The
Hundred Year Hunt for the Sprite
Walter A. Lyons Yucca Ridge Field
Station, FMA Research, Inc. Fort Collins, CO
Russell A. Armstrong Mission Research Corporation, Nashua, NH
E.A. Bering, III Department of Physics, University of Houston, Houston, TX
Earle R. Williams Massachusetts Institute of Technology, Parsons Laboratory,
Cambridge, MA
Science often advances at a deliberate and cautious pace. Over 100 years passed
before persistent reports of luminous events in the stratosphere and mesosphere
associated with tropospheric lightning were accepted by the scientific community.
Since 1886, dozens of eyewitness accounts, mostly in obscure meteorological
publications, have been accompanied by articles describing meteorological
esoterica such as turtles encased in hailstones and toads falling during rain
showers. The phenomena were variously described as “cloud-to-space lightning”
and “rocket lightning.” 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.”
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 possible 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 below the horizon. They
were diffuse, fan-shaped flashes…extending up into a clear sky.” Over the
next three decades, several compendia of similar subjective reports from credible
witnesses worldwide were prepared by Otha H. Vaughan (NASA) and the late Bernard
Vonnegut (The University at Albany). The events were widely dispersed geographically
from equatorial regions to above 50° latitude, with 75% occurring over land.
The eyewitness descriptions 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 6 July 1989. Scientists
from the University of Minnesota, lead 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 which 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. In the early 1990s, NASA scientists searched tapes from the Space
Shuttle’s LLTV camera archives and confirmed at least 17 apparent sprites
above storm clouds. The orbital perspective suggested a relationship between
sprites and tropospheric lightning. The specific lightning flashes associated
with the observed sprites often were among the brightest in the region. The
sprites occurred within milliseconds of the brightest cloud illumination,
and were apparently triggered by especially energetic discharges within the
storm cell which, while larger its neighbors, had otherwise unexceptional
flash rates.
By 1993, NASA developed concerns that this newly discovered “cloud-to-space
lightning” might be fairly common and thus pose a potential threat to Space
Shuttle missions especially during launch or recovery. Based upon the available
evidence, the hunt for these elusive events was directed above the stratiform
regions of large mesoscale convective systems (MCSs), known to generate relatively
few but often very energetic lightning discharges. On the night of 7 July
1993, a borrowed Xybion LLTV was deployed for the first time at the Yucca
Ridge Field Station (near Fort Collins, Colorado). Exploiting an uninterrupted
view of the skies above the High Plains, the LLTV was trained above a large
nocturnal MCS in Kansas, some 400 km distant. Once again, good fortune intervened
as 248 sprites were imaged over the next four hours. Analyses revealed that
almost all the sprites were associated with +CG flashes. Within 48 hours,
in a totally independent research effort, sprites were imaged from a NASA
DC8 aircraft over Iowa. The following summer, the University of Alaska’s flights
provided the first color videos detailing the red sprite body with bluish,
downward extending tendrils (Sentman et al., 1995). The same series of flights
documented the truly bizarre blue jets (Wescott et al., 1998).
During the last seven years the scientific community’s misperception of the
middle atmosphere above thunderstorms as “uninteresting” has completely changed.
Today a host of phenomena have been named: sprites, blue jets, elves, sprite
halos and trolls, with perhaps others remaining to be discovered. Collectively
they have been termed Transient Luminous Events (TLEs). But as research has
progressed, even this appellation appears somewhat inadequate. The Phenomenology
of Transient Luminous Events Much has been learned about the morphology of
TLEs.
Sprites can extend vertically between 95 km and under 30 km. While telescopic
investigations reveal that individual tendril elements may be of the order
of 10 m across, the envelope of the illuminated volume can exceed 104 km3.
Sprites almost invariably follow +CG flashes, with time lags of less than
one to over 100 ms. To date, there are only two documented cases of sprites
associated with negative polarity CGs. The sprite parent +CG peak currents
range widely, 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 suggest that sprites usually initiate around 70 to 75 km with
both downward and upward development at speeds ~107 m/sec. Sprite luminosity
on typical LLTV videos can endure for tens of milliseconds. Photometry suggests,
however, that the brightest elements usually persist for a few milliseconds
(Armstrong et al., 1998), though occasionally small, bright “hot spots” linger
for tens of milliseconds.
