Spatial variability of TEC perturbations and ROTI during the 3 october 2024 X9.0 solar flare: a multi-station GNSS analysis

This study investigates the spatially heterogeneous low-latitude ionospheric response to the 3 October 2024 X9.0 solar flare using GNSS observations from seven equatorial and low-latitude stations. TEC, quiet-time differential TEC ( Δ TEC ), and ROTI were analyzed under relatively quiet geomagnetic conditions ( Kₚ=1 –3). The flare peaked at 12:18 UT, and comparison with quiet-time conditions revealed Δ TEC perturbations of approximately 6–14% at several stations, with pronounced spatial differences. YKRO showed the largest positive perturbation at the flare peak, reaching 12.75 TECU (14.21%), whereas MBAR and CUSV exhibited negative perturbations of $$-10.98$$ TECU ( $$-10.77%$$ ) and $$-6.14$$ TECU ( $$-9.29%$$ ), respectively. However, substantial and sometimes larger Δ TEC variations occurred outside the flare interval, indicating that their magnitude alone cannot establish a flare-driven response. ROTI remained generally below 0.04 TECU min ⁻¹ during the flare, with stronger irregularity activity occurring predominantly after sunset. The results indicate a measurable and spatially heterogeneous ionospheric response temporally associated with the X9.0 flare, while highlighting the substantial contribution of intrinsic low-latitude variability. These findings emphasize the importance of multi-station observations and quiet-time normalization for robustly assessing solar-flare-associated ionospheric disturbances.