Search the Community

Red sprites, blue jets and elves are upper atmospheric optical phenomena associated with thunderstorms ....... Sprites are electric discharges shaped in the form of a carrot or vertical column that form in the mesosphere (at a height of between 50 and 85 km). They often appear with a luminous "halo". Elves are rings that expand and propagate themselves horizontally at the speed of light across the base of the ionosphere (85-700 km). There is also a third group, jets, which are bluish lightning bolts that can electrically connect the top of the clouds with the base of the ionosphere. What is a sprite? A sprite is a very brief, luminous glow that occurs in the middle atmosphere. Sprites often start around 45 miles high (in the mesosphere) and extend upwards to the edge of the ionosphere (around 55-60 miles) and sometimes downwards into the stratosphere below (to as low as 15-20 miles). They are primarily red in color, except for some bluish tinge in the downward extending tendrils. When were sprites discovered? Human beings have probably noticed red sprites out of the corner of their eye since they first began roaming the Earth. Since 1886, scientists have periodically reported in scientific journals that they thought they had seen something they did not understand high above thunderstorms. Yet the “discovery†can be traced to 6 July 1989 when University of Minnesota Physics Professor John R. Winckler was testing a low-light video camera for an upcoming research rocket flight. On playing back the tape, he and his graduate students, Robert Franz and Robert Nemzek, were astounded to find two fields of video showing two giant columns of light towering high above distant thunderstorms in northern Minnesota. They were quick to realize this “accident†might actually explain over a century of unexplained visual reports of strange lights above thunderstorms. How long does a sprite last? Sprites are very brief. The portion of the event that might be visible to the naked eye often lasts less than one hundredth of a second. When using a “night vision†camera, they are visible for longer periods, but rarely more than one tenth of a second. GO TO TOP How big is a sprite? Huge! Aside from stretching vertically for as much as 45 miles, the sprite can often be tens of miles across. Often sprite events comprise a series of individual sprites that occur in clusters stretching 50 miles or more across the sky. The entire volume of atmosphere enveloped by a sprite can cover thousands of cubic miles. What causes sprites? Lightning. Sprites are the result of extremely powerful lightning discharges sometimes occurring within thunderstorms. They are almost always triggered by a powerful positive cloud-to-ground (CG) flash which lowers massive amounts of electrical charge to the Earth. This momentarily increases the electric field in the middle atmosphere beyond the point of “dielectric breakdown.†In other words, a giant spark occurs, usually starting around 45 miles above the ground. Electrical streamers then race both downwards and upwards from that point. Though sprites may look rather “solid†in many images, when viewed through telescopes, many sprites are actually composed of networks of thin channels of electrical streamers. Only a very small percentage (<10%) of positive CGs actually produce sprites, and then, only in certain storms. Do sprites connect with the clouds below? Good question. At least from the low-light camera images obtained to date, it does not appear that the downward extending tendrils actually reach the tops of thunderstorms below. But the brilliant glow from the parent lightning discharge in the cloud makes it difficult to tell for sure. It is important to note that sprites do NOT shoot up from the cloud. The luminous structure actually starts many miles above the storm which contains the parent lightning discharge, and extends both upwards and downwards. It is possible that some tendrils may extend all the way back down to the cloud, but scientists are still working to prove this. What gives sprites their color? Current thinking is that sprites result when free electrons in the thin atmosphere are accelerated by the sudden change in electric field strength caused by the parent lightning discharge far below. When the electrons slam into molecules of nitrogen, they cause the nitrogen to glow. Certain energies result in primarily red optical emissions, but in the lower part of the sprite, blue colors can also be seen. The process is not all that dissimilar from the aurora, where the energetic particles are supplied by the solar wind. Oxygen also is activated in the aurora, accounting in part for the green and white colors that also occur in that phenomenon. Some humans see sprites as white or green in color, but that is a result of the inability of the human eye to distinguish colors at such low intensities. How did the sprite get its name? When people first began reporting sprites, initially from naked eye observations, and then in 1989 on video tape, no one really knew what to call them. Rocket lightning, “flachenblitzâ€, upward lightning, cloud-to-stratosphere, and even cloud-to-space lightning were just a few of the names used. This started getting very confusing very fast, especially as no one knew if the phenomenon was actually lightning (as opposed to be being a result of lightning), whether it was connected to the clouds, which way it went, and whether it reached into “space.†Thus, early on, scientists realized that a name was needed that was descriptive of the phenomenon, but did not imply that we understood the physics before we did. "Sprites†are mythical, fleeting, and playful creatures that appear in mythology and Shakespearean plays. The name was first suggested by Prof. David Sentman of the University of Alaska-Fairbanks in 1994, and it stuck. As new transient luminous events (TLEs) above thunderstorms were discovered, they were given names in the same vein: elves, trolls, gnomes, pixies and blue jets (which just named themselves because they looked just like...blue jets.) What shapes do sprites take? Like snowflakes, no two sprites are alike. Early on, scientists were rather bewildered by the amazing variety of shapes they were seeing in their TV monitors. Soon they started attempting to classify them by shape, and terms like carrot sprites, angel sprites, broccoli sprites and A-bombs emerged. Tall, skinny sprites, now usually called columniform sprites (or c-sprites) were initially called diet sprites (who said scientists don’t have a sense of humor!) Can you see a red sprite with the naked eye? The majority of sprites are probably just a bit too fast and too dim to be easily seen by the naked eye. But given the right conditions, one can visually observe the brighter sprites. (See our section on how to watch for sprites.) It is difficult, though not impossible, to capture a sprite on a consumer camcorder. If you can add a “night scope†attachment, your chances of capturing one on tape