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Eos,Vol. 81, No. 33, August 15, 2000

U N I O N

The Hundred Year Hunt for the Sprite PAGES 373-374,376-377

The scientific community's perception of the middle atmosphere above thunderstorms as "uninteresting" changed completely in the last decade.Today, a host of lightning-related Tran­sient Luminous Events (TLEs) have been identified, including sprites, blue jets, elves, sprite halos,and trolls. Others may remain to be discovered.

Aside from the intrinsic scientific issues arising from this linkage of tropospheric electrical phe­nomena with that of middle atmosphere, a num­ber of practical questions emerge.Whatjf any threats might TLEs pose to aerospace operations above 20 km? Do sprites represent a heretofore undocumented source of middle atmospheric Nox? What role might they play in the global electrical circuit, as well as in the energetics of the upper atmosphere [Bering et ai, 1998]? Might these phenomena impact satellite-based global monitoring and surveillance efforts?

Curiously, given the recent flurry of investiga­tions, over a century has elapsed since the first published reports of TLEs. Since 1886, dozens of eyewitness accounts of TLEs, mostly in obscure meteorological publications, have been accompanied by articles describing meteorological esoterica such as toads falling from the sky during rain showers. A typical description might read,"In its most typical form it consists of flames appearing to shoot up from the top of the cloud or, if the cloud is out of sight, the flames seem to rise from the horizon."

Science often advances at a deliberately cautious pace and such reports were largely ignored by the nascent atmospheric electricity community—even when they were posted by a Nobel Prize-winning physicist. As early as 1925, C.T. R.Wilson proposed mechanisms to explain such phenomena. In 1956,Wilson commented,"It is quite possible that a discharge between the top of the cloud and the ionosphere is a normal accompaniment of a lightning discharge to earth...a diffuse discharge between the top of the cloud and the upper atmosphere...many years ago I observed what appeared to be discharges of this kind from a thundercloud...they were diffuse, fan-shaped flashes...extending up into a clear sky"

Over the next 3 decades, many similar sub­jective observations from credible witnesses worldwide were reported. During the 1980s,

these were documented by Otha H.Vaughan at NASA Marshall Space Flight Center and the late Bernard Vonnegut at the State University of New York at Albany The observations shared one common characteristic: they were perceived as highly atypical of "normal" lightning. The reaction of the atmospheric science commu­nity could be summarized as indifference at best.Then, as so often happens in science, serendipity intervened.

Hard Evidence

The air of mystery began to dissipate at 0414 UTC on July 6,1989. Scientists from the University

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of Minnesota, led by John R.Winckler, were testing a low-light camera system (LLTV) for an upcoming rocket flight when, quite by accident, they captured two fields of video that provided the hard evidence for what are now called sprites [Franz et al, 1990]. From this singular observation emanated a decade of fruitful research into the electrodynamics of the middle atmosphere.

The sprites occurred several milliseconds after especially energetic discharges within the storm cells which, while larger than their neighbors, had otherwise unexceptional flash rates. By 1993, the NASA Shuttle Safety Office developed concerns that this newly discovered "cloud-to-space lightning" might pose a threat to Space Shuttle missions during launch or recovery Based upon the then-available evidence, the hunt for these elusive events was directed above the stratiform regions of large mesoscale convective systems (MCSs),

E L V E (Expanding Disk)

Upward Superbolt

{To Be Conventional Cioud-to-Air

Discharge

o - 1

Negative Cloud-to-Ground Flash Near Convective Core

Stratiform Region

Posi t ive^ loud-to-Ground Flash with "Spider Lightning"

100 DISTANCE (km)

200

Fig. 1. Depiction of a sprite, elve, blue jet, cloud-to-air lightning, and the parent lightning within a large mesoscale convective complex. The complexuspider lightning"event associated with the sprite parent +CG is believed to be centered near the melting layer, typically between 4 km and 6 km. (Figure courtesy of Carlos Miralles,AeroVironment, Inc.). Original color image appears at the back of this volume.

T R A N S A C T I O N S , A M E R I C A N G E O P H Y S I C A L

Eos,Vol. 81, No. 33, August 15, 2000

Fig. 2. GOES infrared cloud-top temperatures (purple being coldest) overplotted with NLDN nega­tive CG flashes (blue) and positive CG flashes (pink) during a 30-min interval centered on 0545 UTC, August 18,1999. Those +CGs associated with sprites are highlighted by a box. The sprite events are restricted to a relatively small portion of the cold cloud tops that overlays the trailing stratiform precipitation region below. Optical confirmation of sprites was made using LLTV cameras at the Yucca Ridge Field Station (YRFS) in Colorado. Original color image appears at the back of this volume.

which were known to generate relatively few but often very energetic lightning discharges.

