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
branching carrots, or tentacled octopi. The luminous body of the sprite can extend as high as 95
km (60 miles) with peak brightness between 50 and 75 km (30 and 47 miles). Downward
draping tendrils often drop below 30 km (19 miles) altitude but do not reach the thundercloud
tops. Rather than forming a narrow channel like the cloud-to-ground lightning with which they
are associated, sprites are estimated to be around 10 metres (30 ft) across and often appear as
clusters that illuminate a large volume, perhaps thousands of cubic kilometres spreading out
over 150 km (93 miles) from their origin.
Sprites emerge high above very large thunderstorm systems, appearing at intervals of up to
several minutes and lasting several milliseconds. They seem to associate with cloud-to-ground
lightning flashes of large positive polarity (most, but not all, lighting strokes are of negative
polarity). A diffuse disk-shaped glow lasting about a millisecond precedes some sprites. These
sprite halos are less than 100 km (62 miles) wide, and propagate downward in altitude from
about 85 to 70 km (53 to 44 miles). Columnar sprites sometimes emerge from the lower portion
of the sprite halo's concave disk.
Researchers have discovered unique radio signals are emitted by the lightning stroke producing
each sprite event. But using this property for detection rather than visual observations,
researchers now believe sprites, once considered rare, appear to form during roughly one in
every two hundred lightning strikes.
Blue Jets
In
1993, while flying above severe thunderstorms in Arkansas, Davis
Sentman and Eugene
Wescott of the University of Alaska's Geophysical Institute noted a new
TLE. They were
surprised to see blue light beams shooting upward directly out of cloud
tops. Rising at speeds of
over 100 km/sec (60 miles/sec), the beams reached heights of 40 to 50
km (25 to 30 miles) two or three times the cloud heights before
fading away. Wescott and Sentman named these
flashes blue jets.
Blue jets propagate from the cloud tops toward the ionosphere 20 to 50 km (12 to 30 miles) high
and last from tenths to a full second. They are always blue and funnel-shaped: 1.6 to 3.2 km (1 to
2 miles) at their base and 8 to 10 km (5 or 6 miles) at the top. Simultaneous blue jets propagate
slowly upward from the cloud tops, but extinguish simultaneously. The blue starter, a related
phenomenon, may actually be a blue jet that fails to completely form. Blue jets appear to be very
rare, but that may arise from the fact their faint blue light is quickly scattering by the
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).
Learn More From These Relevant Books Chosen by The Weather Doctor
Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.
Written by Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
November 1, 2005 A version of this material was previously published by Keith Heidorn on Suite 101: Science of the Sky, 2004
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