Sprites are large-scale electrical discharges that occur high above
thunderstorm clouds, or cumulonimbus, giving rise to a quite varied range of
visual shapes flickering in the night sky. They are triggered by the
discharges of positive lightning between an underlying thundercloud and the
ground. Sprites appear as luminous
reddish-orange flashes. They often occur in clusters within the altitude range
50–90 km above the Earth's surface. Sporadic visual reports of sprites go
back at least to 1886, but they were first photographed on July 6, 1989 by
scientists from the University of Minnesota and have subsequently been
captured in video recordings many thousands of times.
Sprites are sometimes inaccurately called upper-atmospheric lightning.
However, sprites are cold plasma phenomena that lack the hot channel
temperatures of tropospheric lightning, so they are more akin to fluorescent
tube discharges than to lightning discharges.
History Allusions to transient optical phenomena
above thunderclouds can be found in anecdotal reports from as early as 1730.
Nobel laureate C. T. R. Wilson had suggested in 1925, on theoretical
grounds, that electrical breakdown could occur in the upper atmosphere, and in
1956 witnessed what possibly could have been a sprite. They were first
documented photographically on July 6, 1989 when scientists from the University
of Minnesota, using a low-light video camera, accidentally captured the first
image of what would subsequently become known as a sprite. Several years after
their discovery they were named sprites after their elusive nature. Since their
1989 discovery, sprites have been imaged tens of thousands of times, from the
ground, from aircraft and from space, and have become the subject of intensive
investigations. Characteristics
Sprites have been observed over North America, Central America, South America,
Europe, Central Africa, Australia, the Sea of Japan and Asia and are believed
to occur during most large thunderstorm systems.
Three types of sprites have been categorized by Matthew 'Geoff' McHarg
Ph.D. of the US AirForce Research Academy. Using an image intensifier on
the front of super slow motion camera McHarg and his researchers have named
the sprites based on their visual appearance.
Jellyfish sprite - very large, up to 30 by 30 miles.
Carrot sprite C or Column sprite. These are large
scale electrical discharges above the earth that are still not totally
understood. Sprites are colored reddish-orange in
their upper regions, with bluish hanging tendrils below, and can be preceded by a
reddish halo. They last longer than normal lower stratospheric discharges,
which last typically a few milliseconds, and are triggered by the discharges of
positive lightning between the thundercloud and the ground. They often
occur in clusters of two or more, and typically span the altitude range 50 to
90 kilometres, with what appear to be tendrils hanging below, and branches
reaching above. Optical imaging using a 10,000
frame-per-second high speed camera shows that sprites are actually clusters of
small, decameter-sized balls of ionization that are launched at an
altitude of about 80 km and then move downward at speeds of up to ten percent
the speed of light, followed a few milliseconds later by a separate set of
upward moving balls of ionization. Sprites may be horizontally displaced by
up to 50 km from the location of the underlying lightning strike, with a time
delay following the lightning that is typically a few milliseconds, but on
rare occasions may be up to 100 milliseconds.
In order to film sprites from Earth, special conditions must be present:
150–500 km of clear view to a powerful thunderstorm with positive lightning
between cloud and ground, red-sensitive recording equipment, and a black unlit
sky. Sprite halo
Sprites are sometimes preceded, by about 1 millisecond, by a sprite halo, a
pancake-shaped region of weak, transient optical emissions approximately 50
kilometres across and 10 kilometres thick. The halo is centered at about 70
kilometres altitude above the initiating lightning strike. These halos are
thought to be produced by the same physical process that produces sprites,
but for which the ionization is too weak to cross the threshold required for
streamer formation. They are sometimes mistaken for ELVES, due to their visual
similarity and short duration. Research carried out at Stanford
University in 2000 indicates that, unlike sprites with bright vertical
columnar structure, occurrence of sprite halos is not unusual in association with
normal lightning discharges. Research in 2004 by scientists from Tohoku
University found that very low frequency emissions occur at the same time as the
sprite, indicating that a discharge within the cloud may generate the
sprites. Related aircraft damage
Sprites have been blamed for otherwise unexplained accidents involving high
altitude vehicular operations above thunderstorms. One example of this is
the malfunction of a NASA stratospheric balloon launched on June 6, 1989 from
Palestine, Texas. The balloon suffered an uncommanded payload release while
flying at 120,000 feet over a thunderstorm near Graham, Texas. Months
after the accident, an investigation concluded that a "bolt of lightning"
traveling upward from the clouds provoked the incident. The attribution
of the accident to a sprite was made retroactively, since this term was not
coined until late 1993. See also
Aurora Catatumbo lightning
Cosmic ray visual phenomena References
External links "Red Sprites & Blue Jets" A digital
capture of the VHS video distributed in 1994 by the University of Alaska
Fairbanks that popularized the terms. "Red Sprites and Blue Jets" webpage by
University of Alaska Fairbanks Ground and Balloon-Borne Observations of
Sprites and Jets Darwin Sprites '97 Space Physics Group,
University of Otago. Sprites, jets and TLE pictures and
articles Sprites in Europe: European contributors
blog Kanmae, T.; Stenbaek-Nielsen, H.C.;
McHarg, M.G.; Haaland, R.K.. "Observation of blue sprite spectra at
10,000 fps". Geophys. Res. Lett. 37: L13808. Bibcode:2010GeoRL..3713808K.
doi:10.1029/2010GL043739. Mysterious Sprite Photographed by ISS
Astronaut: Big Pic Short Professional Bio of Dr 'Geoff'
McHarg Photography website Petapixel posted a
link to a very rare and very clear photograph of a sprite taken by
photographer Mike Hollingshead. Article at Photographer Captures Rare Photograph
of a Sprite with an Aurora At the Edge of Space A NOVA program that
examines the phenomenon of Sprites