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Blue Jets, Red Sprites and Elves - Page 3

© Keith C. Heidorn

surrounding air and thus difficult to see from the ground.

Elves

Elves were the next TLEs to be discovered, observed in 1995 from Walter Lyon's Yucca Ridge field station in Colorado by scientists from the University of Tohoku (Japan) and Stanford University. Elves appear as giant expanding disks of light between 65 and 95 km (40 and 60 miles) altitude. They are caused by the passage through the ionosphere of an electromagnetic pulse in the form of intense radio waves emitted from powerful lightning flashes. The radiating pulse excites the electrons in the nitrogen gas which then emits light by fluorescence. Though huge, sometimes expanding to more than 400 km (250 miles) in diameter, elves are so transient (less than one-thousandth of a second), it is unlikely the human eye could see them. The lightning that triggers elves can be as far as 50 km (30 miles) away from where they appear.

Trolls

Trolls are another addition to the menagerie similar to the blue jet but generally reddish in colour. Trolls occur following an especially vigorous sprite in which tendrils have extended downward to near cloud tops. The trolls exhibit a luminous head leaving a faint trail and ascend initially at around 150 km/sec (95 miles/sec), before gradually decelerating and then vanishing around 30 km (19 miles) altitude. It is still uncertain whether the preceding sprite tendrils actually extend to the physical cloud tops, or if the trolls emerge from the storm cloud. Researchers have also termed them embers and fingers, but troll has the advantage of being a plausible acronym (for Transient Red Optical Luminous Lineament).

Copyright 2004, Keith C. Heidorn, All Rights Reserved.

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