# Equatorial Plasma Bubbles: A Review
## 2 Observations of EPBs and Associated Irregularities
### Ionospheric Scintillation Measurements
#### 2.3.1 Dependence of Scintillation Parameters on Irregularity Characteristics
An example of these power spectra with detrended weak amplitude and phase scintillations on a 140 MHz signal transmitted from the geostationary satellite ATS-6 and recorded at the low-latitude station Ootacamund (geomagnetic latitude: 1.8\({}^{\circ}\) N) in India is shown in Figure 3. In agreement with the theory outlined in [39], the spectrum of weak amplitude scintillations displays a broad peak centered on the Fresnel frequency \(v_{F}\), which is related to the Fresnel scale \(d_{F}\): \(v_{F}~{}=~{}V_{0}/d_{F}\), where \(V_{0}\) is the drift speed of the ground scintillation pattern. For an assumed height of the phase screen, \(V_{0}\) may be estimated from \(v_{F}\)[42]. It is seen from Figure 3, as suggested by the theory described in [39], that, at frequencies higher than the Fresnel frequency, spectra of amplitude and phase scintillations follow a power-law, which yields information about the power-law spectrum of intermediate-scale irregularities in EPBs observed at that time [42]. For weak scintillations, spatial scales in the ground scintillation pattern correspond to spatial scales present in the irregularities, and the frequencies in the spectra are related to the spatial scales of the irregularities through \(V_{0}\). For strong scintillations, roll-off in the power spectra occurs at a higher frequency, as the larger irregularities focus the incident radio waves to produce short-scale variations in intensity in the ground scintillation pattern, and the one-to-one correspondence between irregularity scale size and frequency is lost [39]. It may be noted that phase variations in the recorded signal also include the original phase perturbation imposed by the phase screen, which contains spatial scales much larger than the Fresnel scale. These large-scale phase variations arise due to large-scale irregularities, which are not involved in scattering the radio waves. Therefore, in computing \(\sigma_{\varphi}\), the standard deviation of phase scintillations, the phase of the received signal must be high-pass filtered to retain only frequencies of the order of or higher than the Fresnel frequency, which depends on \(V_{0}\). Only then does \(\sigma_{\varphi}\) represent the strength of phase scintillations, which vary throughout a scintillation event in the same manner as \(S_{4}\)[43]. Fluctuations in the phase of the recorded signals may be used effectively in studying the break in slope of the irregularity spectrum, even when the break occurs at a scale size larger than or close to the Fresnel scale, as seen in Figure 3. The distinctly different slopes of the linear fits to the spectra of phase fluctuations over different frequency ranges in this figure indicate a two-component irregularity spectrum and also yield an estimate of the scale size where the break in slope occurs [42]. Measurements of amplitude scintillations on a signal transmitted from a geostationary satellite, and recorded by two ground receivers spaced along a magnetic east-west-oriented baseline, yield better estimates of \(V_{0}\)[44]. As the EPB irregularities are aligned with the geomagnetic field, assumed to be in the direction of the y-axis, spatial variations in the ground scintillation pattern are only present in the magnetic east-west direction, which is taken to be along the x-axis. However, it is necessary to take into account the changes in the ground scintillation pattern with time due to fluctuations in the drift velocity of irregularities. As will be seen in the next section, fluctuations in the drift velocity of the irregularities are an important component of the evolution of the irregularities caused by growth of the R-T instability. These give rise to a decorrelation of the signals recorded by spaced receivers, sometimes resulting in the maximum cross-correlation between the two signals falling well below 1. Therefore, a full correlation technique introduced by [PERSON] [45] has been used in the analysis of spaced receiver measurements of scintillations caused by EPB irregularities.