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GEOPHYSICAL RESEARCH LETTERS, VOL. 22, NO. 10, PAGES 1205-1208, MAY 15, 1995
Preliminary results from the Sprites94 aircraft
campaign: 1. Red sprites
D.D. Sentman, E.M. Wescott, D.L. Osborne, D.L. Hampton, and M.J. Heavner
Geophysical Institute, University of Alaska Fairbanks, AK 99775-7320
Abstract. The dual jet aircraft Sprites94 campaign yielded
the first color imagery and unambiguously triangulated
physical dimensions and heights of upper atmospheric
optical emissions associated with thunderstorm systems.
Low light level television images, in both color and in black
and white (B/W), obtained during the campaign show that
there are at least two distinctively different types of optical
emissions spanning part or all of the distance between the
anvil tops and the ionosphere. The first of these emissions,
dubbed "sprites" after their elusive nature, are luminous
structures of brief (< 16 ms) duration with a red main body that
typically spans the altitude range 50-90 km, and possessing
lateral dimensions of 5-30 km. Faint bluish tendrils often
extend downward from the main body of sprites, occasionally
appearing to reach cloud tops near 20 kin. In this paper the
principal characteristics of red sprites as observed during the
Sprites94 campaign are described. The second distinctive
type of emissions, "blue jets," are described in a companion
paper [Wescott et al., this issue].
Introduction and Background
Accounts of brief optical emissions above thunderstorms
go back more than a century [see, e.g., Boys, 1926; Malan,
1937; Vaughan and Vonnegut, 1989], and Wilson [1956] has
discussed the possibility of lightning discharges extending
upward from cloud tops. However, it has not been until the
past few years that images of discharge events have been
obtained. Franz et al. [1990] recorded the first low light level
television images of these events, and estimated that they
extended to altitudes of 34 km or higher. Following this
observation Vaughan et al. [1992] and Boeck et al. [1994]
reported television observations from the space shuttle of a
number, now approaching twenty, of what appear to be similar
events above thunderstorms on the limb of the planet.
During the summers of 1993-1994 there occurred a marked
increase in the quantity and quality of reports of this
phenomenon. Using a low light level All Sky television
system, Sentman and Wesco, [1993] imaged nineteen
examples of these events during a single flight of NASA's DC-
8 Airborne Laboratory over thunderstorms in Iowa, Nebraska
and .Kansas in July, 1993. They estimated the most probable
terminal heights of the events to be 60 km, with error bars
extending to 100 km. The duration was estimated to be 16 ms
or less, and the brightness was calculated from comparison
with stellar brightness to be 25-50 kR, roughly that of
moderately bright aurorae. The occurrence rate was estimated
Copyright 1995 by the American Geophysical Union.
Paper number 95GL00583
0094-8534/95/95GL-00583 $03.00
to be approximately 1 for every 200-300 cloud-to-ground
strokes.
In a more sustained ground-based program, Lyons
[1994 a,b] obtained B/W images of high altitude flashes
above storm systems in Nebraska and Kansas in July and
August, 1993, increasing the inventory of recorded events to
more than six hundred. Similarly, Winckler [1994], using low
light level B/W cameras, recorded about 150 events above
intense thunderstorm systems to the south of his recording
station in Minnesota in July-August, 1993. Recent summaries
of observations before the summer campaigns of 1994 are
given by Boeck et al. [1994] and Lyons [1994a].
The high altitude luminous structures imaged in the above
reports have been variously referred to in the literature as
upward or cloud-to-ionosphere lightning, cloud-to-ionosphere
or cloud-to-stratosphere discharges. Such appellations suggest
possibly unwarranted parallels to normal tropospheric
electrical discharge activity, or imply specific production
mechanisms about which at present little is known. We
therefore adopted the non judgmental name "sprites" for these
events after their elusive qualities. Hence, the name
"Sprites94" for our observation campaign.
