Constraining spectral models of terrestrial gamma-ray flashes from a terrestrial electron beam observation by the Atmosphere-Space Interactions Monitor

Abstract:

Terrestrial Gamma-ray Flashes (TGFs) are short flashes of high energy photons, produced by thunderstorms. When interacting with the atmosphere, TGFs' photons produce relativistic electrons and positrons, and a fraction is able to escape the atmosphere (mostly produced between 40 and 100 km altitude). Electrons positrons are then bounded to Earth's magnetic field lines and can travel large distances in space. This phenomenon is called a Terrestrial Electron Beam (TEB). The Atmosphere-Space Interactions Monitor (ASIM) in-board the International Space Station and dedicated to the study of TGF and associated events, started to operate in June 2018. We present a new TEB event detected on March 24, 2019. The TEB originated from rainbands produced by the tropical cyclone Joaninha, in the Indian Ocean, close to Madagascar. This observation shows, for the first time, a TEB spectrum recorded down to 50 keV, using ASIM's low energy detector. It permitted an unprecedented spectral analysis to constraint the spectrum of the source TGF. For this event, leader models with potential drops lower or equal to 300~MV are not able to explain the observed spectrum, and the compatible fits are attributed to source TGF spectrums in the form $E^{-1}\exp\left(-E/\epsilon\right)$ with cut-off values $\epsilon \geq 6.5$~MeV. We suggest that deeper investigation should be done regarding high potential ($>\approx300$~MV) TGF production leader models able to produce such spectrums.