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SPRITES
Upper Atmospheric Optical Flashes Excited by Thunderstorms
"Now it is the
time of night...
Every one lets forth his sprite..."
Wm. Shakespeare, "A Midsummer Night's Dream," Act 5, Scene 1. *
Check the "Explorations"
link to Scientific American, as well as the article
"Lightning Between Earth and
Space"
in the August, 1997 issue of Scientific American
In addition to their well known effects in the troposphere, large cloud-to-ground lightning discharges produce several kinds of lesser known effects on the middle and upper atmosphere. These effects have only recently been discovered (1989), and are still in the early stages of investigation.
Red Sprites
"Red Sprites" such as shown above are very large flashes of red light excited in the high atmosphere above thunderstorms by positive cloud-to-ground lightning strokes. They were first documented on video tape, from Minnesota, only quite recently [Franz, R.C., R.J. Nemzek, and J.R. Winckler, Television image of a large upward electrical discharge above a thunderstorm system, Science, 249, 48-51, 1990]. Since this initial report many research teams have investigated these and related phenomena. By the end of 1996 about a dozen groups had succeeded in obtaining television images of sprites dancing high above thunderstorms, with observations obtained from the space shuttle, from aircraft, and from the ground. The image presented above was the first* color television picture of a Red Sprite, obtained in the summer of 1994 by the University of Alaska sprites research team from aboard a NASA research jet aircraft. The above example image is reproduced from a research paper reporting some of the initial results from this project. An early and easily digestible article about these and other observations is given in Sentman and Wescott [1996]. An earlier draft of this article, written and posted on the web from Mt. Evans in July, 1995 and laying out the original instructions for sprite viewing and reporting, is included in our main Sprites and Jets web page.
Red sprites are one of several newly discovered phenomena excited in the middle and upper atmosphere by lightning. The locations of the optical components of these phenomena within the atmosphere and their relationship to the causative thunderstorm are shown to scale below. On the right of the figure are shown sections of the temperature and electron density vertical profiles that define the various regions of the atmosphere. Note the height scale on the left - the top of these phenomena lies at the bottom edge of the ionosphere.
This Web Site
This web site contains only a few sample images and pieces of information about sprites. Visit our main Sprites and Jets site where you can find more complete information about this fascinating new subject, including MPEG video clips and links to numerous other resources. Our main web site was originally established from atop Mt. Evans, Colorado (alt. 14,000 ft) in the summer of 1995 by graduate students Matt Heavner and Don Hampton while on a University of Alaska field campaign to study sprites. They used a laptop computer to connect to our Alaska web server via a cellular telephone/modem link through a local ISP in Denver. Operating from their mountaintop perch, Don and Matt obtained the first optical spectra of sprites and were the first to identify nitrogen as the source of the red color. From this web construction site they also initiated the first web based sprite reporting service; it has operated continuously since that time..
Origin of the Term "Sprite" for Upper Atmospheric Optical Flashes
As a historical side note, the name "sprite" was coined in 1993 [over pie and coffee one winter evening in a cabin near Fairbanks, Alaska] in response to the use of other names that had crept into usage within the research community, but which were unsatisfactory in various ways. Descriptive terms such as "cloud-to-stratosphere discharge," "upward lightning," and "cloud-to-ionosphere discharge" were being used, but they were presumptive in their connotations, or they made unwarranted assumptions about physical processes that were not known. For example, following the initial reports of these unusual events there was great uncertainty about their dimensions and terminal heights in the atmosphere. Triangulation using dual aircraft observations of sprites resolved these uncertainties. It was determined that they extend upward to the lower edge of the ionosphere (alt. ~90 km), with the main body spanning the region of the atmosphere known as the mesosphere (altitudes of approximately 50-90 km). Sprites therefore occur primarily above the stratosphere, which lies between approximately 18-50 km, so "cloud-to-stratosphere discharge" incorrectly placed them in the wrong region of the atmosphere. Such terms also implied the direction of development to be upward and that the events are "discharges" of some kind, which may not be true. The events are visually and structurally quite unlike lightning discharge channels, and the physical mechanisms that produce these respective phenomena may be quite different. Thus, use of the term "upward lightning" also seemed inappropriate, or at least misleading in its implications. The descriptive term "cloud-to-ionosphere discharge" was even more presumptive in the way it implied attributes for which there was little or no evidence. Still, the events had to be referred to somehow, preferably using a name that was neutral in its physical connotations.
In light of these many uncertainties, and because of their capricious nature and the eerie, ghost-like qualities of their optical signatures in early video tapes, this researcher** coined the name "sprite" as a neutral and generic reference for the events. The term "sprite" was later adopted by many in the research community and picked up by the popular press. It was purposely chosen because it evokes a sense of the elusive and fleeting quality of these events (duration about 1/100 second or less), it makes no statement about processes of which we are ignorant, and it doesn't have to be continually changed in response to new observations.
