HIGH-RESOLUTION STUDIES OF ATMOSPHERE-IONOSPHERE COUPLING AT ARECIBO OBSERVATORY, PUERTO-RICO

Very accurate measurements of electron density can be made at Arecibo Observatory, Puerto Rico, by applying the coded long-pulse (CLP) radar technique [Sulzer, 1986a] to plasma line echoes from daytime photoele ctrons [Djuth et al., 1994]. In the lower thermosphere above Arecibo, background neutral waves couple to the ionospheric plasma, typically y ielding similar to 1-3% electron density ''imprints'' of the waves. Th ese imprints are present in all observations made to date; they are de cisively detected at 30-60 standard deviations above the ''noise level '' imposed by the measurement technique. Complementary analysis and mo deling efforts provide strong evidence that these fluctuations are cau sed by internal gravity waves. Properties of the neutral waves such as their period and vertical wavelength are closely mirrored by the elec tron density fluctuations. Frequency spectra of the fluctuations exhib it a high-frequency cutoff consistent with calculated values of the Br unt-Vaisala frequency. Vertical half wavelengths are typically in the range 2-25 km between 115- and 160-km altitude, and the corresponding phase velocities are always directed downward. Some waves have vertica l wavelengths short enough to be quenched by kinematic viscosity. In g eneral, the observed electron density imprints are relatively ''clean' ' in that their vertical wavelength spectrum is characteristically nar row-banded. It is estimated that perturbations in the horizontal wind field as small as 2-4 m/s can give rise to the observed electron densi ty fluctuations. However, the required wind speed can be significantly greater depending on the orientation of the neutral wave's horizontal wave vector relative to the geomagnetic field. Limited observations w ith extended altitude coverage indicate that wave imprints can be dete cted at thermospheric heights as high as 500 km.