Nature
This blue jet reached from a cloud top to an altitude of 70 kilometers. Electrical engineer Victor Pasko and three colleagues captured the phenomenon using a low-light camera; their original monochrome image is reproduced here in false color.
In September 2001, Victor Pasko spent 20 nights on a roof in Puerto Rico, waiting for storms powerful enough to spawn the mysterious middle-atmosphere electrical events known as red sprites and blue jets. Pasko, a Penn State electrical engineer, sat on a wooden hatch cover on the flat concrete roof of a laboratory on the grounds of Arecibo Observatory. In front of him, his laptop projected a stream of images from a tripod-mounted low-light video camera, also on the roof. Pasko's gaze shifted from the computer's screen to the dark tropical sky where stars twinkled, clouds billowed and blotted the stars out, and lightning wrinkled across the heavens and arced to earth. Insects stridulated in the jungle around the building. Night birds called. Thunder rumbled.
As Pasko waited, the GOES 8 weather satellite with which he was in contact picked up storms being born hundreds of kilometers away. It tracked cells towering above the Dominican Republic, anvil-headed clouds sending down rain and hail on Haiti, on the waters of the Atlantic Ocean to the north and the Caribbean Sea to the south. When the satellite detected a promising storm, Pasko and Mark Stanley, an atmospheric physicist from New Mexico Tech, would aim the video camera at it. The camera was equipped with a nightvision intensifier similar to the light-gathering telescopes and goggles used by soldiers and police.
At times Pasko stayed up all night, since the potent storms that could give rise to sprites and jets often did not develop until three or four in the morning. Stanley had also deployed a VHF (very high frequency) interferometer and a flat-plate electric field antenna on another Puerto Rican rooftop—this one capping a salmon-colored, ramshackle auto repair shop four miles from the observatory. The interferometer mapped VHF emissions, letting the researchers locate stormcloud lightning, while the field antenna recorded electromagnetic radiation generated by lightning, data the researchers hoped would yield insights into any sprites or jets they might luck into. A third team member was Troy Wood, a graduate student in Stanford's Space, Telecommunications, and Radioscience Laboratory, where Pasko had earned a Ph.D. and done postdoctoral work from 1992 to 2000 before joining the Communications and Space Sciences Laboratory at Penn State. Wood had installed another diagnostic device, a VLF (very low frequency) receiver outside the hotel where he was staying on Vieques Island, east of Puerto Rico. Rounding out the group was Penn State electrical engineer John Mathews, co-principal investigator on the project with Pasko. Mathews had helped plan and set up the detecting and recording array and was responsible for operations related to the observatory's 430 megahertz UHF (ultra high frequency) radar.
The team had been granted two to three hours per day on the UHF radar, one of the world's most powerful radar-radio telescopes. Operated by Cornell University under a cooperative agreement with the National Science Foundation, the radar is in demand from a host of researchers studying everything from extrasolar dust to plasma turbulence to atmospheric electricity. Pasko and his colleagues used their radar time to examine lightning and lightning-caused perturbations in the ionosphere, a region of charged particles surrounding our planet and extending from around 70 to more than 500 kilometers in altitude. The ionosphere, created primarily by ionizing ultraviolet radiation from the sun, deflects radio waves, allowing them to propagate around the curve of the earth's surface.
On the evening of September 14, the weather satellite picked up a storm gathering to the northwest. By 11:20 p.m., the system was poised above the Atlantic, pounding down lightning 200 kilometers away. Pasko and Stanley, who had joined him on the roof, could see the clouds and lightning, but the action was too far off for them to hear any thunder. They directed the camera's field of view above the cloud top.
At 11:25, a blue light rose from the cloud. Its speed suddenly increased, and it split into two brilliant spikes that themselves branched intricately while streaming even higher. The display flashed twice, then subsided, leaving bright spots that glowed for a moment before dying.
