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Wave-like flow at the surface of the Earth’s outer core and
background magnetic field lines. (Felix Gerick.) |
Earth’s interior is a far from quiet place. Deep below our surface activities, the planet rumbles with activity, from plate tectonics to convection currents that circulate through the hot magmatic fluids far underneath the crust.
Understanding the origin of the magnetic field, why it
constantly fluctuates, how it interacts with the solar wind, and, indeed, why
it is currently weakening- all these questions are topics of interest for
scientists. Most of the magnetic field is generated by an ocean of superheated,
swirling liquid iron that makes up Earth’s outer core. The Earth’s outer core
acts like a spinning conductor in a bicycle dynamo. It generates electrical
currents and a continuously changing electromagnetic field.
ESA’s Swarm mission, which comprises three identical
satellites, measures these magnetic signals from Earth’s core. It also measures
other signals from the crust, oceans, ionosphere, and magnetosphere.
Using data from the Swarm mission
scientists have unearthed a hidden secret. They have
detected a new type of magnetic wave that sweeps across the ‘surface’ of
Earth’s outer core. What’s more interesting is- this wave oscillates every
seven years and propagates westward at up to 1500 kilometers a year.
These waves align in columns along the axis of
rotation due to the Earth’s rotation. The motion and magnetic field changes
caused by these waves are most pronounced in the core’s equatorial region.
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Visualization of the waves at the core-mantle
boundary. (Université Grenoble Alpes) |
This discovery
could offer insight into how Earth’s magnetic field is
generated, and provide clues of our planet’s thermal history and evolution –
that is, the gradual cooling of the planetary interior.
While the research shows magneto-Coriolis waves within
seven years, whether such waves exist at different intervals remains
unanswered.
Nicolas Gillet, from the University Université
Grenoble Alpes and lead author of the paper, said, “Geophysicists have long
theorized over the existence of such waves, but they were thought to take place
over much longer time scales than our research has shown.”
Measurements of the magnetic field
from instruments based on the surface of Earth
suggested that there was some wave action, but we needed the global coverage
offered by measurements from space to reveal what is going on.”
“We combined satellite measurements from Swarm and the
earlier German Champ mission and Danish Ørsted mission with a computer model of
the geodynamo to explain what the ground-based data had thrown up – which led
to our discovery.”
“Magnetic waves
are likely to be triggered by disturbances deep within the Earth’s fluid core,
possibly related to buoyancy plumes. Each wave is specified by its period and
typical length scale, and the period depends on the characteristics of the
forces at play. For magneto-Coriolis waves, the period is indicative of the
intensity of the magnetic field within the core.
“Our research suggests that other such waves are
likely to exist, probably with longer periods – but their discovery relies on
more research.”
ESA’s Swarm mission scientist
Ilias Daras, noted, “This current research will
certainly improve the scientific model of the magnetic field within Earth’s
outer core. It may also give us new insight into the electrical conductivity of
the lowermost part of the mantle and also of Earth’s thermal history.”
For now, because waves carry information about the
medium through which they travel, new discovery could be used to probe the
interior of our planet in new ways – including the core, which is difficult to
study, as well as the core-mantle boundary.
Reference: PNAS
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