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Earth's Auroras Don't Mirror
04.04.05
 
Thanks to observations from the ground and satellites in space, scientists know that the North and South Poles light up at night with Auroras because a "solar wind" of electrified gas continually flows outward from the sun at high speed in all directions, including toward the Earth. Recently, however, NASA and university scientists looking at the Earth's northern and southern auroras were surprised to find they aren't mirror images of each other, as was once thought.

This series of near simultaneous auroras were observed between 6:24am and 7:10am ET on October 23, 2002.Image to right: North and South Auroras Aren't Mirrored:This series of near-simultaneous auroras were observed between 11:24am and 12:10pm Universal Time (6:24am and 7:10am ET) on October 23, 2002. Observations were made of the northern (left) and southern (right) hemispheres by IMAGE and Polar satellites, respectively. White dots indicate the geographic poles. Analysis of the spacecraft images showed how the auroras shift depending on the "tilt" of the Earth's magnetic field toward the sun and conditions in the solar wind. The "12" at the top indicates noon (the direction toward the sun), and "0" at the bottom indicates midnight, (the direction away from the sun). Likewise, the "6" indicates dawn or morning side of the Earth, while "18" indicates dusk or evening side of the Earth, thus placing the auroras on a 24 hour clock face. Click on image to enlarge. Credit: NASA

According to scientists, the main cause behind the differences in location appears to be what occurs between the solar wind and Earth's magnetic field.

The Earth's magnetic field, like that of the sun and some of the other planets, is generated by electrical currents flowing inside them. The sun's magnetic field, like that of Earth, has a north and south pole linked by lines of magnetic force.

Still from the animation that depicts how Earth's magnetosphere is stretched from a simple, symmetrical dipole field into a windsockImage to left: The Shape of Earth's Magnetosphere: This animation depicts how Earth's magnetosphere is stretched from a simple, symmetrical dipole field into a windsock-shaped bubble. We start with a simple bar magnet, surrounded by iron filings that trace the invisible magnetic field lines of force. Like a bar magnet, Earth has a dipole magnetic field -- that is, a north and south magnetic pole (not to be confused with the geographic poles). If there were no solar wind, Earth's magnetosphere would make a near-perfect dipole. But as we see at the end of the animation, when the magnetized solar wind blows past the Earth, it compresses the day (or sunlit) side of Earth's field and stretches out the night side into a long tail. Click on image to view animation (no audio--7.8 MB). Credit: Angela Cheyunski/Honeywell Max-Q Digital Group for NASA

Looking at the auroras from space, they look like almost circular bands of light around the North and South Poles. At the North Pole, it's called aurora borealis, or northern lights, and at the South Pole it's called the aurora australis, or southern lights.

From spacecraft observations made in October, 2002, scientists noticed that these circular bands of aurora shift in opposite directions to each other depending on the orientation of the sun's magnetic field, which travels toward the Earth with the solar wind flow. They also noted that the auroras shift in opposite directions to each other depending on how far the Earth's northern magnetic pole is leaning toward the sun.

Still from animation showing the coronal mass ejection erupting from the Sun. Image to right: This animation hints at the complexity and vastness of the magnetosphere as a CME heads toward Earth. The structures present are shaped by the Solar wind, and solar storms that can affect communications, power grids, and other important aspects of modern life on Earth. Click on image to view animation (no audio -- 4.1 MB). Credit: NASA

What was most surprising was that both the northern and southern auroral ovals were leaning toward the dawn (morning) side of the Earth for this event. The scientists suspect the leaning may be related to "imperfections" of the Earth's magnetic field.

"This is the first analysis to use simultaneous observations of the whole aurora in both the northern and southern hemispheres to track their locations," said lead author Timothy J. Stubbs of the Laboratory for Extraterrestrial Physics at NASA's Goddard Space Flight Center (LEP/GSFC), Greenbelt, Md.

This is a view of the Aurora Australis, or Southern Lights, from STS 39 in 1991.Image to left: Aurora Australis: This view of the Aurora Australis, or Southern Lights, which was photographed by an astronaut aboard Space Shuttle Discovery (STS-39) in 1991, shows a spiked band of red and green aurora above the Earth's Limb. Calculated to be at altitudes ranging from 80 - 120 km (approx. 50-80 miles), the auroral light shown is due to the "excitation" of atomic oxygen in the upper atmosphere by charged particles (electrons) streaming down from the magnetosphere above. Click on image to enlarge. Credit: NASA

The Earth's magnetic field provides an obstacle in the flow of the solar wind, and it becomes compressed into what looks like an extended tear-drop shaped bubble known as the "magnetosphere." The magnetosphere protects the Earth by shielding it from the solar wind. However, under certain conditions charged particles from the solar wind are able to get through Earth's magnetic shield and get energized. When this happens, they crash into the Earth's upper atmosphere and create the light which we see as an "aurora."

Stubbs and his colleagues, Richard R. Vondrak, and John B. Sigwarth, both of LEP/GSFC, Nikolai Østgaard at the University of Bergen, Norway and Louis A. Frank at the University of Iowa, used data from NASA's Polar and IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) spacecraft to study the auroras. It was by luck that the orbits of Polar and IMAGE were aligned such that the entire auroras at the north and south poles could be observed in detail at the same time.

NASA's Polar spacecraft took this series of images of the aurora over Earth's northern hemisphere. Image to right: Aurora Over the North Pole of Earth: NASA's Polar spacecraft took this series of images of the aurora over Earth's northern hemisphere. The images were collected by Polar's Visible Imaging System in February 2000, and they reveal the auroral oval around the polar regions in visible and ultraviolet light. The most intense auroral activity appears in bright red or white. Click on image to view movie (no audio--8.6 MB). Credit: University of Iowa/NASA Scientific Visualization Studio

By knowing how auroras react to the solar wind, scientists can better determine the impacts of space weather in the future. The new discovery shows that auroras may be more complicated than previously thought.

For more information about NASA's IMAGE satellite, please visit on the Internet:

http://image.gsfc.nasa.gov

For more information about NASA's Polar satellite, please visit on the Internet:

http://pwg.gsfc.nasa.gov/istp/polar/

For "Secrets of the Polar Aurora: What is an Aurora?" please visit on the Internet:

http://www-spof.gsfc.nasa.gov/Education/aurora.htm

 
 
Rob Gutro
Goddard Space Flight Center