"We just started screaming," Pasko recalls today, sitting in his office on the Penn State campus. "It was the most spectacular thing I've ever seen." Stanley, quoted in a publication of Los Alamos National Laboratory, where he now works, said: "It was the most fascinating color; I can't even describe it, I'd never seen that kind of blue before."
"We had to wait until morning," says Pasko, "to find out if the camera actually recorded the event." In fact, the blue jet showed up on 24 consecutive video frames, each lasting 33 milliseconds, for a total span of eight tenths of a second. Pasko runs the video in slow motion on his office computer; runs it again. "This is the first time a blue jet was caught on video from the ground. It may be a once-in-a-lifetime experience, seeing this with the naked eye. We were very lucky to be looking in the right direction."
Although the blue jet came from a relatively small thunderstorm, it was immense, filling an estimated 6,000 cubic kilometers of the atmosphere. And it did something no one had ever seen a blue jet do before: It reached all the way to the ionosphere, 70 kilometers up.
Hans Stenbaek-Nielsen
Most red sprites last for one to ten milliseconds. They propagate downward, starting at an altitude of approximately 90 kilometers.
For more than a century, especially after humans began flying in airplanes, people have reported strange goings-on above thunderstorms—what one atmospheric scientist and a collector of such observations termed "energetic and anomalous electrical activity."
A Canadian Pacific Airlines pilot saw the following on a night flight near Fiji in 1950: "Out from the top of the cloud shot a burst of light like a firework display. The flash took several seconds to reach its maximum height at about 3,000 feet above the cloud top; it was not just a burst of light but rather a series of streamers extending from a single point at the center of the anvil and spreading out like a water fountain."
A U.S. Air Force pilot recorded this sighting after a 1965 flight from Montana to Washington, D.C.: "The night was clear, and the nearby full moon was very bright. Approaching a line of northeast to southwest thunderstorms in eastern Indiana, there was considerable lightning activity. The clouds were white from the moonlight but fiery inside. . . . About a mile or two from the nearest top, a bolt of lightning came straight out the top and went to a point about 2,000 feet above us and shattered in all directions as an egg would do if it were thrown against a ceiling. . . . We climbed to 41,000 feet, and we observed the phenomenon seven or eight more times. The next day we read in the newspaper that a line of thunderstorms had spawned many tornadoes."
People who have seen these events—which, until recently, were variously called "rocket lightning," "upward lightning," "cloud-to-stratosphere lightning," and "cloud-to-space lightning"—often remarked on their striking, luminous colors, including blue, blue-white, green, purple, and several shades of red. Charles Thomson Rees Wilson was a Scot who received a Nobel Prize in 1927 for inventing a cloud chamber, a device to detect the track from energetic particles in a supersaturated vapor; his conclusions regarding the interactions between air and electricity contributed greatly to our understanding of storms and lightning. In a paper published in 1956, when he was 87 years old, Wilson wrote: "There have been a number of reports of ordinary lightning discharges extending upwards from the top of a thundercloud. . . . Many years ago I observed what appeared to be discharges of this kind from a thundercloud below the horizon. There were diffuse fan-shaped flashes of greenish color extending up into a clear sky." Wilson suggested that "a lightning discharge to earth from the base of the cloud may be initiated by a discharge above the cloud." He also wrote: "The electric field of the cloud may cause ionization at great heights, the result being continuous or discontinuous discharge between the cloud and the upper atmosphere."
Despite the persistent pilots' reports, and even with a Nobel Prize winner weighing in, few scientists gave the sightings credence. Then in 1989, while testing a low-light television camera for NASA, space physicist John Winckler of the University of Minnesota caught a cloud-top discharge on videotape: two giant pillars of light flashing above a distant storm. "It was totally serendipitous," Pasko says. "And it became a threshold for a new field of science."
NASA, concerned that such electrical phenomena might endanger spacecraft during launching and re-entry through the atmosphere, reviewed videotapes taken by automated payload-bay cameras during space shuttle missions. They identified more than a dozen flashes above thunderstorms that seemed to match the Winckler video.
