Responses of the Daytime Low and Equatorial Ionosphere and Thermosphere Over the Indian Region During the Geomagnetic Storm of April 2023.

Study of the response of Thermosphere–Ionosphere (TI) system over the Indian longitude sector during the geomagnetic storm of April 23–24, 2023, is presented. The ionosonde observations at the dip equatorial station, Trivandrum (8.52°N, 77°E, dip lat. = 1.96°N), are found to have unusually high F2 peak plasma density (foF2), with three maxima during the daytime on April 24. The Total Electron Content (TEC) observations from different latitudes showed enhancements temporally progressing from north to south. These features are identified as Traveling Ionospheric Disturbances (TIDs) with speeds ∼400 to 750 m/s. Two major enhancements in TEC observed over Trivandrum were concurrent to the first and third maxima in foF2. Simulations of meridional wind and temperature for this storm were carried out using the Thermosphere–Ionosphere–Electrodynamics General Circulation Model (TIEGCM) that was driven with high‐latitude inputs derived from the Assimilative Mapping of Ionospheric Electrodynamics (AMIE) technique. Simulations show the signatures of Traveling Atmospheric Disturbances (TADs) in ionospheric F‐region, which conform with the TID observations. The second foF2 peak is found to be simultaneous to the modulation in Prompt Penetration Electric Field (PPEF), which is also observed in the drift of the E‐region ionospheric plasma irregularities using an HF radar and the Equatorial Electrojet (EEJ) strength measured by ground‐based magnetometer. Simultaneously increasing electric field and foF2 implies the presence of storm‐time equatorward wind, inhibiting the plasma fountain. This is the first report comparing comprehensive observations and AMIE‐TIEGCM modeling studies of daytime responses of low and equatorial TI during a geomagnetic storm. Plain Language Summary: Geomagnetic storms correspond to temporary disturbances in the chemistry, dynamics, and electrodynamics of the Thermosphere–Ionosphere system. This study focuses on the plasma density modulations in the equatorial ionosphere over the Indian region during the geomagnetic storm of April 23–24, 2023. This was one of the severe geomagnetic storms that occurred during the rising phase of solar cycle 25. Ground‐based observations of the peak plasma density at the equator revealed three distinct daytime enhancements during the storm's initial recovery phase. The Total Electron Content (TEC) measurements from various latitudes in the Indian longitude sector showed that the first and third TEC enhancements corresponded to Traveling Ionospheric Disturbances (TIDs) moving from mid‐latitudes in the northern hemisphere to the equator. One of the TIDs also propagated trans‐equatorially to the southern hemisphere. Model simulations of neutral temperature and meridional wind corroborated these observations by indicating the presence of Traveling Atmospheric Disturbances (TADs). The second enhancement in equatorial plasma density, on the other hand, was likely to be caused by the interaction between the modulations in the equatorial E‐region electric field, brought in by the Prompt Penetration of the Interplanetary Electric Field, and the meridional wind. Key Points: The F‐region peak plasma density is found to have three unusual enhancements during the daytime over TrivandrumThe observations of TEC at multiple latitudes indicate the presence of TIDs that are largely responsible for two of the three enhancementsThe remaining enhancement is due to the modulation in the E‐region electric field owing to PPEF oscillation [ABSTRACT FROM AUTHOR]