# Equatorial Plasma Bubbles: A Review
###### Abstract
Thus, the irregularities that caused backscattering of vertically propagated signals at the JRO were the ones with vertical wave vectors corresponding to a wavelength of 3 m, which is half the wavelength of the radar signal. Measurements made by such radars are displayed as two-dimensional maps of the intensity of the backscattered signal as a function of altitude and time. Observations at the JRO showed that echoes also came from regions above the peak of the F layer, indicating the presence of ESF irregularities on the top side of the post-sunset equatorial F-layer, where the plasma density gradient is downward. [PERSON] et al. [8] pointed out that [PERSON]'s theory [7] could not explain these observations of top-side ESF irregularities. Range-time-intensity (RTI) maps obtained at the JRO showed plume-like features, which led [PERSON] and [PERSON] [9] to postulate that a large scale perturbation on the unstable bottom side of the post-sunset equatorial ionosphere could develop non-linearly into a low-density (compared to the background plasma density) plasma bubble, which could rise to the top side of the equatorial ionosphere, which is otherwise stable. A theoretical breakthrough put this idea on firm ground when [PERSON] and [PERSON] [10] carried out numerical simulation of the non-linear development of collisional R-T instability where an equatorial plasma-depleted bubble was shown to rise to the linearly stable top side of the equatorial ionosphere. The R-T instability involves the interchange of entire geomagnetic flux tubes; therefore, the EPBs would be highly elongated in the direction of the geomagnetic field [11]. A fully steerable radar, ALTAIR, located in the Kwajalein Atoll in the Marshal Islands at a magnetic dip latitude of 4.3\({}^{\circ}\) N was used by [PERSON] [12] to demonstrate the geomagnetic field-aligned nature of EPBs. After decades of study using various observation techniques, and with substantial progress in the numerical simulation of the non-linear development of EPBs and the structures within them, the challenge of understanding day-to-day variability in the occurrence and characteristics of EPBs still remains. After this introduction to the origin of the term 'equatorial plasma bubble' (EPB), in the next section of this paper, the contributions of various techniques to the observation of the different characteristics of EPBs and associated ionospheric irregularities are described. In the third section, theoretical aspects of the linear and non-linear development of R-T instability, as applicable to the generation of EPBs and some of the associated irregularities, are discussed. Although observations of EPBs, associated irregularities, and theoretical developments are described under different sections, in reality, many developments in both areas are closely linked. These connections will be pointed out as they arise in the discussion. The fourth section is devoted to a closer look at more recent observational and theoretical results, with a focus on new results obtained from analysis of ionospheric scintillation data and new three-dimensional simulations of the non-linear evolution of EPBs. This section summarizes the current scenario regarding EPBs and indicates some of the areas where future studies are required.