Monday, July 21, 2014

Bored with lightning from thunderstorms? Try watching for sprites.

Less than ten years ago, nobody had heard of sprites, blue jets and elves, in fact, the terms hadn't even entered the meteorological vocabulary. Recent research, however, has confirmed over a century of frequent but generally ignored observations of an entire menagerie of strange, luminous, lightning-related flashes dancing high above thunderstorm tops. And you can, if you know how, observe some of these with the naked eye. Here's the story of how sprites were discovered and how you might be able to spot some on your own.

In decades past, the textbooks said weather stopped at the tropopause, the layer separating the turbulent troposphere from the quiescent stratosphere above. Not so. As long ago as 1886, people were publishing reports in which they struggled to describe momentary discharges of "lightning," for lack of a better term, that they had observed high above storm clouds.

A 1903 paper discussed 
"rocket lightning ... a luminous tail ... shooting straight up ... rather faster than a rocket..."
From Africa in 1937 came reports of 
"long and weak streamers of reddish hue...some 50 kilometers high..."

English scientific papers from the 1950s detailed what seemed to be flames appearing to shoot above thunderstorms near the horizon. Recently Earle Williams of the Massachusetts Institute of Technology took a closer look at a nighttime photograph of an Australian thunderstorm in his possession since the late-1980s. A lightning channel extending into the clear air above the storm top in turn appears to have "blue flame" fanning upward, perhaps even into the stratosphere. An airline passenger over Texas reported he saw over twenty faint plumes of light "extending from the top of the thunderhead above the pool of light from the lightning discharge..." 

On the night of 7 July 1993, giant thunderstorms were boiling over Kansas and Nebraska, one of the series of mesoscale convective complexes that drowned the midwest in record setting flooding rains far to the east. We aimed a low-light video camera, cousin to the night scopes used by the military, to the east and began taping. For the first two hours, not much happened, except for the almost continuous flashing of lightning within the distant clouds. Then, suddenly, a bright flash occurred high above the storm tops (appearing white on the monochrome television system screen). Over the next several hours, over 240 high altitude flashes were captured. The very next night, University of Alaska scientists obtained similar images from a high-flying NASA aircraft over Iowa. Since then, thousands of flashes have been recorded on low-light video from the ground and from air. While easily visible on the television monitor on that very first night, I was unable to see anything with the naked eye while staring above the distant clouds. But several nights later, when the show started again, with some patience, and dark adapted eyes, there they were, bright reddish curtains dancing a gossamer ballet high above the storm clouds.
In 1994, while flying an extremely sensitive color camera normally used for auroral photography, University of Alaska scientists confirmed that the flashes indeed have a generally reddish color, but which often fades to purple or blue in the downward extending tendrils.

Check out Spaceweather's gallery of sprites!

The flashes were named sprites after the creatures in Shakespeare's "The Tempest," in part because of their transient, ephemeral nature. But unlike the bard's characters, these sprites are very real indeed.
And the sprites were soon found to have company. At least two other distinct phenomena have been discovered to date. While flying near an especially active hailstorm in Arkansas, the University of Alaska team were startled to see blue beams of light shooting upward directly out of cloud tops at speeds over 100 kilometers a second. They reached heights of 40 or 50 kilometers (two or three times the cloud heights) before fading away. Around 50 of these "blue jets" were seen that night. But the blue jet seems to be very rare. In four years of ground monitoring, only one blue jet has been captured on tape. 

After the red sprites and blue jets came the elves. In 1995, scientists from the University of Tohoku (Japan) and Stanford University, working with other science teams at the Yucca Ridge Field Station, confirmed the presence of elves (emissions of light and VLF perturbations from EMP sources). These were actually predicted by theorists before they were ever caught on tape. The elves appear as giant expanding disks of light between 70 and 100 kilometers altitude. They are caused by the passage through the ionosphere of the electromagnetic pulse (EMP), the intense radio waves emitted from powerful lightning flashes. Though huge, sometimes expanding to more than 400 kilometers in diameter, the elves are so transient (less than one-thousandth of a second), it is unlikely the human eye could detect them.
But the red sprites can be seen by the naked eye. They are by far the most common of these mesospheric creatures, and we know where they "live". So a plan for some serious "sprite hunting" is relatively easy to develop.
Sprites come in a bewildering variety of sizes and shapes. They can look like giant red blobs, picket fences, upward branching carrots, or tentacled octopi. The sprite luminosity can extend upward as high as 95 km, with the brightest part usually located between 50 and 75 km altitude. The often bluish tendrils can sometimes extend downward below 30 km, close to, but probably not touching, the cloud tops. Sprites can occur singly or in clusters which sometimes fan out for over 150 kilometers. Sprites appear to be uniquely associated with cloud-to-ground (CG) flashes of positive polarity, usually those having peak currents larger than most of the other positive CG events in the storm. By comparison to the pencil-thin channel of their parent positive CG flash, the volume illuminated by a large sprite can reach hundreds or even thousands of cubic kilometers.

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