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 similar to 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. Plain Language Summary The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) is an experiment onboard the Taiwanese FORMOSAT-2 satellite designed to explore transient luminous events (TLEs) as its main mission objectives. Since most TLEs are driven by intense lightning, this mission can also act as an intense lightning survey. ISUAL recorded nearly 300,000 transient events dominated by lightning in 12 years. An autonomous lightning identification algorithm, developed in this work, successfully registered most of the intense lightning from captured images. The intense lightning occurrence rate of ISUAL derived was 1.29 fl s(-1) , and the land/ocean ratio 1.27, which is lower than typical lightning. The seasonal variation of ISUAL lightning also indicated that intense lightning is active near the equator and peaks in April and September. The spectrophotometer of ISUAL provides spectral features of energetic lightning and shows that most of the intense lightning is accompanied by nitrogen emissions.

Automated summary

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment, conducted onboard the Taiwanese FORMOSAT-2 satellite, recorded nearly 300,000 transient events over a period of 12 years. An autonomous lightning identification algorithm was developed to successfully identify most of the intense lightning from captured images. The observations revealed that intense lightning is most active near the equator, with peak occurrences in April and September, and is often accompanied by nitrogen emissions.