# Equatorial Plasma Bubbles: A Review
## 4 Present Scenario
The later electrodynamic changes occur due to composition changes in the ionosphere-thermosphere system [79]. Sometimes, both prompt penetration and disturbance dynamo electric fields can appear simultaneously, in which case the resultant change in the zonal electric field differs from a simple superposition of the two effects [80]. All these factors have a bearing on the occurrence of EPBs associated with magnetic disturbances. However, in order to relate the occurrence of EPBs with either a promptly penetrated electric field (PPEF) or a disturbance dynamo electric field (DDEF), it is necessary to first identify the time when an observed EPB was initially generated, keeping in mind that, once the perturbative electric field associated with the R-T instability vanishes, the EPBs simply drift with the background plasma. Typically, after the perturbative electric field associated with the R-T instability dies down, an EPB may drift eastward with an initial speed of approximately 100-150 m/s, slowing down with the passage of time until the EPB itself weakens and disappears due to diffusion. Thus, an EPB which was initially triggered 1000 km to the west of the observation site may be observed by an ionosonde, a radar, or a receiver recording scintillations almost 3 h after its initiation. Spaced receiver observations of scintillations at an equatorial station have provided answers to questions related to the age of an observed EPB [47,48]. It is seen in Figure 4 that, for a post-sunset scintillation event recorded on a magnetically quiet day by two receivers spaced along a magnetic east-west baseline at an equatorial station, the two signals have very little correlation in the initial phase around 20:00 LT (local time). The maximum correlation between the two signals, \(C_{1}(x_{0},\ t_{m})\), gradually increases and is close to unity after about 21:00 LT. Decorrelation of the signals in the initial stages has been attributed to the presence of fluctuating perturbative electric fields associated with R-T instability in the initial stages of the development of EPBs by [PERSON] et al. [47,48]. This has provided the means of identifying freshly generated EPBs and 'fossilized' EPBs, which simply drift with the background plasma. In Figure 4, scintillations recorded after 21:00 LT, as represented by the \(S_{4}\) index, are caused by EPB irregularities that were generated more than an hour ago to the west of the observation site (Tirunelveli), which then, in due course of time, drifted across the signal path from the satellite to Tirunelveli. This important development paved the way for proper identification, using spaced receiver scintillation data, of EPBs thatwere generated as a result of modifications to the zonal electric field in the equatorial ionosphere due to magnetic activity, which often happens around midnight or post-midnight hours [81]. The existence of nascent EPBs and their fluctuating zonal and vertical drifts, attributed to perturbative electric fields associated with R-T instability, has been captured in in situ measurements by instruments on board ROCSAT-1, DMSP, and C/NOFS satellites, as has the existence of 'fossilized' EPBs where the plasma velocity inside the density structure was not significantly different from that of the background plasma [25]. These observations support the technique, based on spaced receiver scintillation data, of distinguishing between 'nascent' and 'dead' EPBs, as first suggested by [PERSON] et al. [47,48]. As noted earlier, 3-D simulations of the non-linear evolution of EPBs have been carried out to determine how irregularities develop within the EPBs. This is particularly important because the irregularities that develop within EPBs at different altitudes map along geomagnetic field lines to F region peaks at off-equatorial latitudes.