Science Report 022

Lifting the Curtain on Auroras 04

Why Recreating Auroral Explosions Help Us Better Understand Space

Watching auroras can be an extremely intense experience. Anyone who has ever witnessed an “auroral breakup” would be able to attest to that. Auroras, which typically glow faintly, sometimes explode in brightness in the northern and southern polar skies, touching off random displays of incredible bursts of lights in all directions. Researchers have long tried to understand the geomagnetic disturbances that set off this spectacular phenomenon. Recently, a Japanese team of scientists successfully untangled a part of the mystery through the use of numerical simulations. In this installment, one of the team’s principal investigators will share the team’s findings and what they mean for the advancement of space research.

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Yusuke Ebihara (Kyoto University)

Ask the Expert: Yusuke Ebihara (Kyoto University)

Yusuke Ebihara, associate professor at Kyoto University’s Research Institute for Sustainable Humanosphere, specializes in the study of space physics. He is well known for his research on geomagnetic storms and magnetospheric substorms. In 2015, Prof. Ebihara proposed a simulation model designed to quantitatively explain auroral breakups with the use of the simulation method developed by Prof. Takashi Tanaka, professor emeritus at Kyushu University. After completing his Ph.D. studies at The Graduate University for Advanced Studies (SOKENDAI) in 1999, Ebihara served as a specially appointed professor at the Nagoya University Institute for Advanced Research before becoming an associate professor at Kyoto University.


Simulations as a way to make magnetosphere fit in your palm

There are three interrelated factors that give rise to an aurora: The sun that releases electrons and other particles; the Earth’s magnetosphere that captures those electrons; and the Earth’s upper atmosphere, on which the electrons fall, setting off chain reactions that result in the displays of lights. One of these factors, the magnetosphere, is the focus of Prof. Ebihara’s aurora research.

Between 1999 and 2001, he worked in the northern Swedish city of Kiruna as a local research institute’s postdoc researcher. His experience of repeatedly witnessing auroral breakups in Kiruna sparked his deep interest in the interplay between aurora breakups and the magnetospheric activity, particularly “substorms,” which is a disturbance of the magnetosphere.

“I would suddenly see a bright sparkle in one spot, and then it would quickly spread to form an aurora, brightening up the entire sky. Finer lights are woven into the shape of curtains, and those lights would run across the sky like waves in random directions. There were no rules or patterns to it. I absolutely had no idea of what was causing lights to move like that. I was just in awe of the power of Mother Nature,” Prof. Ebihara said, looking back on his experience.

Back then, he was conducting research on magnetic storms, disturbances of the magnetosphere that typically last for several days. Substorms are much more short-lived than magnetic storms, but are known to cause auroras to suddenly spread.

“The magnetosphere is a very important factor to consider when trying to understand the space environment around the Earth. But, the magnetosphere, which extends as far as the Earth’s magnetic force can reach, is so vast that it’s difficult to research it. Satellites are launched to examine the state of the magnetosphere during an auroral breakup. But that’s like trying to probe the Pacific Ocean from a boat floating on it. You may know what’s happening at a specific site, but it’s impossible to find out how the ocean is behaving as a whole,” Prof. Ebihara said.

“Nowadays, multiple satellites are used simultaneously to look at the magnetosphere, but it’s still difficult. This led to an idea of tackling this aurora and magnetosphere issue through numerical simulations by using a supercomputer. This effort has been going on for the past 20 to 30 years.

And that’s what Prof. Ebihara has also been doing. He uses a simulation that Prof. Takashi Tanaka, a professor emeritus at Kyushu University, developed following his awestruck experience of observing auroral breakups while participating in the Japanese Antarctic Research Expedition at the Showa Station between 1983 and 1984.

Prof. Tanaka created the simulation in the mid 1990s and continues to improve it.

“It’s very effective in recreating the unique characteristics of auroral breakups,” Prof. Ebihara said of Prof. Tanaka’s simulation.

The photo to the left is that of an auroral breakup observed by a satellite. An arc-shaped area (front) in the auroral oval starts shining brightly before the lights rapidly spread to the east and west like a wildfire. The photo to the right is that of a simulation result. The simulation is effective in replicating observed features of auroral breakups.

Storms in invisible magnetosphere that affect us all

There are two types of severe disturbances of the magnetosphere: Magnetic storms and substorms. Magnetic storms refer to a period in which the magnetic force surrounding the globe declines in strength. Once a storm occurs, the magnetic disturbance continues for at least several days, often causing auroras to appear outside the polar regions.

Substorms, on the other hand, last only a few hours. During a magnetic storm, many smaller and intermittent magnetic field variations are observed in the polar regions. Sydney Chapman (1888-1970), a British geophysicist who contributed to the advancement of aurora research, proposed naming these transient storms “substorms,” suggesting that they come together to form a magnetic storm.

Scientists have since learned, however, that a magnetic storm can happen without substorms. Today, the link between magnetic storms and substorms remains a mystery.

