William L. Boeck, Niagara University, New York
Otha H. Vaughan, Jr. and Richard J. Blakeslee, NASA Marshall Space Flight Center, Huntsville, Alabama
Bernard Vonnegut, State University of New York at Albany, Albany, New York
Marx Brook, New Mexico Institute of Mining & Technology, Socorro, New Mexico
Abstract
The sequence of video tape observations of the upper atmospheric optical flashes called sprites, jets, starters, and ELVES are described in the successsive phases of search, discovery, confirmation, and exploration for the years before 1993. Although there were credible eyewitness accounts from ground observers and pilots, these reports did not inspire a systematic search for hard evidence of such phenomena. The science community would instead wait for serendipitous observations to move the leading edge of this science forward. The phenomenon, now known as a sprite, was first accidently documented on ground based videotape recordings on the night of July 6, 1989. Video observations from the space shuttle acquired from 1989 through 1991 provided 17 additional examples to confirm the existence of the sprites phenomenon. Successful video observations from a mountain ridge by Lyons, starting July 7, 1993, and night-time aircraft video observations by Sentman and Wescott on July 8, 1993 established the basic science of the sprite phenomena by acquiring and analyzing data based on hundreds of new events. The 1994 Sprites campaign and the video titled "Red Sprites and Blue Jets" popularized the name sprite and provided a vocabulary of terms to describe the visual attributes. Prior to this video, investigators used a variety of vague descriptive words to describe the individual events. Also, during the 1994 campaign, Wescott and coworkers obtained the first quantative measurements of jets and provided the name "blue jets". A third phenomenon was discovered in video from the STS-41 mission (October 1990) in the lower ionosphere directly above an active thunderstorm. It consisted of a large horizontal brightening several hundred kilometers across at the altitude of the airglow layer. In 1995, Lyons and associates confirmed the existence of this type of very brief brightening which they named Emissions of Light and Very Low Frequency Perturbations From Electromagnetic Pulse Sources (ELVES). Because sprites, jets, and ELVES have appeared for millennia, their discovery was inevitable. The partial history related in this paper outlines the unsophisticated activities using shuttle video and the dissemination of the results by video presentations during the early phases of sprite research. This paper does not attempt to evaluate the advances in the science based on the measurement campaigns of Lyons, Sentman and the many other investigators.
Background
The observation of lightning is one of the oldest human activities on this planet. Yet, it is surprising that a category of lightning phenomena, that are visible to the unaided eye, were not more thoroughly investigated before 1989. Perhaps this stems primarily from the fact that the point-of-view for most observations, explanations, and theories about lightning has been from the vantage point of an earth-bound observer. When the lightning phenomena were viewed from a different vantage point - from above the thunderstorms (e.g., from space, aircraft or mountaintop) - new discoveries were made and insights gained into the upper atmospheric optical flashes now commonly referred to as sprites, jets, starters, and ELVES.
The lightning research summarized in this paper is a modern example of "pure science". These are serendipitous observations of natural events obtained by scientists who were unwilling to dismiss the unknown and the undiscovered as unimportant or random. The phases of an advance in science are comparable to the discovery of a new route over a mountain range. Rumors or stories of new trail motivate the search phase. Then someone with a combination of good luck and effort discovers a crossing. The initial discovery is followed by a trailblazer that confirms the discovery and leaves guidance for those who follow. Finally, exploration of the new trail involves a focused construction campaign and "heavy equipment". This paper reviews the initial investigations into sprites, jets, starters, and ELVES. The sequence of events in obtaining video in the time period before 1993 will be described as the phases of search, discovery, confirmation, and exploration.
Search Phase for Sprites and Jets
Throughout the historical scientific literature, there are sprinklings of eyewitness accounts of unusual "lightning" observed in the clear air above nighttime thunderstorms. The descriptions use phases such as "continuous darts of light... ascended to a considerable altitude, resembling rockets more than lightning." (MacKenzie and Toynbee, 1886), "a luminous trail shot up to 15 degrees or so, about as fast as, or faster than, a rocket" (Everett, 1903), "a long weak streamer of a reddish hue" (Malan, 1937), "flames appearing to rise from the top of the cloud" (Ashmore, 1950), or "the discharge assumed a shape similar to roots of a tree in an inverted position" (Wood, 1951). Partly because these eyewitness reports of unusual "lightning" appearing above thunderstorms were never captured on film, the lightning science community generally ignored them. The lack of an established vocabulary and the existence of several distinctive phenomena contributed to the variation in the verbal descriptions.