On the night of July 7,1993, an LLTV was deployed for the first time at the Yucca Ridge Field Station near Fort Collins, Colorado.The LLTV was trained above a large nocturnal MCS 400 km away in Kansas. Once again, good fortune intervened, as 248 sprites were imaged over the next 4 hours.

On the very next night, in a totally independ­ent research effort sponsored by NASAs Office of Space Sciences (OSS), the University of Alaska imaged sprites over Iowa from on board the NASA DC-8 Airborne Flying Laboratory Their 1994 flights provided the first color videos detailing the red sprite body with bluish, downward-extending tendrils, as well as the first quantitative measurements of sprite altitude range and absolute brightness [Sentman et ai, 1995] .The same series of flights documented the truly bizarre blue jets [Wescottet ai, 1998].

The Phenomenology of Transient Luminous Events

Much has been learned about the morphol­ogy of TLEs. Sprites can extend vertically from 95 km to less than 30 km. While telescopic investigations by Stanford University reveal that individual tendril elements may be on the order of 10 m across, the envelope of the illuminated volume can exceed 104 km3. Sprites almost invariably follow cloud-to-

ground lightning flashes of positive polarity (+CG),with time lags of less than 1 ms to more than 100 ms [Lyons, 1996].To date,only two cases of sprites associated with negative polarity CGs have been documented (by Stan­ford and the New Mexico Institute of Mining and Technology).

The sprite parent +CG peak currents range from under 10 kA to over 100 kA, though on average, the sprite +CG peak current is 50% higher than other +CGs in the same storm. High speed video images obtained by Mark Stanley of the New Mexico Institute of Mining and Technology (NIMIMT) suggest that sprites usually initiate between 70 km and 75 km, with both downward and upward development at speeds of ~107 m/s. Sprite luminosity on typ­ical LLTV videos can endure for tens of milliseconds. Photometry suggests, however, that the brightest elements usually persist for only a few milliseconds [Armstrong et ai,2000].

By 1995, independent spectral measurements by both the University of Alaska and Lockheed Martin confirmed the presence of the N 2 first positive emission lines in sprites. In 1996, pho­tometry and blue-filtered LLTV imagers (Dave Suszcynsky of Los Alamos National Laboratory) provided clear evidence of ionization in some sprites within the tendrils and sometimes the sprite body Peak brightness within sprites is esti­mated to be on the order of 1000 kR.

In 8 years of observations at Yucca Ridge, sprites were typically associated with larger storms, especially those exhibiting substantial

regions of stratiform precipitation. The TLE-generating phase of nocturnal High Plains storms averages about 3 hours, peaking between 0400 UTC and 0700 UTC. The TLE counts observed from single storm systems has ranged from 1 to 776, with 48 being an average count. Sustained rates as high as once every 12 s have been noted, but more typical intervals are on the order of 2-5 min.

In the early 1990s, Stanford University researchers proposed that the electromagnetic pulse (EMP) from CG flashes could induce a transient glow in the ionosphere between an altitude of 80 km and 100 km [Taranenko et ai, 1993]. Evidence for this was first noted in 1994, using LLTVs at Yucca Ridge, and was confirmed the following year by photometric arrays deployed byTohoku University [Fukun-ishi et al, 1996]. Elves, as they are now called, are believed to be expanding, quasi-toroidal structures that attain an integrated width of several hundred kilometers.While relatively bright (1000 kR), their duration is <500 msec. They usually follow by -300 msec very high peak current (often >100 kA) CGs, most of which are positive in polarity.

Stanford researchers, using sensitive photo­metric arrays, documented the outward and downward expansion of the elves' disk.They also suggest many more dim elves occur than are detected with conventional LLTVs. These fainter elves have been suggested to be more evenly distributed between positive and nega­tive polarity CGs.

Recently, it has been determined that some sprites are preceded by diffuse, disk-shaped glows that last about a millisecond and superfi­cially resemble elves. However, these structures, now called "sprite halos," are usually less than 100 km wide and propagate downward from about 85 km to 70 km altitude. Columnar sprite elements sometimes emerge from the lower portion of the sprite halo's concave disk.

