Atmospheric storms constitute one of the most significant disturbance phenomena in the Earth's environment. Two thousand storms are permanently active throughout the world, producing 50 to 100 lightning bolts per second. Recent observations of light emissions in the medium and upper atmosphere and of gamma emissions from atmospheric origins demonstrate that there is impulsive coupling of the Earth's atmosphere with the ionosphere and the magnetosphere above active storm cells. With regards to space plasma and the chemistry and dynamics of the medium atmosphere, this direct coupling and the considerable energy involved leads to the intervention of processes that had not been envisaged until now. It can be triggered by cosmic radiation, solar winds and the meteorological and volcanic processes that affect the lower layers in the atmosphere.

Since the discovery of these phenomena is very recent, current knowledge is limited to light emissions observed in the spectrum visible from the ground or using optical detectors embedded on satellites and directed towards the horizon. Theoretical studies demonstrate that these emissions are just a small part of a much more complex phenomenon that also involves X and rays, electromagnetic rays that extend over a large range (0.1 Hz to several tens of MHz) and atmospheric/ionospheric coupling that results in the generation of intense electronic fields and electron acceleration that can reach very high energy levels (up to 30 MeV).
 

1- Detection and characterisation of TLEs

The objective is to measure lightning and Transient Luminous Events (TLEs), which occur above storm systems at between 20 and 10 km of altitude. These TLEs were observed by focusing on the horizon from the ground or by satellite (ISUAL/FORMOSAT). The figure above is a diagram of different TLEs.

The number of TLEs is not currently known. It is possible to estimate their number from global lightening frequency figures calculated by the LIS and OTD experiments onboard the TRRM satellite. Global lightening frequency stands at 45 flashes per second. This leads to between 5 and 27 sprites per minute, if we assume that there is 1 sprite (1,5 Mb) for 100 to 500 lightening flashes. The number of elves, which occur more frequently than sprites, could reach between 500 and 600 per minute.

These phenomena have different spatial and temporal characteristics. They are all systematically produced after a lightning flash but some last for varying lengths of time. Their characteristics are summarized in the table below:

* : the average delay between a sprite and a lightning flash is 30 ms, but its distribution presents large standard deviation. This delay cannot be estimated as yet.

The sprite spectrum is studied both on a theoretical level and through practical experiments. These spectra demonstrate that light emission from sprites is related to excitation in nitrogen molecules. The table below brings together the different N2 vibrational bands involved in sprite emissions.

* : Lyman-Birge-Hopfield

Unlike previous experiments, TARANIS will observe these phenomena as it passes directly below them to study the different effects on the medium (ionosphere) and couplings with neighbouring mediums (atmosphere and magnetosphere). To this end, it will have an embedded optical detection and observation system for TLEs that is designed to:

Detect TLEs and lightning on board the satellite to send out a warning signal to all equipment.
Differentiate between the lightning signal and that of TLEs.
Locate and characterise lightning and TLEs and their occurrence frequency.
Study any correlations between these characteristics and those of other associated phenomena (TGFs, waves, etc.).
 

2- Detecting TGFs and studying generation mechanisms of TLEs and TGFs

TGFs (Terrestrial Gamma ray Flashes) were discovered during the BATSE experiment. They originate in the upper atmosphere and would appear to be related to storm activity.

The mechanisms behind the creation of TLEs and TGFs have been the subject of numerous, often contradictory, studies. According to certain studies, they could be related to quasi-static electric fields that result in air breakdown; according to others they could be the result of an avalanche of relativistic electrons that are triggered by cosmic radiation and develop as far as the ionosphere and the magnetosphere. This avalanche could produce secondary X and ray through deceleration radiation.

First approximations of the chance observation of X and rays originating from the terrestrial atmosphere by the GRO satellite seem to support the latter model. Current observations from the Rhessi satellite demonstrate that the energies involved reach 30 MeV. Recent observations of X-rays in contact with triggered lightning show that these mechanisms could be much more common than previously thought.
 

3- Characterising runaway electrons that are accelerated upwards from atmosphere to the magnetosphere

Observations of ray have demonstrated the existence of energetic runaway electrons travelling upwards as TLEs occur. Several theoretical studies indicate that these runaway electrons cross the ionosphere and spread across into the magnetosphere. If this hypothesis were confirmed we would have a process that contributes to both:

populating radiation belts through the acceleration towards the magnetosphere of high-energy electrons generated by cosmic radiation at low altitudes,
variations in ionization rates in the atmosphere.

Less hypothetical is how radiation belt electrons are precipitated into the atmosphere. It was established many years ago that at high latitudes (L>5) precipitated electrons are at the origin of Nox production at altitudes of 90 km, and that polar wind transports this Nox to lower altitudes. Although it has never been formally demonstrated, it is widely accepted that at medium and low altitudes - where the highest energy electrons are precipitated (significant electron flows of more than 10 Mev have been observed by the SAMPEX satellite) – effects on NOx and O3 concentration must be observed directly in the stratosphere without using a transport mechanism.
 

4- Identifying the effects of TLEs on coupling between the ionosphere and the magnetosphere and the role of precipitated electrons in the coupling between the magnetosphere and the atmosphere.

Electron acceleration and precipitation are generally accompanied by electromagnetic and electrostatic waves, which either contribute to the process or were generated by it. In both cases, it is essential to research the electromagnetic and/or electrostatic signature to identify the mechanisms at work, or even to detect runaway electrons that are too narrow to be observed directly. A significant example of signatures of this kind is currently provided by the "second peaks" observed from the ground on ELF whistles. They are currently interpreted as the signature of currents produced in sprite cores.

Studies are in progress into the electromagnetic and electrostatic signatures of transient phenomena likely to reveal coupling processes between the ionosphere and the atmosphere or between the magnetosphere and the atmosphere. Models predict the generation of LF/MF electromagnetic waves through runaway electrons accelerated upwards from the atmosphere to the magnetosphere. Although wave generation mechanisms are well documented with regards to electron precipitation at high latitudes, much work remains to be done for medium and low latitudes.