Berkeley - Among the first pictures captured by the Magnetopause-to-Aurora Global Exploration, or IMAGE, satellite, launched a year ago to study the Earth's magnetic shield, is the first global view of the double aurora - the pretty curlicues and shimmering curtains of the electron aurora and the more diffuse proton aurora.
The colored light display most people associate with the aurora or Northern Lights is produced by electrons crashing into the atmosphere as they descend along the Earth's magnetic field lines. Almost equally bright but less structured are the lights produced by positively charged protons - hydrogen nuclei - as they ram the atmosphere.
Though scientists have been able to distinguish the electron and proton auroras from the ground since the 1950s, IMAGE's far-ultraviolet imager, built by a team at the University of California, Berkeley's Space Sciences Laboratory, has obtained the first pictures from space showing the entire proton aurora and its relationship to the electron aurora.
The hour-long series of photos, taken every two minutes, suggest that the proton aurora appears first and may initiate the more spectacular electron aurora.
"These pictures show for the first time that the electron and proton auroras are different and develop differently over time," said Stephen B. Mende, an atmospheric physicist and lead investigator of the far-ultraviolet instrument team. "We've looked at the proton light for some time from the ground, but never seen the proton aurora from a global perspective like this. That is very exciting."
The findings will be published along with nine other papers on IMAGE results in the March 15 issue of Geophysical Research Letters, a publication of the American Geophysical Union. Some earlier results from IMAGE were reported in the Jan. 26, 2001, issue of Science.
The far-ultraviolet imaging team includes research physicists Harald U. Frey and Michael Lampton of UC Berkeley's Space Sciences Laboratory; J.-C. Gerard and B. Hubert of the University of Liege, Belgium; S. Fuselier of Lockheed-Martin Palo Alto Research Laboratories, Calif.; J. Spann of NASA Marshall Spaceflight Center; and R. Gladstone and J. L. Burch of the Southwest Research Institute (SWRI) in San Antonio, Texas.
Captured last June 28, the auroras were the result of a rather puny substorm in the Earth's magnetosphere, the magnetic field region that enshrouds the Earth and protects it from the Sun's periodic particle storms, Mende said. Nevertheless, IMAGE's far-ultraviolet instrument was able to capture the first images of the two distinct auroras, like lopsided halos around the North Pole and slightly offset from one another.
The images show, Frey said, that the diffuse aurora at lower polar latitudes come from both protons and electrons, while the very pretty, structured aurora at higher latitudes near the North Pole is due almost entirely to the electrons.
Substorms are generated when the Earth's magnetosphere for some reason gets charged up with protons and electrons and then discharges, sending ionized particles spiraling along magnetic field lines to where they converge at the pole. Along the way, they hit atoms in the atmosphere and emit light, ranging from colorful visible light to the invisible far-ultraviolet.
Substorms, which may last an hour, are distinct from the day-long storms, which are generated by coronal mass ejections and large flares on the sun, that disrupt global communications.
What makes the two types of auroras distinct are the different behaviors of protons and electrons as they enter the atmosphere. Protons quickly become neutralized as they combine with electrons, and once this happens they ignore magnetic field lines and fly in all directions. Electrons, however, remain free and stick to magnetic field lines.
"Electrons spiral tightly around the magnetic field lines, so even after making many collisions, at the end they're not far from the original field line they were attached to," Mende said. As a result, the light from their collisions with atmospheric atoms has a structure dictated by the field lines, typically shimmering curtains of light.
In the June 28 substorm, the proton aurora started at a lower polar latitude than the electron aurora but gradually moved northward to sit atop the electron aurora - two concentric ovals some 2,000 miles in diameter - until it was outshone by the electron aurora. As the electron aurora continued to expand toward the pole, the proton aurora remained behind.
"The proton aurora is actually very important at the start of the substorm, but the electron aurora takes over, at least as far as brightness is concerned," Mende said. "By studying the two separately we are beginning to understand the dynamics of the Northern Lights."
IMAGE was launched March 25, 2000, carrying five suites of camera systems, among them the far-ultraviolet imager built by Mende's team. The instrument observes the aurora in three far-ultraviolet wavelengths: at very short wavelengths, where mainly hydrogen emissions are seen; at longer wavelengths, where oxygen atoms are visible; and at even longer wavelengths, where nitrogen emits.
As IMAGE swings over the North Pole, it is far enough above the Earth - about seven Earth radii - to be visible from Berkeley. As a result, an antenna built for the HESSI (High Energy Solar Spectroscopic Imager) mission, scheduled for launch in the next couple of months, has been able to download data from the FUV imager directly to the scientific team at UC Berkeley.
This has allowed the team to put on the Web every two minutes a new far-ultraviolet picture of the aurora. Assessment of individual storms and substorms, however, takes months of computer analysis.
The research was sponsored by the National Aeronautics and Space Administration through SWRI.
* Current images of the electron aurora -- http://sprg.ssl.berkeley.edu/image/latest_wic.html
* Mpeg video of a proton aurora -- http://sprg.ssl.berkeley.edu/sprite/ago96/image/mpegs/si12_2000_197_mapped.mpg
* Mpeg video of an electron aurora -- http://sprg.ssl.berkeley.edu/sprite/ago96/image/mpegs/wic_2000_197_mapped.mpg
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