Seeing The Light

In an increasingly satellite-dependent world, understanding the power of the aurora borealis has become critical

Saturday, July 01, 2000


There's a lonesome outpost in the Alaskan interior where scientists work in wool slippers and all-nighters are standard procedure, where sunlight is scorned and darkness cherished for the promises it holds. That outpost is the science center at the Poker Flat Research Range, a rocket-launching facility in the hills 30 miles northeast of Fairbanks. Poker Flat is one of the world's premier centers for studies of the aurora borealis, the electromagnetic light show that graces the subarctic firmament almost every night of the year.

Perched on top of a low rise with a uniform view of sky, snow, and spruce, the center sometimes teems with humanity, as researchers come from around the globe to grab a glimpse of the luminous curtains and rays that dance to a silent madrigal far above the tundra. Tonight, though, it's the regulars from the Geophysical Institute of the University of Alaska at Fairbanks. They'll while away the wee hours swapping stories, watching movies, trading computer tips, and waiting for a phantom stream of light.

Coffee thickens on the burner in the kitchen, sepulchral light spills from exit signs in the darkened hallways, instruments and monitors hum in the control room upstairs, and stars bear down on the glass walls of the center's observation room. At peak aurora season, fanatics like Hans Nielsen, a fiftysomething Dane, keep vigil in this place every night for weeks on end. "When I get on this schedule, I can't remember one night from the next," he confesses. "It all becomes a blur." Not that he's complaining. Like most of his colleagues, Nielsen has a special fondness for the northern lights and an awe of the sort that has inspired thousands of years of folklore and myth.

"No pen nor pencil can portray its fickle hues, its radiance, and its grandeur," polar explorer William H. Hooper said of the aurora, but literal translations of its various names in other cultures hint at its magnificence: The Romans called it "blood rain," the Chinese "candle dragon," Eurasians "wind light." To the Inuit, the aurora borealis was the highest level of heaven, where the dead danced. "Streaks of light toss about with abandon," goes a typical account. "Suddenly, for a second, all light melts away and the sky is full of darkness. Just as quickly the lights blossom again in pulsating waves and arcs, and then, as if to test the credulity of man, giant draperies of it wash by in undulating movements across the whole heavens, sometimes stabbing the ends of their folds toward the earth, dripping with the green of grass or the red of blood." The technical details of auroral activity aren't quite as poetic. In the 1950s, scientists began to understand that the aurora borealis and its southern conjugate, the aurora australis, are generated by the interaction of charged particles streaming from the sun and the magnetic field that surrounds Earth. The geomagnetic field deflects most of the deadly particle stream, but in doing so it gets swept out into a comet-shaped envelope, called the magnetosphere, in which some of the particles are trapped. Electromagnetic forces energize the trapped particles and draw them down into Earth's atmosphere at the polar regions, where they collide with atoms and molecules of gas, producing light.

Such rational explanations in no way diminish the aurora's splendor: Even after 30 years of auroral studies, Nielsen will still pull his car to the side of the road and get out to watch when the lights act up. Nor has science managed to dispel the aurora's mysteries. No one knows, for example, what processes spawn the many guises of the northern lights—the rays and arcs, the folds and filaments, the patches and spirals, the diffuse, pulsating veils. It's still not clear what factors collude to produce auroral substorms, the bright, dynamic global displays that create some of the aurora's most spectacular visual effects. Most important, experts still can't predict exactly when the lights will appear. Despite decades of sophisticated studies and hordes of high-tech gadgetry, auroral prediction remains a practice that is part plasma physics, part atmospheric science, and part voodoo.

And that shortcoming has become a bit of a problem. Although the vagaries of the northern lights may not at first glance seem a matter of critical import, the aurora is a key indicator of solar-terrestrial interactions, better known as space weather. And space weather, like weather closer to terra firma, can pack walloping storms that threaten Earthlings and some of their favorite artifacts.

Space weather is conjured by the solar wind, the plasma stream of charged particles and magnetism that flows continuously from the sun. When disturbances such as flares erupt on the sun's surface, the solar wind goes to gale force, prompting pandemonium in the magnetosphere, particle showers in the upper atmosphere, and surges of current with effects that reach all the way down to the ground.

