High-altitude light show in focus

By Dr David Whitehouse
BBC News Online science editor

The phenomena occur between 50 and 100km above the Earth's surface

Fresh data on sprites, jets and elves - strange flashes of coloured light in the Earth's upper atmosphere - is being returned to Earth by a new satellite.

The Taiwanese Rocsat-2 spacecraft has been in orbit for two months and is studying the high-altitude phenomena.

They are believed to be discharges of electricity from above thunderstorms, part of a global electrical circuit.

Rocsat-2's first goal is to make a map of the distribution of the flashes and how often they occur, say scientists.

Polar orbit

For years, reports of red streamers, blue jets and strange diffuse glows seen in the upper reaches of the atmosphere were not taken seriously.

But in the past decade videos taken from high-altitude aircraft and the space shuttle have convinced scientists that they are real.

The phenomena are difficult to study as they occur between 50 and 100km (30-60 miles) above the Earth's surface, too high for most aircraft and too low for satellites.

With Isual, we are trying to figure out the properties of the global electrical circuit, how the lower and upper atmosphere are coupled electrically Stephen Mende, University of California at Berkeley

To study them, the Taiwanese government built the Republic of China Satellite 2 (Rocsat -2), which includes a sensor built by the University of California at Berkeley, US, to gather information about the lights.

The instrument contributed by the Americans is called Isual - the Imager of Sprites and Upper Atmospheric Lightning.

"With Isual, we are trying to figure out the properties of the global electrical circuit, how the lower and upper atmosphere are coupled electrically," says Stephen Mende, from UC-Berkeley's Space Sciences Laboratory.

Molecular mash

The first Isual image was returned on 4 July. It showed red sprites - short fluorescent "tubes" glowing like neon lights - reaching to the ionosphere.

Another image showed a brilliant lightning flash with a trio of red sprites above it and a sprite halo encircling it.

Researchers believe that the lights occur after a lightning strike which electrically "grounds" the top of the storm clouds, creating a voltage difference between the cloud and the upper atmosphere.

In the thin air at this altitude, the strong electrical fields break down the air molecules.

The red sprites are formed in the regions of molecular disintegration. The blue jets, however, seem to come from the top of thunderclouds.

Elves are rapid bursts of light due to electromagnetic pulses from lightning jolts.

Isual is expected to operate for five years from its polar orbit more than 890km (550 miles) above the Earth's surface.