Diffuse radar aurora: Spectral observations of non-two-stream irregularities

Abstract

Since the two-stream instability was identified as the source of one type of electron density irregularity that is found in the equatorial electrojet, a number of people have suggested that this instability is also responsible for diffuse radar aurora. Consequently, there have been a number of attempts to observe two-streamlike spectra in diffuse radar auroral backscatter. To date, these attempts have produced mixed results. In this paper we review these past spectral studies and present some recent spectral observations of 3-m irregularities (50-MHz radar) that were made from Anchorage and Aniak, Alaska. Our observations indicate the presence of two distinct spectral components, both of which have a number of non-two-stream characteristics. Namely, they have an azimuth-dependent mean Doppler shift that approaches zero when the radar is directed perpendicular to the electrojet current. The broader spectral component, which we observe, has a typical width of 400 Hz, whereas the narrow component has a width of less than 150 Hz. The narrow component often displays a double-peaked structure with a peak separation of 30–60 Hz. Since these spectral observations were made at angles within 30° of perpendicular to the electrojet current vector and since primary two-stream or gradient drift irregularities are preferentially excited in the direction of the electron current vector, we conclude that our observations are of secondary irregularities generated from the primary irregularities and that these irregularities are more likely due, directly, to the gradient drift instability than to the two-stream instability. Recent theoretical studies of small-scale irregularity generation in the diffuse radar aurora support this contention.