# Equatorial Plasma Bubbles: A Review
## 4 Present Scenario
It is well known that, at a given longitude, moderate to strong L-band scintillations are recorded in the vicinity of the EIA crest, while L-band scintillations measured in the dip equatorial region are weak even though scintillations on VHF signals recorded at the dip equatorial station are strong [82,83]. Traditionally, this was attributed to the much higher ambient plasma density near the EIA crest as compared to that at the dip equator. In more recent work [49], the temporal evolution of the dominant scale in the spatial variations of intensity, in the ground scintillation pattern for a VHF signal recorded at an equatorial station during the course of a scintillation event, was studied as discussed in Section 2.3.1. Theoretical modeling of the spatial correlation function of intensity variations in the ground scintillation pattern established the dependence of the dominant scale of the intensity variations on the ionospheric irregularity spectrum. This study showed that, in the growth phase of an EPB, the spectrum of intermediate-scale irregularities near the peak of the equatorial F region fell off steeply beyond the Fresnel scale for the observed VHF signal [49]. In a further study [84], \(S_{4}\) indices computed from VHF and L-band scintillations recorded at a chain of stations extending from the dip equator to beyond the EIA crest were theoretically modeled using different irregularity parameters. This study established that the irregularity spectrum near the peak of the F layer in the EIA crest region must be much shallower than that at the F layer peak in the equatorial region, which explains the much stronger L-band scintillations recorded near the EIA crest. It was shown that, with a steep irregularity spectrum, even a ten-fold increase in ambient plasma density would not produce an L-band \(S_{4}\) index greater than 0.1. Thus, strong L-band scintillations would never be recorded at the EIA crest if the irregularity spectrum was as steep as it is in the equatorial F peak region. This result implied that, during the non-linear growth of the EPB, shorter scale irregularities developed only on the top side of the equatorial F-region, which then mapped down along the geomagnetic field lines to F layer peaks at off-equatorial latitudes near the EIA crest. With a shallow irregularity spectrum, the greater background plasma density in this region resulted in strong L-band scintillations. Development of the irregularity structure within an EPB at different altitudes over the dip equator, and thus the different nature of the irregularity spectra near the F peak at different magnetic latitudes, can also be seen in the results obtained in a 3-D simulation of the evolution of EPBs [85]. However, the spatial resolution used in this simulation limited the information about irregularity spectra to wavelengths greater than 10 km in the magnetic east-west direction. Simulations of EPB development using the 3-D HIRB model of [PERSON] et al. [69], mentioned in Section 3.1, have yielded information down to a scale size of 1 km. Pictures of the evolution of EPBs from these simulations clearly show the development of small-scale structures in the EPBs in the top side of the equatorial F region, which map down to F layer peaks at off-equatorial latitudes. In addition to the development of intermediate-scale irregularities at different altitudes within an EPB, another important aspect of EPB development is the maximum height to which EPBs would rise, which also varies from day-to-day. The latitudinal extent of scintillation producing irregularities depends on the maximum height reached by the EPBs. The Naval Research Laboratory 3-D code SAMI3/ESF has been used to determine when an EPB stops rising [66], as mentioned in Section 3.1. These authors found that an EPB stops rising when the geomagnetic flux tube-integrated ion mass density inside the bubble equals that of the surrounding background ionosphere.