That sort of high-energy chaos makes for great auroras, but it can also play havoc with the radio communications, radar systems, power transmission lines, telephone cables, and satellites on which humanity grows more dependent every day. An onslaught of charged particles in space can disable satellites by damaging solar-cell panels and altering digital signals. Currents induced by the solar wind's fluctuating magnetic fields can provoke heating and expansion of Earth's upper atmosphere, dragging down low-flying satellites. One of the most stunning auroral substorms in recent history has been blamed for a nine-hour-long power outage that darkened all of Quebec. Auroral displays can also disrupt the propagation of radio waves by changing the electrical properties of the upper atmosphere. Currents of electricity in auroras 60 miles overhead are believed to have caused corrosion in oil pipelines, such as the Trans-Alaska.

With hundreds of telecommunications satellites in orbit and a way of life sustained by electrical power and oil, the developed world is more vulnerable than ever before to the hiccups of our neighboring star. Small wonder, then, that space-weather prediction has become a top priority on the nation's research agenda. And with the sun so close to another solar maximum—the peak of its 11-year cycle of activity—fans of the aurora and of high technology alike are anticipating some fireworks. This summer the gauzy tendrils of auroral substorms could reach all the way down to the southern United States. Novice observers might not want to sit up all night for weeks waiting for a stray spark. But if the sky is clear and dark, and the time close to midnight, it may be worth casting an eye northward. Millions are likely to see this enchanting display by chance in the next six months.

At Poker Flat, auroral observation is a bit more complicated. On the second floor of the science center, banks of monitors, keyboards, and computer screens provide links to satellites and ground sensors near and far. One screen displays the feed from the Advanced Composition Explorer, a satellite parked 930,000 miles sunward that detects perturbations in the solar wind an hour before it gets to our magnetosphere. On another screen, graphs generated by the Earth-orbiting GOES satellite chart the energy stored in the magnetosphere as a result of interactions with the solar wind. Readings from ground-based magnetometers relay information about electromagnetic currents in the upper atmosphere. Should the aurora make an appearance, another ground-based device called a photometer will record the brightness of different colors of light in one slice of sky.

And on the science center rooftop, under a Plexiglas dome that looks like something out of My Favorite Martian, sits a low-light-level video camera with a fish-eye lens that takes in the entire sky. Images from this all-sky camera are transmitted live to every room in the science center, where they show up on monitors as a black-and-white disk, with black being sky and white being stars, clouds, or aurora. The all-sky camera is more sensitive to light than human eyesight—so sensitive that moonlight blinds it. And it has a more encompassing vantage than any single human observer could enjoy. Plus, watching the all-sky monitor inside definitely beats standing outside in the cold Alaskan night, turning in circles with your head cocked back.

As Nielsen and his colleague Dirk Lummerzheim arrive at Poker Flat during a clear Tuesday twilight recently, the first thing they look for is the all-sky monitor in the kitchen. Out in the parking lot, the air pierces bare skin like a blanket of frozen needles, and a ribbon of smoky light hangs in the heavens to the northeast, teasing. In the science center foyer, the scientists exchange their bunny boots for boiled-wool slippers that muffle their footsteps on the tile floors. The kitchen all-sky shows a narrow white band at the bottom of the disk—which, confusingly, corresponds to the northern horizon.

"When it's really good, the whole thing gets white," says Lummerzheim, nodding in the direction of the monitor.

Also keeping an eye on the kitchen all-sky is Bob Eather, a former atmospheric physicist turned filmmaker who lives in Brookline, Massachusetts. He's hoping to score some footage for an IMAX movie on the solar maximum. Eather is the father of the keogram, a now-standard photometric plot of the aurora's distribution in space and time. He has come all the way from Brookline because at this latitude, just a few degrees below the arctic circle, auroral displays are a nightly event. Poker Flat lies under the auroral zone, a doughnut-shaped expanse of sky around Earth's geomagnetic pole near Thule, Greenland. The zone occurs where Earth's magnetic field lines converge upon the atmosphere as they loop around the planet from the southern geomagnetic pole to the northern one (there's a southern auroral zone too). The field lines act as channels for charged particles, mostly electrons with some protons mixed in, moving from the magnetosphere into the ionosphere, the near-vacuumlike layer of charged particles that reflects radio waves and is one of the outermost blankets of atmospheric gases. Because the same electromagnetic processes drive the flow of particles to the northern and southern poles, auroras tend to happen simultaneously in both hemispheres.

