|3. Everything / Maths, Science & Technology / Physics|
So you thought you knew a thing or two about sprites then?
Well, there is the well known soft drink, a classic car, some early pixelated images on a screen, and of course the obvious woodland imp, but the sprites of this entry are without doubt the oldest in the history of the planet and the newest to be discovered by the current inhabitants.
These sprites are a fleeting, ethereal and a relatively unknown aspect of lightning storms.
Everyone is familiar with lightning storms and the concept that lightning will 'go to ground' by striking the highest point. This happens when static electricity builds up within the clouds, with the upper cloud storing a positive charge while a negative charge builds in the lower cloud. When the difference between the two charges becomes great, the charge leaps from one area to another causing a lightning bolt. The lightning bolts mostly leap within the cloud or more rarely between clouds, but when a high positive charge is created on the ground the bolt will leap to the highest point on it.
The bolt of lightning can be hotter than the sun and this causes the surrounding air to expand explosively, creating thunder. As the speed of light is greater than that of sound, if you count the seconds between the lightning and the thunder then divide by three for kilometres or five for miles, you will have an estimation of how far the storm is from your present position.
Lightning and the awesome aurora borealis are both plasma-based phenomena. The plasma globe1 uses the same principle on a much smaller scale to create the same effect and illustrate the phenomenon safely in the comfort of your own living room.
Negative or Positive?
In the case of a cloud to ground lightning discharge, a negative charge (known as a 'stepped leader') carries a negative current downwards. When this is within about 100 metres of the ground, a return stroke is initiated, which transfers a current of positive charge upwards2. It is the luminous return stroke that we actually see. Lightning travels in both directions - sometimes several dozen times, hence the 'flickering' effect.
It was thought that all cloud to ground lightning was negatively charged, but in the 1990s a more powerful and destructive positively charged lightning was recorded. It is estimated about 10% of the cloud to ground lightning strikes we experience are positively charged. Positive strikes last twice as long and expel more energy than their negative counterparts therefore posing a real danger to aircraft, for example. Positive strikes are thought to be implicated in at least two major air disasters.
History of Sprites
Since the 1960s, and probably before then, pilots have been seeing but seldom reporting what have become known as sprites and elves above the clouds. Sprites are electrically-charged lightning funnels which shoot up from the top of a cloud as much as 60 miles into the atmosphere. These charges are vivid red and usually occur in clusters of three or more but are only visible for nanoseconds. They are sometimes preceded by lower altitude red flashes known as elves, and can have striking blue tendrils which are easily mistaken for blue jets. While they are a similar visual phenomenon, blue jets are less powerful than the sprites and travel neither as quickly nor as far.
Because 'everyone knows' lightning goes to ground, pilots were naturally reluctant to report this phenomenon in case they found themselves grounded for hallucinating. As a result, serious research was delayed until the last 15 years or so.
While sprites are more common during positively-charged lightning storms, this is not due to any preference on the part of the sprite, but rather due to the greater internal energy of a positively charged storm. It was not until 1999 that the first sprites of a negatively-charged storm were recorded.
During a powerful storm it is possible to see red sprites, elves and blue jets, but the exact atmospheric conditions which create such a show are uncertain.
Mystery of Sprites
As sprites are relatively new to the science world there is still a lot more to learn about them.
It is only with the advent of high speed photography that the existence of these light shows could be confirmed, and even with that they were first photographed by accident in 1989. Amazingly, there have since been more than 10,000 confirmed sightings. They are also known to create a very low-frequency thunder which was only recently captured with the use of specialist listening equipment.
As their energy is spread more thinly than the traditional thunderbolt due to the cone like dispersal from cloud to atmosphere, they are thought to be relatively weak.
The effects of sprites are currently being investigated by various agencies including NASA who seriously addressed them as a possible cause for the tragic loss of the space shuttle Columbia, which was, incidentally, on a mission to record data about the very same sprite phenomenon.
If you're interested in spotting a sprite, try to position yourself on a height at the edge of a flat low plain or a valley and watch an approaching positively charged storm. Total darkness is required for the best effect, but they are visible to the naked eye. Please use caution when attempting this as lightning has been known to travel quite a distance ahead of the storm. All being well, sprites should be visible from your position rising from the thunderhead to the high atmosphere. If you think you see one, keep looking in that direction, because while they appear and vanish again very quickly they usually appear in a trail of maybe five or seven, so you could see quite a show. Good luck.
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