Global ionospheric electron density variations observed by F7/C2 and $\Sigma \mathrm{O}/\mathrm{N}_{2}$ratios measured by GUVI during intense geomagnetic storms in solar cycle 25

In this study, we investigate the global ionospheric response to the four most intense geomagnetic storms of Solar Cycle 25 (SC 25) by analyzing hourly averaged $\Sigma \mathrm{O}/\mathrm{N}_{2}$ratios from the Global Ultraviolet Imager (GUVI) aboard the TIMED satellite, alongside 3D electron density profiles from the FORMOSAT-7/COSMIC-2 (F7/C2) constellation satellites. The analysis focuses on ionospheric behavior at low and mid-latitudes, with further evaluation across three longitudinal sectors: Asia (60°E–120°E), Africa (30°W–60°E), and the Americas (120°W–30°W). The negative effects at mid-latitudes started during the local nighttime in the main phase, prolonged into daytime in the early recovery phase, and continued into the following nighttime. These negative effects also reached low latitudes during similar local times. The extent, intensity, and equatorward expansion of these prolonged negative responses were strongly dependent on storm severity and duration. Short-term positive effects were observed at low latitudes during the early recovery phases of less intense storms. Moreover, positive ionospheric responses were recorded at low and mid-latitudes during daytime in the main phase, and long-lasting positive effects were evident at mid-latitudes during the late recovery phases. This study would be beneficial in enhancing space weather modeling and forecasting capabilities, particularly in understanding ionospheric variability during geomagnetic disturbances.