Drift of auroral absorption observed with a scanning beam riometer at Syowa station

The scanning beam riometer at Syowa station, Antarctica, detects north-south (N-S) and east-west (E-W) movement of auroral absorptions with a time resolution of 10 sec and spatial resolutionof 10 km at the ionospheric height. It is shown that auroral absorption drift eastward in the postmidnight to prenoon sectors, and drift westward in the noon to premidnight sectors, with speeds ranging from 60 to 700 m/s. The E-W drift direction is reversed around the 10 MLT sector where the drift direction is poleward. The E-W drift direction is also reversed in the premidnight sector where the drift is equatorward. This local time dependence of the absorption drift coincides with the ionospheric projection of the magnetospheric convection deduced from radar and balloon observations. The observed belt-shaped auroral absorption suggests an existence of a sheet-like structure of magnetospheric plasma, where the pitch angle diffusion causes the precipitation of energetic electrons responsible for the auroral absorption.