Irregularity drift velocity estimates in radar auroral backscatter

Abstract

It is known that, in VHF radio aurora, the observed irregularity drift velocity V ir is often much lower than the ambient electron drift velocity V d particularly when the latter exceeds ~400 m s −1. The main reason for this discrepancy is believed to be the phase velocity saturation of the two stream instability plasma waves to values near the ion acoustic speed. On the other hand, experimental evidence suggests that additional factors are involved which make the relationship between V ir and V d more complex. In this paper a simultaneous coherent and incoherent scatter data set is used to investigate the irregularity drift vs electron drift relationship in the auroral electrojet plasma. To explain the irregularity drift velocity measurements, an empirical model is introduced which considers, in addition to azimuthal phase velocity anisotropies, altitude integration effects. The latter are due to altitudinal changes of the various plasma parameters and the influences of both electron density and magnetic aspect angle height variations on the mean scattering cross-section which affect indirectly the measured mean irregularity drift. The results suggest that the proposed method is somewhat more effective, compared to previous approaches, but still it cannot account for many discrepancies observed between V ir and V d .