The First Terrestrial Electron Beam Observed by The Atmosphere-Space Interactions Monitor

We report the first Terrestrial Electron Beam (TEB) detected by the Atmosphere-Space Interactions Monitor (ASIM). It happened on September 16th, 2018. The ASIM Modular X and Gamma-ray Sensor (MXGS) recorded a two millisecond long event, with a softer spectrum than typically recorded for Terrestrial Gamma-ray Flashes (TGF). The lightning discharge associated to this event was found in the World-Wide Lightning Location Network (WWLLN) data, close to the northern footpoint of the magnetic field line passing by the International Space Station (ISS). Imaging from a GOES-R geostationary satellite shows that the source TGF was produced close to an overshooting top of a thunderstorm. Monte-Carlo simulations were performed to reproduce the observed lightcurve and energy spectrum. The event can be explained by the secondary electrons and positrons produced by the TGF (i.e. the TEB), even if about 5\% to 10\% of the detected counts may be due to direct TGF photons. A source TGF with a Gaussian angular di...