We got on the subject of sprites and jets in the weather office yesterday... thought you would enjoy this material on the very interesting subject from the University of Alaska in Fairbanks:
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. 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.
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). Possibly associated gamma ray bursts and TIPPS have also recently reported. Together, these phenomena suggest that thunderstorms exert a much greater influence on the middle and upper atmospheres than was previously suspected.
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.
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.
Why Haven't Sprites and Jets Been Reported Before?
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.
How to Look for Sprites and Jets
*A clear view above a thunderstorm is required. This generally means the thunderstorm activity must be on the horizon. Additionally, there must be very little intervening cloud cover.
*Best viewing distance from storm is 100-200 miles (200-300 km). At these distances sprites will subtend a vertical angular distance of 10-20 degrees. This is 2-4 times the separation of the pointer stars in the Big Dipper.
*For observing sprites, it must be completely dark. (i. e. no longer twilight)
*Eyes must be completely dark adapted. Use same criteria for this as for astronomical observing. If you can see the Milky Way, then it is probably dark enough and the eyes have adapted enough to see sprites.
*Fix your gaze on the space above an active thunderstorm. Do not be distracted by underlying lightning activity in the storm. Block out the lightning if necessary using a piece of dark paper in such a way as to still being able to view what is going on above the cloud.
*Sprites will be very brief flashes just on the edge of perceptability. They occur too quickly to follow with the eyes, but their strange vertically striated structure and dull red color may be perceived.
*Patience will be rewarded. If the right kind of storm is present and one's viewing geometry is favorable, then there is a greater likelihood of seeing a sprite than of seeing a shooting star or comet.
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. 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.
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). Possibly associated gamma ray bursts and TIPPS have also recently reported. Together, these phenomena suggest that thunderstorms exert a much greater influence on the middle and upper atmospheres than was previously suspected.
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.
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.
Why Haven't Sprites and Jets Been Reported Before?
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.
How to Look for Sprites and Jets
*A clear view above a thunderstorm is required. This generally means the thunderstorm activity must be on the horizon. Additionally, there must be very little intervening cloud cover.
*Best viewing distance from storm is 100-200 miles (200-300 km). At these distances sprites will subtend a vertical angular distance of 10-20 degrees. This is 2-4 times the separation of the pointer stars in the Big Dipper.
*For observing sprites, it must be completely dark. (i. e. no longer twilight)
*Eyes must be completely dark adapted. Use same criteria for this as for astronomical observing. If you can see the Milky Way, then it is probably dark enough and the eyes have adapted enough to see sprites.
*Fix your gaze on the space above an active thunderstorm. Do not be distracted by underlying lightning activity in the storm. Block out the lightning if necessary using a piece of dark paper in such a way as to still being able to view what is going on above the cloud.
*Sprites will be very brief flashes just on the edge of perceptability. They occur too quickly to follow with the eyes, but their strange vertically striated structure and dull red color may be perceived.
*Patience will be rewarded. If the right kind of storm is present and one's viewing geometry is favorable, then there is a greater likelihood of seeing a sprite than of seeing a shooting star or comet.
on May 16, 2006, 11:11 am
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