Spirits of Another Sort
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and Mysterious Sprites
In our own century, pilots reported seeing strange lights in the sky above thunderstorms. Low-light cameras on board airplanes and the Space Shuttle have recently made possible the documentation of sprites - quick red flashes of light that appear above storm clouds.
Dr. Dave Sentman of the University of Alaska, Fairbanks is one among a small group of researchers who have been studying these mysterious bursts of colored light. Although no one is sure what sprites really are or what causes them, these scientists have learned that sprites contain a great deal of energy.
"Although we're not yet certain, we suspect that the energies from sprites may be sufficient to drive some novel chemical reactions," said Sentman. "The region of the atmosphere where sprites appear typically doesn't contain a lot of energy, so the energy introduced from a sprite could do some really interesting things."
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Left: A red sprite with blue tendrils extending downwards. Sprites are emitted near the tops of thunderclouds and reach up into the ionosphere (40-95 km range). Credit: University of Alaska, Fairbanks.
Sentman speculated on potential sprite-generated chemical reactions at the International Conference for Atmospheric Electricity. He's hoping to entice atmospheric chemists to look at sprites and their electro-chemical byproducts. Sentman also discussed the "energy budget" of sprites, which includes the electromagnetic emissions that sprites may give off and how that energy dissipates over time. Discovered because of their visible light emissions, sprites may also give off heat (infrared) and other forms of energy.
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Last summer, Sentman and other scientists flew above the cloud cover at night to search for sprites. Scientists from the University of Alaska cooperated with the Air Force, the Naval Research Lab and NASA on this joint flight campaign. The scientific team looked at sprite emissions ranging from ultraviolet to visible to infrared. They also looked for gravity wave interactions, carbon dioxide emissions, and nitric oxide production. Using two ground stations - one in Colorado and one in Wyoming - scientists were able to triangulate sprites for stereoscopic views.
"We flew for a two week period during moon-down," said Sentman, referring to a time period when the moon is not in the night sky. "The moon is so bright, if you get it in the field of view it can burn out low-light cameras."
The brightness of sprites is about 500 to 1,000 kiloRayleighs. The cones in the retina, which see color, can barely perceive this light level. Because the more sensitive, achromatic rods of the eye permit night vision, they can see this low light level more easily. Rods are more numerous than cones in the periphery of the retina, so you can see sprites best when you're not looking directly at them.
Left: The wispy colored lights of the aurora borealis are somewhat similar in appearance to sprites.
"Although younger members of the crew saw the sprite's red color, they all looked white to me," said Sentman with a rueful grin.
Sprites are brief - lasting only 3 to 10 milliseconds - and that makes them difficult to study. They are not a constant and predictable phenomena, so scientists are never sure exactly when and where sprites will appear above the storm clouds.
"Sometimes you go an entire night without seeing one - you're intently looking at the instruments all night, and you get nothing," said Sentman. "Other nights, you can see hundreds over a period of several hours. We don't know why one storm will spawn so many, while another storm remains barren."
Sentman named the phenomena "sprites" to reflect their eerie, ghost-like qualities and fleeting, elusive natures. Inspired by Shakespeare, Sentman decided on the name one wintery night over pie and coffee in a cabin near Fairbanks.
Through the house give gathering light,
By the dead and drowsy fire:
Every elf and fairy sprite
Hop as light as bird from brier;
And this ditty, after me,
Sing, and dance it trippingly.
- Wm. Shakespeare, "A Midsummer Night's Dream," Act 5, Scene 1.
Right: Time-elapsed photography shows sprites "dance" across the horizon. Credit: University of Alaska, Fairbanks.
Said Sentman, "I think William Shakespeare would have approved, if he had known of their existence."
(from The UAF website)
For observing sprites, it must be completely dark (not twilight) and your eyes must be dark-adapted. If you can see the Milky Way, your eyes have adapted enough to see sprites.
You need to have a clear view above a thunderstorm. Generally, this means the thunderstorm activity must be on the horizon, without any other clouds to obstruct your view. The best viewing distance from the storm is 200 to 300 km (100-200 miles). At these distances sprites will rise to a vertical distance of 10-20 degrees; 2 to 4 times the separation of the pointer stars in the Big Dipper.
Fix your gaze on the space above an active thunderstorm. To avoid being distracted by underlying lightning activity, you may want to use a piece of paper to block out the area below the clouds.
The bright lights of a city or cloud illumination from lightning may prevent you from seeing sprites.
Sprites are brief - only 3 to 10 milliseconds. They occur too quickly to follow with the eyes, but their vertical structure and red color may be perceived.
Patience will be rewarded. If the right kind of storm is present and one's viewing geometry is favorable, there is a greater likelihood of seeing a sprite than of seeing a shooting star.
If you DO see a sprite (or any other kind of optical emission above a thunderstorm) please report it!Web Links Human Voltage (June 18, 1999) Scientists discuss biology, safety, and statistics of lightning strikes.
News shorts from Atmospheric Electricity Conference (June 16, 1999) Poster papers on hurricanes and tornadoes summarized.
Soaking in atmospheric electricity (June 15, 1999) 'Fair weather' measurements important to understanding thunderstorms.
Lightning position in storm may circle strongest updrafts (June 11, 1999) New finding could help in predicting hail, tornadoes
Lightning follows the Sun (June 10, 1999) Space imaging team discovers unexpected preferences
Spirits of another sort (June 10, 1999) Thunderstorms generate elusive and mysterious sprites.
Getting a solid view of lightning (June 9, 1999): New Mexico team develops system to depict lightning in three dimensions.
Learning how to diagnose bad flying weather (June 8, 1999): Scientists discuss what they know about lightning's effects on spacecraft and aircraft.
Three bolts from the blue (June 8, 1999): Fundamental questions about atmospheric electricity posed at conference this week.
Lightning Leaders Converge in Alabama (May 24, 1999): Preview of the 11th International Conference on Atmospheric Electricity.
What Comes Out of the Top of a Thunderstorm? (May 26, 1999): Gamma-rays (sometimes).
Lightning research at NASA/Marshall and the Global Hydrology and Climate Center.
Los Alamos National Laboratory - computer models of sprites.
More Space Science Headlines - NASA research on the web
NASA's Earth Science Enterprise Information on Earth Science missions, etc.
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|Author: Leslie Mullen
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NASA Official: Ron Koczor