By 1995, sprite spectral measurements confirmed the presence of the N2 first
positive emission lines. In 1996, photometry provided clear evidence of ionization
in some sprites associated with blue emissions within the tendrils and sometimes
the sprite body. Peak brightness within sprites is on the order of 1000 kR.
In seven years of observations at Yucca Ridge, sprites were typically associated
with larger storms (>104 km2 radar echo), especially those exhibiting substantial
regions of stratiform precipitation. The TLE-generating phase of High Plains
storms averages about three hours. The probability of optical detection of
TLEs from the ground in Colorado is highest between 0400 and 0700 UTC. The
TLE counts observed from single storm systems has ranged from one to 776,
with 48 as an average count. Sustained rates as high as once every 12 seconds
have been noted, but more typical intervals are on the order of 2 to 5 minutes.
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 80 and 100 km altitudes (Taranenko et al., 1993).
Evidence for this was first noted in 1994 using LLTVs at Yucca Ridge, and
was confirmed the following year by photometric arrays deployed by Tohoku
University (Fukunishi et al., 1996). Elves, as they are now called, are believed
to be expanding quasi-torroidal structures which attain an integrated width
of several hundred kilometers. Some evidence suggests their intrinsic color
is red due to strong N2 first positive emissions. While relatively bright
(1000 kR), their duration is <500 microseconds. Roughly 5% of the TLEs detected
with standard LLTV imagers are elves. These usually follow by ~300 microseconds
very high peak current (often >100 kA) CGs, most of which are positive in
polarity. Stanford University researchers, using sensitive photometric arrays,
documented the outward and downward expansion of the elve’s 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 a diffuse
disk-shaped glow which lasts about a millisecond and superficially resemble
elves. However, these structures, now called “sprite halos,” are less than
100 km wide, and propagate downward from about 85 to 70 km altitude. Columnar
sprite elements sometimes emerge from the lower portion of the sprite halo’s
concave disk.
Blue jets are the least frequently observed TLE from ground based observatories,
in part due to atmospheric scattering of the shorter wavelengths. Evidence
from aircraft missions shows blue jets emerging from the tops of electrically
active thunderstorms. The jets propagate upwards at speeds of ~100 km/sec
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 al.,
1998). There have been anecdotal associations of blue jets with hail producing
storms. An ER2 pilot over the Dominican Republic flying above Hurricane Georges
in 1998 described seeing luminous structures which resembled blue jets.
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 moving upwards initially around 150 km/sec, then 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. Figure 1 portrays several forms
of TLEs and the lightning within a typical parent MCS. Footnote: (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. These include the larger mid-latitude MCSs tornadic squall lines, tropical
deep convection, tropical cyclones and winter snow squalls over the Sea of
Japan. It appears, however, that the central U.S. may be home to some of the
most prolific TLE producers, even though only a minority of High Plains thunderstorms
produce TLEs. Some convective regimes, such as supercells, have yet to be
observed producing many TLEs and the few are mostly confined to any stratiform
precipitation region which may develop during the late mature and decaying
stages. Furthermore the vast majority of +CGs, even many with peak currents
above 50 kA, produce neither sprites nor elves which are detectable using
standard LLTV systems.
While large peak current +CGs populate both MCSs and supercells, only certain
+CGs possess characteristics which generate sprites or elves. Monitoring in
the Schumann resonance bands 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 generating both sprites and elves (Huang et al., 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 associated with the very largest
+CGs (often >150 kA), exhibits much smaller charge moments (< 300 C-km).
