# Equatorial Plasma Bubbles: A Review
## 2 Observations of EPBs and Associated Irregularities
### Radar Observations  
The important role played by radar observations at the JRO in the initial characterization of EPBs has been described in the Introduction section. Extensive studies of equatorial spread F using the JULIA (Jicamarca unattended long-time studies of the ionosphere and atmosphere) radar have shown the existence of bottom-type layers which are often present as precursors of the plumes that extend to higher altitudes [34]. Sometimes, the bottom-type layers were precursors to bottom-side layers, which are distinct from the bottom-type layers, being thicker and more structured. [PERSON] and [PERSON] [34] found that intermediate-scale plasma depletions often arose from the bottom-side layers and, moving upwards, penetrated through to the top side of the equatorial ionosphere. RTI maps from radars provide detailed pictures of the day-to-day variability in the occurrence of EPBs and in many of their characteristics when they do occur. The challenge is to relate this variability to the state of the post-sunset-coupled ionosphere-thermosphere system, which is determined by forcingsfrom the atmosphere below and the magnetosphere above the ionosphere. An example of the variety displayed by plumes in radar backscatter RTI maps  
Figure 1: Top two panels show fluctuations in transverse components of the magnetic field measured by a vector fluxgate magnetometer on board the CHAMP satellite as it crossed the dip equator (indicated by the dashed vertical lines in each panel). The third panel shows fluctuations in the component parallel to the main geomagnetic field, which is taken to be in the z direction. Electron density measured by the PLP is plotted in the bottom panel, which shows the presence of EPBs. for the post-sunset low-latitude ionosphere is shown in Figure 2. These RTI maps were obtained by the 53 MHz MST (Mesosphere-Stratosphere-Troposphere) radar operating from Gadanki, India, a low-latitude location at a geomagnetic latitude of 6.5\({}^{\circ}\) N. On the two consecutive days for which the RTI maps are displayed in Figure 2, different seed perturbations would have initiated the development of EPBs on the bottom side of the equatorial F region after sunset. The EPBs also evolved differently with time and gave rise to plume structures with entirely different characteristics. While comparing the two RTI maps, it has to be kept in mind that the plume structures that appear in an RTI map arise due to backscattering of the radar signal from a succession of EPB structures that drift into the radar's field of view at different times. Thus, the EPBs on the two days would have been initiated at different locations along the dip equator. Observations using this radar, located away from the dip equator, have indicated the greater presence of multiple plumes compared to those seen in backscatter of a radar signal from meter-scale irregularities at an equatorial location [35]. A 30 MHz radar with scanning capability in a plane perpendicular to the geomagnetic field and an interferometry/imaging system has also been established at Gadanki in order to study drifts and spatial distribution of large- and small-scale irregularities. Observations of F region irregularities with this radar show bottom-type, bottom-side, and plume structures [36] similar to those reported at Ijcamara, an equatorial location [34]. The 47 MHz Equatorial Atmosphere Radar (EAR) located in West Sumatra, Indonesia, at a dip latitude of 10.36\({}^{\circ}\) S, has also been used extensively in investigations of ESF irregularities at an off-equatorial location [37]. The steering capabilities of this radar have enabled it to clearly establish the connection between the onset of equatorial F region irregularities and the sunset terminator [38].