# Equatorial Plasma Bubbles: A Review
###### Abstract  
The equatorial plasma bubble (EPB) phenomenon is an important component of space weather as the ionospheric irregularities that develop within EPBs can have major detrimental effects on the operation of satellite-based communication and navigation systems. Although the name suggests that EPBs occur in the equatorial ionosphere, the nature of the plasma instability that gives rise to EPBs is such that the bubbles may extend over a large part of the global ionosphere between geomagnetic latitudes of approximately \(\pm 15^{\circ}\). The scientific challenge continues to be to understand the day-to-day variability in the occurrence and characteristics of EPBs, such as their latitudinal extent and the development of irregularities within EPBs. In this paper, basic theoretical aspects of the plasma processes involved in the generation of EPBs, associated ionospheric irregularities, and observations of their characteristics using different techniques will be reviewed. Special focus will be given to observations of scintillations produced by the scattering of VHF and higher frequency radio waves while they propagate through ionospheric irregularities associated with  
also scattered radio waves emitted by radio stars and were recorded on the ground, giving rise to fluctuations or scintillations in the intensity of the recorded signal [3,4]. Once the launch of artificial satellites ushered in the 'Space age', radio signals transmitted from these satellites were used to obtain a global picture of the occurrence of ionospheric scintillations [5]. In the equatorial and low-latitude regions, strong scintillations were recorded only during post-sunset hours [6]. Ionosonde observations of the ESF phenomenon also showed it to be a nighttime phenomenon. Theoretical investigations on scintillations have shown that ionospheric irregularities with scale sizes in the intermediate range (100 m to a few km) may give rise to strong scintillations on VHF to L-band trans-ionospheric radio signals. Such irregularities are generated in the course of the non-linear development of EPBs when they become highly structured. Many modern-day technologies are greatly dependent on space-based navigation systems, such as the Global Navigation Satellite Systems (GNSS), which transmit L-band signals. Scintillations on these signals caused by intermediate-scale irregularities within EPBs may cause loss of signal and loss of lock when a ground-based receiver tries to record the signal from a satellite as it comes into view. This is sometimes highly detrimental to the operation of satellite-based communication and navigation systems. Therefore, EPBs are considered to be an important component of space weather and continue to be studied vigorously even after so many decades since their existence was discovered. It was first proposed by [PERSON] [7] in 1956 that growth of the collision-dominated Rayleigh-Taylor (R-T) instability on the bottom side of the post-sunset equatorial ionosphere was responsible for the development of ESF irregularities. Strong coherently backscattered signals during ESF conditions observed at the Jicamarca Radar Observatory (JRO) led to the systematic study of ESF irregularities at this facility. The JRO became operational in the early 1960s with the aim of studying ionospheric electron densities, temperatures, and ion composition using incoherently scattered radar signals. Incoherent scattering of the transmitted radar signal in the ionosphere is due to thermal fluctuations in electron density, which are always present and give rise to a weak scattered signal. Thus, an incoherent scatter radar requires a powerful transmitter and a large antenna. In the presence of ESF irregularities, a strong backscattered radar signal was observed, and this was attributed to coherent scattering by the ESF irregularities. This was considered to be Bragg scattering by irregularities of a particular wavelength depending on the radar signal frequency.