Observational limitations on the role of Langmuir cavitons in ionospheric modification experiments at Arecibo

Abstract

Observations of the time dependence of the enhanced plasma line power during high power ionospheric modification experiments at Arecibo, with a resolution of 1 ms, sometimes showed 5–10 ms long pulses recurring very nearly periodically every 20–50 ms. Other observations of the slower variations of power at Arecibo show the gradual disappearance of the plasma line overshoot as the HF pulsing was changed from 0.5‐s on 5.5‐s off to 0.5‐s on 19.5‐s off, and its gradual reappearance as the pulsing was changed back to 0.5‐s on, 5.5‐s off. Observations of the height dependence of the enhanced plasma line spectrum were also made. They showed that the decay line with its cascade and the OTSI line all originated at the same narrow range of heights which observations by others showed to be near and just below the theoretical height of reflection of the pump wave rather than, as was previously believed, at the considerably lower height where the dispersion relation of the Langmuir waves detected by the radar is satisfied in the unperturbed medium. The possibility of interpretation of the observations in terms of existing theories is considered and the need for further theoretical and experimental work is pointed out.