Sprites are large, brief flashes of light that occur very high in the atmosphere above large thunderstorms. Instead of discharging toward the earth like lightning, sprites soar upward above a thunderstorm and occur immediately following strong lightning strokes.
University of Houston physics professors Edgar Bering and James Benbrook, along with their students, collected sprite data during a balloon campaign in the summer of 1999 when several balloons equipped with special detectors flew high into the atmosphere - around 20 miles up - over Texas and Iowa. The experiments were intended to study the electromagnetic signature of the lightning strokes that produce sprites.
"One of the more interesting things we discovered is that every lightning stroke tries to produce a sprite in the sense that it produces a similar but weaker electrodynamic pulse in the mesosphere," Bering says.
The layers of the atmosphere consist of the troposphere, which extends from the ground to about nine miles up; the stratosphere, beginning just above the troposphere and extending to 31 miles high; the mesosphere, extending from the stratosphere to 53 miles high; and the thermosphere, extending beyond the mesosphere to about 372 miles.
Previous research has shown that most sprites are produced by positive cloud to ground lightning strokes, which are more rare than negative cloud to ground lightning strokes, Bering says. Negative cloud to ground lightning strokes are initiated by a large concentration of negative charge in the cloud base, which tends to induce an area of positive charge on the ground, resulting in a discharge of electricity - lightning. A positive lightning stroke is exactly the opposite, with a positive charge concentration in the cloud inducing a negatively charged area on the ground.
Bering and his colleagues also found that negative cloud to ground strokes produce a phenomenon that is not often observed from the ground, termed a sprite halo, which is basically a sprite precursor.
"We discovered that seven to ten times as many negative cloud to ground strokes produce sprite halos as do positive cloud to ground strokes. That, coupled with the fact that every cloud to ground stroke, positive or negative, tries to produce a sprite or sprite halo, indicates that the amount of energy being deposited in the mesosphere by these sprite processes and related processes exceeds what we thought the sprites did by a factor of 50."
Bering says that amount of energy is comparable to the amount of energy the sun pumps into that same volume of atmosphere above the thunderstorm in daylight hours.
Bering will present an invited talk on the research findings at the World Space Congress 2002, to be held Oct. 10-19 in Houston.
The closest the balloon flights got to sprite-producing thunderstorms was about 300 kilometers, or 186 miles, which limited the amount of useful data the scientists could collect, Bering says. He cautions that the experiments need to be repeated because the results depended only on observations from a few storms. However, he says the results of the balloon flights indicate our understanding of the mesosphere is incomplete.
"It means we actually have at certain times and latitudes about a factor of two discrepancy in the energy budget of the mesosphere. From the standpoint of global understanding of the atmosphere as a whole, a factor of two in an energy budget is nontrivial," Bering says.
While the mesosphere does not directly affect weather on earth, and the altitude is too high for precipitation-producing clouds, some researchers are attempting to use mesospheric weather as a tracker for global temperature change.
Bering says the 1999 data suggests two avenues for future studies.
"We need to get closer to the storms, which requires both more balloons and also is a matter of luck, and put additional data collection equipment on board the balloons," he says.
The National Aeronautics and Space Administration funded the 1999 balloon flights. Bering has submitted a proposal to NASA for additional balloon studies of sprites.
In January, Bering will begin a study of phenomena other than sprites when he sends three balloons into the air over Antarctica to investigate the electrodynamics of the polar ionosphere.
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