The number of reports increased as the view point moved up from the ground, to aircraft, and up towards space. Throughout his lifetime, Dr. Bernard Vonnegut maintained a keen interest in unusual lightning produced by thunderstorms and tornadoes. He was the principal investigator for the first space shuttle lightning observation experiment. He collaborated with O. H. "Skeet" Vaughan, Jr., who used his background as a researcher-pilot to gather information about pilot observations. A number of the pilots were reluctant to officially report the things they had seen, because the scientific community, the Air Force and the airlines were skeptical of "upward lightning". In the 1980's, Vaughan and Vonnegut (Vonnegut, 1980, Vaughan and Vonnegut, 1982; Gales, 1982; Vonnegut, 1984; Vaughan and Vonnegut, 1989) gathered and published reports of the unusual luminous phenomena that pilots saw above thunderstorms. The pilots chose a variety of terms and analogies to draw a verbal picture of "upward lightning". Although these were credible eyewitnesses with a professional interest in severe weather phenomena, their accounts did not inspire a general search for hard evidence of such phenomena. The lightning science community would instead depend on other serendipitous observations to move the leading edge of this science forward.
Discovery of a Lighting Phenomenon Now Known as a Sprite
The discovery of the phenomenon, now known as a sprite, was first documented on video tape recorded the night of July 6, 1989. Dr. John Winckler and associates at the University of Minnesota were conducting a cross calibration experiment of various optical sensors intended for a sounding rocket flight. As part of this experiment, Dr. Robert Franz was testing a low-light-level video camera by recording images of stars and a distant lightning storm. Dr. Franz arrived after dark and set up the camera, recorder and video monitor without lights. A few moments after he had found and replaced a defective cable connector, the monitor displayed a twin flash of light above the horizon. They immediately recognized that they had observed something unusual and noteworthy. After a search of the literature and some analysis, they concluded (Franz et al., 1990; Winckler et al., 1995) that they recorded an "upward lightning flash" that probably originated from a storm that was beyond the horizon. A daylight search of the terrain in the direction of the flash did not reveal twin TV towers or any other local explanation for the observation. Following publication of this result, Dr. Winckler distributed video tapes to a few interested researchers. The video tape contained both real time and slow motion images that accurately convey those transient qualities of the phenomenon that are poorly conveyed by a still picture and verbal description. In addition to documenting the reality of upward lightning discharges that are now called sprites, the group also demonstrated that image-intensified video cameras are required to capture the sprite phenomena.
Confirmation of the Sprites Phenomena: Observations from the Space Shuttle
Before the NASA space shuttle began to fly in 1981, Vaughan and Vonnegut (1979) proposed to use the shuttle as an observational platform to observe and photograph lightning. The main thrust of the research was to develop an understanding of the role of lightning and how it might relate to severe storm development as seen from space. The Night-Time/Daytime Optical Survey of Lightning Experiment (NOSL) hardware was flown on the space shuttle's STS-2, 4,and 6 missions (Vonnegut et al., 1983, Vonnegut et al., 1985). Data from that experiment was the catalyst for proposing a follow-on experiment called the Mesoscale Lighting Experiment (MLE) that used the space shuttle's sensitive payload bay monochrome video cameras to record the Earth's nighttime lightning activity from space (Boeck, 1987). During the next few years the techniques for obtaining and analyzing shuttle video imagery of lightning improved. Vaughan and Blakeslee modified the operational plans for MLE to emphasize the use of the payload bay cameras, rather than the astronaut hand held camcorder. They planned to use the remotely controlled payload bay cameras on a non-interference basis instead of requesting astronaut active participation, because there is a 24-hour mission control crew on the ground. While the shuttle crew members were sleeping, there were hours of opportunities for the ground based Instrumentation and Communications personnel (INCO) at JSC to make nighttime observations of storms. During the STS-30 (May 1989) and STS-34 (October 1989) missions, Vaughan and Blakeslee worked directly with the INCO during the missions as the INCO remotely controlled the TV cameras to track active thunderstorms.
