They were once so elusive that scientists gave them a mystical name. “Red sprites” are short-lived, red flashes that occur about 80 kilometers (50 miles) up in the atmosphere. With long, vertical tendrils like a jellyfish, these electrical discharges can extend 20 to 30 kilometers up into the atmosphere and are connected to thunderstorms and lightning.
These images of a red sprite were captured with a digital camera by Expedition 31 astronauts on the International Space Station as they traveled southeast from central Myanmar (Burma) to just north of Malaysia. The still images are part of a time-lapse movie collected from 13:41 to 13:47 Universal Time on April 30, 2012. (View the entire sequence here.) The sprite occurs about 6 seconds into the video, above a bright, wide lightning flash in the upper right quadrant.
Red sprites are difficult to observe because they last for just a few milliseconds and occur above thunderstorms—meaning they are usually blocked from view on the ground by the very clouds that produce them. They send pulses of electrical energy up toward the edge of space—the electrically charged layer known as the ionosphere—instead of down to Earth’s surface. They are rich with radio noise, and can sometimes occur in bunches.
Cameras aboard the International Space Station captured this intense lightning storm as well as an elusive "red sprite" as it passed over Myanmar (Burma) and Malaysia. The sprite appears at the 6 second mark in this video
For decades, pilots reported seeing ephemeral flashes above storms, but it was not until the 1990s that scientists were able to verify the existence of these electrical discharges. A sprite was first photographed by accident from an airplane in 1989, and observers on the space shuttle captured several more images with low-light cameras in 1990 and in subsequent missions. Viewers on the ground can occasionally photograph sprites by looking out on a thunderstorm in the distance (often looking out from high mountainsides over storms in lower plains.)
Characteristics of Red Sprites
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.
Early research reports for these events referred to them by a variety of names, including "upward lightning," "upward discharges," "cloud-to-stratosphere discharges," and "cloud-to-ionosphere discharges." Now they are simply referred to as sprites, a whimsical term that evokes a sense of their fleeting nature, while at the same time remaining nonjudgemental about physical processes that have yet to be determined.
A movie of a sprite is available (157K mpeg).
Characteristics of Blue Jets
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. Their intensities are on the order of 800 kR near the base, decreasing to about 10 kR near the upper terminus. These correspond to an estimated optical energy of about 4 kJ, a total energy of about 30 MJ, and an energy density on the order of a few mJ/m^3. Blue jets are not aligned with the local magnetic field.
A movie of a jet is available (46K mpeg).
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.
These images of a red sprite were captured with a digital camera by Expedition 31 astronauts on the International Space Station as they traveled southeast from central Myanmar (Burma) to just north of Malaysia. The still images are part of a time-lapse movie collected from 13:41 to 13:47 Universal Time on April 30, 2012. (View the entire sequence here.) The sprite occurs about 6 seconds into the video, above a bright, wide lightning flash in the upper right quadrant.
Red sprites are difficult to observe because they last for just a few milliseconds and occur above thunderstorms—meaning they are usually blocked from view on the ground by the very clouds that produce them. They send pulses of electrical energy up toward the edge of space—the electrically charged layer known as the ionosphere—instead of down to Earth’s surface. They are rich with radio noise, and can sometimes occur in bunches.
Cameras aboard the International Space Station captured this intense lightning storm as well as an elusive "red sprite" as it passed over Myanmar (Burma) and Malaysia. The sprite appears at the 6 second mark in this video
For decades, pilots reported seeing ephemeral flashes above storms, but it was not until the 1990s that scientists were able to verify the existence of these electrical discharges. A sprite was first photographed by accident from an airplane in 1989, and observers on the space shuttle captured several more images with low-light cameras in 1990 and in subsequent missions. Viewers on the ground can occasionally photograph sprites by looking out on a thunderstorm in the distance (often looking out from high mountainsides over storms in lower plains.)
Characteristics of Red Sprites
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.
A red sprite
Early research reports for these events referred to them by a variety of names, including "upward lightning," "upward discharges," "cloud-to-stratosphere discharges," and "cloud-to-ionosphere discharges." Now they are simply referred to as sprites, a whimsical term that evokes a sense of their fleeting nature, while at the same time remaining nonjudgemental about physical processes that have yet to be determined.
Characteristics of Blue Jets
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. Their intensities are on the order of 800 kR near the base, decreasing to about 10 kR near the upper terminus. These correspond to an estimated optical energy of about 4 kJ, a total energy of about 30 MJ, and an energy density on the order of a few mJ/m^3. Blue jets are not aligned with the local magnetic field.
A movie of a jet is available (46K mpeg).
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.
Contacts and sources
NASA
University of Alaska
Related Reading
NASA Gateway to Astronaut Photography of Earth. (n.d.) Crew Earth Observations Videos—Southeast Asia. Accessed July 9, 2012.
NASA. (1997). Sprites and Jets. Accessed July 9, 2012.
Science@NASA. (1999, June 10). Spirits of Another Sort: Thunderstorms Generate Elusive and Mysterious Sprites.Accessed July 9, 2012.
University of Alaska–Fairbanks. (n.d.) Red Sprites and Blue Jets. Accessed July 9, 2012.
Astronaut photographs ISS031-E-10711, ISS031-E-10712, and ISS031-E-10713 were acquired on April 30, 2012. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by Michael Carlowicz.
University of Alaska
Related Reading
NASA Gateway to Astronaut Photography of Earth. (n.d.) Crew Earth Observations Videos—Southeast Asia. Accessed July 9, 2012.
NASA. (1997). Sprites and Jets. Accessed July 9, 2012.
Science@NASA. (1999, June 10). Spirits of Another Sort: Thunderstorms Generate Elusive and Mysterious Sprites.Accessed July 9, 2012.
University of Alaska–Fairbanks. (n.d.) Red Sprites and Blue Jets. Accessed July 9, 2012.
Astronaut photographs ISS031-E-10711, ISS031-E-10712, and ISS031-E-10713 were acquired on April 30, 2012. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by Michael Carlowicz.
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