High‐latitude F region electron concentration measurements near noon: A case study

Abstract

A case study is presented of a ridge of enhanced F region plasma in the afternoon sector near the equatorward edge of the auroral oval. The study for September 26, 1982, principally exploits data from three ionosondes near 62° invariant latitude in the southern hemisphere separated by 5½ hours of local time but only 3½ hours of magnetic time. These observations demonstrate that the ridge structure is better ordered in magnetic time than in local time. Each station observes the ridge for about 4 hours. The poleward part of the ridge is rich in small‐scale (1–10 km) ionospheric irregularities. It is suggested that the entire ridge is formed by the mechanism described by Foster (1993) in which plasma initially corotates past noon at lower latitudes, before being entrained in the convection pattern in the afternoon sector, and brought back toward noon. The plasma thus has an extended trajectory in sunlight leading to high concentrations. The irregularities probably result from the combined effects of high electric fields and energetic particle precipitation. The contribution which this plasma is likely to make to plasma structures in the polar cap is discussed briefly.