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Upward Electrical Discharges from Thunderstorm Tops

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Upward Electrical Discharges from Thunderstorm Tops UPWARD ELECTRICAL DISCHARGES FROM THUNDERSTORM TOPS BY WALTER A. LYONS, CCM, THOMAS E. NELSON, RUSSELL A. ARMSTRONG, VICTOR P. PASKO, AND MARK A. STANLEY Mesospheric lightning-related sprites and elves, not attached to their parent thunderstorm’s tops, are being joined by a family of upward electrical discharges, including blue jets, emerging directly from thunderstorm tops. or over 100 years, persistent eyewitness reports in (Wilson 1956). On the night of 6 July 1989, while the scientific literature have recounted a variety testing a low-light television camera (LLTV) for an Fof brief atmospheric electrical phenomena above upcoming rocket launch, the late Prof. John R. thunderstorms (Lyons et al. 2000). The startled ob- Winckler of the University of Minnesota made a most servers, not possessing a technical vocabulary with serendipitous observation. Replay of the video tape which to report their observations, used terms as var- revealed two frames showing brilliant columns of ied as “rocket lightning,” “cloud-to-stratosphere light extending far into the stratosphere above dis- lightning,” “upward lightning,” and even “cloud-to- tant thunderstorms (Franz et al. 1990). This single space lightning” (Fig. 1). Absent hard documenta- observation has energized specialists in scientific dis- tion, the atmospheric electricity community gave ciplines as diverse as space physics, radio science, at- little credence to such anecdotal reports, even one mospheric electricity, atmospheric acoustics, and originating with a Nobel Prize winner in physics chemistry as well as aerospace safety, to explore the linkages between tropospheric lightning and the middle and upper atmosphere. Given concerns for AFFILIATIONS: LYONS AND NELSON—FMA Research, Inc., Fort possible impacts on aerospace vehicles, the National Collins, Colorado; ARMSTRONG—Mission Research Corporation, Aeronautics and Space Administration (NASA) im- Nashua, New Hampshire; PASKO—The Pennsylvania State University, mediately initiated a review of video tapes from the University Park, Pennsylvania; STANLEY—Los Alamos National space shuttle payload bay LLTV employed to image Laboratory, Los Alamos, New Mexico mesoscale lightning events. Over a dozen events ap- CORRESPONDING AUTHOR: Walter A. Lyons, CCM, FMA pearing to match Winckler’s observation were un- Research, Inc., Yucca Ridge Field Station, 46050 Weld County Road 13, Fort Collins, CO 80524 covered (Boeck et al. 1998). On 7 July 1993, the first E-mail: [email protected] night of observing at the Yucca Ridge Field Station DOI: 10.1175/BAMS-84-4-445 near Fort Collins, Colorado, Lyons (1994) docu- In final form 19 November 2002 mented over 240 events. Evidently they were not a ©2003 American Meteorological Society rare occurrence. The very next night, LLTV cameras onboard the NASA DC-8 detected huge flashes above AMERICAN METEOROLOGICAL SOCIETY APRIL 2003 | 445 complex conventional electric breakdown in the rar- efied middle atmosphere triggered by thunderstorm electric fields of electrostatic origin produced by the transfer of large amounts of charge to ground, follow- ing the basic suggestion of Wilson (1925). Most sprites start with small initiating zones around 70–75 km followed by complex networks of streamerlike dis- charges propagating first downward and then upward (Stanley et al. 1999). Not all thunderstorms produce sprites, including many with substantial numbers of +CGs. Evidence began to accumulate that +CGs occurring with the stratiform regions of large mesoscale convective sys- tems (MCSs) were preferred sprite producers (Lyons 1996). The Severe Thunderstorm Electrification and FIG. 1. The morphology of lightning-related, middle Precipitation Study (STEPS), conducted on the high atmospheric transient luminous events including red plains of Colorado and Kansas during summer 2000, sprites, blue jets, and elves. Also indicated is an addi- tional type of discharge from cumulonimbus tops that further documented that sprites most commonly occur may be a true “cloud-to-stratosphere” lightning or sim- above the stratiform precipitation region of MCS larger ply another manifestation of the blue jet phenomenon. than ~20,000 km2. Conditions there appear to favor Adapted from Lyons et al. (2000), reprinted with per- the formation of unusual +CGs producing extremely mission of the American Geophysical Union. large charge moment changes, the product of the charge lowered to ground and the altitude from which it was removed. As inferred from extremely low frequency a large thunderstorm complex in Iowa (Sentman and (ELF) sferics signatures, charge moments of the order Wescott 1993). of 1000 C km, more than an order of magnitude larger With the