快猫短视频

RED GLOWS called 鈥渟prites鈥� that dance above the clouds safeguard the Earth鈥檚
atmosphere from huge electric fields generated by thunderstorms, researchers
say. This may have been crucial to the survival of early life on Earth.

For many years, pilots have reported seeing carrot-shaped red lights that
linger for only a split second, several kilometres above thunderstorms. The
sprites鈥� vertical red fingers are tens of kilometres long and snake downwards
from the Earth鈥檚 ionosphere.

No one鈥檚 sure what triggers sprites鈥攑ossibly meteors colliding with the
atmosphere just above a thunderstorm. But scientists do know that they are made
of highly ionised glowing gas. They grow downwards and branch into fractal trees
as each patch ionises the patch of air next to it, then fizzle out as the air
becomes denser and harder to ionise lower in the atmosphere.

Victor Pasko of Pennsylvania State University in University Park decided to
work out how far down a sprite could possibly reach. Using measurements gathered
by his sprite-chasing colleagues at Stanford University in California, he
modelled their growth using the maths of fractals.

Pasko found that the most powerful sprites could reach down to within 20
kilometres of the Earth鈥檚 surface, around the height of the tops of
thunderclouds. This means they could join up with lightning to form an
electrical short circuit from the ionosphere to the ground. 鈥淎 sprite is a
highly conducting object,鈥� says Pasko.

The ionosphere is highly ionised by particles streaming out of the Sun, so
sprites could act as a channel for earthing the charge. Pasko thinks this may
help to keep the potential difference between the ionosphere and the ground
constant鈥攁t around 300,000 volts鈥攄espite the immense charge
thunderstorms generate in the lower atmosphere.

If the ionosphere weren鈥檛 defused by sprites, Pasko says, its potential
difference relative to the ground could be much higher. It鈥檚 possible that this
would alter the numbers and compositions of free radicals in the atmosphere.

鈥淭his could affect the chemistry of the atmosphere,鈥� Pasko concludes. On the
early Earth, this altered chemistry might have hindered the prospects for the
evolution of life.

Steve Cummer of Duke University in Durham, North Carolina, says we need to
find out more about the frequency of sprites to discover if they safeguarded
early life: 鈥淚f they only happen once a year they don鈥檛 matter, but ten times a
day would.鈥� Future studies by sprite-chasers will hopefully resolve this
question.