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One of the strangest new findings in Earth's atmosphere are the flashes found high above the thunderstorm clouds. Although the clouds themselves are at low altitudes to be important for ionospheric physics, the flashes can reach altitudes of about 90 km, i.e., the ionospheric E-region, and can thus be relevant also for space physics. The flashes seem to occur all around the world at a rate of at least one flash per minute. They have a life time measured in milliseconds. In addition, it seems that there are two types of flashes:

  • sprites
    • predominantly red
    • reach up to 90 km
  • jets
    • blue
    • narrowly collimated sprays or fans of light
    • reach up to 45 km
    • propagation speed about 100 km/s

Another type of strange flash has also been seen:

  • air glow flash
    • horizontal illumination at the bottom of the ionosphere
    • maximum brightness just before the lightning flash

Observed emissions are most likely byproducts of the ionization processes of nitrogen and oxygen (the main molecules at these altitudes), which takes place because of the flashes. Details of the process are, however, still unknown.

One of the models suggested to explain these observation starts from a single cosmic ray colliding into an air molecule, and sparking a runaway atmospheric breakdown.

Other, possibly related events, are twin bursts of radio energy, or transionospheric pulse pairs, TIPP, and gamma ray flashes. They have also been associated with thunderstorms.

Source: High-Altitude Flashes Mystify Scientists in EOS, 75, Number 51, 1994

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