Recent studies of High Plains MCSs confirm that their electrical and lightning
characteristics are radically different from the textbook “dipole” thunderstorm
model, derived largely from studies of rather small 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 melting
layers (order 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 which produces middle atmospheric optical emissions. Sprites occur
most readily above MCS stratiform precipitation regions with radar echoes
larger than ~104 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 ~3x105
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 only the larger MCS, which contain large stratiform precipitation
regions, give rise to 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 18 August 1999, a massive MCC produced a spectacular
series of sprites over Nebraska. We identified which CGs were associated with
TLEs using data from the National Lightning Detection Network (NLDN). 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 whereas the +CGs were mostly confined to the highest
cloud tops which are usually found above trailing portion of the stratiform
precipitation area. Moreover, only a small subregion of the westernmost trailing
stratiform area 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 which drive TLEs.
The Atmosphere Between the Clouds and Sprites
One of the more important gaps in our knowledge concerns the electrical environment
above large thunderstorms during TLE episodes. Consequently, the 1999 Sprite
Balloon Campaign conducted three high altitude balloon flights. Flight 3 flew
out of Ottumwa, IA between 0039 UTC and 1112 UTC on 21 August 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 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 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.
The Physics of Transient Luminous Events TLEs have captured the interest of
many theoreticians (Rowland, 1998). Several basic mechanisms have been postulated
to explain the observed luminous structures. These include excitation by a
quasi-electrostatic (QE) mechanism, sprite production by runaway electrons,
and elves from electromagnetic pulses (EMP). More than one process may be
operating, but on different temporal and spatial scales, in order to produce
the bewildering variety of TLE shapes and sizes.
Absent from almost all theoretical modeling efforts are specific data on key
parameters characterizing lightning flashes which actually produce TLEs. Many
modelers refer to standard reference texts which, in turn, tend to compile
data taken in storm types and locales which are not representative of the
nocturnal High Plains. 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 midcontinental storms (Williams, 1998).
A survey was made of the range of lightning parameters used in over a dozen
theoretical modeling studies. While the height of the vertical +CG channel
ranges from 4 to 20 km, there is 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 QE mechanism first proposed
by Wilson. The key physics of the problem appear to involve the altitude and
magnitude of the removed charge and the time scale on which this occurs –
parameters about which little agreement exists. Many theorists note that even
with an assumed tall +CG channel (~10 km) this still requires extremely large
(~100 Coulombs) charge transfers, typically ten times larger than in “conventional”
lightning. Some models yield a thousand-fold enhancement in optical intensity
at 75 km for a doubling of lightning charge removal altitude from 5 to 10
km. The use of shorter channels to ground, say 5 km, would imply truly large
charge transfers. Yet evidence is accumulating that indeed such may be the
case. While the various models simulate optical emissions bearing some (though
in many cases rather minimal) resemblance to the observations, such wide ranges
in the lightning source term parameters do not appear physically realistic.
If in fact such a range of lightning characteristics could produce sprites,
why does only a very small subset of +CGs (<1:20 even in active storms) actually
produce observable TLEs (with current sensors)? It appears that most models
have made assumptions about the lightning in order to produce something resembling
a TLE – rather than starting with hard physical constraints on the source
term. The reason, of course, is that there is very little data on the actual
CGs which generate specific TLE occurrences.