Blue jets are rarely observed from the ground, in part due to atmospheric scattering of the shorter wavelengths. Evidence from the University of Alaska's aircraft missions shows blue jets emerging from the tops of electrically active thunderstorms.The jets propagate upward at speeds of -100 km/s, reaching ter­minal altitudes around 40 km.Their estimated brightness is on the order of 1000 kR. Blue jets do not appear associated with specific CG flashes. Curiously, however, CG lightning activity appears to cease for several seconds within a 15-km radius after each blue jet occurrence [ Wescott et ai, 1998]. Blue jets have been anec-dotally associated with hail-producing storms.

The troll is the most recent addition to the TLE family. In LLTV videos, trolls superficially resemble blue jets, yet they are clearly domi­nated by red emissions. Moreover, they occur after an especially vigorous sprite in which tendrils have extended downward to near cloud tops.The trolls exhibit a luminous head leading a faint trail and move upwards initially around 150 km/s, gradually decelerating and disappearing by 50 km. It is still not known whether the preceding sprite tendrils actually extend to the physical cloud tops, or if the trolls emerge from the storm cloud per se.

Eos,Vol. 81, No. 33, August 15,2000

Figure 1 shows several forms of TLEs and the lightning within a typical parent MCS.The May-June 1998 issue of the Journal of Atmos­pheric and Solar-Terrestrial Physics was dedi­cated to TLEs and provides a valuable source of references.

Parent Storms and Lightning

Worldwide, a variety of storm types have been associated with TLEs, including larger mid-latitude MCSs, tornadic squall lines, tropi­cal deep convection and cyclones, and winter snow squalls over the Sea of Japan. It appears, however, that the central United States may be home to some of the most prolific TLE producers, even though only a minority of High Plains thunderstorms produces TLEs. Some convec­tive regimes, such as electrically prolific super-cells, rarely generate TLEs except during their late mature and decaying stages, as stratiform precipitation layers form.

Furthermore, the vast majority of + C G S , even many with peak currents above 50 kA, produce neither sprites nor elves that are detectable using standard LLTV systems. While large peak current + C G S populate both MCSs and super-cells, only certain -HCGS possess characteris­tics that generate sprites or elves.

Monitoring at extremely low frequencies (ELF) in the Schumann resonance bands (3-120 Hz) has provided a clue for what dif­ferentiates the TLE parent CG from "normal" flashes. Real-time visual sprite observations at Yucca Ridge coordinated with ELF transients (Q-bursts) detected at a Rhode Island receiver station clearly demonstrate that Q-bursts are companions to the +CG flashes that generate both sprites and elves [Huang et ai, 1999].

Sprite parent +CGS are associated with exceptionally large charge moments (300 C-km to >2000 C-km).The sprite +CG ELF wave­form spectral color is "red;" that is, peaked toward the fundamental Schumann reson­ance mode at 8 Hz. As Williams [1998] points out, lightning charge transfers of hundreds of Coulombs may be required for consistency with theories for sprite optical intensity, and to account for the ELF Q-burst intensity. Light­ning causal to elves has a much flatter ("white") ELF spectra, and though it is asso­ciated with the largest +CGs (often >150 kA), it exhibits much smaller charge moments (<300 C-km).

Studies of High Plains MCSs confirm that their electrical and lightning characteristics are radically different from the textbook "dipole" thunderstorm model derived from studies of smaller convective storms.Vast hori­zontal laminae of positive charge are found, often near the 0°C layer, and these structures persist for several hours over spatial scales of -100 km.With positive charge densities of 1-3 nC/m3, even relatively shallow layers (on the order of 500 m) covering 104 to 105 km2 can contain thousands of Coulombs.

Some 75 years ago,C.T.R.Wilson postulated that large charge transfers and particularly large charge moments appear to be a necessary condition for conventional breakdown in the middle atmosphere. Sprites occur most readily

above MCS stratiform precipitation regions with radar echoes larger than -10 4 km2.

It is not uncommon to observe rapid fire sequences of sprites propagating above storm tops, apparently in synchrony with a large underlying horizontal lightning discharge. One such "dancer" included a succession of eight individual sprites within 700 ms along a 200-km-long corridor.This suggests a propagation speed of the underlying "forcing function" of -3 x 105 m/s.This is consistent with the propa­gation speed of "spider" lightning—vast hori­zontal dendritic channels tapping extensive charge pools once a +CG channel with a long continuing current becomes established.

