In situ measurements of electron density and electric field fluctuations over low latitudes during equatorial spread-F

A review of in situ measurements of electron density and electric field fluctuations over low latitudes has been presented. The paper deals only with F-region measurements conducted during equatorial spread-F epochs, mainly using rocket-borne Langmuir probe and electric field probes. At long wavelengths (λ ≥ 20 km), spectra of electron density irregularities are best represented by a spectral index of -1.5±0.4. Some of the mechanisms, which are considered to be potential, include a two-fluid mechanism, strong shears in horizontal velocity of plasma and gravity wave organization of plasma. The spectral index of intermediate scales (100 m < λ < 20 km) lies in the range of -2.4±0.2. Regarding the mechanism of generation of intermediate scales, there is a general consensus that the generalized Rayleigh Taylor instability is the main mechanism. There are some minor problems with this mechanism, such as flattening of spectra (n = -1.5) in some regions and an insufficient growth rate, which puts a condition of minimum seed amplitude (≈ 5%) for the initial perturbations. Irregularities in transitional scale (10 m < λ < 100 m) have much steeper spectra with spectral index ranging between -4 and -5, and are believed to be generated by low frequency electrostatic waves with finite wavelength parallel to the magnetic field. Low frequency drift waves appear to be the most plausible mechanism at transitional scales. For short scales (λ ≤ 10 m), the two most probable mechanisms are: (i) lower hybrid drift mode and (ii) a resonant wave-particle interaction.