Employing Optical Lightning Data to identify lightning flashes associated to Terrestrial Gamma-ray Flashes

Terrestrial gamma-ray flashes (TGFs) are bursts of energetic X- and gamma-rays emitted from thunderstorms and observed by the Atmosphere-Space Interactions Monitor (ASIM) mounted onto the International Space Station (ISS) detecting TGFs and optical signatures of lightning. ISS-LIS (Lightning Imaging Sensor) detects lightning flashes allowing for simultaneous measurements with ASIM. Whilst ASIM measures ~300-400 TGFs per year, ISS-LIS detects ~ 10⁶ annual lightning flashes giving a disproportion of four orders of magnitude. Hence, based on the temporal evolution of lightning flashes and their spatial pattern, we present an algorithm to reduce the number of flashes potentially associated with TGFs by ~90%, and we use the ASIM TGF list to ensure that the resulting flashes are those associated with TGFs and thus benchmark our algorithm. We will compare how the radiance, footprint size and the global distribution of lightning flashes of the reduced set relates to the average of all measured lightning flashes. Finally, we will present a parameter study of our algorithm and discuss which parameters can be tweaked to maximize the reduction efficiency whilst keeping those flashes associated to TGFs. In the future, this algorithm will hence facilitate the search for TGFs in a reduced set of lightning flashes.