Flow angle dependence for the asymmetry of broad 50-MHz coherent echoes at large magnetic aspect angles

The skewness of broad Type 2-like spectra has been studied using data collected by two orthogonal CW 50-MHz radio links with co-located scattering volumes. Geometrical aspect angles of observations were about 10°. One short event was considered. For this event, the electron ̄ow direction was changing periodically (period about 9 minutes) presumably due to the passage of a magnetospheric MHD wave through the ionosphere. It was found that for the radar observations along the electrojet ̄ow, the skewness had the same sign as the mean Doppler shift with average absolute values in between 0.5±1.0. For observations perpendicular to the electrojet ̄ow, spectra were more symmetrical (average skewness was around 0) and the sign of the skewness was sometimes opposite to the sign of the mean Doppler shift. These observations are interpreted in terms of contribution from both the Farley-Buneman and gradient-drift instabilities to the resultant spectrum. Di€erences with radar observations at small aspect angles are discussed.