During the 2000 High Plains convection season, many of these questions may
begin to be resolved. Taking the Next STEPS. No lightning flash known to produce
a sprite or elve has ever been well characterized. To simulate complex TLEs,
modelers will require information on the total charge removed and its waveform,
the continuing current characteristics, the rate and altitude from which charge
was removed, and the geometry of the vertical and especially horizontal lightning
channels. Between 22 May and 16 July 2000, a major field observation effort
called the Severe Thunderstorm Electrification and Precipitation Study (STEPS)
will be undertaken. (www.mmm.ncar.edu/community/steps.html contains a comprehensive
overview). The STEPS domain, located 100 to 400 km east-southeast of Yucca
Ridge, is ideally situated for acquiring a wide array of optical measurements
of TLEs above the storm concurrent with the lightning discharges below. With
the inclusion of the 3-D lightning mapping array (LMA) from New Mexico Tech
(see www.lightning.nmt.edu/nmt_lms),
STEPS will facilitate a wide variety of measurements on TLE characteristics
but most importantly that of the unique lightning discharges which give rise
to them. STEPS goals will include documenting the differences between +CGs
which do and do not produce TLEs. STEPS may confirm whether or not massive
“spider” discharges are a necessary condition for sprites, and identify those
convective systems capable of producing TLEs. What is the relevance of these
findings? It has been suggested that there may be significant production of
NOx in the middle atmosphere by sprites. This becomes even more interesting
in light of recent observations that regional smoke palls from biomass burns
radically enhance the percentage of +CGs within storms, and thus increase
sprite counts (and middle atmosphere NOx production?). Schumann resonance
analysis methods (Huang et al., 1999) promise a means to obtain a worldwide
TLE census. There is growing interest in determining the sources of unusual
infrasound emissions detected above sprite-capable MCSs (Al Bedard, NOAA ETL,
personal communication). Along with characterizing the optical signatures
of TLEs, these findings may have important implications for global monitoring
efforts for the Comprehensive Test Ban Treaty. TLEs may contribute in ways
not yet understood to the maintenance of the global electrical circuit (Bering
et al., 1998). To quantify the impacts of TLEs, we require information on
the global frequency (now roughly estimated between one and ten per minute)
and their geographic distribution. Some of the STEPS findings may engender
some broader issues such as aerospace safety above 15 km.
ACKNOWLEDGEMENTS. This work has been supported by a number of agencies including
NASA’s Office of Space Sciences and the Kennedy Space Center, the U.S. Department
of Energy, and the National Science Foundation. REFERENCES Armstrong, R.A.
J.A. Shorter, M.J. Taylor, D.M. Suszcynsky, W.A. Lyons, and L.S. Jeong, 1998:
Photometric measurements in the SPRITES'95 and '96 Campaigns of nitrogen second
positive (399.8 nm) and first negative (427.8 nm) emissions. J. Atmos. and
Solar-Terrest. Phys. , 60, 787-799. Bering, E.A., III, A. A. Few and J.R.
Benbrook, 1998: The Global Electrical Circuit. Physics Today, October, 24-30.
Franz, R.C., R.J. Nemzek and J.R. Winckler, 1990: Television image of a large
upward electrical discharge above a thunderstorm system. Science, 249, 48-51.
Fukunishi, H., Y. Takahashi, M. Kubota, K. Sakanoi, U.S. Inan and W.A. Lyons,
1996: Elves: Lightning-induced transient luminous events in the lower ionosphere.
Geophys. Res. Lett., 23, 215-2160. Huang, E., E. Williams, R. Boldi, S. Heckman,
W. Lyons, M. Taylor, T. Nelson and C. Wong, 1999: Criteria for sprites and
elves based on Schumann resonance observations. J. Geophys. Res., 104, 16943-16964.
Lyons, W.A., 1996: Sprite observations above the U.S. High Plains in relation
to their parent thunderstorm systems, J. Geophys. Res. 101, 29,641-29,652
Rowland, H.L., 1998: Theories and simulations of elves, sprites and blue jets.
J. Atmos. And Solar-Terrestrial Phys., 60, 831-844. Sentman, D.D., E.M. Wescott,
D.L. Osborne, D.L. Hampton and M.J. Heavner, 1995: Preliminary results from
the Sprites 94 aircraft campaign: 1. Red sprites. Geophys. Res. Lett., 22,
1205-1208. Taranenko, Y.N., U.S. Inan, and T.F. Bell, 1993: The interaction
with the lower ionosphere of electromagnetic pulses from lightning: excitation
of optical emissions. Geophys. Res. Lett., 20, 2675-2678. Wescott, E.M., D.D.
Sentman, M.J. Heavner, D.L. Hampton and O.H. Vaughan, Jr., 1998: Blue Jets:
Their relationship to lightning and very large hailfall, and their physical
mechanisms for the production. J. Atmos. and Solar-Terrestrial Phys., 60,
713-724. Williams, E.R., 1998: The positive charge reservoir for sprite-producing
lightning. J. Atmos. Solar-Terrest. Phys., 60, 689-692
