Dayton Daily News

How sun plays a role in auroral asymmetry

- Robin George Andrews

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.

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 largescale 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 Ostgaard, 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-todate 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.

Ostgaard said that this study, for which he is a co-author, disproves a long-standing 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.

 ??  ??
 ??  ??
 ?? DREAMSTIME ?? The northern lights as seen from Alaska. New evidence shows the asymmetry, not collision, occurs because of the angles of the sun and Earth’s magnetic fields.
DREAMSTIME The northern lights as seen from Alaska. New evidence shows the asymmetry, not collision, occurs because of the angles of the sun and Earth’s magnetic fields.

Newspapers in English

Newspapers from United States