Sreeba Sreekumar

1 Introduction Just as forcing from above due to varied geomagnetic conditions, forcing from below also plays a vital role in the ionospheric variability. Various natural events like earthquake, volcanic eruption and severe meteorological events etc cause the generation of acoustic and gravity waves. As these waves vertically propagate upward, they cause significant perturbations in the ionosphere (e.g., Astafyeva et al., 2009; Heki, 2011; Rolland et al., 2011; Thomas et al., 2018).