Small magnetic waves have been discovered at Earth's core, with each oscillation working on a seven year cycle.
Earth's magnetic field is generated by electric currents generated by the movement of molten iron deep in the planet's core. The magnetic field extends far into space, providing the planet with a barrier that deflects harmful particles from the sun. Without the magnetic field, solar radiation would strip the planet of its atmosphere, leaving it a barren land much like our cosmic neighbor Mars.
The magnetic field is dynamic and it waxes and wanes over long, geological timescales. But researchers have now found smaller oscillations right at the planet's core.
Nicholas Gillet, from the Grenoble Alpes University in France, and colleagues examined 20 years' worth of data looking at fluctuations in the magnetic field. Satellites that have continuously monitored the geomagnetic field allowed them to look for tiny—or interannual—changes that have previously been undetected. Their findings are published in PNAS.
"We try to understand the physics that is responsible for the observed evolution of the magnetic field of our planet," Gillet told Newsweek. "It evolves over all time-scales, and the longer periods show the strongest changes. What we discuss here are tiny fluctuations.
"These interannual changes remained unexplained since their discovery in the late 1970s in ground-based records. Understanding them opens a door on the field deep in the core, which we cannot directly probe based on observations alone."
The team found that at Earth's core, there are small oscillations every seven years. These waves would travel westwards at a speed of around 900 miles per hour.
"This partly came as a surprise," Gilet said. "Our community had detected interannual magnetic oscillations for several years now, by analyzing satellite records. But the family of waves that is responsible for the oscillation was believed to operate on much longer—centennial to millennial—time-scales, for observable field variations. We have revisited their physical characteristics, and now understand that they can live on interannual periods. This gives a physical framework for the interannual oscillations that were before unexplained."
The discovery, researchers say, should provide a better understanding of the geomagnetic signals at Earth's core, allowing a better idea of the physical characteristics of the core and the magnetic field. It also knocks the idea there is a stratified layer at the top Earth's core.
"In the 1990's the possible existence of a hidden (stratified) ocean at the top of the core [was proposed]," he said, adding that a stratified layer helps with some models of magnetic fluctuations. "This scenario was put to the fore over the past decade, based on evidence from the seismology and mineral physics communities—but these are currently debated.
"Our findings...mean that there is no need for a stratified layer at the top of the core to understand observed field changes. There is still the possibility that a stratified fluid layer exists, in which case the fluid motions would not be very sensitive to it."
Gilet said he now hopes to look for signatures of other waves in longer-term records. He also wants to look more closely at the magnetic field within Earth's core, to study the relationship between the waves and the electrical conductivity in the lowermost mantle, and to predict interannual changes to the magnetic field.
