Does lightning affect the ozone layer? What causes "sprites?" And why does "messy" lightning follow a simple lightning model?
Martin Uman of the University of Florida asked these questions yesterday at the opening of the International Conference for Atmospheric Electricity. While most of the scientists will share what they have learned in their specific areas of study, Uman instead decided to ask a few general questions about atmospheric electricity. Uman hopes to motivate discussion among all the conference scientists so they will work together to solve these atmospheric mysteries.
"I'm asking these questions because nobody ever worries about putting it all together," Uman said.
Uman's first question was: How much nitric oxide (NO) is produced by atmospheric electric discharges? It is important to study nitric oxide levels because of its impact on the ozone layer in the troposphere and stratosphere. This ozone layer protects us from harmful ultraviolet (UV) radiation, but nitric oxide can destroy ozone.
Increasing concentrations of nitric oxide and other chemicals in the Earth's atmosphere contribute to the ozone hole over Antarctica. Although most of this nitric oxide is produced by human activity, lightning also produces a small but significant amount. Still, estimates vary about exactly how much nitric oxide is produced by lightning.
"The literature about NO production is confusing," said Uman. "Everyone cites different production levels, so it's still an unsolved mystery."
Also, scientists don't really know what sort of electrical discharges produce nitric oxide. Although it is well established that lightning produces some nitric oxide, other electrical discharges such as sprites could also be a source.
Uman then asked about sprites - huge colored emissions coming from the tops of thunderclouds. Once thought to be extremely rare, high-altitude cameras have shown us that sprites are often produced by thunderstorms. While the upper portions of sprites are red, they also have wispy blue tendrils that extend downward.
According to current models, only the most powerful lightning strikes generate enough energy to produce sprites. Uman questioned whether these models are accurate representations of the energy needed to generate sprites.
"The models don't agree with the all the measurements, so there's a big debate over which is wrong," said Uman. "Are the models wrong, or do we need to get better measurements?" "The transmission line model is a case of a model that works, but probably shouldn't," Uman said.
The transmission line model for lightning shows a smooth upward curve of current. Lightning, however, is not a smooth and steady phenomena.
"Lightning is a mess," said Uman. "The transmission line model is the oldest and simplest model about lightning, so it shouldn't work. Lightning is much more complicated than that."
Lightning does not follow a simple path across the sky. For instance, downward lightning can meet upward lighting in the middle of a cloud. Despite the 'messiness' of lightning, it somehow still obeys the simple current curve of the transmission line model.
Uman asked these questions because he believes most theorists are too intent on developing their own models. By posing these questions at the International Conference on Atmospheric Electricity, Uman hopes to stimulate further thought about the physics of lightning. When these questions are answered, they could fundamentally alter our understanding of how electricity interacts with the atmosphere.