In July 1993, the independent researcher Walter Lyons set up low-light cameras at his Yucca Ridge Field Station, a house with a rooftop deck perched on the front range of the Rocky Mountains near Fort Collins, Colorado. At an elevation of 1,690 meters (5,544 feet), the station looks out over an expanse of the Great Plains taking in all or parts of nearly a score of states; it is an excellent place from which to observe the clusters of thunderstorms known as "mesoscale convective complexes," powerful storms spread out over hundred or even thousands of kilometers, storms that spawn intense, repeated lightning. During a single night, Lyons documented 240 cloud-top flashes. He wrote in the Monthly Weather Review: "Cloud-to-stratosphere events occur far more frequently than previously suggested and are perhaps a ubiquitous aspect of many larger convective systems." Most of the flashes were extremely brief and too dim to be seen by human eyes; however, Lyons spotted one and described it as "a gossamer, orangered, aurora-like curtain."
The following summer, researchers from the University of Alaska flew a pair of midsize corporate jets out of Oklahoma City, photographing storms in that vast weather-brewing stretch of middle America from Colorado to Arkansas and from Texas to South Dakota. They captured the first sharp color images of the transient displays, of which there appeared to be two distinct types. In twin papers published in the May 15, 1995, issue of Geophysical Research Letters, Davis Sentman, one of the Alaska investigators, proposed calling one class of discharges "sprites," after the fleeting, mysterious fairies populating Shakespeare's The Tempest; his colleague Eugene Westcott advanced the name "blue jets" for phenomena of the second category.
Pasko was then at Stanford, studying the chemistry of the middle and upper atmosphere. He published a paper in Geophysical Research Letters suggesting a basic theory for the optical emissions now known as red sprites: They are caused by quasi-electrostatic fields that, following lightning flashes, temporarily heat up electrons, producing ionization and light. The theory accounted for the duration, color, and spatial extent of sprites. It became Pasko's Ph.D. topic and helped earn him a National Science Foundation postdoctoral award, as well as a Young Scientist Award from the International Union of Radio Science. And it placed him squarely in a field of atmospheric science where, it seemed, discoveries were being made almost daily.
Victor Pasko and Hans Stenbaek-Nielsen
The theoretical model on the left represents vertical altitude structuring in red sprites. Center, a video observation of a large sprite. At right, a fractal model of a sprite.
What do we know, so far, about sprites and jets? The field is in its infancy, with researchers striving to understand the phenomena based on concepts advanced by scientists such as the Nobel laureate Wilson, who long ago proposed his theory—Pasko terms it "simple and elegant"—linking conventional lightning, luminous discharges above storms, and electrical fields in the middle and upper atmosphere. Worldwide, around 1,800 thunderstorms rumble at any given moment, with lightning striking the earth 100 times a second. Lightning detection systems in the United States record an average of 25 million cloud-to-ground strikes annually. Although they cannot yet forecast the location or timing of lightning, scientists have gained a basic understanding of this awe-inspiring force, which the National Weather Service calls "a random, chaotic, and dangerous fact of nature."
During a typical thunderstorm, one lightning stroke in five hits the ground; the others flash inside a cloud, from one cloud to another, or into the surrounding air. A lightning bolt is a channel of electricity an inch or two across and 200 feet to 40 miles long. Its current can be 30,000 amperes. Its temperature can reach 50,000 degrees F., four times as hot as the surface of the sun. Thunder is the shock wave caused by the sudden expansion of that ribbon of lightning-heated air.
Thunderstorms develop when air rises rapidly, its moisture condensing to form a cloud. Clouds can build to heights of 35,000 to 60,000 feet; cold temperatures at those altitudes freeze water into ice particles ranging in size from small crystals to large hailstones. In the turbulence of a swiftly ascending cloud, ice particles, raindrops, and cloud droplets churn and collide, picking up electrical charges. In general, the lighter, positively charged particles float to the top of the cloud, while negatively charged ones, including large raindrops and hail, sift to the lower zones.
