April 23, 2019

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The Northern and Southern Lights Are Asymmetric Dancers in the Dark

The Northern and Southern Lights Are Asymmetric Dancers in the Dark
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Earth’s auroras, popularly known as the Northern and Southern Lights, are indisputably beautiful. They are also, perhaps surprisingly, not mirror images.

You can imagine the two like mismatched dancers: Viewed from space, the Northern Lights may contort and groove in one direction, while the Southern Lights could perform a routine that doesn’t quite sync up with its partner’s.

Research published in December in the Journal of Geophysical Research: Space Physics, reveals that the cause of this north-south auroral asymmetry is the angle at which the sun’s solar wind and magnetic field approaches Earth.

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Earth has two magnetic poles, a north and a south, much in the same way a bar magnet does. From these two poles, bunched up magnetic field lines — invisible tendrils that represent the direction and strength of this planetary bar magnet — reach out into space as the planet travels on its orbit. Like fishing lines, they catch energetically excitable particles heading our way in the solar wind. These particles slam into our atmosphere, and energy is released in the form of the colorful auroras.

As these two geographically opposed magnetic poles are reflections of each other, it was once assumed that the auroras would be the same too. However, scientists can now see more clearly that they not only have large-scale differences in shapes, but they crop up at slightly different locations around the magnetic poles too.

Earth’s entire magnetic field is a bit like a bubble, one that is constantly being squashed by the sun’s magnetic field and solar wind. This causes the bubble to stretch out on Earth’s nightside in the shape of a magnetic tail. Space physicists have now found that when the sun’s magnetic field is heading toward Earth in a more east-west orientation relative to Earth’s magnetic poles, it compresses our magnetic tail in a peculiar way. This makes it tilt, which triggers distortions that ultimately produce differently shaped Northern and Southern Lights.

Auroral asymmetry has been studied for decades. However, it wasn’t until the early-to-mid-2000s that scientists could use satellites to observe both auroras at the same time, which allowed space physicists to clearly see that they weren’t mirror images.

Nikolai Østgaard, head of the Birkeland Centre for Space Science at Norway’s University of Bergen, and Karl Laundal helped to underscore how different the two auroras could be in 2009. But a conclusive explanation of these differences remained elusive.

This new paper, led by Anders Ohma, a doctoral candidate at Bergen, examined old images of both auroras taken at the same time by two separate satellites. By assessing the most up-to-date information about the two entities’ magnetic fields, they uncovered clear evidence that the asymmetry occurs because of the angle of the sun’s magnetic field compared to Earth’s field.

Prof. Østgaard said that this study, for which he is a co-author, disproves a longstanding hypothesis suggesting that the collision, fracturing and eventual chaotic reassembly of Earth’s and the sun’s magnetic field lines may have caused the auroral asymmetry. But their data found that this process, known as magnetic reconnection, actually reduces the asymmetry somewhat.

Elizabeth MacDonald, founder of the first citizen science network for the auroras and a space physicist at NASA’s Goddard Research Center, said the research was useful, but she added that auroras come in a wide range of flavors. It “doesn’t explain all of the asymmetry possible in different flavors of the constantly evolving aurora,” she said.

Mr. Ohma agreed that smaller phenomena remained enigmatic, but said their work did explain large-scale auroral asymmetries.

Jonathan Eastwood, a senior lecturer of space physics at Imperial College London who wasn’t involved in the study, said the research provides interesting clues as to how planetary magnetic fields work, while also inviting us to more deeply appreciate the auroras’ aesthetic charms.

“It starts to give you a sense of how majestic and big these processes are, and how small we are,” he said.



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