Observations of type 4 VHF radar Doppler spectra during a Ps6 pulsation event

Abstract It has been suggested (Fejer et al. , 1986, J. geophys. Res . 91 , 13583) that type 4 auroral Doppler spectra are caused by a true two-stream instability in regions of enhanced electron temperature and intense electric fields. In such regions, type 4 spectra are expected to be observed when the Hall current direction becomes approximately parallel to the radar wave vector so that the latter is inside the two-stream unstable cone. The validity of this assumption is tested by the use of 50 and 60 MHz CW Doppler radar data during a Ps6 pulsation event. The CW radar data are supported with BARS (Bistatic Auroral Radar System) and MARIA (Magnetometer and Riometer Array) data, both instruments being parts of the CANOPUS program. It is demonstrated that type 4 irregularities are highly directional; most likely along the Hall current direction. The simultaneous observations of type 4 spectra by both CW radars favor the scaling of the Doppler shift according to transmitter frequency, although the observed type 4 Doppler velocity is found to be about 5% higher on 50 MHz than it is on 60 MHz.