| April 5, 2005 NASA STUDY FINDS EARTH’S AURORAS ARE NOT MIRROR IMAGES Scientists looking at the Earth’s northern and southern auroras were surprised to find they are not mirror images of each other, as was once thought. The main cause behind the differences appears to be the interaction between the Sun’s outer atmosphere and the Earth’s magnetic field.
Analysis of the images from NASA’s Polar spacecraft and the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft showed how the auroras move and change, based on the “tilt” of the Earth’s magnetic field toward the Sun and conditions in the solar wind.
By knowing how auroras react to the solar wind, scientists can better determine the impacts of space weather in the future. The new discovery by scientists from NASA, the University of Iowa, Iowa City, and the University of California at Berkeley, shows that auroras may be more complicated than previously thought. The NASA-funded study appeared in a recent issue of Geophysical Research Letters.
The aurora form near-circular bands around both the northern and southern poles of the Earth, known as the auroral ovals. These phenomena also are known as the aurora borealis, or northern lights, and the aurora australis, or southern lights. It was expected that the auroral ovals would be mirror images of each other.
”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, Greenbelt, Md.
The Sun’s outer atmosphere is an extremely thin electrified gas, or “plasma,” better known as the “solar wind,” since it blows constantly out from the Sun at around 250 miles per second. The Earth’s magnetic field provides an obstacle in the solar wind flow and becomes compressed into an extended teardrop-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 penetrate this magnetic shield and become energized. Collisions between these charged particles and the Earth’s upper atmosphere emit light which we observe as an “aurora.”
Stubbs and his colleagues used data from the two spacecraft to study the auroras. By luck the orbits of Polar and IMAGE were aligned so the entire auroral ovals in both hemispheres could be observed simultaneously in detail.
Stubbs and his colleagues noted four important items in their study of auroras observed in October 2002. As predicted, they observed the auroral ovals shift in opposite directions to each other depending on the orientation of the Interplanetary Magnetic Field (IMF). The IMF is the Sun’s magnetic field that travels out into space with the solar wind.
They noted the auroral ovals also shift in opposite directions to each other depending on how far the Earth’s northern magnetic pole is leaning toward the Sun (known as the “dipole tilt angle”).
Following a change in the orientation of the IMF, they observed the southern auroral oval shift toward the Sun while the northern auroral oval remained in about the same location. The scientists believe the southern aurora moved because the solar wind was able to penetrate into the magnetosphere in the southern hemisphere, but not in the northern hemisphere.
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. The Earth has a similar type of magnetic field to that which occurs around a simple bar magnet, which causes iron filings to arrange themselves in loops around it.
”Because Earth’s magnetic field is not a perfect dipole, we think this fact plays some role in causing the auroras to not be mirror images of each other,” Stubbs said.
For more information and images:
http://www.nasa.gov/vision/earth/lookingatearth/ dueling_auroras.html ###
Contact: Rob Gutro
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301/286-4044) | |
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. Credit: NASA 
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. Credit: Angela Cheyunski/Honeywell Max-Q Digital Group for NASA 
Earth’s Magnetosphere
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. Credit: NASA 
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. Credit: NASA 
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. Credit: University of Iowa/NASA Scientific Visualization Studio |