go way up. How do scientists detect sprites? Sprites and related phenomena can be detected in a variety of ways. Aside from the naked eye, low-light cameras and sensitive optical sensors are the main instruments used to detect sprites. One can also detect sprites, or more properly the radio emissions from their parent lightning, by using extremely low frequency (ELF) and very low frequency (VLF) radio receivers. Do sprites make thunder? There is growing evidence that sprites - or their parent lightning - may produce sound waves at extremely low frequencies (around 1 Hertz). Below the range of human hearing, these infrasound waves can be picked up by special receivers at ranges of hundreds if not thousands of miles away. While not thunder in the usual sense of the word, these infrasound emissions are the acoustic signature of the processes which produce sprites. Where have sprites been observed? Sprites have now been observed over much of the world, with the exception of the Arctic and Antarctic regions where thunderstorms are almost non-existent. The High Plains of the United States probably have some of the highest sprite rates due to frequent, large nocturnal thunderstorms during spring and summer. But sprites are likely to be common above storms in northwestern Mexico, Argentina and southern Brazil and central Africa. Sprites have been photographed over Europe, Japan, Peru, China and Australia, to name a few. What is the connection between the Space Shuttle Columbia and sprites? Shortly after Prof. John Winckler observed sprites from the ground in 1989, NASA officials realized that they had been using similar low-light cameras on the Space Shuttle to take pictures of lightning and other events. A review of the payload bay video cameras flown from 1989 to 1991, found at least 18 examples of what appear to be sprites. Then in 2003, the first Israeli astronaut, Ilan Ramon, flew on the STS-107 mission with optical equipment designed to make detailed, calibrated images of sprites. Within several days of launch, successful images of sprites (and elves) were sent back to Earth for further analysis. Did sprites pay a role in the loss of the Space Shuttle Columbia? Shortly after the tragic loss of the Shuttle Columbia on 1 February 2003, wild rumors began to fly that the Shuttle was brought down by a sprite. This was fueled by reports of a picture taken by an amateur astronomer in San Francisco purporting to show some sort of “lightning-like bolt†striking or emanating from the orbiter as it flew over the California coastline. As of this writing, NASA has not released the image for external scientific review. But neither the description of the image nor the weather conditions at the time suggested that a classic sprite could have been involved. If the image does not turn out to be simply an optical illusion, then the possibility of some as yet unknown electrical phenomenon at the base of the ionosphere will have to be investigated. What are the major “layers†of the atmosphere? Earth’s atmosphere is proportionately thinner than the skin on an onion. And like the onion, it comes in several layers. The lowest layer is called the troposphere. Rarely more than 10 miles deep, it contains almost all of Earth’s weather systems. The next layer is the stratosphere, where temperatures slowly increase with height, producing stable conditions. It is also home to much of the planet’s shield of ozone. This gas absorbs most of the harmful ultraviolet radiation streaming in from the sun. At about 30 miles the stratosphere gives way to the mesosphere. This layer is where many incoming meteors burn up, leaving fiery trails across the night sky. Higher yet is the hot thermosphere, where most aurora are found. The outermost layer is called the exosphere which gradually merges into “space,†which as we have come to learn, isn’t quite as empty as once thought. The ionosphere, a region of electrically charged particles, is considered more a part of the thermosphere, and it, too, surrounds the planet like a shell. What is an elve? The elve was first theoretically predicted by scientists at Stanford University in the early 1990s and then observationally confirmed by imagers on the Space Shuttle and the ground (by Tohoku University scientists) several years thereafter. The elve results from an especially powerful electromagnetic radiation pulse (EMP) that emanates from certain lightning discharges. As the energy passes upwards through the base of the ionosphere it causes the gases to briefly glow. Though as bright as a sprite, the elve only lasts for less than a thousandth of a second. This makes elves virtually impossible to see with the naked eye. They are most likely red in color, and if you could see them, they would look like giant expanding doughnuts. They occur at a height of around 60-65 miles, and can expand outward to several hundred miles in diameter. What is a blue jet? The blue jet was caught, again quite by accident, on low-light cameras from the University of Alaska-Fairbanks onboard a NASA research jet in 1994. The jets appear to spurt upwards from cloud tops at speeds of 50-100 miles per second, reaching heights of up to 25 miles before fading. They last generally less than a quarter of a second, but it is possible to perceive their upward motion with the unaided eye. While generated by storms with high lightning rates, unlike red sprites, blue jets do not appear to be related to specific cloud-to-ground lightning discharges. They also appear more likely to occur near the highest portion of intense thunderstorm cells, such as those which produce tornadoes and severe weather. Can you see blue jets with the naked eye? Yes. At least the brighter ones. They emerge from the tops of intense thunderstorm clouds that are highly electrically active. They are a pale blue in color, and often appear as grainy, upward shooting blobs or cones of light leaving a trail behind. It is best to be within 100 or so miles of the storm as blue light does not transmit well through the atmosphere. A moonless night in a rural area away from city lights, and a dark adapted eye will help in spotting the elusive blue jet. What is an upward superbolt? We are beginning to think that true “upward lightning†may well exist. Regular lightning flashes can sometimes jump outside the parent cloud, and rarely extend a short distance (less than a mile) above storm tops. But a growing number of reports describe brilliant white channels extending upwards many tens of thousands of feet above storm tops. They resemble ordinary lightning channels, but also appear to last much longer, up to one or two seconds, and do not flicker. They also seem to grow upward out of the cloud, and upon reaching their maximum height, the entire channel dims away. They may well occur above the tops of explosively growing clouds. They often occur every few minutes, and episodes can last for a half hour or more. Thus, they may not be too hard to photograph.e http://www.sky-fire....ejetselves.html