As a cloud marches across the land, its negatively charged base attracts a pool of positive charge on the ground. From the cloud, a "stepped leader" edges downward—stepping, pausing, stepping again. When it nears the ground, a positive charge rises to meet it. The two opposing charges unite, closing the circuit, and a vivid return stroke rushes back to the cloud. As the return stroke subsides, secondary strokes from the cloud retrace the main channel, striking the earth two, three, as many as a dozen times. The eye can barely make out these speedy strokes, which is why lightning seems to flicker.
Not all lightning is unleashed by the lower, negatively charged cloud region. Lightning can also shoot out of the upper part of a thunderhead, that glowering, fast-rising zone known as the anvil. This sort of lightning typically carries a positive charge. Positive lightning can reach many miles from the storm, well beyond any rain, and its strong electrical current can ignite fires and strike people dead. Lightning kills an average of 73 Americans annually—more than tornadoes or hurricanes—and injures another 300.
"We don't know exactly how red sprites are triggered," says Pasko, "but they're almost always associated with intense positive cloud-to-ground lightning."
According to Pasko, the 1989 event that John Winckler fortuitously photographed in Minnesota was a sprite: "It was relatively short and impulsive. Most sprites last from one to ten milliseconds." A millisecond is a thousandth of a second. To actually see a red sprite, you have to be looking straight at it: there's no time to spot one peripherally, then shift your gaze. The odds of viewing one from the ground are not great. Unless you can get up high, as at the Yucca Ridge Field Station in Colorado, clouds generally stand between you and any sprites or jets.
"The top of a sprite is red, converting to blue or purple below," Pasko says. Sprites propagate downward, starting at an altitude of approximately 90 kilometers. Traveling at 10 million meters per second, they terminate at around 40 kilometers of altitude. Rarely do sprites appear singly; usually they flash in clusters of two, three, or more, and they show a variety of complex shapes. Since the Winckler video, researchers have photographed more than a thousand sprites. During his 2001 vigil in Puerto Rico, Pasko videotaped eight: seven linked to positive cloud-to-ground lightning, as shown by their electromagnetic signatures, and an anomalous eighth coinciding with a stroke of negative lightning.
"While lightning seems to be the root cause of sprites," says Pasko, "no one has discovered a direct link between lightning and blue jets." Blue jets behave differently from sprites. On their upward trajectory, they move more slowly, on the order of 100,000 meters per second. A typical blue jet lasts several tenths of a second, fountaining up as high as 42 kilometers—around the altitude where red sprites cease their downward journeying.
"Blue jets appear following the fast growth of positive charge near the thundercloud top," Pasko says. "Hail may actually trigger the event." When hail falls, it removes a large quantity of negative charge from a thunderstorm. "Positive current then flows upward, creating a strong upwardly directed field at the cloud top. The field may then launch filaments of ionization filling giant cone-shaped volumes of the atmosphere."
From his vantage point 200 kilometers away, the blue jet that Pasko videotaped "looked huge. And in rising to 70 kilometers, it was taller than anything recorded before." A frame from the video made the cover of the 14 March 2002 Nature, and the journal ran a report, "Electrical Discharge from a Thundercloud Top to the Lower Ionosphere," by Pasko and his colleagues.
Says Pasko, "The phenomenon actually displayed traits of both jets and sprites." It started slowly, like a jet, climbing in a single smooth filament. When it reached 42 kilometers, it rebrightened and sped up tremendously while ramifying into branching discharge trees, which dwindled to become hot spots: Scientists have observed a similar sequence following the occurrence of large groups of sprites.
Data from the GOES 8 weather satellite combined with the positions of stars recorded on the video let the researchers accurately gauge the event's altitude, extent, and speed: about 50 kilometers per second in its early phase, increasing to 160, then 270—and finally a blistering 2000 kilometers per second.
