Thundercloud discharges have been imaged in unprecedented detail.
Atmospheric sprites, the mysterious lights that dance above thunderclouds, have dazzled scientists with their sheer speed. Newly captured high-speed video footage shows the strange sparks shooting across the sky at more than 1,600 kilometres per second.
Sprites were first captured on video by accident in 1989, and have subsequently been studied from above by NASA's Atlantis, Columbia and Discovery space shuttles. Although quite common, little is known about them because they last for mere milliseconds, making it difficult to measure how fast they move, or even where they go.
Previous videos have either been unable to resolve the sprites' features, or have operated too slowly to catch more than a single frame of bright light.
But Umran Inan and Robert Marshall, atmospheric scientists at Stanford University in California, say they have the first high-speed observations of sprites that also reveal details of their structure.
The researchers watched sprites over New Mexico in July and August 2004, using a camera that shot 1,000 frames per second, allowing them to see how the sprites evolved from millisecond to millisecond. "We've now put a lower limit on the speed of sprites," says Inan, who analysed the growth of different flashes from one frame to the next to calculate their speed.
The pair also found that some sprites are made of long chains of bright beads, each as small as 10 metres across, separated by dark patches. Others appear to be continuous 'streamers' that taper to a fine point just a few metres across. Inan and Marshall, who publish their work in Geophysical Research Letters1, say that they do not yet know how these structures form.
Scientists think that the sprites' light is produced when fast-moving electrons hit nitrogen molecules in the air. These electrons come from the Earth's ionosphere, a layer of charged particles in the upper atmosphere more than 60 kilometres above the Earth's surface.
The sprites usually appear during thunderstorms. When a thundercloud loses its electrical charge to Earth through a bolt of lightning, it leaves the cloud tops with relatively few electrons, Inan explains. This sets up an enormous voltage between the cloud and the ionosphere, accelerating electrons that subsequently crash into molecules in the air.
Some of the sprite features that the team saw only lasted for one frame, says Inan, which means that some aspects of the flashes are presumably still being missed. The team is now planning observations for this July with a high-resolution camera that takes about 10,000 frames per second.
At this rate, the camera would only miss the evolution of the sprite if it raced across the 2.5-kilometre field of view at better than 30,000 kilometres per second, one-tenth of the speed of light.
MarshallR. A. & InanU. S. Geophys. Res. Lett., published online, doi:10.1029/2004GL021988 (2005).
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Peplow, M. Heavenly light show caught on film. Nature (2005). https://doi.org/10.1038/news050321-6