In this paper we report preliminary results of observations
of sprites recorded during the Sprites94 aircraft campaign of
June-July, 1994. The principal objectives of the research were
to obtain color images of the events, to triangulate their
dimensions and terminal altitudes accurately, to record their
optical waveforms and emission spectra, and to study their
ELF/VLF electromagnetic signatures. During the campaign a
second type of emission, distinct from sprites, was also
unexpectedly observed. In a companion paper Wesco, et al.
[this issue] describe these "blue jets." Sentman and Wescott
[1994] have previously released a short Video containing
selected sequences of television images obtained during the
campaign.
The Sprites94 Aircraft Campaign
Two jet aircraft were used as imaging platforms •to provide
the high altitude observing vantages and angular separation
necessary for triangulation studies. Flight missions were
conducted out of Oklahoma City after dusk on the dates listed
in Table 1 to areas of active thunderstorm activity in the
Midwest. The dates of the campaign interval were chosen to
provide for moon down observing conditions necessary for
low light camera operation during 2200-0200 Local Time
when thunderstorm activity was at a maximum.
Aircraft. The two corporate jet aircraft used to make the
observations were a Rockwell Jet Commander and an Israel
Aircraft Industries Westwind 2, both leased from Aero Air, Inc.
of Hillsboro, Oregon. Observations were conducted at altitudes
40,000-42,000 ft (12.2-12.8 kin) at speeds of 425 knots (790
km/hr). On a typical flight mission the aircraft flew in a loose
1205
1206 SENTM• ET AL.: RED SPRITES
Table I Sprites94 Flight Missions (1994)
UT Date Area of Tl•nderstorm Observation Hr. Data
29 Jun Colorado, Texas P•handl• 1.0
30 Jun Oklahoma to Nabraska 2.5
1 Jul Central Arkansas 2.0
3 Jul Gulf Coast, Florida Panhandle 3.0
4 Jul New Mexico t•o Kansas 3.0
5 Jul Nebraska to Colorado 1.25
6 Jul Oklahoma to Texas Panhandle 2.7 5
7 Jul Iowa, N•braska, Colorado 2.0
10 Jul Texas Panhandle 2.0
11 Jul South Dakoda 2.0
12 Jul Eastern/Central Texas 1.2 5
13 Jul Eastern Colorado 2.0
ß
trail formation, with the Jet Commander leading the Westwind
2 by 10-75 km.
Instrumentation. The aircraft were both fitted on the
right side with factory new plastic windows. Above 450 nm
their transmissivity was about 90%, and at 400 nm and 390
nm the transmissivity fell to 75% and 51%, respectively. The
primary camera systems utilized during the campaign are
described below. Additionally, the Jet Commander had a pair
of baffled photomultiplier tubes to detect flash waveforms and
propagation velocity, and a television slit spectrograph. The
Westwind 2 carried an intensified B/W All Sky camera as a
backup system. Finally, an ELF/VLF detector was deployed on
the ground northwest of Oklahoma City to record
electromagnetic wave forms associated with sprites.
Both aircraft were equipped with Trimble model Six-Vee 6
Global Positioning Satellite (GPS) navigation systems to
track aircraft position and speed. GPS information was
recorded directly onto the video images using a Horita model
GPS-2 SMPTE time code generator. The GPS signals were fed
to a master sync generator to synchronize the horizontal and
vertical sweeps of all camera systems. This permi.tted accurate
triangulation and stereoscopic visualization of images
simultaneously captured from the separate aircraft. Position,
altitude, and dimension measurements of the sprites
subsequently obtained from triangulation calculations were
accurate to about 2-3 km. Measured: GPS timing and camera
synchronization accuracy was +32 •, absolute.
The measurements reported here were obtained using the
following instruments.