It is suspected that the stratiform precipita­tion regions of large MCSs favor the +CGs associated with the spider lightning networks able to lower the necessary charge to ground. The majority of sprite parent +CGS are concentrated in the trailing MCS stratiform regions [Lyons, 1996].The radar reflectivities associated with the parent +CGs are relatively modest, 30-40 dBZ or less. On August 18,1999, a massive MCC produced a spectacular series of sprites over Nebraska. CGs associated with TLEs using data from the National Lightning Detection Network (NLDN) were identified.

Plotting the CGs by polarity on a GOES satel­lite, infrared cloud top temperature map revealed a distinct pattern (Figure 2). Predominantly CGs were present along the leading edge of the MCS while the +CGS were mostly confined to the highest cloud tops, which are often found above the trailing stratiform precipita­tion area. Moreover, only a small subregion of the trailing stratiform produced sprite and elves. It would appear that this portion of the MCS possessed, for several hours, the requisite dynamical and microphysical processes favor­able for the unique electrical discharges that drive TLEs.

The Atmosphere Between the Clouds and Sprites

One of the more significant gaps in our knowl­edge concerns the electrical environment above large thunderstorms during TLE episodes. Consequently, the 1999 NASA OSS Sprite Bal­loon Campaign conducted three high-altitude balloon flights to obtain such ambient data. Flight 3 flew out of Ottumwa, Iowa, between 0039 UTC and 1112 UTC on August 21,1999. The balloon floated at 32 km and drifted westward at -30 knots.The payload was instrumented with dual, three-axis electric field detectors, three-axis fluxgate and induc­tion magnetometers, an x-ray scintillation counter, a Geiger-Mueller tube, upward-look­ing high-speed photometer, vertical current density meter, conductivity measurements, and an ambient thermometer.

Ground-based LLTV observations were made from three sites in Wyoming, South Dakota, and Colorado. All three stations had clear skies. There were two small TLE-producing storms, one in eastern South Dakota and one in central Kansas. Of 67 TLEs seen by at least one station or the balloon, five were seen by two or more stations. The balloon data at a

typical range of 300 km show that the sprite is accompanied by a positive vertical electric field pulse of -0.2 V/m. Curiously, no perturba­tion in any component of the electromagnetic fields was observed during the several milliseconds between the lightning flash and a sprite. Also, two very bright elves or halos were detected by ground and balloon optical sensors.The NLDN, however, failed to compute an associated CG.

Analysis of the raw network sensor data did, however, reveal that the elve parent lightning events were each observed by over 100 sen­sors. This powerful sferic was so complex as to prevent classification by the NLDN algorithms. Preliminary results from this flight have indi­cated that more data are required before we understand the complex physics involved.

Linking Lightning to Stratospheric and Mesospheric Events

Several mechanisms have been postulated by theoreticians at Stanford University, Los Alamos National Laboratory, the U.S. Naval Research Laboratory, and the University of Maryland, among others, to explain the rela­tionships between lightning and the observed luminous structures [Rowland, 1998].A key element is likely excitation of sprites by con­ventional breakdown in quasi-electrostatic (QE) fields produced by the transport of large quantities of charge.

A potential role for runaway electrons pro­ducing x-rays and gamma-rays (Bremsstrahlung) in the lightning-induced fields has also been advanced.The role of the lightning electromagnetic pulse (EMP) in triggering elves has been extensively modeled. Blue jets have been related to streamer mechanisms above highly electrified cloud tops by Victor Pasko at Stanford and others. More than one process may be involved, but on different tem­poral and spatial scales to produce the bewil­dering variety of TLE shapes and sizes. Absent from the models are specific data characteriz­ing the flashes that actually produce TLEs, most using standard references from lightning data that are not representative of nocturnal High Plains storms.

Specifically, many invoke the conventional view that the positive charge reservoir for the lightning is found in the upper portion of the cloud at altitudes of -10 km.The positive dipole (or tripole) storm model has been found wanting in many mid-continental storms [Williams, 1998]. In over a dozen theo­retical modeling studies, the assumed height of the vertical +CG channel ranges from 4 km to 20 km, with a clear preference for 10 km and above.The amount of charge lowered varies over three orders of magnitude, as does the time scale over which the charge transfer occurs.