In the spring of 1989, Vaughan found an image of a streak of light in the MLE video from the space shuttle. It may have been a sprite or it may have been video noise. In either case, the long axis of this event aligned with a video scan line, and therefore the image was indistinguishable from a product of random video noise. Vaughan continued to search for an unambiguous event in the space shuttle video. Three shuttle missions produced several hours of good data during the fall of 1989 and the spring of 1990. Some anomalous events accompanying lightning were seen in the video from the October 1989 STS-34 mission. In the summer of 1990, Dr. William Boeck began a sabbatical leave at the NASA Marshall Space Flight Center (MSFC). At that time, he and Vaughan worked on the analysis of the MLE data. During the fall of 1990, the MLE data archives expanded further with the addition of tens of hours of new lightning video. A copy of the Franz tape was also available for study.
Vaughan, Boeck, and other researchers at MSFC studied each MLE video tape using an editing VCR in 1990 and 1991. They watched hundreds of lightning storms searching for interesting examples of lightning. Early in this process they positively identified two or three "upward lightning" events (i.e., sprites) in the stratosphere. The researchers found that when playing the video tapes at normal playback speed, the sprites appeared and disappeared before a solid mental impression was formed. However, if the observer slowly played and replayed the video, the presence of a sprite image could be confirmed. At the 1990 Fall Meeting of the American Geophysical Union (AGU), Boeck and Vaughan (1990) presented a video documenting the first sprite found in shuttle data. The video format for the paper was chosen because information about transient optical phenomena is more accurately conveyed by a videotape replay rather than a description accompanied by a series of freeze frames. The freeze frames are contaminated with video snow that the human vision system can easily disregard during video playback.
In retrospect, researchers have determined that development time is a defining characteristic for classifying the type of upper atmospheric optical flashes. In the sprite video from Minnesota, as well as the seventeen examples from space shuttle video acquired between October 1989 and November 1991 (Boeck and Vaughan, 1990; Boeck et al., 1991a,b; Vaughan et al., 1992; Vaughan, 1993a,b; Vaughan, 1994a,b; Boeck et al., 1995), the sprites appeared in a single video field without a precursor at the sprite location. Researchers now know the characteristic development time of a sprite is on the order of 10 milliseconds (ms). This short duration fit the eyewitness descriptions of "flames" or "darts of light".
These 17 examples established that the sequence of visible events leading up to a sprite began with a discharge in the thundercloud. After a typical delay of a quarter to a half-second there was a large increase in both the horizontal extent and brightness in the cloud luminosity accompanied by the appearance of the sprite. Later work (Boccippio et al., 1994) has shown that these bright discharges are associated with large amplitude return strokes bringing positive charge downward. In fact, a positive return stroke accompanied the only MLE sprite recorded within range of a ground based lightning detection network. The videos showed that additional discharges continued in the clouds after a sprtie for a total mean time of a second, which can be interpreted as evidence for a continuing current. All together, this was strong evidence that the sprite above the thunderstorm was caused, directly or indirectly, by an energetic lightning discharge.
There were severe limitations in the use of shuttle video camera recordings as scientific data. The monochrome shuttle video did not provide any information on the optical spectrum of the sprites. Also, because the range to the sprites was well over 1000 kilometers only the brightest examples stood out from the video noise and the finer structure was poorly resolved in the images. Because there was no readout of the zoom lens focal length, only scenes with identified ground targets or star fields could be calibrated for sprite size and geolocation. The most severe limitation was an inability to track a storm for a time span of minutes to hours. This is primarily due to the speed at which the shuttle passes over the earth compounded by the fact that in INCO did not detect the presence of sprites while controlling the camera. The later work of Lyons (Lyons and Williams, 1993) proved that sprite producing storms can be tracked for hours.
