Transient Luminous Events (TLE) were discovered “by chance” in 1989 during night-time video camera tests by a team from the University of Minnesota. They all occur above thunderstorms, but can be distinguished from one another by their shape, duration, conditions of production and regions of development. There are several types of TLE: sylphs, elves, jets, halos and trolls.
Sprites
Sprites were first observed in 1989. They are associated with mesoscale convective systems (MCS), i.e. thunderstorms with a horizontal dimension of at least 150 km and a lifetime of a few hours. They are produced above the stratiform zone of these thunderstorms, a few milliseconds to a few tens of milliseconds after a positive cloud-to-ground flash, and correspond to a streamer-type discharge process that starts at around 70 km altitude and lasts a few milliseconds. They can be found in a variety of forms, hence their classification as “column”, “carrot”, “angel” or “jellyfish”.
ELVES
ELVES (Emission of Light and Very low frequency perturbations due to Electromagnetic pulse Sources) are large rings of light (100-300 km) at the base of the ionosphere. They are produced after cloud-to-ground flashes of high amplitude and of both signs, with a very short delay (around 150 µs) and are very brief. They are caused by electromagnetic pulses generated by the arc of the flash, which illuminate the base of the ionosphere through electronic excitation.
Jets
Jets are luminous electrical discharges that develop above the storm cloud. There are blue jets, which reach altitudes of 30 to 40 km, and gigantic jets, which reach the ionosphere at an altitude of around 90 km at night. Their discovery is very recent: 1994 for the former and 2001 for the latter. The blue jets emerge from the cloud tops in the form of cones of bluish light. The giant jet is broken down into 2 phases: the leading jet, which initially traces the course of the discharge with more or less upwardly developed branches, and the trailing jet, which appears as a secondary discharge that is brighter but less vertically developed.
They are difficult to see with the naked eye, and can be recorded using highly sensitive video cameras, typically in the millilux range in black and white. At the Pic du Midi, for example, a camera of this type has been installed for observations over thunderstorms within a radius of around 150 km to 700 km. The camera can be remotely controlled via the Internet. It is mounted on an elevation and azimuth orientation system to target the zone most likely to trigger high altitude discharges. Video recording is managed by capture software sensitive to light variations in the camera’s field of view. The videos can be transferred via the Internet for analysis and combined with other videos from cameras installed at other sites, to triangulate these light phenomena and pinpoint their location. They can also be combined with other types of observations on the storm itself, notably on the cloud’s microphysical and dynamic characteristics, and on the characteristics of associated lightning. Other recordings of the electromagnetic radiation produced can also provide information on the physics of the light discharge and its effects on the environment and the upper atmosphere. These discharges provide a natural laboratory for exploring the physics of electrical discharges. The research programs (Eurosprite) and European space projects (Taranis, ASIM) being set up to study these phenomena bring together several European teams with expertise in different aspects of physics: cloud physics, discharge physics, particle physics…
