Red sprites high above Earth


High above Earth in the realm of meteors and noctilucent clouds, a strange and beautiful form of lightning dances at the edge of space. Researchers call the bolts "sprites"; they are redfleeting, and tend to come in bunches. Martin Popek of Nýdek in the Czech republic photographed these specimens on August 27th:

"Sprites develop in mid-air around 80 km altitude, growing in both directions, first down, then up. This happens when a fierce lightning bolt draws lots of charge from a cloud near Earth's surface. Electric fields to the top of Earth's atmosphere–and the result is a sprite. The entire process takes about 20 milliseconds."

Although sprites have been seen for at least a century, most scientists did not believe they existed until after 1989 when sprites were photographed by cameras onboard the space shuttle. Now "sprite chasers" routinely photograph sprites from their own homes. (SpaceWeather)

Sprites are large-scale electrical discharges which occur high above a thunderstorm cloud, or cumulonimbus, giving rise to a quite varied range of visual shapes. They are triggered by the discharges of positive lightning between the thundercloud and the ground. They normally are colored reddish-orange or greenish-blue, with hanging tendrils below and arcing branches above. They can also be preceded by a reddish halo. They often occur in clusters, lying 50 miles (80 km) to 90 miles (140 km) above the Earth's surface. Sprites were first photographed on July 6, 1989 by scientists from the University of Minnesota and have since been witnessed tens of thousands of times. Sprites have been held responsible for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms. (wiki)

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. (http://elf.gi.alaska.edu/)


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