Pulsating aurora spotted in Canada helps explain northern lights’ origin

The origins of intense flickering displays of light high up in Earth's atmosphere are now revealed after a decades-long hunt, a new study finds.

Similar auroras may take place high above Jupiter and Saturn, according to the scientists behind the new research.

The dramatic light shows known as the northern and southern lights, also called auroras, are as varied in nature as the colors they display in the sky. The most familiar kind, known as discrete auroras, are renowned for shimmering ribbons and streamers of color. In contrast, pulsating auroras are giant blinking patches of light. [Aurora Guide: How the Northern Lights Work (Infographic)]

Auroras result when streams of high-speed particles from the sun — collectively known as the solar wind — slam into Earth's magnetosphere, the shell of electrically charged particles trapped by the planet's magnetic field. Whereas discrete auroras originate a few thousand miles above Earth's surface, pulsating auroras arise about 10 times farther away.

Previous research suggested that pulsating auroras were triggered by electromagnetic fluctuations known as chorus waves that arise in the magnetosphere at the equator. The idea was that chorus waves send electrons in the magnetosphere hurtling along the planet's magnetic-field lines toward the upper reaches of Earth's atmosphere, generating light when they collide with molecules of air.

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However, for decades, scientists could not gather sensitive-enough ground-based and space-based observations to line up at the right time and place to prove this model. Now, researchers have finally collected direct evidence of the chain of events behind pulsating auroras.

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The scientists analyzed data from the Arase spacecraft, which was launched by the Japan Aerospace Exploration Agency in late 2016. This satellite could both detect chorus waves and investigate their effects on magnetospheric electrons within a narrow window around a magnetic-field line.

However, prior to the work by Professor Kasahara and his colleagues, it was unclear whether these theories played out in the real world. Specifically, scientists had no direct evidence that chorus waves were powerful enough to sufficiently excite electrons and drive them into the atmosphere.

The researchers also pinpointed where the magnetic-field line the Arase spacecraft examined made contact with Earth. They searched for any pulsating auroras matching electron activity triggered by chorus waves.

To overcome this, the research team led by Professor Kasahara designed a specialised electron sensor – mounted on Japan’s Exploration of energisation and Radiation in Geospace (ERG) satellite – which was capable of observing the exact movement of electrons participating in auroras.

The scientists identified an aurora in 2017 in central Canada that was apparently generated by magnetospheric electrons scattered by chorus waves. 

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"Observational results are usually very complex, and tests of theoretical predictions often result in ambiguous results, which was not the case here," said study lead author Satoshi Kasahara, a space and planetary physicist at the University of Tokyo.

The researchers noted that similar activity may occur in the auroras of Jupiter and Saturn, where prior work detected chorus waves. "Application for other planets would be exciting," Kasahara told Space.com.

“We, for the first time, directly observed scattering of electrons by chorus waves generating particle precipitation into the Earth’s atmosphere,” said Professor Satoshi Kasahara, a planetary scientist at the University of Tokyo who led the research.

The researchers detailed their findings online today (Feb. 14) in the journal Nature.

Charles Q. Choi is a contributing writer for Space.com and Live Science. He covers all things human origins and astronomy as well as physics, animals and general science topics. Charles has a Master of Arts degree from the University of Missouri-Columbia, School of Journalism and a Bachelor of Arts degree from the University of South Florida. Charles has visited every continent on Earth, drinking rancid yak butter tea in Lhasa, snorkeling with sea lions in the Galapagos and even climbing an iceberg in Antarctica.

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Using data taken from the Exploration of energization and Radiation in Geospace (ERG) satellite – also known as the Arase spacecraft – the scientists observed that pulsating aurorae are caused by interactions in the Earth’s magnetosphere between electrons and plasma – a gas that has been energised to the point that electrons break free from their host molecules. (Plasma is one of the four fundamental states of matter, along with solids, liquids and gases).

Pulsating auroras are a special type of ‘northern lights’ where patches of light blink in the night sky. (ERG Science Team)

Emily Chung covers science and technology for CBC News. She has previously worked as a digital journalist for CBC Ottawa and as an occasional producer at CBC’s Quirks & Quarks. She has a Ph.D. in chemistry.