Only a few papers consider the possible role of horizontal components of the parent discharge.The charge moment (in C-km), not the peak current as measured by the NLDN, is the key parameter in the basic quasi-electro­static mechanism first proposed by Wilson. The key physics of the problem appears to involve the time scale, altitude, and magnitude

Eos, Vol. 81, No. 33, August 15,2000

of the charge removal—parameters for which few measurements existed. Many theorists note that even with an assumed tall +CG channel (-10 km), this still requires extremely large (-100 Coulombs) charge transfers, typi­cally 10 times larger than in "conventional" lightning.The use of shorter channels to ground—say, 5 km—would imply truly large charge transfers.Yet evidence is accumulating that such may be the case.

Taking the Next STEPS

The various TLE optical emission simulation models employ such wide ranges in the light­ning source term parameters as to suggest a very large population of TLEs. If such a spec­trum of lightning characteristics could gener­ate TLEs, why does only a very small subset of +CGS (<1:20 even in active storms) actually produce sprites and elves detectable with cur­rent sensors? This divergence arises from the fact that until the summer of 2000, no lightning flash known to have generated a TLE has been well characterized.This has now changed. From May 22 to July 16,2000, a major NSF-fund-ed field observation effort called the Severe Thunderstorm Electrification and Precipita­tion Study (STEPS) was undertaken. (For an overview, see Web site: http://www.mmm. ncar.edu/community/steps.html).

The STEPS domain, 100-400 km east-southeast of Yucca Ridge, was ideally situated for monitor­ing TLEs above the storm concurrent with the lightning below.The deployment of the new three-dimensional lightning mapping array (LMA) from NMIMT (www.lightning.nmt.edu/nmt_lms) yielded detailed measurements of the unique lightning discharges that gave rise to TLEs.

Once analyzed, the STEPS data will allow documentation of the differences between CGs that do and do not produce TLEs. Modelers will be provided with information on the altitude, rate, and total charge removed; the continuing current characteris­tics; and the geometry of the vertical and especially horizontal lightning channels. STEPS may confirm whether or not massive "spider" discharges are a necessary condition for sprites and discriminate those convective systems that are capable of producing TLEs. Of course, other issues related to TLE forma­tion remain,such as the influence of the underlying storm and other factors on the mesosphere. But a deeper understanding of how lightning discharges produce TLEs is at hand.

Acknowledgments

This work has been supported by a num­ber of agencies, including the Office of Space Sciences and the Kennedy Space Center of the National Aeronautics and Space Administration, the U.S. Department of Energy, the U.S. Air Force, and the National Science Foundation.

Authors

Walter A. Lyons, Russell A. Armstrong, E.A. Bering III, and Earle R. Williams For more information, contact Walter A. Lyons, Yucca Ridge Field Station, FMA Research, Inc.,Fort Collins,Colo., USA; E-mail: walyons@ frii.com

References

Armstrong, R. A., D. M. Suszcynsky, W. A. Lyons, and T. E. Nelson, Multi-color photometric measurements of ionization and energies in sprites, Geophys. Res. Lett, 27,653-656,2000.

Bering, E. A., Ill, A. A. Few, and J. R. Benbrook,The global electrical circuit,Physics Today, 57,24-29, October 24-30,1998.

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.

Fukunishi, H.,Y.Takahashi, M. Kubota, K. Sakanoi, U. S. Inan, and W A. Lyons, Elves: Lightning-induced transient luminous events in the lower ionosphere, Geophys. Res. Lett., 23,215-2160, 1996.

Huang, E., E.Williams, R. Boldi, S. Heckman, W Lyons,M.Taylor,! Nelson,and C.Wong,Criteria for sprites and elves based on Schumann resonance observations,/. Geophys. Res., 104,16,943-16,964, 1999.

Lyons, W. A., Sprite observations above the U.S. High Plains in relation to their parent thunderstorm systems,/ Geophys. Res., 707,29,641-29,652,1996.

Rowland, H.L.,Theories and simulations of elves, sprites and blue jets, J. Atmos. Solar-Terr. Phys., 60, 831-844,1998.

Sentman, D. D., E. M. Wescott, D. L. Osborne, D. L Hampton, and M. J. Heavner, Preliminary results from the Sprites 94 aircraft campaign: 1. Red sprites, Geophys. Res. Lett., 22,1205-1208,1995.

