Home | Welcome | What's New | Site Map | Glossary | Weather Doctor Amazon Store | Book Store | Accolades | Email Us

Weather Phenomenon and Elements

Blue Jets, Red Sprites and Elves

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.

Red Sprite

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


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


Blue Jets, Red Sprites and Elves ©2004, 2005 Keith C. Heidorn, PhD. All Rights Reserved.
Correspondence may be sent via email to: see@islandnet.com.

For More Weather Doctor articles, go to our Site Map.




I have recently added many of my lifetime collection of photographs and art works to an on-line shop where you can purchase notecards, posters, and greeting cards, etc. of my best images.

To Purchase Notecard,
Greeting Cards and Posters
featuring my images, visit
The Weather Doctor's
Nature Gallery


In association with Zazzle.com


Now Available! Order Today!

NEW! Now Available in the US!
And Now...The Weather

by Keith C. Heidorn
To Order in Canada:
And Now...The Weather
by Keith C. Heidorn

The BC Weather Book:
From the Sunshine Coast to Storm Mountain

by Keith C. Heidorn










Home | Welcome | What's New | Site Map | Glossary | Weather Doctor Amazon Store | Book Store | Accolades | Email Us
In association with Zazzle.com


Weather Doctor Bookstore