Wide Anele Low Light Level Black and White C•tncr• We
used Dage-MTI VE-1000 SIT B/W cameras, which have a
sensitivity of 1.6 x 10 '7 W-m '2 at 550 nm and 300 lines
resolution. The cameras were equipped with Kinoptik 5.7 mm
f/1.8 TEGEA wide angle lenses, with 92øH x 69øV fields of
view (FOV). One Dage camera was on each aircraft. The video
images from these cameras were recorded at 60 fields per
second (fps) on Sony U-Matic 3/4 inch tape recorders, along
with GPS timing and navigational information.
Intensified Color Camera The color camera on the
_
Westwind 2 was an Ikegami HL-51S with 2,000,000 ISO
equivalent and sensitivity of approximately 2 kR [lkegami,
1984; Hallinan, 1988]. The camera has three separate SIT
subsystems with red, green and blue responses that maximize
at approximately 600 nm, 530 nm, and 450 nm, respectively
[see Wescott et al., this issue]. For Sprites94 the camera was
equipped with a Canon PV 10X 12B2 12-120 mm zoom lens,
operated at 12 mm and f/2.0, yielding a 55øH x 43øV FOV.
The images from the Ikegami camera system were recorded at
60 fps on a Sony Betacam SP recorder, along with GPS timing
and navigation information.
A ground support station was established at the National
Severe Storms Laboratory (NSSL) for the Sprites94 campaign
to provide real-time updates from the National Lightning
Detection NetwOrk (NLDN). Radar and GOES 7 infrared weather
maps from Weather Services, International (WSI) and
lightning maps from the NLDN were collected periodically and
uploaded to the Jet Commander via the AGRAS and SKYHOOK
communications systems.
Preliminary Results
An estimated 500 separate upper atmospheric optical events
were recorded during the twelve flight missions listed in Table
1. More than half of these events were imaged simultaneously
from both aircraft, and more than half by the color camera.
Figure 1 shows two events recorded during the campaign.
One of the first newly documentable results of the
observations, as shown here in the right panels, is that the
main upper portion of sprites is predominantly red. In the
images from the color camera the main body of the sprite
registers primarily in the red channel. Very weak or no
signatures are present in the blue and green channels.
The luminous regions appear to range from small single or
multiple vertically elongated spots to spots with faint
extensions above and below, to bright groupings which
extend from the cloud tops to altitudes up to 95 km. The
concentration of the luminosity into distinctive spatial
features tend to occur within a limited range of altitudes and is
sufficiently repeatable to warrant consideration as "unit"
elements. We refer to these unit elements as "sprites" for
reasons mentioned above.
A preliminary anatomy of sprites may be constructed based
on these features. Results of detailed triangulations of the
positions of the features of 29 separate sprites or clusters of
sprites reveal that the brightest region, the "head," lies within
the bracketed range of altitudes 66-74 km (+ 4 km). Above the
head there is often a faint red glow or wispy structure ("hair")
separated from the bright head by a dark band ("hairline"), and
extending to 88:•.5 km. On two of the sprite clusters studied in
detail the terminal height of the hair exceeded 95 km. The hair
may occasionally exhibit a weak horizontal dark bar•. •t 79+4
km. On some of the brighter sprites, as shown in ?'•igure 1,
there is a dark band ("collar") beneath the head at 66ñ 4 km,
below which extend tendril-like filamentary forms. The color
of the tendrils is red just below the collar, gradually merging
into blue with distance downward. The tendrils have been
triangulated to reach downward to as low as 40 km, below
which Rayleigh scattering and camera blooming from
intracloud lightning prevents identification of structural detail
necessary to perform a triangulation. Figure 2 shows our
preliminary anatomy of the basic sprite form, and its altitude
relationship to atmospheric temperature and typical night
time electron density profiles.
