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Blue Jets, Red Sprites and Elves

© Keith C. Heidorn

Blue jets, red sprites, elves, sprite halos and trolls may sound like the melding of Tolkein characters with the psychedelic Sixties, but these entities comprise the newest members in the pantheon of atmospheric electrical phenomena, joining lightning and St Elmo's Fire. Though first reported in 1886 as unidentified oddities, it was not until the last decade that the meteorological community accepted their existence.

Part of the reason for their slow acceptance is that these very short-lived phenomena, collectively termed Transient Luminous Events (TLEs), appear above the clouds where they are usually hidden from ground-based operations. With the advent of manned orbital platforms and regular high-altitude aircraft operations, reports of the phenomena increased, engendering an interest in their cause and nature. First described as cloud-to-space lightning and rocket lightning, TLEs have now acquired a variety of colourful names. According to one of the pioneers in the field, Dave Sentman of the University of Alaska, Fairbanks, the first of the group to be studied, the sprite, was named after the creatures in William Shakespeare's play A Midsummer Night's Dream, because of their transient, ephemeral nature.

Early descriptions of the TLEs noted that their most typical form consisted of flames appearing to shoot up from the top of the cloud or, if the cloud is out of direct sight (such as below the horizon), flames rising from the horizon. Pilots flying above thunderstorms occasionally noted strange flashes rising from the tops of the thunderclouds into the darkness of the upper atmosphere. But it was not until 1990, when John R. Winckler and colleagues at the University of Minnesota video-taped these mysterious apparitions, did anyone seriously consider the discovery of a completely new configuration of lightning. Like many newly documented discoveries, the seemingly rare became more commonplace over the next years as the phenomena were observed from the space shuttle and station, high flying aircraft and even ground observations.

The properties and underlying physics of the TLEs are just starting to be discovered. There appear to be four distinct categories: the sprites and elves - forms of high-altitude lightning - blue jets, and gamma ray events. The latter two being extremely rare, and thus still poorly understood.

A red sprite with blue tendrils extending downwards emitted near the tops
of thunderclouds. Sprites reach up into the ionosphere (40-95 km range).
Courtesy: NASA/University of Alaska, Fairbanks.

Sprites

Though first called red sprites, it was latter observed that sprites also contained faint tendril-like elements of blue and purple. As the catalogue of observations grows, we now see that sprites come in a menagerie of sizes and shapes described as giant red blobs, picket fences, upward

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