But auroras are not created equal. Magnetic field lines can get warped, Lummerzheim explains, by interactions with the solar wind. That's why the aurora borealis hugs the northern horizon when it's quiescent; when it becomes more active, the aurora expands to lower latitudes, sometimes spreading as far south as Mexico. As it moves over greater areas, it becomes brighter, more restless, and more colorful. Scientists would love to know why the aurora is more active on some nights than on others, and in the most general terms, they do. A souped-up solar wind pointed in our direction is a necessary, but not sufficient, condition. The magnetic field carried by the solar wind must also be aligned opposite of Earth's magnetic field, which flows from south to north. Only then can the charged particles break into the magnetic sheath that usually deflects them.
Illustration by Nigel Holmes




Once they've entered, the particles stream along the outermost field lines of the magnetosphere, stretching them into a leeward tail that extends far beyond the orbit of the moon. Finally, the field lines snap back toward Earth or out into space. Then the magnetic field on the earthward side of the plasma stream rebounds, accelerating particles and tossing them into the atmosphere like a slingshot. It's the energy boost of this rebounding that drives the most memorable auroral events—the substorms that can last for hours and cause the most costly electromagnetic carnage.

That's the theory, anyway. Then there's the practice.

So far, this night's aurora is about as active as a vapor trail, and the all-sky monitor has been dark. After conferring with Eather on his video selection for the evening (The Proposition, with Kenneth Branagh), Nielsen and Lummerzheim head upstairs to check the satellite feeds. It looks as if all systems are go. The satellite data shows an incoming solar wind that is fast and dense, with a magnetic field pointing due south, counter to Earth's. The GOES graph indicates that the tail of the magnetosphere is stretched out and ready to shoot back. When Lummerzheim consults the keogram from the lab's photometer, he finds a surfeit of blue light reaching down to the south—a signature of hydrogen atoms and evidence that more protons than usual are being pumped into the ionosphere. More protons in the ionosphere equals more energy in the magnetosphere, says Lummerzheim, because protons are vastly heavier than electrons and therefore much more difficult to hurl earthward.

"There's something coming in that's transferring energy to the particles," he says. "It's definitely building." Such optimism is the blessing and the bane of auroral observers. There's a bronze plaque in the entrance lobby honoring T. Neil Davis, the former acting director of the Geophysical Institute. An inscription cites Davis's credo during long nights at Poker Flat: "I think we are in a building situation."

Lummerzheim moves across the hall to set up his equipment. Up a steep steel ladder, behind a thick curtain, on a platform beneath a broad skylight hums his spectrometer, an instrument that records the brightness of light at various wavelengths. Like the blue light of hydrogen atoms, the other colors of an auroral display correspond to the amount of energy propelling the incoming particles. "Exactly how this acceleration works is one of the areas we don't understand very well," Lummerzheim concedes. It's known that the energy of electron and proton beams can vary by many orders of magnitude. Such variations determine how far into the ionosphere the particles penetrate, as well as the kinds of collisions that ensue.

The most common auroral color, a pale green, results when electrons collide with oxygen atoms below 250 miles of altitude. More elevated encounters with low-energy electrons may also elicit a red glow from oxygen at the upper edges of a green curtain. Nitrogen molecules near the bottom of the ionosphere sometimes produce red light, too, but far fewer electrons have enough energy to get that far. Charged molecules of nitrogen at the very top or bottom of the ionosphere can emit deep violet light, but it's usually too faint to be seen.

