# Equatorial Plasma Bubbles: A Review
## 3 Development of EPBs and Associated Irregularities: Theoretical Aspects  
### Electrostatic Rayleigh-Taylor Instability  
After sunset, due to rapid recombination, plasma density in the E region decreases faster than that in the F region, giving rise to a steep upward-directed density gradient on the bottom side of the F region. In the equatorial ionosphere, where the geomagnetic field is horizontal, an unstable situation exists where higher density plasma rests on lower density plasma, as shown by the shaded and unshaded regions in a schematic representation in Figure 6.  
Figure 5: Scatter plot of \(C_{I}(x_{0},\ t_{m})\) vs. \(x_{0}\ V_{C}/\sqrt{\left(V_{C}^{2}\ +\ V_{0}^{2}\right)}\), which is equivalent to a scatter plot of the spatial correlation function \(C_{I}(x,\ 0)\) of the ground scintillation pattern vs. spatial lag \(x\). The parameters \(V_{0}\), \(V_{C}\), and \(C_{I}(x_{0},\ t_{m})\) have been computed for each 3-min interval of all the scintillation events recorded by spaced receivers at equatorial station Tirunelveli on a 251 MHz signal transmitted from a geostationary satellite during magnetically quiet days in September 2002. Separation of the receivers, \(x_{0}=540\) m. The points in the plot are color-coded according to the local time (LT) of the 3-min interval for which the parameters have been computed. Under the influence of the downward gravitational force in the presence of the horizontal, northward-directed geomagnetic field at the dip equator, ions move eastward and electrons move westward. As the ions are much heavier than electrons, the gravitational force experienced by ions is much greater than that experienced by electrons. As a result, a net eastward current \(\mathbf{J}\ =\ N\ M\ \mathbf{g}\ \times\mathbf{B}/B^{2}\) flows, where \(N\) and \(M\) are the ion density and ion mass, respectively. In the presence of a perturbation on the bottom side of the equatorial F layer, as shown in Figure 6, this current has non-vanishing divergence since greater current flows in a region with higher plasma density than in a region of lower plasma density. This causes charges to pile up, as shown in Figure 6, giving rise to fluctuating perturbative electric fields \(\Delta\mathbf{E}\), which are eastward in low-density regions and westward in high-density regions. Thus, there is an upward \(\Delta\mathbf{E}\ \times\ \mathbf{B}\) movement of the low-density region, causing it to rise from the bottom side. This is how the electrostatic Rayleigh-Taylor (R-T) instability initially grows in the equatorial ionosphere, giving rise to EPBs [54]. In the E and F regions of the ionosphere, electrical conductivity is a tensor, with the specific or parallel conductivity being much larger than the Pedersen or Hall conductivities [54]. Thus, the geomagnetic field lines may be considered to be equipotential, so that a zonal perturbative electric field in the equatorial F region maps down to the conjugate E regions along the geomagnetic field lines that connect the equatorial F region with the off-equatorial E regions. During daytime, Pedersen conductivity of the conjugate E layers is so large that the equatorial F region's perturbative zonal electric fields \(\Delta\mathbf{E}\), which are mapped down to the E regions, are short-circuited, and an EPB cannot grow as a result [55; 56]. Thus, EPBs are a nighttime phenomenon. Ion-neutral collisions play an important role in the growth of R-T instability, particularly in the bottom side of the equatorial F region. In his original theory, [PERSON] [7] suggested collisionless R-T instability as the source of EPBs. [PERSON] [57] considered a situation where the ion-neutral collision frequency \(\
u_{in}\) is high enough that it may not be neglected. This is the case on the bottom side of the equatorial F region, depending on the height of the F layer.