50-MHz auroral Doppler spectra dynamics during the Harang discontinuity

In August 1983, 50-MHz CW bistatic radars were operated in Canada so as to measure the Doppler spectra from a common volume of aurora near Southend, Saskatchewan. The radar sites were chosen so that complete spectra could be measured simultaneously in roughly the magnetic north-south and east-west directions. This paper describes the spectra obtained in a presubstorm period on August 8 (0100-0310 UT) when the magnetometer and radar data showed that the radar viewing area passed from being equatorward of the Harang discontinuity to being directly within the Harang discontinuity. Some unusual spectral features occurred, such as a complete absence of echoes from directions nearly parallel to the eastward electrojet, the simultaneous presence of ion-acoustic and ion-cyclotron-type spectra, and strong zero-shifted Doppler spikes.