# Equatorial Plasma Bubbles: A Review
## 4 Present Scenario
As the background ionosphere starts descending when the zonal electric field in the equatorial ionosphere turns westward from eastward during post-sunset hours, the height at which the above condition would be met depends on the rise velocity of the EPB. According to a linear theory describing the rise of an EPB, in the initial phase, the magnitude of the rise velocity in the equatorial region is given by [86]:  
\[V=\ \frac{\Delta N}{N_{0}}\left(\frac{g}{v_{in}}\ +\ \frac{c\,E_{0}}{B}\right) \tag{6}\]  
Here, an ambient zonal electric field, \(E_{0}\), which is generally present in the post-sunset equatorial ionosphere, has also been included. As \(V\) depends on the ion-neutral collision frequency \(v_{in}\), it is determined not only by the ambient ionospheric conditions, but by thermospheric conditions as well. This is particularly important in the altered thermospheric conditions that sometimes follow a major magnetic storm. Thus, a quiet day and another day a few days after a major magnetic storm, both may have the same ionospheric conditions prevalent after sunset that are conducive to the growth of EPBs. However, the altered thermospheric conditions after the magnetic storm, for instance an enhanced atomic oxygen concentration, may be such that \(v_{in}\) increases and thus lowers the EPB's rise velocity. The maximum height reached by EPBs on the second day is lower than that on quiet days, and this is reflected in the latitudinal extent of scintillations [87]. All the above aspects of the development of EPBs are important from the point of view of assessing the effects of space weather on the operation of space-based communication and navigation systems. The above discussion of the present scenario regarding EPBs shows that there is much greater complexity involved in predicting low-latitude ionospheric scintillations than simply using the linear growth rate of EPBs over the dip equator [46]. It is necessary to have information about the ambient ionosphere-thermosphere system not only to determine if the R-T instability will grow on the bottom side of the post-sunset equatorial F region, but also to predict how the EPB will evolve in the non-linear phase of development of the R-T instability. It is necessary to know how the background conditions would impact the maximum height to which EPBs could rise and the development of irregularities, particularly the intermediate-scale irregularities which give rise to scintillations, at different altitudes within the EPBs. The latter is required for predicting the latitudinal distribution of scintillations, as the irregularities map along the geomagnetic field lines from different altitudes over the dip equator to different off-equatorial latitudes. Thus, it would be very useful if 3-D simulations of the development of EPBs used different realistic background conditions in order to study their effects on the various characteristics of EPBs and the associated irregularities mentioned above. Improving the resolution of these simulations to sub-km scales in the magnetic east-west direction in order to simulate the development of irregularities that give rise to scintillations on L-band signals remains a significant challenge. As the ionosphere-thermosphere system is forced from the atmosphere below and magnetosphere above, it is challenging to understand its day-to-day variability. In particular, there is a dearth of observations related to the state of the thermosphere. This gap is now being filled by NASA's Ionospheric Connection (ICON) explorer mission and the Global-scale Observations of Limb and Disk (GOLD) mission of opportunity. GOLD measures densities and temperatures in the thermosphere and ionosphere. Using coordinated observations of thermospheric winds and plasma motions obtained by instruments on board ICON, [PERSON] et al. [88] have shown that vertical plasma velocities at an altitude of 600 km in the equatorial ionosphere are related to thermospheric winds at much lower altitudes.