Sprites rarely appear singly, usually occurring in clusters of
two, three or more. Some of th e very large events, such as in
Figure 1, seem to be tightly packed clusters of many individual
sprites. The brightest regions of these "compound sprites"
typically saturated the B/W images, and in some cases the
color images, as well (e.g., the white central portion of the
sprites in Figure 1). Many sprite events consist of multiple,
SENTMAN ET AL.: RED SPRITF_• 1207
! 0354:12(26) Jet Corn UT 0354:12(26) W W 2 UT 0354:12(26)
4 Jul 94 4 Jul 94 4 Jul 94
U Ma U Ma
KI Kt
b c b
U Ma
Kt
WW2
100 km
8O
' UAF UAF
UT 0400:20(0) Jet Com UT 0400:20(0) W W 2 UT 0400:20(0)
0 4 Jul 94 4 Jul 94 4 Jul 94
U Ma e
• t UMa
-- Kt
c d
UAF UAF UAF
km
lOO
80
60
40
20
a b c d
W2
Figure 1. Two separate sprite events observed on 4 July, 1994. On each row the left-hand B/W image was taken from the
Jet Commander, while the center B/W image and the fight hand color images show the same event from the Westwind 2
aircraft. Selected stars within the respective fields of view have been labeled. The altitude scale in the color image refers t o
the brightest feature. Top Row UT 0345:12(video frame 25). A cluster of sprites. The parallax of stars in the constellation
Ursa Major relative to the sprites may be seen by comparing the B/W images. Bottom Row UT 0400:20(video frame 0)
This large compound sprite was one of the largest observed. Note the blue/purple tendrils. Aircraft separation for both
events was about 50 km.
spatially distinct sprites, which may be laterally distributed
across distances 40 km or more. Figure 3 shows an example of
a cluster of three distinct sprites that were imaged
simultaneously from the two aircraft while studying a storm
over the Texas panhandle on the night of 6 July, 1994. The
point on the ground below each of the individual sprites is
indicated in the accompanying map. The sprites occurred on
the backside of the frontal storm system that was moving
100 --'
80 --
60 --
40--
20--
0
Anatomy of a Sprite
Hair
(Red)
1tead
(Red)
Tendrils
(Red/
Purple/
Blue)
Optical Brightness -40 kR
Optical Energy 1-5 kJ
Optical Power 0.3-1.5 MW
Thllnderstorm ' -.-'-::
T osphere
e
re
Temperature
Nighttime /
E-Region/
Ledg•
D-Region
Electron
Density
Figure 2. Anatomy of a "unit" sprite and its altitude
relationship to atmospheric temperature and typical
night time electron density profiles. More
complicated compound sprites and sprite clusters
consist of groupings of the unit sprite at various
densities.
Texas Oklahoma
UT 0529:52(25)
a b
", .,'! .i .. ++. ..' ": ..
'.".:•'X ' .:':: :'i"2 " '. • ,
.l': .I. ß . ß • J• ' :• -• ' '•,,•:. '.
,.' .::..'.. ,",• . .,½- •. '- .•. ".,
.• -..,, ... -.-, • ?- '•-..,_
'" :•' ß . '1•. • ß - -=- r ß
ß .:.: . :..-r - - "-"-'"L .,..;,..
ß $' ,' ß a' ,r • . ."1 .'-' '
ß . ,•..•.. -1..,, ,_ . ,-..
, .. '-. v ß
..... •" ß ' UT 6 July, 1994 0440-0540 - ,,.,I '. '-',.-,,•...t r
, .' National Lightning Detection Network
Figure 3. Spatial location of the individual sprites in
the event of UT July 6 0529:52(video frame 25). The
location of the sprites, indicated by large red dots, is
projected onto a time/polarity color coded map of
cloud to ground lightning as recorded by the National
Lightning Detection Network. Positive ground
strokes are indicated by pluses, negative strokes by
dots. Strokes occurring, relative to the time the map
was stored (UT 0540), are coded as white (0-15 min),
yellow (15-30 min) or red (30-60 min). The sprites
are seen to be laterally distributed over distances
comparable to the terminal heights of the events
(-100 km).
1208 SENTMAN ET AL.: RED SPRI'IES
through the Texas panhandle. The majority of sprites observed
were similarly located well behind the associated storm fronts,
in regions where positive cloud-to-ground strokes were
dominant.
