Reconstruction of 2D/3D Ionospheric Disturbances During the X9.0 Solar Flare on 3 October 2024 Using Multi‐Instrument Observation

The ionospheric response to the record‐breaking X9.0 solar flare of 3 October 2024, is comprehensively analyzed by integrating multi‐instrument observations (Global Navigation Satellite System (GNSS), Swarm, COSMIC‐2, GUVI) with Thermosphere‐Ionosphere Electrodynamics General Circulation Model simulations. Global detrended electron content results indicate a rapid dayside intensification, peaking at ∼2 TECU over Europe at 12:20 UT. Based on high‐resolution 3D tomography (2° × 2° × 50 km), the transient ionospheric vertical structure reveals that the background electron density state influences its vertical evolution. This mechanism is shown to govern the distinct vertical structures observed across different local times. In the European noon sector, a bulk electron density enhancement across 150–550 km is attributed to strong photochemical‐thermodynamic coupling. Conversely, in the North American morning sector, thermal expansion is limited by the low‐density background, confining the photoionization‐driven enhancement largely below 250 km, despite a relative increase of up to 225%. Furthermore, both ionosonde and geomagnetic observations reveal a flare‐induced surge in the equatorial zonal electric field and equatorial electrojet (EEJ). This enhancement is found to strengthen the upward drift, thereby intensifying the equatorial ionization anomaly (EIA). Thermosphere‐Ionosphere Electrodynamics General Circulation Model simulations consistently reproduce the flare‐time enhancement of upward E × B drift and the associated strengthening of NmF2/EIA. The results highlight that pre‐flare background states critically modulate ionospheric responses through coupled electrodynamic, photochemical, and thermodynamic processes.