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Researchers have solved an old mystery linked to the formation of the Northern Lights

For years, scientists have been unable to confirm how the Northern Lights cast their spectral glow across the sky. But they’ve long had a theory: First, flares on the Sun release a stream of charged particles called the solar wind. These particles interact with Earth’s magnetosphere – the region around the planet controlled by its magnetic field.

In the process, the field launches powerful electromagnetic waves towards the Earth’s surface. The electrons then cling to these waves and travel to the Earth’s upper atmosphere. They then collide with atoms and molecules in the brilliant light show known as the aurora.

Read also – Northern lights hunters film a new type of aurora that stretches across the sky like fingers

However, researchers have struggled to prove this theory for decades. The distances in space involved are far too vast to be recreated in a laboratory, and spacecraft technology only allows scientists to measure electrons and electromagnetic waves separately at different altitudes.

Recently, however, specialists have succeeded in simulating the conditions that produce an aurora inside a vacuum chamber. In a new study, they have indicated that the prevailing theory is indeed correct. Electromagnetic waves, called Alfvén waves, transfer energy to electrons, which gives these particles an accelerated push towards Earth. The electrons can then surf the waves, eventually reaching speeds of up to 72 million kilometers per hour.

“The idea that these waves can energize the electrons that create the aurora goes back over forty years, but this is the first time that we have been able to definitively confirm that it works,” Craig Kletzing said in a statement, professor of physics at the University of Iowa and co-author of the study.

A mockup shows how the Northern Lights light up the sky. Austin Montelius/University of Iowa

A giant vacuum chamber to simulate “electron surfing”

It was in 1946 that Russian physicist Lev Landau first proposed the idea that electrons gain speed while riding electromagnetic waves – a process today known as Landau damping. Lev Kletzing set out to test this theory about twenty years ago, but before that, scientists had to recreate the conditions of the Earth’s magnetosphere.

Their solution was the Large Plasma Device at the University of California at Los Angeles, a nearly 20-meter-long vacuum chamber that produces enough plasma (the ionized gas that makes up a large part of our universe) to withstand waves. d’Alfvén. “They thought it should take a few years,” Gregory Howes, associate professor at the University of Iowa, told Insider. “Well, it turned out to be a much harder problem to do in the lab than was originally expected.”

After launching the Alfvén waves into the chamber, the researchers had to locate a small group of electrons (less than 1 in 1,000) that were moving at about the same speed as the waves – a needle in a boot. hay. This would be the indicator that the electrons are gaining speed by riding the waves. “It had never been proven directly in the lab that it actually worked,” said Gregory Howes. “The major challenge was therefore to be able to show in a real plasma that this theoretical idea materializes.”

Finally, the simulation showed – and mathematical models confirmed it – that this process of “surfing” electrons gives rise to brilliant light shows on Earth. “This solves the key piece of the puzzle that was missing to understand what are known as discrete auroral arcs,” said Gregory Howes. “These are the shimmering curtains of light that you think of when you think of dawn.”

The large plasma device at UCLA (Basic Plasma Science Facility). Basic Plasma Science Facility/UCLA

Auroras don’t form until electrons are close to Earth

However, the researchers failed to recreate the light show of the Northern Lights. Indeed, this phenomenon occurs at a much lower altitude than the Alfvén waves, and under different conditions. The Earth’s magnetic field launches the Alfvén waves about 128,000 kilometers above the surface of the planet, explains Gregory Howes. Electrons in the magnetosphere then begin to surf these waves at an altitude of about 16,000 kilometers.

But auroras don’t form until the electrons are about 100 miles from Earth, he added. At this point, electrons collide with molecules of oxygen and nitrogen, releasing photons – particles of light. In this process, the oxygen atoms emit a red or green tint, while the nitrogen atoms emit a blue or purple light.

The researchers were satisfied with a much dimmer light show inside the lab. “The plasma itself glows – it’s very pretty,” said Gregory Howes. “But the glow that we think of as that of those accelerated electrons hitting the plasma is not what we see.”

Version originale : Aria Bendix/Insider

Read also – The Northern Lights and others are disappearing from parts of the Earth – but scientists are already anticipating their big comeback

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