An aurora is one of the best visual entertainments that nature has to offer. But, what many scientists find even more fascinating than these striking luminary displays is the nearly invisible version, known as a “pulsating aurora.”
As its name suggests, a pulsating aurora comes in the form of patches of light that fade in and out. The light typically stays on for a few seconds at a time, contributing to the obscure glowing type of “diffuse aurora” that is fainter than the usual displays of distinct lights.
As our daily lives on Earth become increasingly intertwined with what goes on in space, researchers from around the world are trying to decode the mysteries surrounding space plasma physics. And, pulsating aurora is a big part of the unknown.
A team of Japanese researchers has been making an all-out effort to shed light on the subject through their “Pulsating Aurora Project” since its inception in 2015. After deciphering the basic mechanism of a pulsating aurora, the team has continued to pull together all expertise and novel technology to push for the initiative. One of the researchers involved in the project will share the latest of their activities and discoveries.

Japan’s Chase of Fleeting Lights

Pulsating auroras are hard to catch sight of without a camera. They are often too dim to identify with the naked eye, according to Prof. Yoshizumi Miyoshi of Nagoya University.

“When I see something hazy in the night sky, I take a picture of it with a digital camera. It often turns out to be a green aurora,” Prof. Miyoshi said.

The mysterious appearance of a pulsating aurora isn’t simply due to its simmering; Even while the light is shining, there is another cycle of luminosity modulation taking place within it. Researchers call the former the “main pulsation,” and the latter the “internal modulation.”

For example, in an aurora that pulsates every 5 seconds, the light’s luminous intensity may change every few hundred milliseconds during the 5-second cycle.

“The repetition of these main pulsation and internal modulations gives a pulsating aurora different shapes and makes it look like dancing to a rhythm,” Prof. Miyoshi said. 

But, a pulsating aurora’s feeble appearance can be deceiving; it carries an average of tens kilovolts of energy -- higher than several kilovolts for the typical, clearly visible aurora, or the “discrete aurora.”

“The difference in voltage may not seem that big. But, we’ve found that there are electrons carrying much higher voltage -- 100 kilovolts or even 1,000 kilovolts -- that are coming down along with pulsating auroras,” Prof. Miyoshi said. “This was a big surprise to us. We are making more discoveries in this area.”

Prof. Miyoshi and his colleagues have participated in the “Pulsating Aurora Project” since its launching in 2015. They’ve used super-high-sensitivity cameras to track pulsating auroras’ changes in detail from the ground.

They also continue to analyze observational data from Arase, a geospace exploration satellite launched by the Japan Aerospace Exploration Agency (JAXA), to develop a higher-level understanding of aurora phenomena.

 “We are hopeful that, over the next several years, Arase will provide us with invaluable first-hand observation data from space. It will be the chance of our lifetime for us to uncover what is happening in space to kindle an aurora,” Prof. Miyoshi said.

In addition, the team is preparing to launcha sounding rocket f rom the polar regions into Earth’s atmosphere to make observations from the same altitudes as auroras. They hope the combinations of various observation techniques and simulation technologies will help advance research on pulsating aurora in a holistic way.

Satellite Arase Keeps Watchful Eye on Van Allen Belt

To understand pulsating auroras, it’s important to pay attention to Van Allen Belts, the zones of highly charged energetic particles, according to Prof. Miyoshi. Pulsating auroras often causes these electrons to fall into the atmosphere from the Van Allen Belts. Prof. Miyoshi and his team know that because they have the data to show just that from Satellite Arase, which launched in December 2016, specifically to observe the activities of electrons in the Van Allen Belts.

Van Allen Belt is named after James Van Allen, a U.S. space physicist who discovered trapped radiation around Earth in 1958 from the observation results of the United States’ first satellite. These particles carry extremely high energy -- ranging from several hundred kilovolts to tens of megavolts, which translates into hundreds of billion degrees Celsius in heat. In other words, Van Allen Belts are exceptionally hazardous zones for any artificial satellite to operate in, and overcoming the difficulty is Satellite Arase’s mission.

When the electrons precipitating from Van Allen Belts hit the middle atmosphere at an altitude of 60 to 80 kilometers above Earth’s surface, the impact causes ionization of nitrogen and nitric oxide in the mesosphere. The ionization manifests physically in the form of a pulsating aurora.

“The ionization that results in a pulsating aurora has the potential to destroy a part of the ozone layer in the mesosphere. It’s possible that there is a connection between pulsating auroras and Earth’s climate change,” Prof. Miyoshi said.

Auroras’ Dance to Space’s choir

So, why do pulsating auroras fade in and out?

The answer is in the chorus waves of geospace, Prof. Miyoshi said.

Chorus waves are a type of very low frequency (VLF) plasma waves that sound like birds’ chirps to the ears of those who use an audio device to listen to it. The “chorus” could be heard by using antennae to capture electromagnetic waves in space ranging between 2-3 kHz and reproducing the data with a speaker, according to Prof. Miyoshi.

“When you hear the chirps through the speaker on the ground, an aurora begins to pulsate above you,” he said. (You can listen to the chorus reproduced from the Satellite Arase’s observation data here.)

In 2015, Prof. Miyoshi and his team became the world’s first scientists to successfully demonstrate that the chorus waves cause modulations of electrons, creating pulsating auroras. They used data on high-energy electrons captured by a satellite, along with some simulation techniques for the research. (Click here for the press release. Click here for the sound data.) 

Then in 2017, Arase captured chorus itself at an altitude of tens of thousands of kilometers above Earth’s surface. At the same time, they observed and recorded a pulsating aurora from the ground, demonstrating the direct correlation between the pulsation of the aurora and the chorus waves. This research was collaboratively conducted by the University of Tokyo, JAXA and Kanazawa University, among some other academic institutions.

“We already have many satellites in geospace and many observation stations on the ground, including the NIPR’s polar research points. We can put together data from all these observation resources and develop a comprehensive picture of what goes on in space and how it happens. And, such a holistic understanding will become increasingly important in years to come,” Prof. Miyoshi said.

“Geospace has become a part of our milieu. There’s a growing desire among all of us to have deeper and more detailed knowledge about geospace and changes happening in it. Such knowledge includes real time figures, as opposed to an average, and we will ultimately need to become able to forecast space weather. Research on pulsating auroras is important for us to reach that goal, as well,” he said.

Interviewer: Rue Ikeya
Photographs: Yuji Iijima unless noted otherwise
Released on: Sept. 10, 2019 (The Japanese version released on Sept. 10, 2018)