Global Ionospheric Responses to Four X-class Solar Flares in November 2025

We present the first comprehensive investigation of global ionospheric responses to four X-class solar flares in November 2025 using multi-instrument observations, including GNSS TEC, ionosonde measurements, 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 minutes and TEC magnitudes increasing with flare intensity. A consistent time delay of 30-60 seconds is identified between peaks in EUV irradiance rate and rate of TEC (ROT), whereas no clear relationship is found with X-ray variations for these three flares, demonstrating that EUV radiation primarily governs TEC enhancement. In contrast, no detectable TEC response is observed for flare #1 X1.7 despite its high X-ray classification and the same flare source location. This discrepancy is attributed to weaker EUV emissions for flare #1 relative to flare #2 and a 1-2 orders of magnitude smaller EUV irradiance rate than the other three flares, resulting in insufficient ionization to overcome background losses or concurrent natural variability, suppressing detectable TEC signatures. 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, a simultaneous increase in TEC and decrease in foF2 (NmF2) is observed during the X1.2 flare, likely associated with weakened upward E × B drift and/or increased electron temperature. These results provide new insights into the distinct roles of different spectral components in governing ionospheric responses to solar flares and their impacts on communication and navigation systems.