Investigations of equatorial plasma bubbles as observed in the OI 630 nm nightglow emissions over off-equatorial and low-latitudinal locations over Indian longitudes

Equatorial plasma bubbles (EPBs), generated over the dip-equator during the sunset time, get mapped over to the low-latitudes through the geomagnetic field lines and contribute to structures of different scales in the nighttime thermosphere-ionosphere system. In the present study, the EPBs are observed in the OI 630 nm nightglow emissions over Mt. Abu and Kolhapur using a High Throughput Imaging Echelle spectrograph (HiTIES) and All-Sky Imager (ASI), respectively. Similar periodicities obtained in these measurements consistently corroborate the presence of the EPBs at both these locations. The strength of the Pre-Reversal Enhancement (PRE) in the zonal electric field has been investigated using the ionosonde data over Trivandrum, a dip-equatorial location. The strength of PRE decides the latitudinal extent of the EPBs. The eastward movement of the EPBs has been estimated to be in the range of 190–90 ms⁻¹, decreasing in magnitude from post-sunset to midnight. The wave number analysis carried out using the observed OI 630 nm emissions in the zonal direction has resulted in a contrasting behaviour during the presence/absence of the EPBs. Based on the analysis of around 1300 images of data, it is revealed that the scale sizes in the range of 250–300 km are omnipresent, whereas, shorter scale sizes (50–250 km) are present only during the presence of EPBs. It is inferred that these shorter scale size gravity waves played a significant role in the seeding the perturbation of the EPBs.