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Terrestrial gamma-ray flashes (TGFs) are bursts of energetic X- and gamma-rays emitted from thunderstorms. The Atmosphere-Space Interactions Monitor (ASIM) mounted onto the International Space Station (ISS) is dedicated to measure 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. Based on the temporal evolution of lightning flashes and the spatial pattern of the constituing events, we present an algorithm to reduce the number of space-detected flashes potentially associated with TGFs by ∼ 60% and of associated LIS groups by ∼ 95%. We use ASIM measurements to confirm that the resulting flashes are indeed those associated with TGFs detected at approx. 400 km altitude and thus benchmark our algorithm preserving 70–80% of TGFs from concurrent ASIM-LIS measurements. We 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 and do not observe any significant difference. Finally, we present a parameter study of our algorithm and discuss which parameters can be tweaked to maximize the reduction efficiency whilst keeping flashes associated to TGFs. In the future, this algorithm will hence be capable of facilitating the search for TGFs in a reduced set of lightning flashes measured from space.

Natural lightning is both frequent and variable and thus a good subject for statistical studies. A typical negative cloud-to-ground (CG) flash consists of multiple individual return strokes. The spatial and temporal distributions of various lightning events throughout the discharge provide a surrogate look inside the CG flash and offer insight into the underlying physical processes. In this study, we combine 8 years of National Lightning Detection Network (TM) (NLDN) and North Alabama Lightning Mapping Array (NALMA) data to compute the spatial and temporal distributions of (i) subsequent NLDN-reported return strokes and (ii) LMA-reported sources around NLDN-reported CG strokes. Subsequent strokes are separated into those with the same contact point as the first stroke and those flowing along new lightning channels. Statistically, the distribution of strokes along new channels evolves deterministically, with similar to 200 km/s propagation speed from the original channel, comparable to the speed of a stepped leader. This suggests that the -CG subsequent strokes forming new channels may be directly linked to the initial one by a propagating leader inside the cloud. We present LMA case studies and a multiyear analysis of NLDN-LMA data that support this behavior. Our results are supported by ground-truth measurements and video recordings from previous field studies., Defense Advanced Research Project Agency (DARPA)