Nowadays, that has all changed, however. In the very hard-core atmospheric research arena, the luminous discharges are taking center stage. And scientists no longer think that the elusive flashes are rare or isolated events. In fact, in the past couple of years, scientists from four institutions have put more than 1000 sightings of the so-called "sprites" in the files.
"There's no reason to believe this isn't a worldwide phenomenon," says Bill Boeck of Niagara University. In addition to these well-documented sightings of flashes over the Midwest, the Space Shuttle has recorded flashes over Australia, South America, the South Pacific Ocean, the Atlantic, and Africa.
Likely, the sprites occur around the world at a rate of at least one flash per minute, estimates Boeck, who along with other investigators in pursuit of the flashy phenomena presented the latest findings at AGU's Fall Meeting here.
However, the acceptance of these events by the mainstream atmospheric research community hasn't made unlocking the secrets of Earth's heavens any easier. In fact, the new findings have only revealed how complex the atmosphere and its inherent energetics are, further perplexing scientists.
The significance of the optical flashes seems to run the gamut of possibilities. Some scientists believe that the flashes may be a key to understanding the ionosphere. Others say that the flashes may affect production of oxides and nitrates in the ozone equation. And there is even the possibility that the flashes may be hazardous to some space- and aircraft, says meteorologist Walter Lyons of the Mission Research Corporation (MRC) in Fort Collins, Colo. Although the question remains wide open, Lyons says that he "wouldn't volunteer to be the first astronaut to fly through one."
So, just what are these flashes? "Quite frankly we don't know," admits Steve Goodman of NASA's Marshall Center's Space Sciences Laboratory. But the investigators reported at AGU's Fall Meeting that progress is being made toward putting the pieces in place.
In late June and early July of this year, a team of scientists was able to capture the sprites in color for the first time with fine-tuned, low light-level cameras loaded aboard two jet aircraft that flew over storm clouds in the Midwest—the only part of the world to date where the flashes have been filmed in this manner. The color pictures revealed that the flashes were not merely some type of upward electrical discharge but a dazzling array of fireworklike constructs, which seem to dance for milliseconds above the clouds.
From the flight data, two distinct types of flashes seem to stand out: sprites, which are predominantly blood red in color and occur about 90 km above storm clouds, and blue jets, which are narrowly collimated sprays or fans of light that begin at the top of the anvil and propagate upward to about 45 km at speeds of about 100 km/s (Figure 1).
Fig. 1. A family of sprites captured over the Midwest on July 6 at 12:29 a.m. with a wide angle, low-light level, black and white television camera. By using triangulation of the images of the same sprites taken from the two aircraft, scientists were able to determine the height and size of the sprites. The thunderstorm's cloud top is about 42,000 feet (12,805 m) high. The sprites reach an altitude of about 90 km. (Photo credit: Dan Osborne, Geophysical Institute, University of Alaska, Fairbanks)
Although there are visual accounts of sprites of many different colors, including salmon, green, yellow, and pink, "it's difficult to know whether that is their real color or a trick of the human eye," MRC's Lyons says.
Atmospheric chemistry models suggest that enough energy is deposited in the stratosphere and mesosphere to accelerate electrons in the fields between the thunderstorm and ionosphere to the point that the electron distribution of the energetic tail is sufficient to induce ionization of atmospheric elements, explains MRC's Russ Armstrong.
Nitrogen and oxygen are the main molecules found at these altitudes. It just so happens that ionization of oxygen produces a red emission, according to conventional knowledge of energetic chemical channels. The first positive of nitrogen also generates a red emission. And the first negative of nitrogen is blue, while weaker or early signals are green. Certain densities of oxygen also emit green light when they are ionized. This type of green emission is commonly observed in auroral phenomena, Armstrong says.
However, at this point there is not enough data for atmospheric chemists to be able to differentiate the various emissions.
What's more, scientists do not know whether there is a physical difference between sprites and jets. But they do know that there is not a direct relationship between the two, explains Eugene Wescott, a member of a leading sprite-chasing team at the Geophysical Institute at the University of Alaska, Fairbanks.
Scientists have not had time to study the ultraviolet and infrared emissions of the flashes, which they suspect will provide the most telling analysis of all. From existing data, scientists have determined that most sprites—but not jets—seem to have a unique—though not strongly distinctive—radio signature. However, not all sprites have exhibited a distinctive radio signature.
In early January of 1992, scientists also described another type of showy species as an "air glow flash," or a horizontal illumination at the bottom of the ionosphere that reaches it's maximum brightness just before the lightning flash [Geophys. Res. Lett. 19(2), 1992].
Meanwhile, other teams of scientists have been observing unusual phenomena—besides optical flashes—that are also associated with thunderstorms.
