Gravity waves cause goblin flashes

Researchers reveal atmospheric conditions above thunderclouds

False-Color Shot of Sprites over a Thunderstorm in Kansas in 2000 © Walter Lyons, FMA Research, Fort Collins, Colorado
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Enigmatic Sky Lights: Lightning strikes on the ground can trigger spectacular discharges even up to 100 kilometers above storm clouds. Scientists have now identified an important factor in the emergence of these "sprites": Heavy waves in the upper atmosphere thus amplify disturbances in electric fields and thus cause the lightnings to swing far up, the researchers write in the journal "Nature Communications".

The lightning in a strong thunderstorm already offer a spectacular sight from the ground. Despite intense thunderstorm research, however, they still raise many questions - and even more mysterious is what happens above a storm cloud: In strong lightning strikes there also electrical discharges flicker up to altitudes of 80 kilometers in the mesosphere, above the stratosphere. As they are so elusive and enigmatic, these flashes were given the name "sprites, " English for "goblins." It has only been scientifically proven since 1989. How it is produced is still largely unclear.

Fireworks in the mesosphere

Sprites are sometimes visible to the naked eye from the ground at night, but most of the images are from aircraft, space shuttles or the International Space Station. They are usually reddish in color and appear like a jet of flame that branches upwards. Many also have beamlike foothills down and are reminiscent of an exploding firework rocket - but with a play of lights of up to 50 kilometers in the vicinity.

High-speed video of a sprite at about 20 seconds, before the faintly glowing halo is visible. Florida Institute of Technology

Scientists led by Ningyu Liu of the Florida Institute of Technology have now shed some light on the ridiculous sprites and their origins: they analyzed images of a sprite taken from research aircraft using two high-speed cameras. Using this information, she compiled and validated a computer model of what happened in the upper atmosphere during these electrical discharges.

The conditions must be right

According to the model, gravity waves in the upper air layers make an important contribution to the sprites. The waves cause disturbances and vortices, which are amplified by the electric discharge of a strong lightning. In the electric field above a storm cloud, these faults can then ignite a sprite. display

However, the conditions must be right: "If the disruption is too large, it's impossible to make sprites, " Liu says. "If the magnitude of the disturbance is small, it takes a relatively long time for sprites to deploy." The model also predicts that the sprites are preceded by a kind of glow. Also this so-called halo can be confirmed with previous observations.

Recolored image of a sprite taken with a high-speed camera from an airplane. Florida Institute of Technology

Many more phenomena?

How and under what conditions sprites are formed is not only interesting for thunderstorms: electrical discharges in the mesosphere can also affect the ionospheres above it. This could interfere with some communication signals or even interrupt them. Even for particularly high rising weather balloons, sprites pose a potential hazard.

The generated model could also help to better understand the responsible turbulence in the mesosphere. These are so far almost impossible to directly observe. Furthermore, according to Liu, the computer model points to faint and hitherto unobserved heavenly lights: "There could be many more interesting phenomena awaiting discovery with more sensitive devices." (Nature Communications, 2015; Press)

(Florida Institute of Technology, 29.06.2015 - AKR)