Analyses of the MLE videotapes (see this discussion in Boeck et al., 1995, first draft of that paper submitted to JGR on 12/31/1992) produced several general classes of events, including, blobs, columns and filaments, and, distinction (e.g., single or multiple breaks) between the upper and lower parts of the sprite. In effect they had identified many of the larger features including the bright "head", weaker extensions (now referred to as "tendrils") below the "head", and occasional events showing weaker filaments above the main luminosity. Using star fields and the shuttle orbital elements, it was possible to establish the vertical and horizontal dimensions and stratospheric height (i.e., bright "head" located at 60 to 75 km) for several of the events. The width of the sprites varied considerably from very thin or even several thin filaments to broad columns some kilometers across, while the bright "head" (when visible) had dimensions on the order of kilometers. Figure 1 is an example of a video image of a sprite from a thunderstorm that is below the limb of the Earth, with a caption showing the vertical dimension and position for this case. This sprite image is very similar to the sprite images acquired from ground-based observations where the storm is often below the visible horizon. Lyons (1993) received a complete set of the unpublished shuttle videotapes that he used to obtain timing and other associations in an independent analysis. Lyons and Williams (1993) state that "After intensive and repeated viewing of the Space Shuttle video imagery and compilation of these statistics, one is left with the distinct impression that there is a rather high degree of repeatability in many aspects of the CS phenomena from event to event." (note, at this time sprites were called CS events by Lyons and Williams.) Subsequent work has confirmed and extended in great detail and with high resolution what was seen in low resolution shuttle video. The early estimates of the relative frequency of sprites were much too low. The large number of sprite images obtained in the summer campaigns of 1993 and 1994 by ground based and airborne camera established that the blobs, columns and filaments were in fact samples of the wide natural variation in appearance of this phenomenon.
The 17 examples that were eventually cataloged demonstrated that the occurrence of sprites is a global phenomenon, which occurs over land and sea (Boeck et al., 1995). The MLE videotapes provide the only evidence to date of sprite events in Australia, Africa and the south Pacific.
Exploration of Sprite Phenomena
The video confirmation of the existence and general appearance of sprites caught the attention of many investigators. The video was shown and distributed to audiences at the 1991 spring AGU meeting, the 1991 International Aerospace Lightning Conference, the February 1991 69th USAF and Navy Range Commander Council Meeting (Meteorology Group) and at seminars at Los Alamos National Laboratory in October 1991 and Stanford University in December of 1991. Copies of the videos were sent to NASA Headquarters, TV and news media, individuals as well as Air Force and other investigators.
Sentman and Wescott (1993) begin their article with "An exciting recent finding in middle atmospheric research is the confirmation that 'electrical discharges' occasionally appear to extend upward from thunderstorm regions substantial distances toward the ionosphere. Television observations from the ground (Franz et al., 1990;Winckler et al., 1993) and from space shuttle (Boeck et al., 1990; Vaughan et al., 1992; Boeck et al., 1993) of what has been called 'cloud-stratosphere lightning' have confirmed the existence of a previously reported (Boys, 1926; Malan, 1937; Vonnegut, 1980; Vaughan and Vonnegut, 1982; Vaughan and Vonnegut, 1989), but apparently rare, form of atmospheric discharge." (NOTE Boeck et al., 1993 in press was eventually published as Boeck et al., 1995) Dr. Davis Sentman and Dr. Walter Lyons actively pursued NASA funding and secured the level of support that allowed them to make detailed quantitative measurements of sprite phenomena in 1993.
Successful observations from the ground (Lyons, 1993; Lyons and Williams, 1993) starting July 7, 1993 and night-time aircraft observations (Sentman and Wescott, 1993; Sentman et al, 1993; Wescott et al., 1993) on July 8, 1993 established and rapidly matured the science of the sprite phenomena by acquiring and analyzing data based on hundreds of new events. Lyons and Williams (1993) report "These images almost certainly show the same phenomenon as observed by Franz et al. (1990), in the Space Shuttle imagery and the 'plume' type discharge described by Ashmore (1951), Wilson (1956), Fisher (1990), Malan (1937) and others". On the night of July 6-7 Lyons obtained 248 images, accompanied by NLDN data to demonstrate that this phenomena was an order of magnitude more frequent than the previous estimate based on shuttle observations. Sentman and Wescott (1993) state that "We recently performed a series of aircraft flights using NASA's DC-8 Airborne Laboratory in an attempt to capture additional video images of these discharges from a closer range than has been the case to date. In this letter we report results of a preliminary analysis of video data showing unambiguous optical signatures of numerous upper atmospheric optical flashes similar to those reported by earlier investigators. We present new quantitative data on their occurrence locations, physical dimensions, optical intensities and rates of occurrence relative to tropospheric lightning."