“Auroral breakups are always followed by a severe disturbance of the Earth’s magnetosphere,” Prof. Ebihara said, stressing the importance of understanding the magnetospheric activity. He reminded how auroral breakups can cripple satellites and the grid systems.

“Substorms occur rapidly, and you never know in advance when they’ll happen. It’s similar to earthquakes in the sense that they’re hard to forecast,” Prof. Ebihara said.“ The process of substorms development is an intriguing research topic because knowing more about it would help shed light on the mechanism of the magnetic field variations.”

“You can forecast magnetic storms to a degree, as long as you can determine how solar wind is going to blow. But when it comes to substorms, it’s not only a challenge to forecast, but it also drastically changes the state of the magnetosphere once it happens. That’s the interesting part for me as a researcher,” Prof. Ebihara said. “And above all, the beauty of auroral breakups is the biggest appeal for me. Substorms are attractive in many different ways.”

Power plants of magnetosphere

Prof. Tanaka’s simulation has enabled Prof. Ebihara to replicate an auroral breakup through its entire process in the form of a clear visual presentation.

“You have lines of magnetic force extending from the Earth’s North and South poles. When you simulate solar wind, or electrically charged particles (plasma), to be blown toward those lines, the lines of magnetic force closer to the Sun get pushed toward the Earth and become skewed. Those lines continue to stretch to the opposite side of the Earth from the sun, as if blown by the wind, eventually forming a magnetosphere with a long, contour,” Prof. Ebihara explained.

This magnetosphere’s tail on the other side of the Earth from the sun is called the “magnetotail.” Some magnetic field lines run in the opposite direction from the others. Magnetic field lines running in one direction can sometimes meet and merge with the lines going in the other direction. This is called “reconnection” of magnetic field lines. Once the reconnection occurs, because the lines have the characteristics to shrink themselves, they begin to bounce back toward the Earth, taking the plasma along with them. This causes plasma pressure to build up near the Earth, which, in turn, creates “field-aligned current” -- strong currents that run along the magnetic field lines in relatively high latitudes -- to develop. When this field-aligned current connects with the ionosphere near the Earth’s surface, it causes auroral breakups.

This simulated beginning-to-end flow of events provided secure footing for his endeavor to illuminate the dynamics of the Earth’s magnetosphere, Prof. Ebihara said.

“When particles that make up the plasmas move, it generates electricity, just as thermal power plants generate electricity by burning oil and natural gas to create vapor, which moves turbines. In space, when plasmas form near the earth as the result of a magnetic reconnection, the particles begin to move fast, causing the “generator” to kick on. When strong field-aligned currents coming out of those generators connect with the ionosphere, electrons begin to precipitate, setting off auroral breakups,” Prof. Ebihara said.

“Through this simulation, we were able to see power being generated in three locations at the time of auroral breakups,” he said.

Because electrons and electric currents run in opposite directions from each other, a bright aurora indicates that strong currents are flowing upward and increase in conductivity as they go higher, according to Prof. Ebihara. The patterns of currents running through the ionosphere also change near bright auroras, and that change generates electricity, as well. This is believed to result in a fiery bright aurora display to the west, he said.

“In short, aurora breakups aren’t just caused by what goes in space. Earth plays a major role in their development. Earth and space work together to create the unworldly phenomena. That’s what the simulation taught us.”

The blue sphere in the center at which Prof. Ebihara is pointing indicates the Earth. The simulation has the solar wind blowing from the left back of the image. It shows the magnetotail’s spread to the right of the image, which depicts the reconnection.

Improving simulation to unlock mystery of space

Even though Prof. Ebihara and his team were able to replicate the developmental flow of auroral breakups through the simulation, it’s only the framework of a big picture they are trying to put together.

“We still haven’t been able to recreate the detailed structure of auroral breakups,” Prof. Ebihara said.

That’s due to the simulation’s design. While it presents simulation results in a clear, visual format, it doesn’t show how it reached the end result in an easy-to-understand manner. In analyzing the results of the simulation, Prof. Ebihara said the team has tried to validate established theory, but that “all attempts failed miserably.”

So, the team changed up the analysis method, looking at the magnetotail using a three-dimensional point of view, instead of two-dimensional. This enabled them to understand how auroral breakups occur. In that process, they also discovered that pulsating auroras -- which often appear following auroral breakups -- happen as the plasma energy built up in the magnetosphere near the Earth becomes unstabilized for some reason and come falling down on the Earth.

Prof. Ebihara and his team continue to work toward replicating the multiple small and bright elements of the auroras that appear during the first phase of auroral breakups.

“We need to keep improving the simulation to increase the accuracy. At the same time, we also need to verify what the simulation results really mean, and compare them with observational data to deepen our understanding of what’s happening in space,” he said.

Prof. Ebihara at Kyoto University’s Uji campus.

Interviewer: Rue Ikeya
Photographs: Yuji Iijima unless noted otherwise
Released on: Nov. 11, 2019 (The Japanese version released on Nov. 12, 2018)

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