On the visibility and zenithal confinement of 150 km ( F 1 ) radar echoes

Five‐beam radar measurements of the so‐called 150 km (or F 1 ) echoes, from Pohnpei (7.0°N, 158.2°E, geographic; 0.6°N dip latitude), are used to show, for the first time, that they are weaker and occur much less frequently at zenith angles of ±30°, than those that are observed overhead or at zenith angles of ±14°, in the vertical east‐west plane. This confinement of F 1 echoes to a narrow range of zenith angles is interpreted in terms of an angular spectrum of plane plasma waves with wave vectors that are similarly restricted in zenith angle. Another manifestation of this viewing aspect sensitivity could be the appearance of F 1 echoes as bands, when plotted as a function of altitude and local time. Although the exact nature of this process is not known, a two‐step interchange process acting on a solar‐produced gradient in plasma density is suggested as a working hypothesis.