Sprites and Elves

 

[July 2003] Red sprites and blue jets are uper atmospheric optical phenomena, strange rocket-like flashes of light from the tops of some lightning clouds, that have been recorded for over a century. Only in the last few decades have scientists been able to study them through low-light level photography.

The first images of a sprite were accidently obtained in 1989. Beginning in 1990, about twenty images have been obtained from the space shuttle. Since then, video sequences of well over a thousand sprites have been captured. These include measurements from the ground and from aircraft. Numerous images have also been obtained from aircraft of blue jets, also a previously unrecorded form of optical activity above thunderstorms. Blue jets appear to emerge directly from the tops of clouds and shoot upward in narrow cones through the stratosphere. Their upward speed has been measured to be about 100 km per second. Sprites appear to have a duration of only a few (3-10) milliseconds. This is too brief to permit shifting one's gaze to obtain a visual fix. Sprites occur randomly with only about one percent of lightning strokes. The mere occurrence of lightning therefore cannot be used as an event marker to indicate that a sprite has occurred above a thunderstorm. When all of these factors are taken together it is not surprising that sprites have been so elusive. So, they can be seen with the unaided human eye...if you are lucky to be looking at the right time and in exactly direction!

The optical energy is roughly 10-50 kJ per event, with a corresponding optical power of 5-25 MW. Assuming that optical energy constitutes 1/1000 of the total for the event, the energy and power are on the order of 10-100 MJ and 5-50 GW, respectively.

What is even stranger is that NASA's Gamma Ray Observatory used to see flashes of gamma-ray light from the atmosphere over regions that had powerful thunderstorm activity. What ever is going on to make sprites and jets can produce more than ordinary light!

Matt Heavner has a wonderful, and thorough, page about this fascinating lightning phenomenon over at his pages at the Geophysical Institute of the University of Fairbanks. There you will find movies and the results of research studies.

There was an August 1997 article in Scientific American about sprites.

The Stanford University VLF Group also has some pages on their research into Sprites. They have also assembled a very nice bibliography of online popular articles.