Preferential phase velocities for type 4 irregularities in the auroral E region plasma

This paper presents additional evidence on the nature of 50-MHz type 4 auroral backscatter. By using a large number of events, recorded during several periods of continuous wave Doppler radar operation in the past few years, and a data sequence of long lasting type 4 echoes, the mean phase velocity of type 4 waves and its relation to type 1 phase velocity have been studied statistically. On the average, type 4 phase velocities range between 800 and 1200 m/s with the great majority grouping near 1000 m/s. The mean velocities of simultaneous type 4 and type 1 spectrum components are well separated in the Doppler shift range with an approximate factor of 2 in the type 4 to type 1 velocity ratio. Also, both spectrum components have the same Doppler polarity, which is indicative of a direct or indirect role for the electric field in type 4 wave generation. On the basis of the widely accepted conviction that type 1 waves have velocities saturating near the ion acoustic speed, it is concluded that type 4 waves propagate with velocities at about twice the acoustic speed in the plasma. The evidence presented here strongly suggests that type 4 waves propagate at preferential velocities near 1000 m/s, an experimental fact which must be explained in theoretical models.