Bottomside sinusoidal irregularities in the equatorial F region: 2. Cross‐correlation and spectral analysis

Abstract

Equatorial bottomside sinusoidal (BSS) irregularities have been studied by applying techniques of cross‐correlation and spectral analysis to the Atmosphere Explorer data set. In order to determine the spatial displacements between the density and transverse velocity components of the irregularities, we have cross‐correlated the ion density data from the retarding potential analyzer with the pitch and yaw ion velocity components as measured by the ion drift meter. The analysis indicates that the pitch velocity component is displaced by somewhat more than 90° (referred to the wavelength of the spectral peak) to the west with respect to the ion density fluctuations. However, the yaw drift component is almost in phase with the density fluctuations. Spectra of the pitch velocity fluctuations show a sharp peak centered at about 1 km E‐W wavelength, and a power law spectral index near +2 in the wavelength range from 10 km to 1 km. Yaw velocity and ion density spectra are also peaked at 1 km, but have more power at long wavelengths than do the pitch velocity spectra. The occurrence frequency of these irregularities maximizes at the solstices, and has a seasonally‐dependent longitude distribution. BSS events are detected most frequently in the local summer hemisphere. Many of these new results can be explained by fairly simple theoretical arguments that emphasize the importance of the ion Larmor flux, interhemispheric plasma flows, and variations in the lower F region Pedersen conductivity. The latter are thought to provide an important boundary condition on the Rayleigh‐Taylor instability dynamics.