What isn't clear is how and where the particle beams receive their energy boost. By studying the spectral signatures of auroral events, Lummerzheim hopes to refine models of energy transport and coupling between the magnetosphere and the ionosphere. Such models are still very crude: Space-weather prediction is said to be as primitive as the prediction of ordinary weather was 40 years ago. If aurora forecasters were doing the TV weather, they could state categorically that, on average, there'll be more snow in winter than in summer. But on any given winter day they couldn't really tell you whether it might snow.

The trouble is, scientists still don't fully understand what generates the fine structure and moment-to-moment changes in auroral activity. In order to decipher those details, they need instruments with better resolution in both space and time. For years, the best satellite observations of auroral displays provided 20-minute composite images every 102 minutes; with that method, many generations of auroral phenomena were conflated and others went entirely undetected. For the past decade, the National Aeronautics and Space Administration (NASA) and space agencies in other countries have deployed dozens of satellites to probe short-term interactions between the solar wind, the magnetosphere, the ionosphere, and the aurora. The IMAGE satellite NASA launched in March, for example, will take instantaneous snapshots of particle flows and auroral displays every five minutes.

Poker Flat also hosts many projects that explore small-scale and short-term auroral characteristics. Since the 1970s, researchers from the Geophysical Institute have shot rockets filled with barium ions into the aurora in order to trace out, in a luminescent blue-green visible to the naked eye, the magnetic field lines and electrical fields guiding and accelerating charged particles. With space scientists from the University of California at Berkeley, a team from the institute has measured specific properties of individual auroral events simultaneously from ground-based instruments, jet aircraft, and a satellite. Comparing these measurements should help to reveal connections between different levels of auroral structure and movement, from mini to mega and from fleeting to phlegmatic.

Nielsen is studying very fast variations in auroral structure: the so-called flickering aurora. With a suite of narrow-field and high-speed cameras mounted under a dome on the science-center roof, he tracks pulses that speed along the lower edges of auroral arcs like a curtain blowing in a very strong wind. The pulses can occur at rates as high as 200 cycles per second, so they can't be captured with ordinary cameras, which film 30 frames per second. So Nielsen built a camera that films 1,000 frames per second. When the aurora shows up, he scans it from a control room downstairs by moving a joystick that turns the equipment platform. He stops when he sees something interesting on the monitors. His quest involves a lot of time sitting in the darkened control room with his arms folded over his chest, waiting.

"The question is, what role does the ionosphere play in this game?" says Nielsen. His observations have led him to suspect that, like auroral colors, many auroral forms and motions result from atmospheric conditions rather than processes in the magnetosphere. While physicists have looked mostly to space to explain the aurora's behavior, Nielsen suspects some answers may lie much closer to home. "It's so interesting that, even though we've been at it a very long time, we still don't really understand why auroras look the way they do."

At Poker Flat, the situation has been "building" for hours, with little to show for it. Yet all the portents and omens persist. So the equipment stays on standby, while Nielsen works out a software glitch and Lummerzheim catches up on some reading.

Down in the kitchen, Eather is halfway through The Proposition when suddenly, just before midnight, the aurora takes off. Without preamble the all-sky lights up, and the experts, after checking their instruments, hasten to the observation room like eager schoolchildren. The sparkling firmament writhes with seven gray-green bands that snake from east to west and stack up north to south. On the ground, Eather forges out into the cold to man his IMAX camera. The aurora drops hints of red along its lower edges, then gently pulses as its bands begin to diffuse. It doesn't last more than a few minutes, and Lummerzheim maintains that it rates only a five on a scale from one to 10. But the display is enough to energize its not-so-tireless devotees for the next few hours. At this time of year, even the unadorned darkness seems precious, for in April Alaskan skies begin to blaze with round-the-clock sunlight, and auroral physicists are out of business.

Ultimately, and inexplicably, tonight's aurora will disappoint, retreating to the north and glowing dimly there despite the hopeful vigilance of the Poker Flat crew. And so finally, at 3:30 a.m., there's a general agreement to pack it in. As his colleagues don their boots in the foyer, Nielsen decides to make one last visit to the observation room upstairs. He's just in time to greet a wavering filament of light that has reached southward to stir the sky."I should have had the high-speed camera on this stuff," he says, in the wistful tone of a man often outwitted by electromagnetic mischief. "Well, tomorrow is another night."







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