LOW-FREQUENCY MAGNETIC TURBULENCE IN THE HIGH-LATITUDE TOPSIDE IONOSPHERE - LOW-FREQUENCY WAVES OR FIELD-ALIGNED CURRENTS

A detailed analysis of the ULF magnetic field fluctuations observed on board the ARCAD-Aureol-3 satellite during a crossing of the northern evening auroral zone is presented. It is shown that magnetic turbulence occurs at the interface between region 1 and region 2 large-scale field-aligned currents. From a minimum variance analysis, the main directions of the fluctuations are determined. The magnetic field turbulence is generally polarized in a plane perpendicular to the Earth's magnetic field. In this plane, the polarization is highly variable, alternatively linear and circular. A spectral analysis allows us to study the frequency variation of the power spectrum, wave normal direction and polarization. The main characteristics of the turbulence agree quite well with a parallel current interpretation, with a maximum intensity as large as 350-mu-A m-2. However, in the region where the turbulence is most intense the presence of oblique Alfven waves, superimposed on the current sheets, may explain the observed signals.