Scientists find long-sought electric field in Earth’s atmosphere

For the first time, scientists have measured a long-sought global electric field in Earth’s atmosphere. This field, called the ambipolar electric field, was predicted to exist decades ago but was never discovered until now.

“That’s the big noise,” says atmospheric scientist Glyn Collinson of NASA’s Goddard Space Flight Center in Greenbelt, Md.

The field is weak, just 0.55 volts – about as strong as a watch battery, Collinson says. But it is strong enough to control the shape and evolution of the upper atmosphere, features that may have implications for our planet’s suitability for life.

“It’s essential to the DNA of our planet,” says Collinson, who reported the new measurement in Nature August 28.

The existence of the ambipolar electric field was first predicted in the 1960s, at the dawn of the space age. Early spacecraft flying over Earth’s poles detected a supersonic outflow of charged particles from the atmosphere, called the polar wind.

The most reasonable thing to explain the fast wind would be an electric field in the atmosphere. The idea is that sunlight can strip electrons from atoms in the upper atmosphere. Those negatively charged electrons are light and energetic enough that they want to float around in space. The positively charged oxygen ions left behind are heavier and tend to sink under Earth’s gravity.

But the atmosphere wants to remain electrically neutral, maintaining an even balance between electrons and ions. The electric field is formed to keep the electrons attached to the ions and prevent them from escaping.

Once established, the field can act as a booster for lighter ions such as hydrogen, giving them enough energy to break free from Earth’s gravity and expand as the polar wind. It can also pull heavier ions higher into the atmosphere than they would otherwise reach, where other forces can also remove them into space.

That was the hypothesis. But until recently, the technology to detect the field did not exist.

“It really was thought impossible to do,” says Collinson. “[The field] so weak, it was just assumed you would never measure up.”

Collinson realized that this measurement had not been obtained after he and his colleagues tried to measure a similar field on Venus. A search for a paper reporting Earth’s field strength for comparison came up empty.

“It turned out, funny story, it’s never been done,” he says. “We were like, ‘Game on!'”

Collinson and colleagues developed a new instrument called a photoelectron spectrometer specifically to detect the electric field. The team mounted the spectrometer on a rocket called the Endurance, after the ship that carried Ernest Shackleton to explore Antarctica in 1914.

Reaching the launch site in Svalbard, Norway was a journey worthy of the rocket’s name. The team traveled by boat for 17 hours to reach the Svalbard archipelago, located just a few hundred kilometers from the North Pole. Several members of the team contracted COVID-19 along the way. And the war between Russia and Ukraine had started only a few months earlier.

“At the time, there was some nervousness about launching missiles,” says Collinson. “Polar bears were the fewest. We had war and pestilence.”

Two more days of storms kept the Endurance grounded. When the rocket finally launched on May 11, 2022, it went straight into the atmosphere at about 770 kilometers, measuring the energies of the electrons every 10 seconds. The entire flight lasted 19 minutes. In the end, the rocket was thrown into the Greenland Sea.

Endurance measured a difference in electrical potential of 0.55 volts between altitudes of 248 kilometers and 768 kilometers—just enough to explain the polar wind on its own, without any other atmospheric effects.

The measurement is solid and exciting, says planetary scientist David Brain of the University of Colorado Boulder, who was not involved in the new work. But it’s just one data point from a rocket. “I think this result is a really big result that argues that there should be more measurements like this,” he says.

Collinson agrees. He and his colleagues recently won NASA approval for a follow-up rocket—this time called Resolute, for an Arctic exploration ship that launched in 1850.

Because the ambipolar electric field helps control how quickly a planet’s atmosphere escapes into space, it probably plays a role in making a planet hospitable to life, Collinson says. Scientists think Mars was once more like Earth, but lost much of its atmosphere to space over time (SN: 11/27/15). Venus may once have been much wetter than it is today (SN: 8/1/17).

Both of these planets also have ambipolar electric fields, but they may have been better off without them.

“If this process didn’t exist on Venus and Mars, then I think it’s possible that Venus and Mars would have lost less oxygen, and therefore less water,” says Brain.

Earth’s ambipolar electric field helps push its oxygen into space, too. But Earth has one major advantage over Mars and Venus: a global magnetic field to guide charged particles around the planet. “The electric field is the engine that makes the particles move,” says Brain. “The magnetic field is a kind of path along which particles move.” The Earth’s magnetic field means that oxygen can only escape near the poles, and not from every part of the atmosphere. This may help explain why Earth has retained its habitable atmosphere for much longer than Venus or Mars.

“Basically, what makes a planet habitable is going to be a lot of things,” Collinson says. “But I think comparing these different energy fields across different planets is one way to answer the question, why is Earth habitable? Why are we here?”