The ambipolar electric field was first proposed over 60 years ago as the primary driving force of the polar wind, a steady outflow of charged particles into space that occurs over Earth’s poles. This electric field lifts charged particles in our upper atmosphere to higher altitudes than they would otherwise reach, and may have shaped the evolution of our planet in ways that have yet to be explored.
Collinson et al. report the existence of an electric potential drop of +0.55 ± 0.09 V between 250 km and 768 km due to a planetary electrostatic field generated solely by the outward pressure of ionospheric electrons; they experimentally show that the Earth’s ambipolar field controls the structure of the polar ionosphere, increasing the scale height by 271%. Image credit: NASA.
Since the 1960s, space probes flying over the Earth’s poles have been detecting a stream of particles flowing from our atmosphere into space.
Theorists predicted this outflow, which they called the polar wind, and spurred research to understand its causes.
Some outflow from our atmosphere was to be expected. Intense, unfiltered sunlight should cause some particles from our air to escape into space, just as steam evaporates from a pot of water. The observed polar wind, however, was more mysterious.
Many of the particles it contained were cold and showed no signs of warming – and yet they moved at supersonic speeds.
“Something must have pulled these particles out of the atmosphere,” said Dr. Glyn Collinson, Endurance principal investigator and researcher at NASA’s Goddard Space Flight Center.
It was assumed that the hypothetical electric field generated at the subatomic level would be incredibly weak and its effects would only be felt over hundreds of kilometers.
For decades, detection was not possible with existing technology.
In 2016, Dr. Collinson and his colleagues went to work and invented a new instrument that they thought would be suitable for measuring the Earth’s ambipolar field.
The team’s instruments and ideas were best suited to a suborbital rocket flight launched from the Arctic.
In reference to the ship that carried Ernest Shackleton on his famous voyage to Antarctica in 1914, the researchers called their mission “Endurance.”
They set course for Spitsbergen, a Norwegian archipelago just a few hundred kilometers from the North Pole, where the northernmost missile launch base in the world is located.
“Svalbard is the only rocket launch site in the world where you can fly through the polar wind and make the measurements we needed,” said Dr Suzie Imber, a space physicist at the University of Leicester.
On May 11, 2022, Endurance launched and reached an altitude of 768.03 km (477.23 miles) and splashed down in the Greenland Sea 19 minutes later.
Over the 518.2 km (322 miles) altitude range in which it collected data, Endurance measured a change in electrical potential of only 0.55 volts (V).
“Half a volt is almost nothing – it’s only about as powerful as a watch battery. But that’s just the right amount to explain the polar wind,” said Dr Collinson.
In this field, hydrogen ions, the most common type of particle in the polar wind, are subject to an outward force that is 10.6 times stronger than gravity.
“That’s more than enough to counteract gravity – in fact, enough to shoot it into space at supersonic speeds,” said Endurance project scientist Dr. Alex Glocer, a researcher at NASA’s Goddard Space Flight Center.
Heavier particles are also amplified. Oxygen ions at the same altitude that are immersed in this half-volt field weigh only half as much.
In general, the scientists found that the ambipolar field increases the so-called scale height of the ionosphere by 271%, meaning that the ionosphere remains denser at higher altitudes than it would be without the field.
“It’s like a conveyor belt that carries the atmosphere into space,” said Dr. Collinson.
The discovery of Endurance has opened many new avenues for exploration.
The ambipolar field, a fundamental energy field of our planet along with gravity and magnetism, may have continuously shaped the evolution of our atmosphere in ways that we can now begin to explore.
Since they are caused by the internal dynamics of an atmosphere, it is assumed that similar electric fields exist on other planets, including Venus and Mars.
“Any planet with an atmosphere should have an ambipolar field. Now that we have finally measured it, we can begin to learn how it has shaped our planet and others over time,” said Dr Collinson.
The team’s results appear in the journal Nature.
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GA Collinson et al. 2024. The Earth’s ambipolar electrostatic field and its role in the escape of ions into space. Nature 632, 1021-1025; doi: 10.1038/s41586-024-07480-3
This article is a version of a press release from NASA’s Goddard Space Flight Center.