During the years of 1990 through 1993, authors used a number of different descriptive names for these flashes including "large upward electrical discharge" (Franz et al., 1990), "vertical light pulse" (Boeck et al. ,1991a,b), "cloud-to-stratosphere electrical discharges" (Lyons, 1993a; Lyons and Williams, 1993b; Lyons and Williams, 1994), "upper atmospheric optical flashes" (Sentman and Wescott, 1993; Wescott et al., 1993), "cloud to space lightning" (Vaughan et al., 1992; Vaughan et al., 1993a,b), "stratospheric flash" (Boeck et al., 1995), and "cloud-ionosphere electrical discharges" (Winckler, 1995). The extremely successful 1994 Sprites campaign (Lyons et al., 1994) and the video titled "Red Sprites and Blue Jets" (Sentman and Wescott, 1994) popularized use of the name sprite and established an "industry standard" vocabulary that was used in publications after that summer.
The optical and RF measurements collected during the 1994 field campaign rapidly uncovered the basic properties of sprites (Lyons, 1994; Lyons and Williams, 1994; Lyons et al., 1994; Sentman et al., 1994; Sentman et al., 1995; Wescott et al., 1994; Lyons et al., 1995a,b). Other workers (e.g., Boccippio, 1994) established the causal association of sprites with positive cloud-to-ground lightning discharges. The articles in this special issue describe the many advances in the understanding and measurement of the sprite phenomena that have since taken place. The shuttle video observations served a role as a starting point for the sophisticated observations, campaigns and analyses that now are the norm in this field of research.
Discovery of Rocket Lightning (now known as Blue Jets)
The initial shuttle images did not match the duration or the description of an upward moving "rocket-like" jet of light rising from the thunderstorm. The search for these "rockets" continued. In 1990, Vaughan, Boeck and associates discovered an upward propagating column within the shuttle video data that was recorded as the shuttle was passing over Australia on October 21, 1989. This column appeared to be an atmospheric phenomena that had been see before but had not been captured on film or video tape (e.g., MacKenzie and Toynbee, 1886; Everett, 1903). This video of a very active storm included a scene of jet of light rising like a rocket from the cloud anvil (Boeck et al., 1991a,b). Copies of this video were widely distributed prior to the publication of the discovery (Lyons, 1993). The image could not be calibrated with the result that size and velocity measurements were not made. The monochrome shuttle cameras provide no color or spectral information. The next recording of this phenomenon was made in 1994. In Boeck et al. (1995), there was sufficient data present to note that sprites are typically associated with low flash rate cells whereas this single rocket lightning was observed rising from a very active thunderstorm complex.
Confirmation of the Jet Phenomena
In the summer of 1994, Wescott and associates (Wescott et al., 1994; Wescott et al., 1995a,b) confirmed the existence of jets and named the phenomena blue jets when they recorded a very active thunderstorm in Arkansas, USA using both a low-light-level monochrome and a color video cameras. The video was collected during a nighttime research flight using two aircraft that were flying around the thunderstorm. During this flight, color video imagery established that jets are blue in color and sprites are red. A total of 52 jets were seen during a 20-minute time span. The jets developed over several video frames, with a characteristic time of the order of 100 ms and propagation speeds similar to that of a step leader process (i.e., ~ 105 m/s). The video released after this flight proved to be a turning point in establishing wide interest in these phenomena. The spectacular multiple close-up images of these jets completely overshadowed the single, poorly resolved jet observation from the space shuttle. Also discovered during this flight were examples of blue starters (Wescott et al., 1995), an upward moving luminous phenomenon closely related to blue jets. It is our belief that "rocket lightning" reported by (MacKenzie and Toynbee, 1886; Everett, 1903) and (Boeck et al., 1991a,b) is the same phenomenon as "blue jets" (Wescott et al., 1994; Wescott et al., 1995a,b).
