The French satellite Taranis will be launched in November

Sprites, elves, jets … few people know that scientists routinely use such otherworldly words to describe transient light events or TLEs, flashes of light that occur during active storms a few tens of kilometers above our heads. Few people also know that storms can act as particle accelerators by generating very short bursts of X-rays and gamma rays. But what are the physical processes and mechanisms underlying these phenomena discovered just 30 years ago? Do they impact the physics and chemistry of the upper atmosphere, the environment, or even humans? These are the questions facing the French satellite Taranis which will fly in the night from 16 to 17 November aboard a Vega launcher of the Guiana Space Center, an all-French mission that involves researchers from CNES, the national scientific research center. CNRS, the CEA commission for atomic energy and alternative energies and several French universities.

TLE and terrestrial gamma-ray bursts (TGFs) are seen all over the world where storms occur. But because we don’t know enough, they are not in the toolbox of climatologists and meteorologists. Are they implicated in the growing number of extreme weather events? If so, they could be modeled and factored into real-time forecasts. Although Taranis is primarily a fundamental research satellite, the data it is expected to provide on the Earth’s thermal and climatic mechanisms could serve more operational applications such as climatology and weather forecasting.

Elves, goblins, goblin halos, blue jets, and even goblins or gnomes are just some of the whimsical names given to the range of phenomena in the generic TLE family – a poetic lexicon that contrasts sharply with their violence. These ephemeral upper atmosphere events occur between the tops of storm clouds and an altitude of 90 kilometers. First predicted as early as 1920, their existence was not confirmed until the 1990s. They have since been recorded by numerous ground and space observations. The elves take the form of an expanding glow of light, appearing at an altitude of 90 kilometers and lasting no more than a millisecond; an active storm can produce thousands in a matter of hours. Occurring between 40 and 90 kilometers above the earth’s surface, sprites have a complex structure of branches and tendrils and can last up to 10 milliseconds. Blue jets appear atop storm clouds and propagate at altitudes of up to 50 kilometers. Occasionally, “gigantic” jets can propagate up to 90 kilometers.

TGFs were first observed scientifically in 1994 by the Compton Gamma-Ray Observatory (CGRO), a NASA spacecraft deployed by the American space shuttle Atlantis. Under certain conditions, storms generate a very short burst of gamma photons. TGFs have for a time been considered a rare event accompanying sprites; we now know that they are generated by electrical activity in clouds. For lack of adequate instruments, the Italian satellite AGILE (2007) and the US space telescope Fermi (2008) have not been able to fully confirm the current hypotheses on the mechanisms that generate them or to estimate their number. Taranis will therefore bring new information on how they are generated and their impact on radiation, which has never been measured before.

In France, the atomic energy agency CEA first turned its attention to these transient events and their impact in 1993. On 9 December 2010, the project received official green light from the Board of Directors of the CNES. Taranis is an all-French mission with scientific objectives set by French research laboratories. In addition to the CEA, the CNRS is closely involved through many of its affiliated research laboratories1: the LPC2E spatial and environmental physics and chemistry laboratory coordinates the development of the science payload, is responsible for the science mission center and provides tools; The IRAP research institute for astrophysics and planetology, the LATMOS laboratory for atmospheres, environments and space observations and the astrophysics and cosmology laboratory of the APC contribute to the payload.

Other tools on Taranis include external contributions from Stanford University and the Goddard Space Flight Center (GSFC) in the United States, the Institute of Atmospheric Physics (IAP) and Charles University in the Czech Republic, and the Space Research Center of the Polish Academy of sciences (CBK).

Taranis has a slightly different appearance, as instead of the aluminized or gold-plated Mylar insulation traditionally used on satellites it is coated with a special black and white paint. This is not just attention to aesthetic details, the purpose of the paint is to avoid interference with the surrounding electric field and prevent reflected light from interrupting the optical sensors. A less visible but fundamental feature is the original design of its payload, consisting of eight instruments operated as a single unit thanks to MEXIC, the brain of Taranis that powers and synchronizes the instruments and manages the payload, executes the trigger strategy for capture an event and also manages the transfer of selected data to mass memory.

Taranis payload close-up:

• XGRE: three X-ray and gamma-ray detectors to measure high-energy photons (from 50 keV to 10 MeV) and relativistic electrons (from 1 MeV to 10 MeV) – APC / IRAP / CNES
• MCP (MC-U and PH-U): two cameras (10 images per second) and four photometers to measure luminance in different spectral bands – CEA / CNES
• IDEAS: two high energy electron detectors (70 keV to 4 MeV) – IRAP / Charles University
• IMM: three-axis magnetometer to measure the alternating magnetic field (5 Hz to 1 MHz) – LPC2E / Stanford University
• IME-HF: HF antenna for measuring the high frequency electric field (from 100 kHz to 35 MHz) – LPC2E / IAP
• IME-BF: instrument for measuring the low frequency electric field (DC at 1 MHz) – LATMOS
• SI: ionic probe to determine the thermal fluctuations of the plasma – GSFC / LATMOS
• MEXICO: two electronic units made up of eight analyzers, each connected to an instrument. It powers each instrument, manages payload modes and interfaces with the mass memory and on-board computer. MEXIC will also have the task of synchronizing the instruments when events are detected (TLE from MCP photometers, TGF from XGRE, electron beams from IDEE, wave burst from IME-HF) – LPC2E / CBK

For two to four years, Taranis will explore regions of the sky where storm activity is intense and the likelihood of seeing TLE and / or TGF is high. Although it may be a national program, its results are eagerly awaited by the wider international scientific community. In atmospheric chemistry and physics, environmental science, climatology, high energy astrophysics and many other fields, Taranis is ready to reveal new insights and scientific endeavors will not end there, as the mission will undoubtedly lead the way. to future investigations.

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