New Insights into the Hybrid Propagation Characteristics of Lightning-Induced Whistler Echo Trains from CSES-01 Measurements at Low-Earth Orbit

As electromagnetic pulses from atmospheric lightning propagate into the ionosphere and inner magnetosphere, they evolve into whistler-mode waves with characteristic descending-frequency tones. Subsequent multiple reflections within the magnetospheric plasma further increase the dispersion of these waves, ultimately forming multi-hop echo trains. This study investigates such whistler echo train events using high-resolution electromagnetic field observations from the China Seismo-Electromagnetic Satellite (CSES-01). Wave vector analysis based on the Singular Value Decomposition (SVD) method reveals that these echo train waves are well structured, right-hand polarized whistler-mode emissions. They propagate obliquely with wave normal angles of 45°–60°. Poynting vector analysis further confirms a predominantly perpendicular energy flux with an earthward component. A key finding from both theoretical and observational analysis is that the dispersion values of successive echoes follow proportional sequences (e.g., 2:4:6:8 and 1:3:5:7…). Combined analysis with the Worldwide Lightning Location Network (WWLLN) establishes clear spatiotemporal correlations between these specific echo trains and causative lightning discharges. Our analysis shows that the observed echo trains originate from ducted whistlers reflected between hemispheres. However, a critical modification occurs near the reflection region at satellite altitude (~507 km), where wave packets are deflected. This deflection results in a hybrid propagation feature, characterized by the coexistence of non-ducted propagation features within a global ducted structure. This work sheds new light on the complex propagation of lightning-induced whistlers and highlights the utility of CSES-01 measurements for probing fine-scale magnetospheric wave phenomena.