Spirits of Another Sort

Thunderstorms Generate Elusive
and Mysterious Sprites

One of a series of stories covering the quadrennial International Conference on Atmospheric Electricity, June 7-11, 1999, in Guntersville, Ala.

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.

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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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.

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.

"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.

But because 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).

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."

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.

Left: Black & white images of multiple sprites seen in October, 1997. Credit: New Mexico Tech/NASA.

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!

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.

NASA's sprite page

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 Curator: Linda Porter NASA Official: Ron Koczor