E-region

Abstract. We present evidence for the observation of high-aspect angle HF radar backscatter from the auroral electro-jets, and describe the spectral characteristics of these echoes. Such backscatter is observed at very near ranges where iono-spheric refraction is not sufficient to bring the sounding radio waves to orthogonality with the magnetic field; the frequency dependence of this propagation effect is investigated with the Stereo upgrade of the CUTLASS Iceland radar. We term the occurrence of such echoes the “high-aspect angle irregularity region ” or HAIR. It is suggested that backscatter is observed at aspect angles as high as 30◦, with an aspect sensitivity as low as 1 dB deg−1. These echoes are distinguished from nor-mal electrojet backscatter by having low Doppler shifts with an azimuthal dependence that appears more consistent with the direction of the convection electric field E than with the expected electron E×B drift direction. This is discussed in terms of the linear theory dispersion relation for electrojet waves. Key words. Ionosphere (ionospheric irregularities; plasma waves and instabilities; auroral ionosphere)