All day long, a steady stream of ionized gas called the solar wind blows outward from the Sun in all directions. Mostly the solar wind flows around Earth's own magnetic field, but sometimes it penetrates the field, where it become energized. The energized particles strike the Earth's upper atmosphere near the poles, creating the aurora—the northern and southern lights.
Scientists have long thought that the two hemispheres' auroras were essentially mirror images of one another. However, data collected by two NASA satellites reveal a surprising asymmetry, which is illustrated in the images above. Each row shows ultra-violet observations of the aurora captured over either the Northern (top) or Southern (bottom) Hemisphere over a 17-minute period on October 23, 2002. The globes are centered on the Earth's magnetic poles, with white dots showing the location of the geographic Poles. The parts of the Earth experiencing noon local time when the observations were collected are at the top of the globes; areas experiencing dawn are at right; areas experiencing midnight are at the bottom; and areas experiencing dusk are at left. Colors range from black, where no auroras were observed, to blue, green, yellow and red, where the auroras were most intense. (The very red areas in the top part of the Southern Hemisphere globes are not part of the aurora but are just image processing artifacts left over from the removal of the “dayglow” caused by sunlight.)
Looking just at the aurora ring, two types of asymmetry are obvious. The first asymmetry is common to both hemispheres: namely, a shift toward the night side of the Earth (bottom parts of the globes). Scientists have known about this shift for a while. The part of the Earth's magnetic field that faces the Sun gets compressed by the solar wind, while the part experiencing darkness gets stretched out, causing the auroral rings to shifted toward the bottom, or midnight part of the images.
The second asymmetry is that the Southern Hemisphere aurora is shifted to the right, or dawn side of the globes. This shift toward dawn means that people who were experiencing dawn in their local time zone at the moment of this aurora would have seen aurora much farther from the magnetic poles than those observing at dusk.
The scientists who published these results think that the shifting of the Southern Hemisphere aurora toward the dawn side may be caused by the fact that the Earth's magnetic field isn't a perfect "dipole," which describes a magnetic object that has two equally strong electric charges of opposite signs (negative and positive) separated over a distance.
These data were collected by NASA’s Polar and IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellites. For more on these non-mirror-image auroras, read the following story on the NASA home page: Earth's Auroras Don't Mirror.
Images courtesy Timothy J. Stubbs, NASA GSFC Solar System Exploration Division
