Report Observations
Red sprites and blue jets are upper atmospheric optical phenomena
associated with thunderstorms that have only recently been documented
using low light level television technology.
The first images of a sprite were accidently obtained in 1989 (Franz et al., 1990).
Beginning in 1990, about twenty images have been obtained from the
space shuttle
(
Vaughan et al., 1992;
Boeck et al., 1994
).
Since then, video sequences of well over a thousand sprites have been
captured.
Most of these images have been obtained during summer campaigns in
1993 and 1994. These include measurements from the ground
(
Lyons, 1994,
Winckler, 1995
)
and from aircraft (
Sentman and Wescott, 1993;
Sentman et al., 1995).
Numerous images have also been obtained from aircraft of blue jets (
Wescott et al., 1995),
also a
previously unrecorded form of optical activity above thunderstorms.
Blue jets appear to emerge directly from the tops of
clouds and shoot upward in narrow cones through the stratosphere.
Their upward speed has been measured to be about 100 km per second.
In addition to sprites and jets, but possibly related, there have
recently been observed from space two other types of unexpected
emissions that appear to originate in thunderstorms. Short duration
(~1 ms) gamma ray (>1 MeV) bursts of terrestrial origin have been
detected by the Compton Gamma Ray Observatory
.
They are observed to occur over thunderstorm regions, and their source
is believed to lie at altitudes greater than 30 km. Finally,
extremely intense pairs of VHF pulses (Trans-Ionospheric Pulse Pairs,
or TIPPS (TIPP
Paper Postscript Source )
originating from thunderstorm regions,
but some 10,000 times stronger than sferics produced by normal lightning
activity, have been observed by the ALEXIS satellite.
Anecdotal reports of "rocket-like" and other optical emissions above
thunderstorms go back more than a century (
Lyons, 1994), and there have
been several pilot reports of similar phenomena (Vaughan and Vonnegut, 1989).
The associated gamma ray
bursts and TIPPS were only recently reported. Together,
these phenomena suggest that thunderstorms exert a much greater
influence on the middle and upper atmospheres than was previously
suspected.
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Sprites are massive but weak luminous flashes that appear directly
above an active thunderstorm system and are coincident with
cloud-to-ground or intracloud lightning strokes. Their spatial
structures range from small single or multiple vertically elongated
spots, to spots with faint extrusions above and below, to bright
groupings which extend from the cloud tops to altitudes up to about 95
km. Sprites are predominantly red. The brightest region lies in the
altitude range 65-75 km, above which there is often a faint red glow
or wispy structure that extends to about 90 km. Below the bright red
region, blue tendril-like filamentary structures often extend downward
to as low as 40 km. Sprites rarely appear singly, usually occurring
in clusters of two, three or more. Some of the very large events,
such as shown in Figure 1, seem to be tightly packed clusters of many
individual sprites. Other events are more loosely packed and may
extend across horizontal distances of 50 km or more and occupy
atmospheric volumes in excess of 10,000 cubic km.
High speed photometer measurements show that the duration of sprites
is only a few ms. Current evidence strongly suggests that sprites
preferentially occur in decaying portions of thunderstorms and are
correlated with large positive cloud-to-ground
lightning strokes. The optical intensity of sprite clusters,
estimated by comparison with tabulated stellar intensities, is
comparable to a moderately bright auroral arc. The optical energy is
roughly 10-50 kJ per event, with a corresponding optical power of 5-25
MW. Assuming that optical energy constitutes 1/1000 of the total for
the event, the energy and power are on the order of 10-100 MJ and 5-50
GW, respectively.
If sprites are only barely detectable by the unaided human eye, in
intensified television images obtained from the ground and from
aircraft they appear as dazzlingly complex structures that assume a
variety of forms.
Figure 1
A movie of a sprite is available (157K
mpeg).
Early research reports for these events referred to them by a variety
of names, including
"upward lightning," "upward discharges,"
"cloud-to-stratosphere discharges," and "cloud-to-ionosphere
discharges." Now they are simply referred to as sprites, a whimsical
term that evokes a sense of their fleeting nature, while at the same
time remaining
nonjudgemental about physical processes that have yet to be determined.
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Blue jets are a second high altitude optical phenomenon, distinct from
sprites, observed above thunderstorms using low light television systems.
As their name implies, blue jets are optical ejections
from the top of the electrically active core regions of
thunderstorms. Following their emergence from the top of the
thundercloud, they typically propagate upward in narrow cones of
about 15 degrees full width at vertical speeds of roughly 100 km/s
(Mach 300), fanning out and disappearing at heights of about 40-50 km.
Their intensities are on the order of 800 kR near the base, decreasing
to about 10 kR near the upper terminus. These correspond to an
estimated optical energy of about 4 kJ, a total energy of about 30 MJ,
and an energy density on the order of a few mJ/m^3. Blue
jets are not aligned with the local magnetic field.
A movie of a jet is available
(46K mpeg).
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Sprites appear to be elusive for several reasons.
(1) Sprites only occur above active thunderstorm systems.
To see them requires visual access to the region above the storm,
unobstructed by intervening clouds, and viewing against a
dark stellar background. In most locations these conditions occur
only rarely.
(2) Sprites are dim and can only been seen with the dark adapted eye.
On average, their brightness compares to moderately bright aurorae, 10-50
kiloRayleighs. In the human eye, this corresponds approximately to
the crossover threshold intensities of cones of the retina, which
respond to color, and the somewhat more sensitive but achromatic
parfoveal rods, which
permit night vision. The dark adapted eye most readily sees sprites
in parfoveal vision, when not directly looking at them. Thus, they
may quite literally appear only as flashes out of the corner of the
eye. Because of their dimness, sprites cannot be viewed in the
presence of nearby bright lights, as would be found in a city.
(3) Cloud illumination from sprite-producing cloud-to-ground or
intracloud lightning activity is often orders of magnitude brighter
than sprites. This lightning activity can easily distract the casual
observer from noticing the fleeting and delicate dance of red sprites
high in the sky above the storm raging below.
(4) Sprites appear to have a duration of only a few (3-10)
milliseconds. This is too
brief to permit shifting one's gaze to obtain a visual fix.
(5) Sprites occur randomly with only about one percent
of lightning strokes. The mere occurrence of lightning therefore
cannot be used as an event marker to indicate that a sprite has
occurred above a thunderstorm.
When all of these factors are taken together it is not surprising
that sprites have been so elusive. However, they can be seen with the
unaided human eye.
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