Studies of Terrestrial Gamma-Ray Flashes and Gamma-Ray Bursts with the XGRE instrument on- board the TARANIS satellite; operational monitoring of the instrument.

Background: Thunderstorms are present all over the world. They produce lightning that emits electromagnetic waves in the gamma ray, optical and radio wavelength. Since the 1970s, optical flashes have been observed from space, but it is only since the mid-1990s that imagers on low-orbit satellites have drawn the first global maps of thunderstorms electrical activity. In addition, very brief and large optical phenomena, called sprites, were observed at great distances over thunderstorms from the 1990s. They are the result of the electrostatic coupling between the thundercloud, in which a lightning strike occurs, and the lower layers of the ionosphere. Thus, observations of “sprites”, “jets”, “elves”, optical events called Transient Luminous Events (TLE) and observations of Terrestrial Flash Gamma (TGFs) have shown the existence of sudden energy transfers between Earth atmosphere and space.

The first detections of TGF over active thunderstorms by the BATSE instrument on board the CGRO satellite provided evidence of the acceleration of relativistic electrons in the Earth's atmosphere. TGFs are also monitored by the RHESSI satellite, which has so far observed 10 to 20 TGFs per month. The geographical distribution of TGF roughly corresponds to the geographical distribution of lightning on the continents at low altitude and to the distribution of sprites.

However, many questions on TGF remain. Their emission are rarely detected in the southern United States where numerous sprites are observed on the ground. The origin of most TGFs could be linked to lightning at the top of thunderstorms. However, differences in the temporal and spectral signatures of several TGFs observed by BATSE and RHESSI suggested the existence of sources at higher altitude.

The XGRE gamma-ray instrument on-board the CNES TARANIS microsatellite mission has been optimized to answer these questions. Developed at APC laboratory in Paris with the help of IRAP, Toulouse, it can detect electrons between 1 MeV and 10 MeV, and X-ray / gamma-rays between 50 keV and 10 MeV, making high resolution light curves and spectra. Its excellent temporal resolution and low instrumental dead time allow it to measure fast and bright events like TGFs. The TARANIS mission will be launched from the Kourou space centre in November 2020.

Moreover, even if it was not its main objective from the beginning, XGRE has the capability to observe and study numerous short Gamma-Ray Bursts with its unprecedented 350 ns timing resolution. Having a larger effective area below 200 keV than the Fermi/GBM (but a lower field of view, being directed to Earth), it may detect up to 20 short GRB by year, potentially contemporaneous with Multi-messenger gravitational or neutrinos events. The XGRE observations will be thus included in the future campaigns to look for coincident photons/GW or photons/neutrinos events.

Goals: The main objectives of the post-doctoral fellow will be :

1) To participate in the performance monitoring of the instrument. This work will be divided in two main phases :

a. Participation to the Performance Verification and Calibration phase: this phase of the satellite's life will last 6 months from launch. The first 2 months will be dedicated to a functional validation of the entire payload. The XGRE instrumental performance optimization and calibration will occur in the next four months.

b. Participation in the daily monitoring of the performance instrument, in direct connection with the XGRE team at APC.

2) Study TGF and GRB using XGRE unique timing and spectral possibilities. XGRE is actually the best instrument to study TGF properties, in conjunction with observations made by the other TARANIS instruments at longer wavelength (optical, radio). XGRE is expected to detect about 200 TGFs and 100 TEB (“Terrestrial Electron Beams”, electrons flux produced during the thunderstorm, possibly in conjunction with a TGF or a TLE) by year. It will have also a good sensitivity to short GRBs, which it could study with its unprecedented timing resolution. The instrument data being not open to the whole community, working at APC will give the fellow a unique opportunity to access these data.

These analyses will be done on one side within the framework of the scientific community working on atmospheric and space electricity in France and around the world (collaboration with Japanese laboratories) but also with connexion with the GRB/Multi-messenger astronomy community, the APC team having strong link with the VIRGO observatory and with the ESA/INTEGRAL satellite projects. The postdoctoral work will have a duration of one year (once renewable) and will take place at APC (close to Paris centre). The post-doctoral fellowship can begin as early as December 2020.

To apply, please send a resume, cover letter, two recommendation letters and a summary of your PhD thesis by e-mail to Cette adresse e-mail est protégée contre les robots spammeurs. Vous devez activer le JavaScript pour la visualiser. or by standard mail to:

P. Laurent
Laboratoire Astroparticule et Cosmologie Bâtiment Condorcet
10 rue Alice Domont et Léonie Duquet 75205 Paris
France