Arecibo's thermospheric gravity waves and the case for an ocean source

Wave‐like disturbances in electron density n e have been observed in the thermosphere above Arecibo Observatory, Puerto Rico throughout its 45 year history. However, only recently has it become evident that these waves are continuously present in the Arecibo thermosphere. The wave characteristics are fairly constant between day and night and from season to season. High‐resolution electron density measurements obtained by applying the coded long‐pulse radar technique to photoelectron‐enhanced Langmuir waves are presented. These new observations strongly suggest that the perturbations in electron density are the result of internal acoustic‐gravity waves (AGWs) propagating through the Arecibo thermosphere. The AGWs appear to be broadbanded in wave number space. The downward phase trajectories of Δ n e / n e between 400 and 120 km combined with the low horizontal phase velocities obtained from airglow measurements support the idea that the AGWs are not ducted but rather are locally produced. In addition, the altitudes at which major peaks in Δ n e / n e are observed follow theoretical estimates for nonducted waves. The nominal period of the AGWs is ∼60 min at 250 km altitude, but periods of ∼20 min are also evident at lower attitudes. Classic sources of AGWs do not appear to be consistent with the Arecibo observations of a continuous flux of background AGWs. Ray tracing of the AGWs combined with 630.0 nm airglow observations point to a source location in the Atlantic Ocean that is roughly 2100 km east northeast of Arecibo. Internal ocean waves generated in response to the internal tide at the mid‐Atlantic Ridge are the most likely source of Arecibo's thermospheric AGWs.