Thunderstorms Generate Elusive and Mysterious Sprites
June 10, 1999: For centuries, man has been transfixed by the spectacular lightning displays of thunderstorms. But after all those years of gazing at the sky, we never realized there was an equally amazing light show going on above the clouds.
Credit: University of Alaska, Fairbanks. Full size image available through contact |
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 Dr. 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."
For instance, there is some speculation that sprites may create nitric oxide (NO) in the upper atmosphere. Nitric oxide destroys ozone, so sprites may have some impact on the Earth's protective ozone layer.
Dr. 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. Dr. 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.
Night Sprite Flight
Last summer, Dr. 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.
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University of Alaska, Fairbanks - Sprite research home page. Red Sprites and Blue Jets - an older page maintained by UAF. Los Alamos National Laboratory - computer models of sprites. |
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.
But because Dr. Sentman was so close to the sprites, he saw them head on. To him, the sprites were as bright as the aurora borealis (the Northern Lights).
"Although younger members of the crew saw the sprite's red color, they all looked white to me," said Dr. Sentman with a rueful grin.
The wispy colored lights of the aurora borealis are somewhat similar in appearance to sprites.
Full size image available through contact |
"Sometimes you go an entire night without seeing one - you're intently looking at the instruments all night, and you get nothing," said Dr. 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."
Dr. Sentman named the phenomena "sprites" to reflect their eerie, ghost-like qualities and fleeting, elusive natures. Inspired by Shakespeare, Dr. 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.
Said Dr. Sentman, "I think William Shakespeare would have approved, if he had known of their existence."
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! |