Global Distribution and Spectral Features of Intense Lightning by the ISUAL Experiment

During the 12 years of the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment onboard the Taiwanese FORMOSAT-2 satellite (2004/07/01–2016/6/20), 297,349 transient events were recorded, but only 48,414 events (16.3%) were manually classified as transient luminous events (TLEs). Since the main target of ISUAL is to observe TLEs, only the top ∼0.82% of intense lightning can meet the trigger threshold, which means the ISUAL experiment can also serve as a space-borne intense lightning survey. An autonomous lightning identification algorithm was developed in this work to screen and locate lightning flashes in images, and 292,248 intense lightning discharges were identified successfully. Combining the statistical analysis of lightning events and unique spectral observations of ISUAL, the global distribution of intense lightning with spectral characteristics was constructed. The intense lightning occurrence rate of ISUAL was 1.29 fl s⁻¹, and the land-to-ocean ratio 1.27. Intense lightning is most active near the equator, and the bimodal pattern peaks in April and September, different from the typical lightning behavior that occurs in the northern hemisphere during the summertime from July to August. In addition, the spectral statistics show that more than 95% of intense lightning is accompanied by nitrogen band emissions (1PN2, 2PN2, 1NN2+). Far-ultraviolet photons mainly come from the radiation processes in the mesosphere to the ionosphere, and its radiation flux may correlate with lightning energies. Further, a significant 777.4 nm background enhancement is recognized in South America, which may affect the trigger of space-borne 777.4 nm lightning surveys.