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.
Allusions to transient optical phenomena above thunderclouds can be found in anecdotal reports from as early as 1730 (see Johann Georg Estor
). 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... Read More