# Equatorial Plasma Bubbles: A Review
## 3 Development of EPBs and Associated Irregularities: Theoretical Aspects
### Electromagnetic Rayleigh-Taylor Instability  
For interchange instabilities in the ionosphere which act to interchange high and low plasma density regions, such as the GRT instability, it has been the general practice to consider plasma fluid equations in the electrostatic limit since ionospheric plasma is a low \(\beta\) (ratio of plasma pressure to magnetic field pressure) plasma. However, in a study of the electromagnetic interchange mode in a partially ionized collisional plasma, [PERSON] and [PERSON] [11] pointed out that the interchange mode is fundamentally electromagnetic in nature. They found that the coupling to the intermediate Alfven mode has a stabilizing effect for finite parallel wavelengths, which would tend to restrict interchange instability to the lowest-order flute perturbations of an entire fluxtube. However they did not consider the E regions at the feet of the flux tubes, which are stable and have a pivotal role to play in the growth of an EPB, since E regions with significant electrical conductivity would impede the interchange of flux tubes. Hence, the effect of E regions needs to be considered separately. Following the observations made by [PERSON] et al. [26], a transmission line analogy was proposed by [PERSON] and [PERSON] [70] to model the growth of EPBs, including field-aligned currents (FACs) carried by Alfven waves that are launched in the equatorial F region when an EPB starts to grow there. In this model, for those regions along geomagnetic field lines connecting the equatorial F region with conjugate E regions where the Pedersen conductivity is much smaller than that in E and F regions, closure of the FACs would occur through polarization currents associated with shear Alfven waves. The E region's Pedersen conductivity is reduced after sunset but is still large enough to be necessarily taken into consideration in the modeling of the development of an EPB [59]. As established by [PERSON] and [PERSON] [11], in the transmission line model the wavelength along the geomagnetic field is also so large that the EPBs are still nearly geomagnetically field-aligned. In the linear regime of electromagnetic R-T instability, coupling between the equatorial F region and the conjugate E regions at the feet of the geomagnetic field lines introduces a condition for the existence of non-vanishing density perturbations in the equatorial F region, which is that the field line-integrated E region Pedersen conductivities in the two hemispheres should not only be small, but they should also be identical. This implies that the sunset terminator should be closely aligned with the magnetic meridian [70]. In order to estimate the strength and characteristics of the magnetic field fluctuations, R-T plasma modes in the equatorial ionosphere have been studied without assuming them to be entirely electrostatic in nature [71]. Characteristics of magnetic field fluctuations, which are produced by fluctuating FACs, were found to be in agreement with those associated with shear Alfven waves damped by ion-neutral collisions. However, the author found that the magnetic field fluctuations diffuse away due to parallel resistivity so that their amplitude always remains much smaller than the geomagnetic field. This would justify the assumption that EPBs are produced by electrostatic R-T instability operating in the post-sunset equatorial ionosphere. [PERSON] et al. [28] used a 2-D model of EPBs and considered the eastward current density to be the sum of gravity-driven current density and ion polarization current density arising from low frequency electric field fluctuations, which was shown by [PERSON] et al. [72] to be important in the non-linear development of the electrostatic GRT instability that would carry EPBs to the top side of the equatorial F region. In the model of [PERSON] et al. [28], at the edge of the EPBs, these currents had to be closed by FACs, and this physical process generated kinetic Alfven waves. In the 2-D simulation by [PERSON] et al.