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.

On the diurnal variation of the E-region coherent HF echo occurrence

“…The exact definitions have varied from study to study and have been mainly dependent on temporal and spatial resolutions of a particular radar system (e.g. Presnell et al, 1959;Leadabrand et al, 1965;Greenwald et al, 1975;Kustov et al, 1996).…”

A review of electrostatic wave measurements on auroral magnetic field lines.

“…feature has recently been reported by GURNETT and FRANK (1977) at slightly higher altitudes and dubbed broadband electrostatic noise. An example is reproduced in Fig.…”

“…This agrees well with the S3-3 low frequency electrostatic wave observations which are seen on both the high-and low-latitude sides of the electric potential discontinuities. GURNETT and FRANK (1977) pointed out that more intense waves were probably located at the slightly lower altitude where S3-3 operated. Although the wave mode is not yet clear, the intensities are, according to FREDERICKS et al (1973), sufficiently high to create anomalous resistivity and potential drops the order of 1-2keV.…”

Flow angle dependence for the asymmetry of broad 50-MHz coherent echoes at large magnetic aspect angles

“…At small aspect angles, a broad type 2 spectrum is a superposition of scattering from weak gradient-drift waves, distributed isotropically in all¯ow directions, and strong Farley-Buneman waves dominating in the wide cone of¯ow angles along the electrojet direction. This explanation was given ®rst by Greenwald et al (1975) and later con®rmed in several studies (e.g., Haldoupis et al, 1984). At large aspect angles, according to Osterried et al (1995), the contribution from the Farley-Buneman waves to the resultant spectrum is not so great in power, and the Farley-Buneman spectral line itself is rather broad.…”