Ionospheric Disturbances Associated With Typhoon Ragasa in 2025 Observed Using GNSS and Satellite Measurements

Abstract

Typhoon Ragasa in September 2025 provides an opportunity to examine ionospheric responses to tropical cyclone forcing from the lower atmosphere. We combine Global Navigation Satellite System (GNSS) observations over Taiwan and Hong Kong with Himawari‐9 thermal infrared (TIR) data, COSMIC‐2 radio occultation profiles, TIMED/GUVI thermospheric composition measurements, and Horizontal Wind Model 2014 (HWM14) winds. Under stable solar and quiet to weakly disturbed geomagnetic conditions, persistent medium‐scale traveling ionospheric disturbances were detected, with phase speeds of 160–271 m/s and detrended total electron content (dTEC) amplitudes of ±0.4–0.5 TECU. TIR ripples were aligned with dTEC wavefronts 560–1,120 km from the typhoon center, supporting a connection between deep convection and the observed ionospheric disturbances. Regional mean dTEC variability during the typhoon was comparable to later variability, and the rate of TEC (ROT) excursions was similar before, during, and after the event, whereas the rate of TEC index (ROTI) transiently increased to about 1.4 TECU/min. Enhanced ROTI along PRN 08 and PRN 27 occurred when ionospheric pierce points sampled the 600–800 km region containing organized TIR perturbations. COSMIC‐2 profiles showed electron density enhancement at 15°–30°N and depletion at 0°–15°N, indicating a latitude dependent plasma response. HWM14 showed weaker and smoother winds in the mesosphere and larger vertical variability above 90 km, providing background context for the propagation and dissipation of gravity waves. TIMED/GUVI showed no pronounced large scale O/N2 depletion. Together, these observations support Ragasa associated wave forcing as an important contributor to multiscale ionospheric response.