Simultaneous Space-Based Observations of TGFs and Lightning Optical Emission: Characteristics of Lightning

A previous study of the relative timing between optical lightning emission detected by the Geostationary Lightning Mapper and a sample of 24 simultaneous Terrestrial Gamma-ray Flashes (TGFs) triggering the Fermi Gamma-ray Burst Monitor has shown that the gamma-ray signal appears at the same time as or slightly before the lightning stroke optical signal. A similar result has been demonstrated using simultaneous optical and gamma-ray data from the Atmosphere-Space Interactions Monitor. In this work, we add 95 additional untriggered (offline) TGFs to further investigate the correlations between the gamma-rays and the associated lightning optical emission. Analysis of radio sferics has shown that TGFs are associated with relatively high lightning peak currents. The current results show that TGFs are associated with short duration, relatively low intensity optical signals in low-to-moderate flash rate thunderstorm cells, consistent with lightning leaders that produce TGFs occurring some distance below the cloud top and/or expending a fraction of their energy in accelerating relativistic electrons rather than in producing bright optical emission. Plain Language Summary Based on simultaneous observations of Terrestrial Gamma-ray Flashes (TGFs) detected by the Fermi Gamma Ray Burst Monitor and lightning optical emission observed by the Global Lightning Mapper on Geostationary Operational Environmental Satellite R series 16, we find that (a) TGFs are accompanied by lightning with lower observed intensity optical flashes, fewer lightning strokes, and shorter flash duration compared to background; (b) TGFs are produced in thundercloud cells characterized by low-to-moderate flash rates; and (c) discharges associated with TGFs are characterized by high absorption and/or expend their energy in accelerating relativistic electrons rather than generating the optical signal. This may mean that TGFs occur at greater depths in the thundercloud than typical lightning, and/or that TGFs are associated with storms with intrinsically weaker hot lightning channels and associated optical emission.