Since its launch on March 25, 2000, a new far-ultraviolet camera onboard NASA's Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft has captured several titanic auroras so intense that they could be seen as far south as Georgia, Florida and Texas.
But it wasn't just the visible beauty of these undulating sheets of icy green light that intrigued scientists at the University of California, Berkeley. It was the camera's ability to image the invisible ion auroras - writhing, flickering and dimming - in the unseen world of a special far-ultraviolet wavelength.
"On several occasions, we've been able to see very minute details in the structure and shape of these electrified halos over the Earth's North Pole," said Harald Frey, a UC Berkeley research physicist on the IMAGE far-ultraviolet camera team. "The far-ultraviolet imager takes a snapshot every two minutes of the auroral ovals over the North Pole and depicts the aurora, the zone of fire created by the collision of protons and electrons with Earth's upper atmosphere. We are able to image the evolution of the aurora borealis over four hours each time the IMAGE spacecraft passes over the North Pole and turn those snapshots into a quick-time movie for the world to see."
The orbiting far-ultraviolet camera, along with five other camera systems onboard IMAGE, has recorded the full range of visible and invisible light in the most vivid and intense auroras observed this year. Many more light shows - caused when huge eruptions of fast-moving, multimillion-degree plasma from the Sun crash into the protective windsock of Earth's magnetic shell - are expected to occur during the apex of solar activity now through the summer of 2001.
One of the most dramatic light shows - the July 14-15 so-called "Bastille Day" aurora borealis - was seen all the way to Georgia, said Frey, who is presenting a video of the electrified halo as it was observed from space at a Dec. 17 session of the American Geophysical Union meeting in San Francisco, titled "First Light from IMAGE." Frey will show videos of other equally dramatic auroral displays: an interplanetary shock wave on Aug. 12 that struck Earth's magnetosphere just before the peak of the Perseid meteor shower and measured about 80 percent of the magnitude of the Bastille Day aurora; and an aurora captured by all six IMAGE cameras on June 8 that measured about one-third, or 30 percent, of the magnitude of the Bastille Day event.
"Our first ultraviolet images of the Bastille Day aurora are a good example of the camera's new capability to see fine structure in an aurora that was invisible before," said Frey, who processes and analyzes data from the spacecraft as it passes within range of UC Berkeley's 11-meter (36-foot) satellite dish outside the Space Sciences Laboratory. "The pictures show an oval-shaped aurora extending more than 4,000 kilometers (2,500 miles) across in a complete ring, with a richness of structure that we did not expect to see, as the geomagnetic storm progressed.
"The oval could be easily traced into the daylight portion of the Earth, where it narrowed considerably into a thin ribbon only a few hundred miles thick. On the night side, we saw a dramatic broadening of the oval as the aurora evolved over time."
Portions of these iridescent halos were invisible to scientists before launch of the IMAGE space weather satellite. Using all known imaging techniques available today, IMAGE is the first spacecraft dedicated to imaging the full extent of the Earth's magnetosphere - an enormous, invisible sea of plasma beyond the ionosphere - rather than photographing conditions in the immediate vicinity of the magnetosphere from a fixed point in space. (An animation of the magnetosphere is available at http://www.digitalradiance.com/sng/magnetosphere.htm.)
The satellite is designed to photograph the glow of an aurora, caused when charged particles are excited to high energies by the Sun's activity and smash into atoms in the upper atmosphere. When the Sun's activity increases, auroras become more intense and plentiful. Following a highly eccentric orbit over the poles of the planet, IMAGE orbits far enough away from the Earth to capture the whole planet and its plasma in a single photographic frame.
Much of the magnetic interference that stirs the Earth's upper atmosphere is spawned by coronal mass ejections on the Sun. These eruptions, which are occurring with great frequency during solar maximum, tear huge holes in the Sun's outer atmosphere, or corona, and shoot billions of tons of highly energetic particles at planets at speeds of almost 2 million kilometers per hour (more than 1 million miles per hour). When these blasts of high-energy particles strike the Earth's tear-shaped magnetosphere and activate its gigantic power generator, the collision sends enormous currents into the Earth's atmosphere, producing a shimmering ring of whitish-blue and pink fluorescence in the night sky.
"A significant gap in our understanding of auroras has come from our inability to image proton auroras, which make up a large part of the aurora, because they are very diffuse and are almost invisible to the naked eye," said Stephen Mende, an atmospheric physicist at UC Berkeley and lead investigator of the far-ultraviolet instrument team. "Now, we are able to make them distinctly visible and track them in far-ultraviolet wavelengths."
A variety of imaging techniques is being used during IMAGE's two-year primary mission to give scientists the first comprehensive pictures of the full extent of the Earth's magnetosphere. Most charged particles are invisible on their own, but helium charged particles glow in the ultraviolet when illuminated by sunlight, and most types make the atmosphere luminous when colliding with it, causing visible and ultraviolet light. Or, they will neutralize themselves and form neutral particle beams, which can also be observed by IMAGE's many cameras. In addition to the two-dimensional imagers aboard the satellite, IMAGE carries a radio sounder to obtain global images of the primary plasma regions and boundaries of the Earth's inner magnetosphere. This radio sounder is a specialized "radar" that detects charged particle clouds.
The far-ultraviolet imager observes the aurora in three wavelengths: at very short wavelengths of 120 to 124 nanometers, where mainly hydrogen emissions can be seen; at 135.6 nanometers, where oxygen atoms are visible; and at 140 to 180 nanometers, where nitrogen emissions are best seen. (A nanometer is one billionth of a meter.) By comparison, the human eye can only see visible wavelengths that are slightly longer than 400 nanometers but do not exceed 660 nanometers.
Over the next two years, the far-ultraviolet imager and its coterie of complementary cameras will gather data while the spacecraft orbits the poles, passing within 1,000 kilometers (620 miles) of the southern hemisphere at its closest approach to the Earth and 44,650 kilometers (27,680 miles or 7 Earth radii) from the northern hemisphere at the farthest point in its orbit.
Data from the half-ton satellite - the first weather satellite for space storms - will be used in parallel with data from several Earth-orbiting probes currently in flight, such as NASA's Polar spacecraft and Japan's Geotail spacecraft, to describe how the magnetosphere changes in shape or configuration and how it processes the charged particles it has acquired from the Sun, Mende said.
"It's important to know the mechanisms that turn these particle showers on and off," Mende said. "As we continue to explore space weather at the edge of Earth's magnetic field, we are able to study first-hand how particles are being captured, stored, then lost in the atmosphere and farther out in space. We are actually watching the magnetosphere fill up and empty before our very eyes."
NOTE: Harald Frey can be reached at (510) 643-3323, by fax at (510) 643-2624, or by e-mail at hfrey@ssl.berkeley.edu. Stephen Mende is available by phone at (510) 642-0876, by fax at (510) 643-2624, or by e-mail at mende@ssl.berkeley.edu.
Videos of the auroras will be available at http://sprg.ssl.berkeley.edu/image/ following the conclusion of the Dec. 15-19 fall meeting of the American Geophysical Union.