A study of the ionospheric response during intense geomagnetic storms over the Indian low-latitude region during the period 2017–2023

This study investigates the low-latitude ionospheric response to five intense geomagnetic storms using Total Electron Content (TEC), Rate of VTEC Index (ROTI), and S4 index observations from a NavIC receiver observations at an Indian low-latitude station Sangli (Latitude 16.5°N, Longitude 74.3°E). We analyze variations in the Interplanetary Magnetic Field (IMFBz), solar wind parameters (Vsw, Nsw), geomagnetic indices (SYM-H, Dst), and Equatorial Electrojet (EEJ), along with O/N₂ data from the Global Ultraviolet Imager (GUVI). Among the selected storms, September 2017 and August 2018 exhibited positive ionospheric storm effects, while April 2023 showed strong negative effects. The February and March 2023 storms demonstrated both positive and negative responses. VTEC variations during the main and recovery phases were influenced by eastward (positive storm) and westward (negative storm) EEJ, along with thermospheric O/N₂ changes. CME-driven storms caused abrupt VTEC enhancements, whereas CIR-HSS-driven storms led to prolonged disturbances due to sustained southward IMFBz. Daytime VTEC enhancements in September 2017, August 2018, February, and March 2023 were linked to eastward electric fields or increased O/N₂ ratios, while nighttime VTEC depletions in February, March, and April storms resulted from prolonged equatorial electric field suppression. Strong ionospheric irregularities and plasma bubble occurrences were observed, particularly during the 2023 storms with higher ROTI and S4 index values. The extreme EEJ suppression in April 2023 and pre-storm irregularities suggest ionospheric preconditioning.