rush of discoveries, confusion soon arose than “normal” lightning flashes, are produced by regarding scientific terminology. Winckler and his sprite-parent +CGs (Williams 1998; Huang et al. 1999; colleagues initially termed their discovery a “cloud- Cummer and Stanley 1999; Hu et al. 2002). It is esti- to-stratosphere (CS) flash.” Press reports frequently mated, based on ELF data, that several sprites occur referred to “upward lightning” or “cloud-to-space each minute on a worldwide basis (Huang et al. 1999). lightning.” But little was known of the underlying Another phenomenon, called elves—toroidal- physics of these transient illuminations. Was it “light- shaped luminous regions near the base of the iono- ning?” In which direction did it really propagate? Did sphere—were confirmed in 1995 by coordinated ob- it connect the cloud top with “space”? To avoid as- servations at Yucca Ridge (Fukunishi et al. 1996). signing a name that might later need revision, Davis Theoretical modeling by scientists from Stanford Sentman of the University of Alaska proposed call- University had suggested the electromagnetic pulses ing them “sprites” (mysterious and fleeting charac- from some CGs should cause brief (< 1 ms), outwardly ters populating Shakespeare’s The Tempest). expanding glows between 80 and 100 km (Inan et al. Sentman’s team also provided the first color images 1996). The impetus for this research was a 1990 LLTV showing sprites to be primarily red with blue high- observation from the space shuttle in which there was lights on their lower extremities (Sentman et al. 1995), an apparent transient brightening of the “airglow so the term red sprites has become widely used. layer” at 85 km subsequent to a lightning flash in Sprites, often as bright as an intense auroa, can on the clouds below (Boeck et al. 1998). Elves, while hav- some occasions be seen with the naked eye. They ex- ing a peak optical output of ~1000 kR, are so brief tend from near the base of the ionosphere (~90 km), (< 0.5 ms) they are not visible to the human eye and with tendrils sometimes reaching below 40 km, but are difficult to detect even with LLTVs. there is no direct evidence of a conductive current During 1994, aircraft-mounted LLTVs searching path to the cloud tops below. Sprites appear within for sprites made a serendipitous discovery reminis- several to perhaps 200 ms after a powerful positive cent of reports of “rocket lightning” reported from the polarity cloud-to-ground lightning discharge (+CG). ground (MacKenzie and Toynbee 1886) and from the A variety of theories arose to explain the observations space shuttle LLTV (Boeck et al. 1998). Tenuous (Rowland 1998). Current thinking favors sprites as fountains of blue light, appearing as grainy and dif- 446 | APRIL 2003 fuse, trumpet-shaped ejections spurted upward from mately 120,000 feet . they were generally straight electrically active thunderstorm tops at speeds compared to most lightning bolts . .” >105 m s−1, reaching heights of 40 km or more • “at least ten bolts of lightning went up a vertical (Wescott et al. 1995). These rare events appear not blue shaft of light that would form an instant be- to be directly associated with any specific CG light- fore the lightning bolt emerged . .” ning flash. Lasting only 100–200 ms, though relatively • “a beam, purple in color . then a normal light- intense (~500–1000 kR), blue jets are difficult to see ning flash extended upwards at this point . after with the naked eye at night even under ideal viewing which the discharge assumed a shape similar to conditions. Jets typically appear to be several kilome- roots in a tree in an inverted position . .” ters wide at their base, becoming wider with height. • “a brilliant, blue-white spire . .” Also noted were blue “starters,” blue jets which would • “ an ionized glow around an arrow-straight finger attain no more than a few kilometers of vertical ex- core . .” tent (Wescott et al. 1996). We note that extensive • “a lightning spike” analyses of the video indicate that theoptical emissions of blue jets/starters are confined exclusively to blue These above observations simply do not describe wavelengths. Thus, no “white” component of the dis- sprites, elves, blue jets, or blue starters. Eyewitnesses charge is anticipated. Though blue jets propagate have, however, reported what appear to be blue jets. upward from the storm top into the stratosphere, their Heavner (2000) compiled numerous sightings, in- video appearance does not resemble “lightning” in cluding that of a pilot flying at 45,000 ft above a very any normal usage of the word. active thunderstorm at night who commented, Several competing theories of blue jets include both conventional and runaway breakdown in the I saw a blue ejection from the top of a storm cloud stratosphere above cumulonimbus cloud tops in the shape
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