A few months after spotting a jet stream of molten iron in the Earth's outer core, the European Space Agency's (ESA) Swarm satellites have found a similar system at work in the upper atmosphere. There, the electrical fields created through solar winds interacting with the planet's magnetic field have been found to drive supersonic plasma jets, which can heat the ionosphere to temperatures as high as 10,000º C (18,032º F).
The Swarm mission, made up of a constellation of three satellites, was launched in 2013 to study the Earth's magnetic field. By sorting out signals from different parts of the planet between the core and the magnetosphere, the program has tracked how that field changed over a two-year period, and found the magnetic influence of the fast-moving iron in the core.
The new findings build on the previous understanding of Birkeland currents. First theorized in the early 20th century and eventually confirmed through satellite observations in the 1960s, these electric currents occur between the magnetosphere and the ionosphere, created by solar winds hitting the Earth's magnetic field and being funnelled along the geomagnetic lines. The most striking result is the auroras that light up the skies near the poles.
As they zip around the Earth – also between the magnetosphere and ionosphere – the Swarm satellites observed that huge electric fields would form in the areas where Birkeland currents heading upwards would connect through the ionosphere with those moving downwards. As they do, they force jets of plasma to squeeze between the layers at supersonic speeds.
"The jets, which we call 'Birkeland current boundary flows,' mark distinctly the boundary between current sheets moving in opposite directions and lead to extreme conditions in the upper atmosphere," says Bill Archer, a University of Calgary researcher working on the project. "They can drive the ionosphere to temperatures approaching 10,000°C (18,032º F) and change its chemical composition. They also cause the ionosphere to flow upwards to higher altitudes where additional energization can lead to loss of atmospheric material to space."
Among the project's other recent discoveries are that the Birkeland currents tend to be at their strongest in winter and in the Northern Hemisphere.
"These recent findings from Swarm add knowledge of electric potential, and therefore voltage, to our understanding of the Birkeland current circuit, perhaps the most widely recognized organizing feature of the coupled magnetosphere–ionosphere system," says David Knudsen, another researcher on the project.
The Swarm mission is due to officially wrap up late this year.
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