HIGH-LATITUDE F-REGION ELECTRON-CONCENTRATION MEASUREMENTS NEAR NOON - A CASE-STUDY

A case study is presented of a ridge of enhanced F region plasma in th e afternoon sector near the equatorward edge of the auroral oval. The study for September 26, 1982, principally exploits data from three ion osondes near 62 degrees invariant latitude in the southern hemisphere separated by 5 1/2 hours of local time but only 3 1/2 hours of magneti c time. These observations demonstrate that the ridge structure is bet ter 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 i n small-scale (1-10 km) ionospheric irregularities. It is suggested th at the entire ridge is formed by the mechanism described by Foster (19 93) in which plasma initially corotates past noon at lower latitudes, before being entrained in the convection pattern in the afternoon sect or, and brought back toward noon. The plasma thus has an extended traj ectory 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 i s likely to make to plasma structures in the polar cap is discussed br iefly.