On the dynamics of Travelling Ionospheric Disturbances and Background Ionospheric changes over low-latitude Indian sector during December 2015 Geomagnetic Storm

Geomagnetic disturbances perturb the ionosphere-thermosphere (IT) dynamics, modify the background densities, compositions, etc. on the global scale. Sudden energy deposition in the auroral region during geomagnetic events can generate wave-like disturbances which further propagate through the IT system under favourable background condition, known as travelling ionospheric disturbances (TIDs). The disturbances can be of different scale sizes, ranging from few kilometres (kms) to thousands of km. Large-scale TIDs (LSTIDs), having typical horizontal scale sizes of several thousands of km and periodicities of a few hours, propagate with a speed of 400-1000 ms^-1. In this study, we have investigated a LSTIDs present over Indian longitudes during a geomagnetic storm of 20-21 December 2015. The imprint of the TIDs is seen in the OI 630.0 nm nightglow emissions, height variation of F-layer, heights of ionospheric iso-electron densities, and total electron count (TEC) over low-latitude Indian longitudes. The variation was wave-like with a period of around 2-3 hours. The detailed study carried out using the TEC variation obtained by 12 International GNSS Service (IGS) stations located at Indian and Australian sector. The LSTIDs originated around the onset of the geomagnetic storm on 20 December in the southern hemisphere near Australian sector, propagated northward, crossed the equator, and then dissipated in the low-latitudes of the Indian longitudes. The LSTIDs were found to be propagating with a speed of around 800 ms^-1 at the Australian sector, but their speeds are reduced to around 200 ms^-1 near the equator. Further, the background changes in the low-latitude IT system are investigated using the measurement of equatorial electrojet, O/N₂ variation. These results will be discussed.