The red, bright portions of sprites and their extensions
above and below occur predominantly within the altitude range
50-95 km, thus making sprites primarily a mesospheric D-
Region phenomenon. The lateral dimension of "unit" sprites
is typically 5-10 km, corresponding to volumes that may
exceed 103 km 3. Compound sprites and sprite clusters may
occupy volumes greater than 104 km 3.
Detailed analysis of the apparent brightness of several
separate unit sprites determined by comparison to stellar
brightness references [Wescott et al., this issue] yields a
maximum brightness of roughly 600 kR within the sprite.
When averaged over the dimmer outer portions of the sprite
the mean brightness is about 50 kR, consistent with our
original brightness estimates [Sentman and Wescott, 1993] of
10-50 kR. If we assume the optical emission is centered at
600 nm and spatially distributed uniformly throughout a mean
estimated emission volume of 2000 km 3, this translates into a
total optical energy of 1-5 kJ per event. Assuming a duration
of 3 ms the instantaneous optical power of these unit sprites is
therefore in the range 0.3-1.5 MW. Energy and instantaneous
optical power of large sprite clusters and compound sprites my
exceed these figures by an order of magnitude or more, i.e., 10-
50 kJ and 3-15 MW, respectively.
Summary
Results of the present study may be summerized as follows.
1. Sprites are a mesospheric/D-region phenomenon. They
reach upward to an average maximum height of
approximately 88 km, with an rms altitude spread of
about 5 km. In the largest and brightest examples
observed terminal heights exceeded 95 km, placing the
tops of these events firmly within the lower regions of
the ionospheric E-region.
2. Sprites are predominantly red. The brightest region of a
unit sprite, its "head," lies between characteristic
altitudes of 66 km and 74 km. Above the head there is
often a dark band ("hairline") above which a faint, wispy
red glow ("hair") is observed to splay upward and outward
toward terminal altitude. Beneath the head there
sometimes occurs a dark band ("collar") at 66+ 4 km
below which faint tendrils may extend downward to
altitudes of 40-50 km, changing from red near the collar
to blue at their lowest extremities.
3. Sprites may occur singly, but more typically occur in
clusters of two or more. Clusters may be tightly packed
together into large structures 40 km or more across, or
loosely spread out into distended structures of spatially
separated sprites. Onset of luminance occurs
simultaneously (within video resolution, 16 ms) across
the cluster as a whole, coincident with the occurrence of
cloud lightning below. All elements of the cluster
typically decay in unison over 3-5 video frames (100-160
ms), but most of this decay time may be attributed to
image lag of the SIT cameras.
4. Sprites seem preferentially to occur on the backside of
frontal storm systems, and occur in regions of positive
lightning ground strokes.
5. The total optical energy in a typical unit sprite is
power of compound sprites and sprite clusters may exceed
these values by an order of magnitude.
Acknowledgments: We thank Mary Mellott and Rick Howard of Code
SS, NASA Headquarters, for support and encouragement in this project,
and Andy Cameron of the Earth Sciences Office for his invaluable
assistance. We thank Aero Air, Inc., Hillsboro, Oregon for use of their
aircraft; the enthusiasm and professional skills of Jeff Tobolski, Norm
Ralston, Mark Satterwhite, and Chuck McWilliams made the success of
the aircraft operations possible. We thank Dave Rust of the National
Severe Storms Laboratory, Norman, Oklahoma for his assistance and
ground station support. We are grateful to H. Stenbaek-Nielsen for
providing us with triangulation routines. Finally, we acknowledge useful
discussions with Walter Lyons, ASTER, Inc., Ft. Collins Colorado, and
Earle Williams, MIT. This research was supported by NASA Grant
NAG5-5019 and National Science Foundation Grant ATM-9217161.
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9977.5-7320
estimated to be 1-5 kJ, and the corresponding (Received November 11, 1994;revised February6, 1995;
instantaneous power is 0.5-2.5 MW. The energy and acc'epted February 17, 1995)
