Cross-equatorial travelling ionospheric disturbances and changes in background ionospheric densities over Indian longitudes during geomagnetic storm of 20–21 December 2015

Geomagnetic disturbances modify the ionospheric densities and composition following the neutral and electrodynamic processes globally. Wave-like perturbations can be generated at the high latitudes due to enhanced energy deposition through the particle precipitation and auroral heating. They can propagate through the ionospheric medium which is known as travelling ionospheric disturbances (TIDs). In this study, we have investigated the changes in the background ionospheric densities and a large-scale TID (LSTID) using the measurements of OI 630.0 nm nightglow emissions, altitudinal profiles of ionospheric electron densities, and Total Electron Content (TEC) during a major geomagnetic storm on 20 December 2015. Wave-like variations in OI 630.0 nm nightglow emission intensities with a period of around 2–3 h were observed on 20–21 December 2015 over Mt. Abu, a low-latitude location over Indian longitudes. The F-layer peak heights and critical frequencies exhibit similar periodic variations. Signature of LSTIDs was studied using the TEC variation obtained from 12 available International GNSS Service (IGS) stations located in the Indian and Australian longitudinal regions. It has been found that the LSTIDs originated in the Australian latitudes, propagated towards and crossed the equator, and then dissipated while moving further northwards. Further, the roles of the equatorial electrodynamics, O/N₂, and interhemispheric wind, responsible for the modification of the background ionospheric densities, are also discussed.