NASA scientists have for the first time detected an electric field across Earth that is thought to be as fundamental to our planet as gravity and magnetic fields.
This 'ambipolar field' was first hypothesised over 60 years ago, and was thought to explain how Earth's atmosphere escapes so rapidly above the north and south poles.
More about Earth in our guide to Schumann resonances, Earth's 'heartbeat'
Now a NASA rocket mission, named Endurance, has confirmed the existence of the ambipolar filed and also measured its strength.
This, say the team, reveals the role the ambipolar field plays in driving atmospheric escape and shaping Earth's ionosphere.
Ambipolar field explained
A stream of particles is escaping from Earth's atmosphere out into space, and this has been detected by spacecraft flying over the poles since the 1960s.
The phenomenon was named the 'polar wind', and scientists have been trying to understand the exact mechanism behind it.
Some amount of atmospheric escape is expected, as sunlight heats particles in Earth's air and causes them to escape into space.
Yet observations of the polar wind showed many particles within it were cold, but were nonetheless escaping rapidly.
Scientists inferred there must be an unknown electric field at work.
It was theorised this electric field should begin at about 150 miles (250 km) altitude, as this is where atoms in our atmosphere break apart into negatively charged electrons and positively charged ions.
"Something had to be drawing these particles out of the atmosphere," says Glyn Collinson, principal investigator of Endurance at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the paper.
Black in 2016, Collinson and a team of scientists began working on an instrument they hoped would detect and measure the ambipolar field.
About the NASA Endurance mission
Named after Ernest Shackleton's 1914 voyage to Antarctica, the NASA Endurance mission launched from Svalbard in Norway on 11 May 2022, just a few hundred miles from the North Pole.
"Svalbard is the only rocket range in the world where you can fly through the polar wind and make the measurements we needed,' says Suzie Imber, a space physicist at the University of Leicester, UK, and co-author of the paper.
It reached an altitude of 477.23 miles (768.03 km), splashing down 19 minutes later in the Greenland Sea.
Endurance collected data at a 322-mile altitude range and detected a change in electric potential of 0.55 volts.
"A half a volt is almost nothing — it’s only about as strong as a watch battery," says Collinson. "But that’s just the right amount to explain the polar wind."
The team says hydrogen ions are the most abundant type of particle in the polar wind, and they undergo an outward force from this field 10.6 times stronger than gravity.
"That’s more than enough to counter gravity — in fact, it’s enough to launch them upwards into space at supersonic speeds,” says Alex Glocer, Endurance project scientist at NASA Goddard and co-author of the paper.
"It’s like this conveyor belt, lifting the atmosphere up into space," says Collinson.
The Endurance scientists say the ambipolar field could have shaped the evolution of Earth's atmosphere in ways we don't yet know, but now the window of opportunity is open for further exploration.
What's more, similar electric fields could exist on other planets of the Solar System, including Venus and Mars.
"Any planet with an atmosphere should have an ambipolar field," Collinson says.
"Now that we’ve finally measured it, we can begin learning how it’s shaped our planet as well as others over time."
