BY DAWN LEVY
Lightning dazzles and amazes as it lights a night sky. Related displays of atmospheric electricity -- 'red sprites' and 'blue jets' -- inspire awe too. But less is known about these natural illuminations, which occur at high altitudes, where the thin atmosphere makes them ghostly and special instrumentation is sometimes needed to view them.
On Sept. 14, 2001, scientists saw -- and caught on video -- a gigantic flash of electricity that discharged from the top of a thundercloud and reached all the way up to the lower ionosphere. This image is enhanced with false color, as the event was recorded using a monochrome low-light video system that does not display the blue color that the researchers saw. Image courtesy of Pasko et al. 2002, reprinted courtesy of Nature.
Though sprite sightings are now common, blue jets have remained elusive -- until now. In the March 14 issue of the journal Nature, scientists at Penn State, New Mexico Tech and Stanford report that they caught one on video. It's the first direct evidence that electricity can discharge from the top of a thundercloud and reach all the way up to the lower ionosphere, the electrically conductive layer that encircles Earth, bends electromagnetic waves and influences global communications. This previously undocumented route may prove to be a common path in the global electric circuit.
"It's the first time that a blue jet has been observed from
the ground," says Stanford electrical engineering Professor Umran
Inan, a pioneer in the use of very low frequency radio waves to
study the electrical environment of the Earth's atmosphere.
"Previous observations of blue jets fizzled earlier in altitude.
Previously, we've seen a blue jet start from the cloud and reach
maybe 40, 45 kilometers [25, 28 miles]. The ionosphere is at 75, 80
kilometers [47, 50 miles]. This one reaches all the way up."
The lead author on the Nature paper is Victor Pasko, an associate professor of electrical engineering at Penn State who completed his Ph.D. and postdoctoral work on sprites with Inan at Stanford. His co-authors are Inan, John Matthews of Penn State, Mark Stanley of New Mexico Tech, and fifth-year doctoral student Troy Wood of Stanford. The research was sponsored by the National Science Foundation.
When clouds discharge static, only a small fraction of electricity travels to the ground as a lightning bolt, Wood says. It's more common for lightning to shoot between clouds, and even more common for it to crackle within clouds.
"The first recorded image of an optical flash above a thunderstorm was obtained serendipitously on July 5, 1989," says Pasko. "However, pilots and others reported observations of sprites and blue jets long before the first one was captured on video, and numerous undocumented reports of similar phenomena have appeared in scientific literature for over a century."
Sprites happen. So do blue jets, such as this one, caught on video for the first time ever. Electricity discharges from the top of a thundercloud, traveling in filamentary paths that branch more and more with increasing altitude. Hot spots are apparent when the electricity reaches the lower ionosphere. Image courtesy of Pasko et al. 2002, reprinted courtesy of Nature.
Over the past decade, many researchers have looked for sprites and blue jets from the ground, air and space. Of the two types of vertical flashes that have been identified above thunderclouds, sprites start near the base of the ionosphere and shoot downward at speeds that can exceed 10 million meters per second (about 33 million feet per second). They are predominantly red and come in a variety of shapes. Blue jets, in contrast, shoot upward from cloud tops at 100,000 meters per second (about 330,000 feet per second) and are predominantly blue and conical.
People used to think that blue jets could only be seen from airplanes. Blue light has the shortest wavelength of all colors, making it easiest for air to scatter -- that's why the sky is blue. But that also makes it difficult to spot blue jets from the ground, as air scatters blue light coming from the upper atmosphere. Special equipment made it possible for the researchers to view their blue jet from the ground. A video camera equipped with a light intensifier provided by ITT Night Vision Industries allowed them to spot a storm 201 kilometers (125 miles) away at night.
On Sept. 14, the researchers were about to experience one of those "happy accidents" for which science is famed. They had set up an experiment at the Arecibo Observatory in Puerto Rico to study the electromagnetic behavior of sprites. To monitor different parts of the electromagnetic spectrum in the upper atmosphere, the Penn State researchers employed optical instruments, and the Stanford and New Mexico Tech researchers, radio instruments.
That night, Pasko and Stanley were looking for a sprite above a small thunderstorm 201 kilometers (125 miles) out at sea. Instead, they were surprised to catch a jet.
Starting at a cloud top 16 kilometers (10 miles) in altitude, a tree of fire shot up, its lightning branching more and more as it rose to a terminal height of 70 kilometers (43 miles) -- the point at which electrical conductivity drops off sharply in equatorial regions. It looked like an electrified palm tree, with a bottom that was smooth and filamentary and a top that was dominated by hot spots.
Says Inan of the radio portion of the experiment: "Every lightning discharge is like an antenna, and it radiates an electromagnetic pulse. We can measure the electromagnetic pulse both locally and also at very far distances." One of their data detectors, at Palmer Station in Antarctica, identified the blue jet's electromagnetic "fingerprint" from more than 10,000 kilometers (about 6,000 miles) away.
The images revealed the electrical structure of a blue jet in intricate detail, and that data will inspire computer modeling to better understand atmospheric dynamics, Inan says.
At high altitudes the atmospheric conductivity is not very well known. "That's one of the reasons why these things are interesting," Inan says. "These altitudes are not easy to measure because satellites cannot stay in orbit -- the friction of air is too high. And it's much higher than airplanes or balloons can fly because the air is too thin. So you can't get there from here, except for these remote measurements."
The finding reported in the Nature paper shows that researchers do not fully understand the global electric circuit, Inan says. And it's hard to draw conclusions about blue jets based on a single captured event. "It might be that they are produced every night in every storm, or only in very special types of storms," he says. Blue jets may be specific to the tropics, Pasko suggests.
That may be why scientists never caught one before. Since the
early '90s, they have looked for sprites extensively in Colorado,
New Mexico and Mexico. "A lot of cameras were looking for hours and
hours every night at sprites and phenomena that are related," Inan
recalls. "We didn't see anything like this (blue jet)."
"These altitudes are not easy to measure because satellites
cannot stay in orbit -- the friction of air is too high."
Stanford Report, March 20, 2002