Taranenko.Y N., U. S. Inan, and T. FBell,The interac­tion with the lower ionosphere of electromagnet­ic pulses from lightning: Excitation of optical emissions, Geophys. Res. Lett., 20,2675-2678,1993.

Wescott, E. M., D. D. Sentman, M. J. Heavner, D. L. Hampton, and O. H.Vaughan Jr., Blue Jets:Their relationship to lightning and very large hailfall, and their physical mechanisms for the produc­tion, J. Atmos. Solar-Terr. Phys., 60,713-724,1998.

Williams, E. R.,The positive charge reservoir for sprite-producing lightning,.//Ifrrcos. Solar-Terr. Phys., 60,689-692,1998.

NSF Considers Recommendations for Marine Seismic Reflection

PAGES 373-374 As marine seismic reflection programs have

become more complex, the number of active investigators has diminished because of the extensive technical expertise required. Estab­lishing national facilities for at-sea seismic acquisition is one way to shift much of the operational burden from individual scientists, enabling more scientists to use modern seismic methods and data. In addition, creating a seis­mic reflection data center would relieve acces­sibility issues and impediments to efficient multidisciplinary research and education.

The U.S. National Science Foundation (NSF) funded a workshop to address marine seismic capabilities in the academic fleet, and the rec­ommendations described above were part of the result. If implemented, the recommenda­tions—which are now before NSF—would engage a broader segment of the scientific community in seismic acquisition and inter­pretive activities by simplifying fieldwork and access to data collected by other investi­gators. There are currently no official national facilities for seismic acquisition.The establish­ment of one or more NSF facilities would be

a significant step, since many shortcomings could be addressed by consolidating manage­ment and incorporating coherent community-based input at facilities operations.

The objectives of such a national facility should be to 1) provide improved performance and reliability; 2) make access easier for more scientists; 3) advance acquisition technology through system upgrades; and 4) reduce the need for detailed knowledge of seismic data acquisition procedures.

For example, reducing the need for a scien­tist to have detailed expertise in single-chan­nel reflection methods should greatly benefit the non-seismologists who need these data as part of other geologic investigations. Below is one example of how two facilities could be organized. A number of other models would work just as well.

Portable-seismic facility. A single academic facility could provide support for convention­al high-speed,single-channel acquisition now conducted on RVs Hatteras,Revelle, Melville, Thompson, Wecoma, and others.This facility could also include high-resolution,short-streamer, two-and three-dimensional multichan­

nel systems. A facility would be responsible for scheduling, equipment pooling, dedicated technical support, and management. A set of standard deliverables of navigation and field seismic data should be arranged for delivery to the investigator and a data center.

Single-ship multichannel seismic facility (for example, RVEwing). Another academic facility could support a multichannel-capable ship by providing the preponderance of tech­nical services during acquisition for naviga­tion and seismics. At-sea support should include providing the tools and knowledge base to help the investigator make decisions about the acquisition performance relative to the goals of the project and producing stan­dardized navigation and field seismic products for the investigator and data center.

Presently, there are no agreed-upon data for­mats or central archives dedicated to seismic reflection data. Simplifying access to seismic data sets is probably the most valuable com­ponent of increasing the use and impact of seismic data in research. Improved access to seismic reflection and related data can best be served with a formal data center structure that identifies a responsible data manager, assures data quality, and simplifies monitoring of compliance. An academic institution or the Incorporated Research Institutions for

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Eos,Vol. 81, No. 33, August 15,2000

Fig. }. Depiction of a sprite, elue, blue jet, cJoud-to-air lightning, and the parent lightning within Q

large mesoscale convective complex. The complex "spider lightning" event associated with the sprite parent +CG is believed to be centered near the melting layer, typically between 4 km and 6 km. (Figure courtesy of Carlos Miralles,AeroVironment, Inc.).

Fig. 2. GOES infrared cloud-lOP temperatures (purple being coldest) ouerp{olted with NWN nega­tive CG flashes (blue) and positive CG flashes (pink) during a 30-min interval centered on 0545 UTC,August 18, 1999. Those +CGs associated with sprites are highlighted by a box. The sprite events are restricted to Q relatively small portion of the cold cloud tops that overlays the trailing stratiform precipitation region below. Optical confirmation of sprites was made using UN cameras at the }itcca Ridge Field Station (YRFS) in Colorado.