BOTTOMSIDE SINUSOIDAL WAVES, A NEW CLASS OF EQUATORIAL PLASMA IRREGULARITIES

We describe a new category of equatorial F region plasma irregularities having properties easily distinguished from those of plasma bubbles, depletions or broadband turbulence. This new class of irregularities has been identified in the data from the Retarding Potential Analizer (RPA) and the Ion Drift Meter (IDM) on the Atmosphere Explorer satellites AE-C and -E. They are observed in the bottomside at night in a narrow belt extending approximately (+OR-)12(DEGREES) from the dip equator, and up to 7500 km or more in the east-west direction. High resolution data from the RPA reveal nearly sinusoidal waves in the ion number density N(,i) with peak-to-peak (DELTA)N(,i)/N(,i) amplitudes that span more than 3 orders of magnitude, from values less than 0.1% to more than 100%. These distinctive "Bottom Side Sinusoidal" (BSS) irregularities typically have a central wavelength lying in the range from 300 m to 3 km, and relatively narrow spectra. The power spectral index is often large (5 or 6) at the shorter wavelengths but the spectrum usually decays much less rapidly on the longer wavelength side. The vertical drift velocity also has a wavelike structure that appears to be anticorrelated with the N(,i) variations, such that regions of lower density plasma move upward and higher density regions move downward with respect to the mean ionospheric drift. Anticorrelated wavelike drift and density fluctuations are consistent with plasma that is Rayleigh-Taylor (R-T) unstable. Considerations of altitude and latitude localization and cross-field ambipolar diffusion effects introduce a peak in the R-T growth rate near 1 km and also restrict the range of wavelengths that becomes unstable.;Ionograms from Huancayo display a typical midlatitude or frequency spread signature when BSS is present. Statistics of the BSS occurrence reveal a single maximum at midnight coincident with the local distribution of frequency spread ionograms. The presence of BSS irregularities is seasonally dependent with the highest occurrence during the solstices.