Scientists have discovered that Earth has a third field. We all know about the Earth’s magnetic field. And we all know about Earth’s gravity field, though we usually just call it gravity.
Now, a team of international scientists have found Earth’s global electric field.
It’s called the ambipolar electric field, and it’s a weak electric field that surrounds the planet. It’s responsible for the polar wind, which was first detected decades ago. The polar wind is an outflow of plasma from the polar regions of Earth’s magnetosphere. Scientists hypothesized the ambipolar field’s existence decades ago, and now they finally have proof.
The discovery is in a new article in Nature titled “Earth’s ambipolar electrostatic field and its role in ion escape to space.” The lead author is Glyn Collinson from the Heliophysics Science Division at NASA Goddard Space Flight Center.
“It’s like this conveyor belt, lifting the atmosphere up into space.”
Glyn Collinson, Heliophysics Science Division, NASA Goddard Space Flight Center
The Space Age gained momentum back in the 1960s as the USA and USSR launched more and more satellites. When spacecraft passed over the Earth’s poles, they detected an outflow of particles from Earth’s atmosphere into space. Scientists named this the polar wind, but for decades, it was mysterious.
Scientists expect some particles from Earth to “leak” into space. Sunlight can cause this. But if that’s the case, the particles should be heated. The wind is mysterious because many particles in it are cold despite moving at supersonic speeds.
“Something had to be drawing these particles out of the atmosphere,” said lead author Collinson.
Collinson is also the Principal Investigator for NASA’s “Endurance” Sounding Rocket Mission. “The purpose of the Endurance mission was to make the first measurement of the magnitude and structure of the electric field generated by Earth’s ionosphere,” NASA writes in their mission description. Endurance launched on May 22nd, 2022, from Norway’s Svalbard Archipelago.
“Svalbard is the only rocket range in the world where you can fly through the polar wind and make the measurements we needed,” said Suzie Imber, a space physicist at the University of Leicester, UK, and co-author of the paper.
Svalbard is key because there are open magnetic field lines above Earth’s polar caps. These field lines provide a pathway for ions to outflow to the magnetosphere.
After it was launched, Collinson said, “We got fabulous data all through the flight, though it will be a while before we can really dig into it to see if we achieved our science objective or not.”
Now, the data is in, and the results show that Earth has a global electric field.
Prior to its discovery, scientists hypothesized that the field was weak and that its effects could only be felt over hundreds of kilometres. Even though it was first proposed 60 years ago, scientists had to wait for technology to advance before they could measure it. In 2016, Collinson and his colleagues began inventing a new instrument that could measure the elusive field.
At about 250 km (150 mi) above the Earth’s surface, atoms break apart into negatively charged electrons and positively charged ions. Electrons are far lighter than ions, and the tiniest energetic jolt can send them into space. Ions are more than 1800 times heavier, and gravity draws them back to the surface.
If gravity were the only force at work, the two populations would separate over time and simply drift apart. But that’s not what happens.
Electrons and ions have opposite electrical charges. They’re attracted to one another and an electric field forms that keeps them together. This counteracts some of gravity’s power.
The field is called ambipolar because it’s bidirectional. That means it works in both directions. As ions sink down due to gravity, the electrical charges mean that the ions drag some of the electrons down with them. However, at the same time, electrons lift ions high into the atmosphere with them as they attempt to leave the atmosphere and escape into space.
The result of all this is that the ambipolar field extends the atmosphere’s height, meaning some of the ions escape with the polar wind.
After decades of hypothesizing and theorizing, the Endurance rocket measured a change in electric potential of only 0.55 volts. That’s extremely weak but enough to be measurable.
“A half a volt is almost nothing — it’s only about as strong as a watch battery,” Collinson said. “But that’s just the right amount to explain the polar wind.”
Hydrogen ions are the most plentiful particles in the polar wind. Endurance’s results show that these ions experience an outward force from the magnetic field that’s 10.6 times more powerful than gravity. “That’s more than enough to counter gravity — in fact, it’s enough to launch them upwards into space at supersonic speeds,” said Alex Glocer, Endurance project scientist at NASA Goddard and co-author of the paper.
Hydrogen ions are light, but even the heavier particles in the polar wind are lifted. Oxygen ions in the weak electrical field effectively weigh half as much, yet they’re boosted to greater heights, too. Overall, the ambipolar field makes the ionosphere denser at higher altitudes than it would be without the field’s lofting effect. “It’s like this conveyor belt, lifting the atmosphere up into space,” Collinson added.
“The measurements support the hypothesis that the ambipolar electric field is the primary driver of ionospheric H+ outflow and of the supersonic polar wind of light ions escaping from the polar caps,” the authors explain in their paper.
“We infer that this increases the supply of cold O+ ions to the magnetosphere by more than 3,800%,” the authors write. At that point, other mechanisms come into play. Wave-particle interactions can heat the ions, accelerating them to escape velocity.
These results raise other questions. How does this field affect Earth? Has the field affected the planet’s habitability? Do other planets have these fields?
Back in 2016, the European Space Agency’s Venus Express mission detected a 10-volt electric potential surrounding the planet. This means that positively charged particles would be pulled away from the planet’s surface. This could draw away oxygen.
Scientists think that Venus may have once had plentiful water. However, since sunlight splits water into hydrogen and oxygen, the electric field could’ve siphoned the oxygen away, eliminating the planet’s water. This is theoretical, but it begs the question of why the same thing hasn’t happened on Earth.
The ambipolar field is fundamental to Earth. Its role in the evolution of the planet’s atmosphere and biosphere is yet to be understood, but it must play a role.
“Any planet with an atmosphere should have an ambipolar field,” Collinson said. “Now that we’ve finally measured it, we can begin learning how it’s shaped our planet as well as others over time.”