PRN-Wise Investigation of Storm-Time GNSS S4 Scintillations and Wave-Like Ionospheric Structures over Agra during Solar Cycle 25

Abstract This study investigates storm-time ionospheric scintillation variability over Agra, India, using GNSS-derived S4 index observations during 2024–2025 under Solar Cycle 25 conditions. PRN-wise analysis revealed enhanced scintillation activity mainly during post-sunset and nighttime hours (1700–2300 UT), with strong scintillation events (S4 ≥ 0.6) observed during severe geomagnetic disturbances. Two major geomagnetic storms, namely the May 2024 superstorm and the November 2025 G5 storm, were analyzed using Dst and Kp indices. The May 2024 event exhibited comparatively stronger scintillation activity with peak S4 values approaching 1.2, whereas the November 2025 storm showed a broader distribution of weak-to-moderate scintillation events. Continuous Wavelet Transform (CWT) analysis identified dominant periodicities ranging from 20 to 180 min, indicating the presence of multi-scale wave-like ionospheric structures and traveling ionospheric disturbances. The results suggest that storm-time electrodynamic processes, including prompt penetration electric fields and post-sunset plasma instabilities, significantly enhanced low-latitude plasma irregularities during disturbed geomagnetic conditions. The study demonstrates a strong coupling between geomagnetic storm intensity and GNSS scintillation variability over the Indian low-latitude region.