Scientists have figured out what causes pulsating auroras, a rarely seen form of northern lights in the night sky. And it was partly thanks to a lucky observation of the phenomenon in Canada.

From a Japanese satellite, researchers measured these factors from a point near the Earth:

The ERG spacecraft observed the origin of pulsation auroras in the Earth’s atmosphere 2018 ERG science team. The Northern Lights – with their dazzling display of green, red and purples – are one of the most beautiful and alluring natural phenomena on Earth.

Then they tracked the particles to The Pas, Man., where an all-sky camera had captured images of the resulting pulsating aurora. The researchers obtained the data with the help of the Canadian Space Agency.

Now, an international team of researchers have directly observed the cause of one type of aurora – known as pulsating aurorae – for the first time. Their findings are published in the journal Nature.

These are the images captured by the all-sky camera at The Pas, Man., that correspond to the signals detected by the spacecraft. The arrows show the aurora blinking on (filled) and off (empty). The red cross represents the location of the spacecraft in the sky. (Kasahara et al./Nature)

The pulsing is generated by the intermittent release of electrons into the upper atmosphere, but the physical mechanisms driving this process has never been observed until now.

Satoshi Kasahara, the University of Tokyo planetary scientist who led the study, and his colleagues compared the data from Earth and from space.

They confirmed that pulsating auroras are generated when clumps of plasma waves in the magnetosphere, the region of space affected by the Earth’s magnetic field, intermittently herd and shove aurora-causing electrons into the Earth’s magnetic field lines.

That causes bursts of electrons to rain down into the Earth’s atmosphere, where they can interact with particles there to produce a colourful light show. Normally, they just remain in space.

Many Canadians have been dazzled by the aurora borealis that they see as colourful curtains and ribbons of light that dance across the sky.

Such “active” auroras are actually less common than blinking patches of light known as pulsating auroras, which can last all night, every night, said Allison Jaynes, a plasma and space physicist at the University of Iowa who was not involved in the new study, but wrote an analysis accompanying it in the journal Nature.

You may never have seen the pulsating kind, though, because they tend to be intermittent and faint, she said. “They don’t necessarily provide a good show for the human eye.”

Scientists already knew that active auroras happen when a blast of charged particles from the sun hits the Earth’s magnetic field. That causes the release of electrons that excite particles in the Earth’s upper atmosphere. Those particles light up when they “relax” back to a less excited state.

But they had only a theory about what causes pulsating auroras — that they were the result of clumps of plasma waves called “chorus waves” (so named because they sound like birds chirping when converted into sound waves) interacting with electrons in the space surrounding Earth. It was a theory that had been proposed more than half a century ago, but never proven, Jaynes said.

An artist’s impression shows the Exploration of energization and Radiation in Geospace (ERG) satellite, nicknamed ‘Arase,’ orbiting the Earth. It launched on Dec. 20, 2016 and started taking measurements on March 23, 2017. (ISIS/JAXA)

That’s because proving it required very precise measurements of both plasma and electrons in space that could be compared with measurements on Earth.

Kasahara said he spent 10 years developing an electron analyzer that could take the space measurements. It was launched into space on Dec. 20, 2016 on board a Japanese spacecraft called Exploration of energization and Radiation in Geospace (ERG), nicknamed “Arase” after a river near Japan’s Uchinoura Space Centre.

The satellite began taking measurements on March 23, 2017, and captured the plasma waves and electrons from the pulsating aurora in The Pas just four days later.

Kasahara added that the new discovery may help researchers understand how the space environment and interaction between the sun and planetary magnetospheres affect planets’ atmospheres.

Jaynes said the process that generates pulsating auroras transfers enormous amounts of energy from space into the Earth’s atmosphere. That could have a significant effect on things like ozone in the atmosphere or atmospheric circulation that generates phenomena like the polar vortex.

Scientists don’t yet know how much of an effect it has, but knowing how this energy transfer occurs could help them figure that out.

Such processes could also happen on Jupiter and Saturn, where chorus waves have also been detected, the researchers suggest.

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