocks recovered from ocean beds, scientists can examine iron-rich — and therefore magnetic — minerals that have had their orientations “frozen” in time, mirroring the directions that Earth’s magnetic field lines were pointing during specific volcanic eruptions. Analyses of these rocks has demonstrated that the direction of our magnetic field has changed over time, so we know pole reversals have occurred in the past. But they also reveal a very disturbing fact: no reversal has occurred in the past 780,000 years.
Considering that Earth’s magnetic field has decreased 10% since it was first measured by Carl Friedrich Gauss in 1832, scientists are undestandably scrambling to figure out what triggers magnetic pole reversals; after all, we may be currently experiencing one in its early stages. In order to understand how a pole reversal would affect life on Earth, scientists need to understand both what causes the poles to flip and why.
Earth’s magnetic field, like all magnetic fields, is produced and maintained by what is known as a geodynamo, which resides in our planet’s core. As Earth’s rotation swirls liquid iron in the outer core, the electrically charged fluid generates a magnetic field. Typically, a geodynamo needs a large volume of conducting fluid, a supply of energy, and a rotational component to operate. These conditions are essential for a magnetic field’s survival. If any of these fluctuate, so does the magnetic field.
Researchers place a rough mathematical approximation of 100,000 years before Earth’s dynamo shuts down on its own (and we hope it doesn’t). The long timeframe is mainly due to the enormous size of Earth’s core and its magnetic diffusivity. Geologic records, on the other hand, indicate reversals occurred over shorter periods, roughly every 4,000 to 10,000 years. These relatively quick transitions indicate that instability can destroy the dynamo’s original polarity while quickly generating a new one.
To learn more about how and why pole reversals occur, satellites such as Magsat and Oersted have been taking snapshots of the magnetic field strength at Earth’s surface. Analysis of the data from these satellites has revealed a process that might cause a reversal: a potential instability occurs when the normal direction of a magnetic field is opposite to what is normally expected in the region.
“The magnetic flux distribution at the top of the core (or surface of the Earth) is determined by the very time and space dependent turbulent convection, influenced by the Coriolis forces, inside the fluid core. The same process produces the small patches of locally reversed magnetic field at the top of the core,” said Gary Galtzmaier, a professor at the University of California, Santa Cruz.
The largest of these reversed-flux patches stretch from South Africa to South America. Here, the local flux is unexpectedly inward, whereas most of the flux in that hemisphere is directed outward. Moreover, older patches are seen moving towards the poles, accounting for the drop recorded in the dynamo’s dipole strength.
While this does explain how the poles might reverse, we do not yet know why it happens, or if we can even expect a reversal soon.
Electric currents in the Earth’s core prevent satellites from probing any further than the surface of our planet. Because of this, it is not quite possible to accurately reconstruct the core of our planet, which is where the flux originates. This is where supercomputers come to the rescue, helping scientists delve a little deeper. And what better way to understand the magnetic field than to simulate the dynamo itself?
To do this, Galtzmaier developed 3-D computer models to study the structure and dynamics of the interiors of planets. Over the course of a year, the researchers ran the models on UC-Santa Cruz’s supercomputers for 12 hours every day, simulating 300,000 years of Earth’s evolving geodynamo. In the simulation, reversed flux patches just like those observed in real life were seen to form. Instead of completely destroying the original field, these patches created a new weak field, and it took over 9000 years for this weak field to gain absolute control of the core.
Despite the realistic simulation, there are still variations between the models and what is really happening deep within Earth’s core. This is because supercomputers cannot yet accurately determine turbulences down there.
“Even our best theory and numerical models can’t resolve the question of exactly what happens to cause a reversal in the Earth. Scientists have worked out several different ways that such systems can undergo reversals, but it isn’t clear whether any of those mechanisms are actually occurring in the Earth,” said Jonathan Mound, a professor at University of Leeds in the UK.
What we do know is that the same symptoms are happening right now underneath us. The tardy pole reversal, the South Atlantic Anomaly, and the global decrease in the intensity of Earth’s magnetic field all urge us to ask one question: Is a magnetic pole reversal around the corner?
“The geodynamo process is very chaotic and seems to be continually trying to reverse, but very seldom succeeds. At the current rate that our global geomagnetic field intensity is decreasing, the soonest it might start to reverse is likely a couple thousand years from now. And it's just as likely that it won't reverse at that time,” said Galtzmaier.
The problem faced by both satellites and supercomputers is the same. They are unable to show us the real-time the conditions in Earth’s core. The magnetic pole exhibits a multi-polar nature during a reversal, and simulating this requires detailed records of both the geometry of the poles and their evolution with time, which we do not yet know. Though the recent numerical simulations have documented hundreds of pole reversals, they are still a far cry away from accurately representing Earth’s dynamo. Thus, we simply do not have a simple yes or no answer.
If the poles did reverse, the magnetic field strength would drop during the transition period (the average duration of a reversal is about 5000 years), exposing Earth to severe space weather that would affect much of our electronic infrastructure. Birds and butterflies would likely be affected as they use magnetism for migration. Since the human body is vulnerable to cosmic radiation, the prevalence of cancers would see a sharp increase. Our brains also contain tiny amounts of inhaled magnetite, which when found in abundance has been linked to Alzheimer’s disease. However, we can’t really predict the exact impact of a pole reversal because life and technology at the time it occurs will likely be very, very different. Hopefully, by then, humankind will figure out a way to deal with it.
By combining observational data with numerical models, researchers have made valuable insights into the life and future our Earth’s geodynomo. As computational power increases, these two fields will undoubtedly continue to work symbiotically, improving our knowledge of the nature of pole reversals. And if our understanding of Earth’s magnetic field keeps progressing at this rate, predicting this seemingly intrinsic feature of our planet is not a long way off.