In May of this year, a team of investigators reported that the Burst and Transient Source Experiment—purely by accident— observed about 14 gamma ray flashes in the upper atmosphere that could also be linked to storm patterns [Science, 264, 1250, 1994]. Since the discovery, other experiments aboard the orbiting Compton Gamma Ray Observatory have been recording the bursts on a weekly basis in the region on both sides of the equator.
Most curiously, Los Alamos scientist Bill Feldman reported at AGU's meeting that gamma ray bursts in equatorial latitudes increased in frequency after Mt. Pinatubo erupted. However, the retrospective study revealed there was no change in the frequency of bursts after an enormous solar disturbance in March 1991. "This tells us that Earth's atmosphere must be doing something," Feldman says.
Another set of phenomena may also be a part of this mystery. About a year ago, a radio detection experiment aboard a Los Alamos National Laboratory satellite first picked up twin bursts of radio energy, named transionospheric pulse pairs (TIPPs).
"These are the strongest signals we've ever seen from Earth," says Los Alamos' Dan Holden, who believes that there is a link between thunderstorms and TIPPs. Over the past year, the satellite has detected hundreds of these bursts, which are about 10,000 times as strong as the typical radio signals associated with conventional lightning. Like the optical flashes, the radio signals can be found worldwide, Holden says.
Generally, scientists tend to be leaning toward the idea that there is some overriding mechanism connecting the unusual flashes and bursts to each other and to thunderstorm activity, but it is equally likely that the events are distinct, unrelated phenomena, says Marshall's Goodman.
From the start, the leading challenge has been to explain how not only the optical flashes are generated, but how the electrons could reach gamma ray energy levels, which are about 30 times more potent than those normally found in Earth's atmosphere. Essentially, three classes of models solidified at the meeting.
The first group suggests that a single cosmic ray colliding into an air molecule might spark a runaway atmospheric breakdown. The resulting high-energy electron then propagates the energetic avalanche, as it races up along electric field lines and continues to collide with other air molecules, freeing other electrons which in turn join the process, explain Los Alamos scientists Yuri Taranenko and Bob Roussel-Dupre, who are championing a leading runaway model. In less than a thousandth of a second, the upward cascade climbs more than 50 km and extends about 20 km across. When the electrons hit the ambient molecules, the collateral effect is the generation of optical emissions, X rays, gamma rays, and radio emissions, Taranenko says.
The other two types of models center around a regular type of heating process that changes the conductance fields of the storm clouds. In a large convective system, a huge buildup of positive charge may occur within the anvil that is independent of the convective core, explains MRC's Lyons. The spiderlike network of charge is more like a tree than a channel, Lyons says. The electrostatic component of the models are then coupled to the electric field in a couple of different ways.
Another mystery has been what happens to all the energy in the system. In the case of the optical flashes, a lot more energy—likely in the form of heat—must be released than what is seen in the visible spectrum.
"If it's similar to aurora, it could be that a lot of heat is released causing changes to atmosphere," Alaska's Wescott hypothesizes.
Scientists are also trying to figure out how much energy is involved and where it is deposited in the atmosphere.
Initially, scientists thought that the sprites extended only to stratosphere, but now they know that they extend to the ionosphere.
"This in itself is quite a surprise," says Davis Sentman of Fairbanks Geophysical Institute.
Scientists are also debating whether sprites can be successfully predicted. A strong correlation between sprites and Q bursts, or electromagnetic transients, was reported at the meeting by researchers at the Massachusetts Institute of Technology who used Schumann resonance detection. Yet many questions surround the Q bursts themselves, such as what meteorologic conditions are necessary for the bursts to occur.
There are also other factors that seem to be indicative of sprites. Generally, they are associated with large convection storm systems that are 25,000 km2 or more. However, pilots have reported flashes associated with small storms. The flashes have also been linked to positive cloud-to-ground strokes of lightning that occur when positive ions reverse direction, typically during the later parts of a storm, MRC's Lyons explains. When the strokes were negative, the scientists saw sprites only about 11% of the time.
At the AGU Fall Meeting, scientists from many disciplines, including space physics and atmospheric physics, began planning a field experiment for 1996 that would take a comprehensive look at the strange and mysterious phenomena.
In the meantime, the sprite watchers from the University of Alaska are planning to use their special all-sky camera and aircraft to look for sprites and jets over Brazil in February of 1995. Later on in the year, the Los Alamos researchers are planning to launch a satellite that will combine Blackbeard radio experiment technologies with various imaging and magnetic signal detecting devices.
"Hopefully, two years from now we'll have a lot more answers," says Marshall's Goodman. And indeed, it seems that even a few answers would be welcome news in the face of the mounting mysteries presented by Earth's atmosphere.
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