Search for ELVES
There are no historical reports from eyewitnesses describing the phenomenon that is now called "Emission of Light and Very Low Frequency Perturbations From Electro-magnetic Pulse Sources" or ELVES (Lyons and Nelson, 1995). The one millisecond lifetime of this phenomenon explains why there have been no eyewitness accounts describing a brief flash that would fill the entire night sky for any observer within a 100 km radius from the causative lightning flash. Inan (1990) and Inan et al. (1991) predicted the existence of strong Joule heating of the base of the ionosphere by the electromagnetic pulses of natural lightning. They, however, did not expect that the heating would be sufficient to excite an optical emission.
Discovery of ELVES
After fifteen sprites and one jet had been identified in the shuttle video, a distinctively different event was discovered in shuttle video acquired on October 7, 1990 directly above an active thunderstorm off the coast of French Guyana (Boeck et al., 1992). A large horizontal flash appeared at the altitude of the airglow layer. It occurred in the video field before the appearance of main lightning flash in a thunderstorm that was near the limb of the Earth. They concluded that the causative lightning flash occurred slightly after the video scan passed the location of the storm image. There was a clear view of the mesosphere below the airglow layer, but there was no indication of a sprite in the video sequence (although a sprite event was captured 4½ hours earlier under similar moonlight conditions). A search of the shuttle video failed to produce a second example of this type of horizontal flash. Since it was clear that this was not an example of a sprite or a jet, the observations were published on the basis of this single example. The video was presented at the 1991 Spring AGU meeting (Boeck et al., 1991a) as well as at the Aerospace Lightning Conference (Boeck et al., 1991b). To promote a better understanding of these new phenomena as seen from space, Vaughan distributed a number of video tapes to various researchers who had express an interest in the phenomena. Seminars based on the video tape observations were given at Los Alamos National Laboratory and Stanford University. Researchers at both of these institutions have made major contributions to the theory of Sprite and ELVES phenomena.
Confirmation of ELVES
Several years passed before there was a second successful measurement of the ELVES phenomenon. On June 23, 1995 Lyons et al. (1995) and Fukunishi et al. (1996) confirmed the existence of a flash similar to the airglow flash seen earlier in the shuttle data, and Lyons et al. (1995) gave it the name Emissions of Light and Very Low Frequency Perturbations From Electromagnetic Pulse Sources (ELVES). Lyons presented video images captured by a low-light-level TV camera sited near Ft. Collins, Colorado. The ELVES phenomenon has a characteristic event duration of one millisecond (Fukunishi, 1996).
Conclusion
This brief partial history has outlined the unsophisticated activities based on the video tapes obtained from the space shuttle at the beginning of sprite research. Because these luminous phenomena have appeared and will continue to appear for millennia, their discovery was inevitable. This description barely touched the importance of good luck in obtaining the early videotapes. Progress from the search, to discovery, then to confirmation and exploration phases was achieved by a combination of serious investigation and luck. The element of luck became less important as the number and sophistication of the investigations increased. By that time an "industry standard" vocabulary of descriptive terms came into use.
Video evidence for the reality of sprites was first obtained in 1989. Although low-light-level TV cameras were produced before then, they had not been utilized to observe distant thunderstorms. The low-light-level monochrome TV cameras mounted on NASA's space shuttle fleet were never designed to be scientific instruments. The camera operators did not notice these rare and unusual events in the sky. Nevertheless, these video tapes (both ground based and shuttle) provided the early evidence of sprite, jet and ELVES phenomena. Analysis of the tapes by many investigators pointed the way toward better experimental approaches and deeper understanding of phenomena in the then very immature field. We think that the research sponsors of Sentman and his associates as well as Lyons had very little doubt about the reality of sprite phenomena.
The gallery of sprite, jet and ELVES images is now filled with excellent examples obtained using sensitive instruments mounted on high mountains or aircraft. The shuttle sprite images are now considered small, distant images in a noisy background but they served a purpose for a time. The high oblique view from the shuttle provided the only unambiguous simultaneous records of a strong lightning discharge in the clouds and the sprite in the mesosphere directly above the flash. The shuttle videos established that lightning directly or indirectly causes sprites.
The early video observations did much to encouraged others to take an interest in this subject area. This paper does not attempt to review the theoretical and experimental advances that followed the pioneering investigations. We are indebted to all who follow the trail bringing new insight and experiences to this subject.
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