The science

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Scientific essay papers objectives
The observations of red sprites, blue jets, elves, sprite halos, gigantic jets, etc, named Transient Luminous Events (TLEs) and the observations of Terrestrial Gamma ray Flashes (TGFs) have pointed out the existence of impulsive transfers of energy between the Earth atmosphere and the space environment. Recent measurements by the FORMOSAT 2 satellite have shown that TLEs are fairly common in most regions of the globe. Measurements at thousands of kilometres distance of infrasounds produced by sprites give an idea of the amount of energy released. RHESSI observations show that TGFs, which are more frequent over tropical or equatorial regions, involve very high energies, reaching 30 MeV. Processes that contribute to these phenomena and potential consequences on the Earth’s electrical and chemical environment are far from being fully understood. However, it is clear that the source of TGFs is itself a fundamentally new physical process that could have much broader implications and manifestations throughout the universe.

Models based on quasi-static electric fields explain the observed optical features of most TLEs, but they do not predict TGFs. The most likely models for the generation of TGFs involve the production of runaway electron beams accelerated in an avalanche process by thundercloud associated electric fields. Both approaches are integrated in the most recent models where diffuse air glow or streamers at the origin of TLEs, that are driven by conventional breakdown, are followed by the runaway discharge. These runaway discharges appear at the top of the cloud and develop a relativistic electron avalanche at high altitudes producing TGFs. The process is more efficient at latitudes where the magnetic field is not horizontal. Runaway breakdown is predicted to begin with secondary energetic electrons produced in the atmosphere by cosmic rays. However, numerous observation tests failed in the recent years. This lack of success is further exemplified by the data set obtained by the low altitude SAMPEX satellite which despite over ten years of continuous monitoring of the radiation belts, shows no evidence for runaway relativistic electron beams in association with lightning. In short, our present understanding of TLEs and TGFs is in a state of confusion. While many of the details regarding the optical characteristics of sprites and elves are presumed to be well in hand, fundamental issues regarding the association of TLEs or lightning with TGFs and the nature of the source of penetrating radiation itself remain a mystery. This mystery is further deepened in light of growing evidence for the production of penetrating radiation by all forms of lightning. What then is the link between TLEs and TGFs and why have measurements of relativistic electron beams at high altitudes and their associated emissions (other than gamma rays) been so elusive? The question of how TGFs fit into the overall scientific picture of high-altitude discharges begs for a dedicated mission such as TARANIS. It will provide combined measurements holding the keys to unlock this mystery and, at the same time, will provide a global coverage with improved instrument capabilities to shed new light on the remaining uncertainties related to TLEs.



Although the space domain involved, and in particular the altitude at which the runaway electron beams may originate and propagate, has not been established, one may imagine that the processes that generate TLEs and TGFs are not local. Lightning-induced Electron Precipitation (LEPs) may be triggered by the same lightning flash that led to the production of sprites. Ionization produced by the precipitation of electrons from the radiation belt may affect the stormy conditions. New results, obtained onboard Demeter satellite, clearly show that the interaction of inner radiation belt electrons with whistler waves are extremely important for the dynamics of this particle reservoir. A complete set of phenomena, including the mid-latitude atmosphere, ionosphere, and the radiation belts, has to be considered.

Generated within the atmosphere, the TLEs and TGFs, as well as the physical mechanisms that produce them, may affect the chemical constitution and the dynamic of the atmosphere. Although at too high altitude to measure them, TARANIS may complement ground-based, plane-based and balloon-based experiments: (i) by providing simultaneous observations at the nadir of lightning flashes, TLEs and TGFs, (ii) by measuring the accelerated and precipitated electrons and, (iii) by monitoring the electromagnetic environment. Synchronizations between ground-based, balloon-based and satellite based experiments are needed to measure all relevant parameters over time scales compatibles with the physical and chemical processes which take place.

Scientific questions to be addressed
TARANIS aims at providing a sufficiently complete package of novel instrumentation to answer specific questions raised by the many ground-based campaigns to observe TLEs, and by the highly successful FORMOSAT 2 and RHESSI missions. The science objectives of the TARANIS mission are into three broad categories:


 * Advance physical understanding of the links between TLEs and TGFs, in their source regions, and the environmental conditions (lightning activity, variations in the thermal plasma, occurrence of extensive atmospheric showers, etc);
 * Identify the generation mechanisms for TLEs and TGFs and, in particular, the particle and wave field events which are involved in the generation processes or which are produced by the generation processes;
 * Evaluate the potential effects of TLEs, TGFs, and bursts of precipitated and accelerated electrons (in particular lightning induced electron precipitation and runaway electron beams) on the Earth atmosphere or on the radiation belts.

To achieve these scientific objectives, the questions to be addressed are:


 * 1) TLEs and TGFs observations
 * 2) Environmental conditions
 * 3) Transfers of energy between the radiation belts and the Atmosphere
 * 4) Generation mechanisms for TLEs, TGFs, and runaway electron beams
 * 5) Effect on the Atmosphere and on the global electric circuit

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