|DOUBLE AURORA CAPTURED ON FILM||
|Among the first pictures captured
by the Magnetopause-to-Aurora Global Exploration (IMAGE) satellite 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.
Mpeg video of an 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 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.
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 substorm recorded, 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.
Northern Lights: The Science, Myth, and Wonder of Aurora Borealis by Calvin Hall and Daryl Pederson
Sasquatch Books, 2001
The stunning photographs of the aurora borealis in this book were taken without special filters or digital enhancing. The dancing braids of light and multi-hued waves of color seem to quiver on the page, their movement somehow captured and translated into two-dimensional form.
Captured in the dark and enormous skies over the Alaskan Interior, the photographs are identified by shooting locations in or near the Talkeetna Mountains, Bard Peak, Mount Alyeska, Turnagain Pass, Cook Inlet and dozens of other sites over a period of 20 years.
"Scientists tell us that the northern lights -- the aurora borealis -- are always present in the far north, even when we can't see them due to daylight or cloud cover," explains George Bryson in the essay accompanying the photographs. "Usually they remain withdrawn inside the auroral zone, a vertical corridor that encircles the Northern Hemisphere along an oval track about a thousand miles from the magnetic North Pole.
"When this auroral oval is relatively calm, its centerline transects the northern third of Alaska almost directly above the Arctic village of Fort Yukon. On dark, cloudless, winter nights in Fort Yukon, the chances of observing the northern lights are virtually 100 percent. In Fairbanks, about 125 miles south of Fort Yukon, the odds under the same conditions are around 80 percent. In Anchorage, still farther south, they dip to 40 percent, in Ketchikan to 20 percent, in Seattle to 5 percent, in San Francisco to 1 percent."
The Peterson First
Guide to Astronomy
A simplified field guide to stars, planets and the universe.
The Book of Clouds
by John A. Day
See the sky as you never have before. Using a series of his awe-inspiring images, photographer and scientist John Day -- who has a Ph.D. in cloud physics and is known round the world as "The Cloudman" -- introduces us to earth's great skyscape.