VHF Doppler spectra of radar echoes associated with a visual auroral form: Observations and implications

Detailed analysis of a relatively uncomplicated premidnight auroral event shows that there can be a close correspondence between visual auroral and radar auroral forms. The radar echoes are not exactly spatially coincident with the visual form, but they occur near the equatorward edge. High-resolution Doppler radar data are consistent with the picture of a horizontal current flowing near the equatorward edge of the form. There is some evidence that the two-stream instability is a causal mechanism for producing some of the radar auroral irregularities; there is additional evidence indicating that the gradient drift mechanism may also play a major role. (auth)