Red Sprites and Blue Jets

Jet Image

This is primarily a description of the Univ. Alaska research into middle and upper atmospheric optical and electrical phenomena. I have recently (May 12, 2004) begun to update the descriptions on this web page. Please email me with any corrections, additions, or suggestions

  • Introduction
  • Characteristics of Red Sprites
  • Characteristics of Blue Jets
  • Why Haven't Sprites and Jets Been Reported Before?
  • How to Look For Sprites and Jets
  • Current Research Focus
  • Speculations
  • Sprite Bibliography (~200 kbytes) (BiBTeX format here
  • 1998 Fall AGU Bibliography (BiBTeX format here
  • 1999 Fall AGU Bibliography (BiBTeX format here (incomplete)
  • Related Topics
  • Report Observations

    Introduction

    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.

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    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.

    Large Color Sprite Image

     Figure 1 

    A movie of a sprite is available (157K mpeg).

    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.

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    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).

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    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.

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    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.

    If you have observed a sprite or any other optical emission above a thunderstorm, please report it.

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    Current Research Focus

    Intense efforts, both experimental and theoretical, are presently underway to determine the full extent to which these new phenomena form a part of the terrestrial electrical environment. Although optical images seem likely to remain the principal experimental form of "ground truth" in sprite detection, focus has already shifted to employing other diagnostics that will yield more specific information about the detailed physical mechanisms. These include optical spectra, including height profiles, radio (ELF-HF) measurements of the electromagnetic emissions from sprites and their accompanying tropospheric lightning strokes, VLF measurements of associated ionospheric heating effects, and continuous wave radar probes of sprites to determine electron densities.

    Interest has also emerged in the possible electrochemical effects of sprites and jets on the mesosphere and stratosphere, respectively.

    Investigations are underway to ascertain whether they may create locally or globally significant long lived electrochemical residues within the upper atmosphere. The production of ionized or electronically excited species, by RF electrolysis or other means, could conceivably lead to the creation of reactive species or to the activation of catalytic species that would otherwise be absent.

    The recent MEIDEX experiment was used to studying sprites from the shuttle. An upcoming satellite experiment, called ISUAL, on the ROCSAT-3 satellite will be studying sprites from space. The groups at New Mexico Tech and Stanford are both active in sprites research and maintain web pages with details of their work and some great photos and data. FMA Research has hosted many research groups and has produced educational materials (see the "100 Year Search for Sprites") and maintains a web page here Return to top of page

    Speculations

    From what is known to date, it may be speculated that sprites or jets, or both, are an integral feature of every thunderstorm system of moderate size or larger in the terrestrial system, and may be an essential element of the earth's global electrical circuit. Further, it seems likely that they have been a part of thunderstorms that have occurred over previous millions of years or longer. One may speculate about the possible occurrences of similar phenomena associated with lightning on other planets where lightning has been detected, most notably Jupiter and Venus.

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    Related Topics

    In addition to sprites and jets, but possibly related, there have recently been observed from space two other types of unexpected emissions that appear to originate in thunderstorms. Short duration (~1 ms) gamma ray (>1 MeV) bursts of terrestrial origin have been detected by the Compton Gamma Ray Observatory They are observed to occur over thunderstorm regions, and their source is believed to lie at altitudes greater than 30 km. Finally, extremely intense pairs of VHF pulses (Trans-Ionospheric Pulse Pairs, or TIPPS (TIPP Paper Postscript Source ) originating from thunderstorm regions, but some 10,000 times stronger than sferics produced by normal lightning activity, have been observed by the ALEXIS satellite.

    Some of the other groups making sprite observations include:

    Some of the recently completed dissertations describing aspects of sprites research include:

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    This research on red sprites and blue jets is being conducted by researchers at the Geophysical Institute of the University of Alaska Fairbanks.

    The URL for this page is: http://elf.gi.alaska.edu/

    If you know of other references please bring them to my attention.

     
    Matt Heavner