## 1 Introduction  
The equatorial plasma bubbles (EPBs) are the regions of depleted electron density in the post-sunset equatorial and low-latitude ionosphere surrounded by enhanced electron density areas, leading to ionospheric electron density irregularities with scale sizes ranging from a few meters to thousands of kilometers [1, 2, 3, 4, 5]. Based on horizontal scale sizes, the EPBs are generally classified as large (>100 km), intermediate (10 km-100 m), and small (<100 m) scale structures, respectively [6]. Irregularities in the intermediate range constitute an important part of EPBs and primarily affect the very high-frequency (VHF) radio waves whereas those in the small-scale ranges are responsible for the disturbances in the L-band transmissions. There is a particular interest in the characteristics of these irregularity scale lengths as it may critically affect the operation of satellite-based communication and navigation systems by introducing degraded signal strengths or even loss of lock [7]. Plasma irregularities and depletions associated with EPBs are generally observed during the nighttime, shortly after sunset, and may even persist until a few hours after midnight. The general morphology of EPBs including the source mechanism, evolution processes, spatial structures, and global distributions have been presented in the past through ground- and satellite-based observations. From the analysis of these observations, it has been established that the EPBs are often initiated in the post-sunset hours under the action of pre-reversal enhancement (PRE) in zonal electric field by suddenly uplifting plasma to a higher altitude with a steep plasma density gradient in the bottomside F-layer. This favors the growth of the irregularities by the Rayleigh-Taylor instability (RTI) in the bottomside ionosphere and their subsequent transportation to topside ionosphere and higher latitudes along the magnetic field tubes [8; 9; 10; 11; 12]. The eastward post-sunset electric field enhances the RTI, while the westward electric field may suppress the instability. The structures that form due to this instability can grow to become large-scale ionospheric depletions, often called equatorial plasma bubbles. The variability due to the uplift of the F-layer after sunset by the PRE may dictate the onset or inhibition of these ionospheric depletions [12; 13]. Radio wave propagation, satellite communication, and Global Navigation Satellite System (GNSS) signals are substantially affected by the EPBs, making them of great interest to researchers in the field of space weather and ionospheric physics. Although the spatiotemporal occurrence distributions of EPBs are strongly controlled by local time, season and geographic location of observation, they are often driven by the disturbances of solar origin such as solar flares, coronal mass ejections, geomagnetic storms, etc. [14; 15]. The plasma density irregularities within the EPBs can scatter and diffract the traversing radio waves, causing abrupt fluctuations in the amplitude (amplitude scintillation) and phase (phase scintillation) of the received signal. Based on the severity, the scintillations can interrupt or degrade GNSS receiver operation by potentially introducing fading, cycle slips, ranging errors or even loss of tracking and may take considerable amount of time for reacquisition [16]. Further, during extreme space weather events, the probability of failures in measurements of GNSS navigational parameters increases and the positioning error could increase up to 10 times compared to its quiet time background level [17]. The significant magnitude of TEC gradients associated with the inner edges (very steep with large magnitudes) or the plasma density irregularities in the interior (lesser magnitude but more prevalent) of the bubble could potentially cause malfunctioning of GNSS receivers by affecting the functional accuracy and reliability [18; 19; 20].