Direct Observations of Large‐Scale Wave Structure and Post‐Sunset Rise of the Ionosphere in the Generation of Equatorial Spread F

Abstract Large‐scale wave structure (LSWS) and the post‐sunset rise (PSSR) are two key factors in the generation of equatorial spread F (ESF). In this study, we reported continuous and direct observations of LSWS and PSSR and their effects on seeding ESF by Sanya incoherent scatter radar (SYISR) and ionosonde in September–October 2024. SYISR data indicated distinct day‐to‐day variability in LSWS, characterized by variable zonal wavelengths and periods with mean values of 436 km and 52 min, respectively. Ray‐tracing simulations demonstrated that the upwelling and propagation of LSWS greatly affect signature features of frequency range and virtual height of satellite traces in ionograms. Further analysis revealed that ESF occurrence rate reached ∼92.3% when the vertical drift exceeded 30 m/s in the presence of LSWS. These results suggested that the presence of LSWS constitutes a precondition for ESF generation during this studying interval, whereas a strong PSSR acts as a favorable condition.