# Equatorial Plasma Bubbles: A Review
## 4 Present Scenario  
After decades of study on the phenomenon of EPBs, using observations and theoretical modeling as discussed in the previous sections, considerable progress has been made in some areas related to the development of EPBs and associated irregularities. There is particular interest in the development of intermediate-scale (\(\sim\)100 m to few km) irregularities in EPBs, as these give rise to scintillations on VHF and higher frequency radio waves propagating through equatorial and low-latitude ionospheres. It is also important to identify the ambient conditions that play a role in determining the height to which an EPB would rise, as this determines the latitudinal extent of scintillations through mapping of the irregularities along geomagnetic field lines. In this section, results that are pertinent to various stages of the development of EPBs are discussed to summarize what is known at present. In theoretical calculations of the linear growth rate of R-T instability on the bottom side of the equatorial F layer, the initial perturbation on the bottom side is considered to be small enough that only linear terms in the deviations from the unperturbed state are retained in the relevant equations. Thus, the linear growth rate does not depend on the amplitude of the initial perturbation. However, observations of ESF indicated that its occurrence often exhibited a decorrelation with variation in longitude beyond a few degrees. This has been attributed to variability in the seeding of R-T instability, which would depend on local conditions. Appearance of a local upwelling or quasi-periodic large-scale wave structure (LSWS) on the bottom side of the post-sunset equatorial F layer prior to the occurrence of ESF has been considered as a good candidate for studying variability in the seeding of R-T instability [77] and there have been several studies on this aspect of the occurrence of EPBs. The source of the upwelling itself has to be established in order to identify the local conditions that lead to the upwelling. On the basis of observed seasonal and longitudinal patterns of the occurrence of equatorial scintillations, [PERSON] [56] suggested that the close alignment of the sunset terminator with the geomagnetic meridian provides a favorable condition for the development of intermediate-scale ESF irregularities in the post-sunset equatorial ionosphere. This condition was clearly seen in the seasonal and longitudinal pattern of occurrence of EPBs detected in electron density measurements carried out by DMSP satellites during the 1989-2002 period [23]. As mentioned in the last section, the height of the base of the post-sunset equatorial F region has emerged as a key factor in the occurrence of EPBs, as observed in a study on the role of post-sunset equatorial F region vertical plasma drift velocity in the generation and evolution of EPBs using long-term incoherent scatter radar data from the JRO [74]. Solar cycle and seasonal variations of the vertical plasma drift velocity in the post-sunset equatorial F region during magnetically quiet times [78] are a major cause of the observed solar cycle and seasonal variations in the occurrence of post-sunset EPBs. Day-to-day variations in the pre-reversal enhancement of the zonal electric field in the post-sunset equatorial ionosphere play a critical role in day-to-day variability in relation to the occurrence of EPBs during quiet times. Magnetic storms alter the zonal electric field in the equatorial ionosphere, either through prompt penetration of magnetospheric electric fields into the equatorial ionosphere or through the setting up of a disturbance dynamo due to changes in global thermospheric winds driven by Joule heating and particle precipitation in the high latitude ionosphere during magnetic storms [79]. The effects of the disturbance dynamo are seen in the equatorial ionosphere, with a delay of about 3 h, and last longer than prompt penetration effects, from a few hours to 1 or 2 days.