Type 4 VHF radio aurora observations during low to moderate geomagnetic activity

Abstract

A continuous wave (CW) VHF bistatic Doppler radar was operated in conjunction with instruments belonging to the CANOPUS array of instrumentation, including BARS (bistatic auroral radar system), MARIA (magnetometer and riometer array), and MPA (meridian photometer array). The measurements were obtained during a period of low to moderate geomagnetic activity in August 1988. Type 4 auroral Doppler spectra were observed by the CW radar while BARS was detecting a series of high velocity events located at the poleward edge of an arc observed by the MPA. Most of the CW radar type 4 spectra and the BARS high velocities were observed while the mean electrojet current was flowing approximately westward. The observations showed that type 4 waves are highly directional and can be generated within regions extended as much as 200 km along the arc boundary. The regions of high velocities appeared to be drifting eastward along the auroral arc at a speed ≈ 1 km s −1; because the occurrence of type 4 echoes was correlated with brightenings and/or distortions of the arc we concluded that the optical forms themselves were drifting eastward. The mean BARS flow was eastward in general, but a series of sharp northward turnings of the flow indicated that the type 4 instability “cone” could rotate by as much as 60° during a BARS integration period (30 s). Because of all these properties it is difficult to explain the presence of type 4 waves in terms of two-stream waves generated in regions of highly elevated electron temperatures. We propose instead that density gradients with scale lengths of the order of one hundred to a few hundred meters in size lead to marginally unstable conditions at 50 MHz. These threshold conditions are associated with electric fields that are significantly stronger than required for the production of a normal two-stream instability. The proposed explanation and the observations themselves are consistent with the results of ray tracing in a realistic model of an arc with moderate density gradients. It is shown that when the rays enter the arc at grazing incidence angles, they can be significantly bent in both the elevation and azimuth and/or guided by the curved surfaces of constant density gradients.