After more than half a century of speculation, NASA scientists have finally detected and measured Earth's elusive "polar wind," an invisible electric field responsible for the rapid escape of atmospheric particles from the planet's poles. This discovery, made possible by the international Endurance sounding rocket mission, provides crucial insights into the dynamics of Earth's upper atmosphere and its evolution over time.
The Mystery of the Polar Wind
In the late 1960s, spacecraft flying over Earth's poles detected a stream of particles escaping into space at supersonic speeds. While scientists understood that sunlight could cause atmospheric particles to leak into space, these particles showed no signs of being heated, suggesting another force was at play.
"Something had to be drawing these particles out of the atmosphere," explained Glyn Collinson, principal investigator of Endurance at NASA's Goddard Space Flight Center in Maryland. The hypothesised force was an invisible, weak electric field, but detecting it was beyond the technological capabilities of the time.
Endurance Mission to the Rescue
In 2016, Collinson and his team began developing specialised sensors for the Endurance mission, which involved launching sounding rockets from the Svalbard Rocket Range in Norway, near 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," said study co-author Suzie Imber, a space physicist at the University of Leicester in the UK.
During its 20-minute flight in May 2022, the Endurance rocket reached an altitude of 477 miles (768 kilometres), gathering data across a vast swathe of the atmosphere. The sensors detected a subtle 0.55-volt change in the electric field, a seemingly insignificant fluctuation but enough to explain the polar wind.
How the Polar Wind Works
The electric field forms at an altitude of roughly 150 miles (250 kilometres) where atmospheric atoms split into charged particles: electrons and ions. The field acts as a tether, counteracting gravity and allowing some particles to escape into space.
"This field is so fundamental to understanding the way our planet works — it's been here since the beginning alongside gravity and magnetism," Collinson emphasised in a NASA video. "Despite being weak, it's incredibly important — it counters gravity and it basically lifts the skies up."
The researchers discovered that hydrogen ions, abundant in the polar wind, experience an outward force 10.6 times stronger than gravity, propelling them into space at supersonic speeds. Even heavier oxygen ions receive a significant boost from the polar wind. "It's like this conveyor belt, lifting the atmosphere up into space," said Collinson.
Implications and Future Research
The discovery of the polar wind has implications for understanding atmospheric escape on other planets, including Venus and Mars. Further research could reveal its role in the evolution of Earth's atmosphere and its influence on our oceans.
"This field is a fundamental part of the way Earth works, and now that we've finally measured it, we can start to ask some of these bigger and exciting questions," said Collinson.
The findings of the Endurance mission were published in the journal Nature on August 28, 2024.