"During the blue jet initiation," says Pasko, "the electrical field antenna did not detect an electromagnetic pulse indicating that lightning was taking place." But when the jet split into two columns and raced upward, "the data tell us that a negative charge was going up—we're still discussing what that might mean. We don't know if the charge came from the blue jet, or from lightning beneath the storm."
The VLF receiver operated by Troy Wood on Vieques Island shed no light on the event. Pasko smiles somewhat sheepishly as he explains: "On the nights that we videotaped the sprites and th jet, the power happened to be down on Vieques, so we lost a chance to pick up some interesting data. Next time we'll take along an uninterrupted power supply."
These days, Pasko and his graduate students are studying the vertical propagation of sprites and jets. By integrating data from lab experiments with videos of actual events, they can model the potential electrical properties of the phenomena, down to the microphysics of their streamers.
"We're looking at the process on the level of individual electrons," Pasko says. "It's like an avalanche that can go both ways, down or up. But instead of gravity, there's an electrical field powering the process." The electrical field causes electrons to accelerate and collide with neutral air molecules. The molecules become ionized, producing new electrons that also are accelerated. These electrons excite various states of nitrogen and oxygen molecules, which then emit light.
"The same types of molecules are excited in both sprites and jets," Pasko says. "But at the lower altitudes where jets occur, the excited molecules have a greater number of ambient gas molecules to run into." These "quenching collisions" reduce the number of molecules responsible for red light production, which is why jets are blue and why blue or purplish colors appear at the bottoms of sprites.
Sprites and jets occupy volumes of the atmosphere many orders of magnitude greater than those of conventional lightning. Because they're so diffuse, and because the middle and upper atmosphere is much thinner than the lower atmosphere, sprites and jets cannot superheat localized areas, as lightning does, and therefore do not create the shock waves that reach our ears as thunder. According to Pasko, only the lowest portions of blue jets, sometimes called blue starters, may heat the air enough to produce sound.
Silent though they may be, sprites and jets "probably have an immense impact on the chemistry of the atmosphere," Pasko says. By now, scientists have observed them all over the globe: in Europe, Japan, Australia, North America, Central America, South America—just recently, above the ocean near the Philippines. The blue jet that Pasko videotaped came from a run-of-the-mill thunderstorm, implying that many such events may take place daily.
Scientists believe the electrical currents of sprites and jets may spur reactions between atmospheric gases. One theory suggests that they affect the planet's protective ozone layer, possibly contributing to ozone production. Says Pasko, "The beauty of this field is that it's bringing together a range of specialists from around the world—electrical and computer engineers, geophysicists, earth and space scientists, meteorologists. And it's linking research into different atmospheric regions, from the lower atmosphere to the ionosphere."
Pasko is particularly curious about how sprites and jets fit into the global electrical circuit. "Think of the ionosphere and the surface of the earth as plates of a giant spherical capacitor," he says. "The two plates are oppositely charged, with a 300,000-volt difference between them. The atmosphere in between is a weak conductor that is constantly trying to solve the disparity between the two plates.
"Sprites and jets may help to discharge the system. Or they may play a role in charging it. We don't know yet. We don't know how frequently they occur, and we're just beginning to study their internal physics. But we can be certain now that these phenomena establish a connection between storms and the ionosphere. The blue jet that we caught on video is evidence of a direct link."
Victor Pasko, Ph.D., is associate professor of electrical engineering in the College of Engineering, 211B Electrical Engineering East, University Park, PA 16802; 814-865-3467; vpasko@psu.edu. To see his videotape of the blue jet, and for links to other pertinent information, go to http://pasko.ee.psu.edu/Nature/. The National Science Foundation supported Pasko's 2001 campaign in Puerto Rico with a Small Grant for Exploratory Research; in 2002, the foundation awarded Pasko a $350,000 Faculty Early Career Development Grant. ITT Night Vision Industries provided light-intensifying equipment. For more information on sprites, jets, and other storm-related phenomena, see an article, co-authored by Pasko and four colleagues, "Upward Electrical Discharges from Thunderstorm Tops," in the April 2003 Bulletin of the American Meteorological Society.