Upper-atmospheric lightning

Upper-atmospheric lightning or ionospheric lightning are terms sometimes used by researchers to refer to a family of short-lived electrical-breakdown phenomena that occur well above the altitudes of normal lightning and storm clouds. Upper-atmospheric lightning is believed to be electrically induced forms of luminous plasma. The preferred usage is transient luminous event (TLE), because the various types of electrical-discharge phenomena in the upper atmosphere lack several characteristics of the more familiar tropospheric lightning. Upper-atmospheric lightning or ionospheric lightning are terms sometimes used by researchers to refer to a family of short-lived electrical-breakdown phenomena that occur well above the altitudes of normal lightning and storm clouds. Upper-atmospheric lightning is believed to be electrically induced forms of luminous plasma. The preferred usage is transient luminous event (TLE), because the various types of electrical-discharge phenomena in the upper atmosphere lack several characteristics of the more familiar tropospheric lightning. There are several types of TLEs, the most common being sprites. Sprites are flashes of bright red light that occur above storm systems. C-sprites (short for “columniform sprites”) is the name given to vertical columns of red light. C-sprites exhibiting tendrils are sometimes called “carrot sprites”. Other types of TLEs include sprite halos, blue jets, gigantic jets, blue starters, and ELVESs. The acronym ELVES (“Emission of Light and Very Low Frequency perturbations due to Electromagnetic Pulse Sources”) refers to a singular event which is commonly thought of as being plural. TLEs are secondary phenomena that occur in the upper atmosphere in association with underlying thunderstorm lightning. TLEs generally last anywhere from less than a millisecond to more than 2 seconds. The first video recording of a TLE was captured accidentally on July 6, 1989. Researcher R.C Franz had set up the camera to be used on a future rocket launch and pointed the camera to view the night sky at zenith to capture images of star fields and/or whatever appeared. He left the camera running for the duration of the night The next morning when the video was viewed there were 2 finger like vertical images, appearing in only two frames of the film. The next video recordings of a TLE in 1989 was during the Shuttle Mission STS-34 while conducting the Mesoscale Lightning Observation Experiment. On October 21, 1989 TLE'S were recorded during the Shuttle orbits 44 and 45. TLEs have been captured by a variety of optical recording systems, with the total number of recent recorded events (early 2009) estimated at many tens-of-thousands. The global rate of TLE occurrence has been estimated from satellite (FORMOSAT-2) observations to be several million events per year. In the 1920s, the Scottish physicist C.T.R. Wilson predicted that electrical breakdown should occur in the atmosphere high above large thunderstorms. In ensuing decades, high altitude electrical discharges were reported by aircraft pilots and discounted by meteorologists until the first direct visual evidence was documented in 1989. Several years later, the optical signatures of these events were named 'sprites' by researchers to avoid inadvertently implying physical properties that were, at the time, still unknown. The terms red sprites and blue jets gained popularity after a video clip was circulated following an aircraft research campaign to study sprites in 1994. 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. The phenomena were named after the mischievous sprite, e.g., Shakespeare's Ariel or Puck, and is also an acronym for Stratospheric/mesospheric Perturbations Resulting from Intense Thunderstorm Electrification. 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 kilometres (31 mi) to 90 kilometres (56 mi) above the Earth's surface. Sprites have been witnessed thousands of times. Sprites have been held responsible for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms. Although jets are considered to be a type of upper-atmospheric lightning, it has been found that they are components of tropospheric lightning and a type of cloud-to-air discharge that initiates within a thunderstorm and travel upwards. In contrast, other types of TLEs are not electrically connected with tropospheric lightning—despite being triggered by it. The two main types of jets are blue jets and gigantic jets. Blue starters are considered to be a weaker form of blue jets. Blue jets are believed to be initiated as 'normal' lightning discharges between the upper positive charge region in a thundercloud and a negative 'screening layer' present above this charge region. The positive end of the leader network fills the negative charge region before the negative end fills the positive charge region, and the positive leader subsequently exits the cloud and propagates upward. It was previously believed that blue jets were not directly related to lightning flashes, and that the presence of hail somehow led to their occurrence. They are also brighter than sprites and, as implied by their name, are blue in color. The color is believed to be due to a set of blue and near-ultraviolet emission lines from neutral and ionized molecular nitrogen. They were first recorded on October 21, 1989, on a monochrome video of a thunderstorm on the horizon taken from the Space Shuttle as it passed over Australia. Blue jets occur much less frequently than sprites. By 2007, fewer than a hundred images had been obtained. The majority of these images, which include the first color imagery, are associated with a single thunderstorm. These were taken in a series of 1994 aircraft flights to study sprites. Blue starters were discovered on video from a night time research flight around thunderstorms and appear to be 'an upward moving luminous phenomenon closely related to blue jets.' They appear to be shorter and brighter than blue jets, reaching altitudes of only up to 20 km. 'Blue starters appear to be blue jets that never quite make it,' according to Dr. Victor P. Pasko, associate professor of electrical engineering. Where blue jets are believed to initiate between the upper positive charge region and a negative screening layer directly above this region, Gigantic jets appear to initiate between the upper positive and lower negative charge regions in the thundercloud. In a similar process to how blue jets form, the higher charge region is discharged by the leader network before the same occurs in the lower charge region, and one end of the leader network propagates upward from the cloud toward the ionosphere. Gigantic jets reach higher altitudes than blue jets, and the upper portion of the jet changes color from blue to red.