Previous incoherent radar studies at Arecibo Observatory, Puerto Rico have demonstrated that ∼1–3% electron density “imprints” of internal gravity waves are routinely present in the Arecibo thermosphere (∼118–500 km). A special radar technique involving photoelectron‐enhanced plasma waves (PEPWs) was used for these observations. Recently, it was discovered that the trails of the gravity waves can be detected in standard incoherent scatter power profiles when properly filtered. This result was validated using simultaneous PEPW observations. This new development opens up the possibility of monitoring thermospheric gravity waves day and night. Preliminary studies indicate that gravity waves are continually propagating through the Arecibo thermosphere, and that “sets” of waves separated by approximately 20–60 min are typically present. With the aid of additional radar tests, it may be possible to unlock Arecibo power profiles recorded over the past 30 years for gravity wave studies. The precise origin of the waves is currently unknown.

Abstract. Very accurate measurements of electron density can be made at Arecibo Observatory, Puerto Rico, by applying the coded long-pulse (CLP) radar tectmique [Sulzer, 1986a] to plasma line echoes from daytime photoelectrons [Djuth et al., 1994]. In the lower thermosphere above Arecibo, background neutral waves couple to the ionospheric plasma, typically yielding ~1-3% electron density "imprints" of the waves. These imprints are present in all observations made to date; they are decisively detected at 30-60 standard deviations above the "noise level" imposed by the measurement technique. Complementary analysis and modeling efforts provide strong evidence that these fluctuations are caused by internal gravity waves. Properties of the neutral waves such as their period and vertical wavelength are closely mirrored by the electron density fluctuations. Frequency spectra of the fluctuations exhibit a highfrequency cutoff consistent with calculated values of the Bmnt-V/iis/il/i 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 vertical wavelengths short enough to be quenched by kinematic viscosity. In general, the observed electron density imprints are relatively "clean" in that their vertical wavelength spectrum is characteristically narrow-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 density 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 with extended altitude coverage indicate that wave imprints can be detected at thennospheric heights as high as 500 kan.

Langmuir/ion turbulence excited with the upgraded high‐power (1.2‐GW effective radiated power) HF heating facility at Tromsø, Norway, has been recently studied with the European Incoherent Scatter VHF and UHF incoherent scatter radars. In this report we focus on the altitudinal development of the turbulence observed at the highest HF power levels available. Quite remarkably, the observed plasma turbulence plunges downward in altitude over timescales of tens of seconds following HF beam turn‐on; the bottom altitude is generally reached after ∼30 s. This phenomenon has a well‐defined HF power threshold. It is most likely caused by changes in the electron density profile brought about by HF heating of the electron gas. If this is the case, then the heat source must be nonlinearly dependent on HF power. Overall, the characteristics of the Tromsø turbulence are quite distinctive when compared to similar high‐resolution measurements made at Arecibo Observatory, Puerto Rico. After HF transmissions have been made for tens of seconds at Tromsø, billowing altitude structures are often seen, in sharp contrast to layers of turbulence observed at Arecibo.

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Recently, the coded long‐pulse radar technique was tested at Arecibo Observatory, Puerto Rico using photoelectron‐enhanced plasma lines in the daytime ionosphere. The technique immediately proved to be a powerful diagnostic tool for studying natural ionospheric phenomena. Our initial observations indicate that extremely accurate measurements of absolute electron density (0.01 to 0.03% error bars) can be achieved with an altitude resolution of 150 m and a temporal resolution of ∼2 s. In addition, the technique provides information about electron density structure within a 150‐m altitude cell and yields parameters from which the energy spectrum of suprathermal electrons (≥ 5 eV) can be deduced. Our earliest measurements are used to illustrate applications of the coded long‐pulse technique to several aeronomic/ionospheric areas of current interest. These include studies of neutral wave motions in the lower thermosphere, measurements of ion composition in the F1 region/upper ionosphere, and investigations of electron‐gas thermal balance and photoelectron energy loss processes. The technique can be utilized to examine irregularity formation in the F region, probe electron acceleration processes in ionospheric modification experiments, verify the magnetic field dependence of Langmuir wave damping, and more generally test higher order corrections suggested for the Langmuir dispersion relation. It is anticipated that the latter tests will facilitate measurements of ionospheric currents.