Wilson had suggested in 1925, on theoretical grounds, that electrical breakdown could occur in the upper atmosphere, and in 1956 he witnessed what possibly could have been a sprite. Another early report is by Toynbee and Mackenzie in 1886. The earliest known report of transient optical phenomena above thunderclouds is from Johann Georg Estor in 1730. Sprites are associated with various other upper-atmospheric optical phenomena including blue jets and ELVES. 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. Sprites are sometimes inaccurately called upper-atmospheric lightning. Sporadic visual reports of sprites go back at least to 1886 but they were first photographed on July 4, 1989, by scientists from the University of Minnesota and have subsequently been captured in video recordings many thousands of times. They often occur in clusters above the troposphere at an altitude range of 50–90 km (31–56 mi). Sprites appear as luminous red-orange flashes. They are usually triggered by the discharges of positive lightning between an underlying thundercloud and the ground. Sprites or red sprites are large-scale electric discharges that occur high above thunderstorm clouds, or cumulonimbus, giving rise to a varied range of visual shapes flickering in the night sky. A sprite seen from the International Space Station (top right, faint red above the lightning).
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