Global Distribution and Seasonal Variation of Transient Luminous Events Observed by ISUAL Over 12 Years

The Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard the FORMOSAT-2 satellite studied global Transient Luminous Events (TLEs), which occur between the troposphere and ionosphere. Using deep learning techniques, the ISUAL TLE event list from July 2004 to June 2016 was systematically constructed, containing 66,586 events. ISUAL is the satellite dedicated to TLE observation, mapping the global distribution of TLEs, and revealing distinct spatial patterns. Elves were most frequent over oceans, sprites over land, halos over coastal areas, and blue jets and gigantic jets predominantly in low-latitude regions. This study first constructed a global TLE occurrence density map through the fusion of ISUAL TLE observations and World Wide Lightning Location Network intense lightning distributions. The corrected global TLE occurrence rate is 47.08 events per minute, notably exceeding earlier studies, with individual rates of elves (32.74), sprites (5.94), halos (2.98), blue jets (5.40), and gigantic jets (0.02). Furthermore, TLE occurrence rates were examined across different climate zones using the Köppen classification system. Higher rates were found in tropical and temperate climates, with sprites particularly frequent in hot summer regions. To further examine potential external drivers, this study for the first time analyzed a full 11-year solar cycle activity in relation to nighttime TLE occurrence, revealing only a weak correlation (R ≤ 0.3) and favoring meteorological factors instead of solar activity as the primary drivers of TLE variability.