On the global occurrence and impacts of transient luminous events (TLEs)

Based on the first four years of the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) experiment onboard the FORMOSA!-2 sateUite, ELVEs are found to be the most abundant type (-80%) of TLEs (transient luminous events), whereas sprites and halos only combine to account for -20%. The distribution of sprites and halos closely resemble that of the cloud-to-ground lightning. However, nearly 60% of ELVEs occurred over the ocean, a feature indicating that the high peak current lightning is more abundant over the ocean. The global TLE occurrence rates are inferred to be 72, 3.7, and -1 events/minute, respectively, for ELVEs, halos, and sprites. Comparing with the results from the first three years of the ISUAL experiment reported by Chen et al. [1], the global occurrence rates for ELVEs and halos are higher due to the adoption of different correction factors. Using these updated TLE rates, the free electron content over an ELVE hot zone is estimated to be elevated by more than 10%. From analyzing ISUAL TLE events without lightning emission contamination, the average deposited energy in the upper atmosphere by sprites, halos, and ELVEs was found to be 22, 14, and 19 MJ per event, respectively. After factoring in the occurrence rates, in each minute sprites, halos, and ELVEs deliver 22, 52, and 1370 MJs of troposphere energy to the upper atmosphere, which are also higher than those reported in Kuo et al. [2]. However, the global occurrence and energy rates for ELVEs are likely the lower-bound values. A past observation [3] and our ensuing analyses both indicate that the ELVES occurrence rate and, thus, their energy deposition rate in the upper atmosphere should be higher.