I think William Shakespeare would have approved, if he had known of their existence.
Additional information about sprites/jets/elves can be found at the following web addesses:
[*Note: The first recognized in our color video tapes, that is. Later we found additional events that had occurred earlier. The sprite shown above is one of the brightest and best resolved examples we captured in color.]
[**Note: A recent (July, 1997) article in Discover Magazine erroneously credits another source.]
A Few Sprite Images
A Temporary Repository of a Few Images From the University of Alaska 1996 Field Campaign
The following images are frames taken from video sequences made using a low light level television camera. Except where noted otherwise, the field of view (FOV) is about 15 degrees horizontally in these images. The sprites shown in these examples occurred above thunderstorms that were typically several hundred km from the observers.
"Every elf and
fairy sprite...
Sing, and dance it trippingly."
Wm. Shakespeare, "A Midsummer Night's Dream," Act 5, Scene 1.
Dancing Sprites
This sequence of video images, covering more than 200 ms, shows a series of successive sprites that appeared to "dance" across the screen from right to left. A few minutes later a second series of sprites danced across the screen in the opposite direction. The thunderstorm producing these events was beyond the horizon. Note that the screen images shown here are not evenly spaced in time. The dashed line is electronically added to the image to mark the location of the spectrograph slit. The tops of these events are estimated to lie at about 85-90 km altitude.
Sprite Fireworks
A spectacular video image of a massive set of sprite columns and associated tendrils. The image was obtained near Fort Collins, CO. Some of the columns appear curved, with the curvature increasing near the bottom of the columns. Several appear to break up into "beads" or "balls," detached blobs of luminosity, near their bottoms. The bottoms of many of these columns appear to be originating points of intricate, downward branching tendril structures. The optical spectrum obtained at the position of the slit (dashed line) exhibited primarily N2 1PG bands.
Fireworks and Columniform Sprites
The "Fireworks" event above bears some resemblance to another distinctive event recorded a year earlier from Mt. Evans in 1995. In the image below the fireworks event is shown (bottom) along with a "Columniform" sprite (top) that exhibits a set of narrow vertical columns. Images of the columniform sprite simultaneously obtained from Mr. Evans and Ft. Collins permit the three-dimensional structure of the features to be determined. It is speculated that these may be the same type of events, with the tendril features of the more distant columniform sprite (top) reduced below visible detectability by Rayleigh scattering and limited camera resolution.
Wishbone Sprite
This sequence of three successive sprite images reveals within the same event interval several distinctive features. First, in the top image are seen several curved columns, remnants of an event initiated several video frames earlier. These columns are similar to what is seen in the "fireworks" and "columniform" sprites shown above. Here, there is no consistent sense to the curvature (left or right) between columns. The bright bottom ends of these curved structures lie at altitudes of about 70 km. In the second image a "classic" sprite has suddenly appeared left of center, with traces of upward diffuse "hair" and downward branching tendrils. The region below one of the curved columns has broken up into discrete patches of luminosity. In the bottom image the sprite has brightened, the diffuse hair has intensified, and the tendrils have become more pronounced.
Dawn Sprite
On the last night of the University of Alaska Sprites96 campaign a sprite-producing storm over Nebraska/Iowa, to the east of the Wyoming observing site, was tracked into dawn. The faint sprite seen in this enhanced and enlarged section (~3 deg HFOV) of a television frame occurred just as the dawn terminator swept over its upper regions. The label "Sprites" lies just beneath the line of the distance horizon.
Sprite Spectrum
This optical spectrum of a sprite extends into the near infrared and exhibits some new features in addition to these recorded from Mt. Evans in 1995. Below, a spectrum is shown that was obtained from the Wyoming Infrared Observatory during a University of Alaska 1996 field campaign to observe sprites. Two plots of the same data are presented. In the top plot the vertical scale is linear. In the bottom plot the same information is plotted on a logarithmic scale to enhance the fine structure at low amplitudes and near the instrument threshold. In these plots, the peak near 870 nm corresponds to the delta-v=1 bands of N2 1PG. The two large peaks near 750 and 775 nm are part of the same delta-v=2 system of N2 1PG, and the smaller sets of peaks 650-690 nm and below 620 nm correspond to the delta-v=3 and delta-v=4 systems, respectively. The black vertical lines mark the wavelengths and intensities of auroral emissions of N2 1PG, while the red lines mark the wavelengths and relative intensities of the N2 ion Meinel bands. There is some evidence for a Meinel feature at 808 nm. For a report of the first measurements of sprite spectra see the paper by Hampton et al. [1996].
Optical Spectrum
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For More Sprite Images, click here.
*Thanks to Dr. Sid Ossakow (Naval Research Laboratory) for pointing out the Shakespeare references to sprites.
Copyright (C) 1997, University of Alaska Fairbanks. For permission to use items from this page please contact dsentman@gi.alaska.edu.
Page last updated 9/1/97.
