Earth’s Magnetic Field Isn’t As Stable As You Think
Our planet’s magnetic field is in a constant state of flux in more ways than one. Geologists have been able to use the magnetic footprints left behind in iron oxides, magnetite, and other fragments of magnetized rock when they formed to track the Earth’s magnetic field throughout history. Ever since it evolved some 3,450 million years ago, our magnetic field has been migrating and changing. Now, in the age of technology, it has become our one natural defense against particle bombardment from space that could knock out our grids and compromise our livelihoods.
One of the more bizarre attributes of the geomagnetic field is its tendency to flip flop polarization. Every so often, the poles change positions. Magnetic north becomes magnetic south and vice versa. Statistically, these changes in polarity have been random, and while most appear to have happened gradually, the most recent complete reversal, the Laschamp Event, which took place during the last ice ace around 41,000 years ago, may have occurred within a human lifetime. That shift in global polarity lasted about 220 years, and the reversal itself took nearly double that to correct. During the Laschamp Event, overall field strength decreased by 5%, relative to present measurements.
Other times, the locations of magnetic north and south may shift and migrate without fully reversing. These phenomena are called geomagnetic excursions. Since at least the 1400s, Earth’s dipoles have been on the move, traveling westward at an increasing rate every year. Our northern pole has been drifting from northern Canada toward the upper reaches of Russia at about 0.2 degrees per year or about 13.7 miles annually. For the past 200 years, the strength of our planet’s dipole has been weakening at a steady rate of about 6.3% per century, a rate that would render it negligible in roughly 1,600 years. The good news is that our current rate of fluctuation is right on par with the expected strength and rate of change that’s been seen for the last 7,000 years.
What A Jerk
Occasionally, predictive models of the magnetic field’s evolution get skewed by phenomena called geomagnetic jerks. These jerks cause the progress of the weakening magnetic field as well as the migrating poles to jump suddenly, leading to a lot of disgruntled head-shaking as scientists worldwide have to adjust their maps and satellites. The phenomenon was first observed in 1978, but it wasn’t until April of 2019 that someone finally proposed an explanation for the bizarre acceleratory events. To understand the answer, we first need to understand the literal core of the problem.
A rotating metallic core occupies the center of the Earth. It has long since been proposed that our magnetic field is generated by the movements of this massive hunk of metal, but until recently, we had little evidence to back that claim. The convection currents caused by the pressure and friction in the center of the planet generate electric currents. Scientists believe that these currents interact with the metallic elements in the core to create a magnetic field.
Earth’s liquid outer core is believed to be responsible for these bizarre jerks in the magnetic field. Sudden changes within the flow patterns of the metallic slurry that envelops the solid inner core leads to significant changes in the magnetic field. These temporary alternations in the outer core’s convection currents and surface flows orient along lines of intense magnetic flux, messing with the magnetic field and effectively giving the whole system a big shove. The discovery resulted in a series of computer simulations to recreate the core activity leading up to these jerks, which finally yielded an answer to what makes our magnetic field tick. Going forward, scientists hope to use this new information to better understand and predict the ebb and flow of the magnetosphere.