Global Ionospheric Responses to Four X‐Class Solar Flares in November 2025

Abstract We investigate global ionospheric responses to four X‐class solar‐disk flares that occurred in November 2025 from the same solar active region, using multi‐instrument observations including GNSS, ionosonde, geomagnetic and GOES solar irradiance data. Pronounced dayside TEC enhancements are observed for three flares (#2 X1.2, #3 X5.1, and #4 X4.0), with comparable durations of 20–30 min and TEC magnitudes increasing with flare intensity. A consistent time delay of 30–60 s is identified between peaks in EUV irradiance rate and rate of TEC (ROT), supporting the primary role of EUV radiation in the E/F‐region TEC enhancements. In contrast, no detectable TEC response was observed for flare #1 (X1.7), despite its higher GOES X‐ray class, common active‐region origin, and similar disk location to flare #2. We interpret this absence as mainly associated with its weak EUV irradiance‐rate enhancement and potentially insufficient to produce E/F‐region responses distinguishable from background ionospheric variability. Concurrent ionosonde observations reveal consistent increases in fmin and disappearance of foF2 across multiple sunlit stations, indicating strong high‐frequency (HF) radio wave absorption in the lower ionosphere. Additionally, GNSS TEC increased while ionosonde foF2/NmF2/hmF2 decreased during flare #2 (X1.2). This contrasting response coincided with short‐lived negative H‐component perturbations at nearby geomagnetic stations, suggesting a role for regional electrodynamic forcing.