Radar auroral echo characteristics as seen by a 398-MHz phased array radar operated at Homer, Alaska

Abstract

In this paper we present the first results of the mapping of the spatial distribution of auroral echoes by a 398‐MHz phased array radar recently installed at Homer, Alaska. The results are shown in the form of two‐dimensional spatial maps of auroral echoes. These maps are classified into types that are extensions of the ‘diffuse’ and ‘discrete’ echo types introduced by Leadabrand et al. (1959) and Presnell et al. (1959) for narrow beam radars. The auroral echo signatures typically seen during the afternoon and evening sectors are then described and discussed in relation to radar auroral echo characteristics found by previous investigations and other auroral processes. We show that interpretation of the occurrence of auroral echoes in terms of control by an electric field and by electron density in the radar scattering volume produces a consistent morphology of the radar aurora in relation to the other observed auroral processes. Finally, we include a dynamic auroral echo signature that may be related to the growth phase of an auroral substorm.

Plasma irregularities adjacent to auroral patches in the postmidnight sector

“…Small‐scale plasma structures, or so‐called field‐aligned plasma irregularities, in the auroral E region ionosphere, whose scale size ranges from a few meters to a few tens of meters, have mainly been studied using ground‐based radar measurements. In particular, the spatial collocation of optical auroral forms and plasma irregularities (i.e., radar aurora) in the auroral E region was investigated extensively using coherent radars at VHF frequencies [ Balsley et al , 1973; Greenwald et al , 1973; Tsunoda et al , 1974]. These studies in the 1970s demonstrated that, rather than being co‐located, backscatter echoes from meter‐scale irregularities were observed to be adjacent to auroral arcs.…”

The spectral characteristics of E-region radar echoes co-located with and adjacent to visual auroral arcs

“…Considerable work was conducted in the 1970s concerning the relationship between optical auroral arcs and VHF coherent radar backscatter echoes from the E-region Greenwald et al, 1973Greenwald et al, , 1975Romick et al, 1974;Tsunoda et al, 1974Tsunoda et al, , 1976Hunsucker et al, 1975;Tsunoda and Fremouw, 1976a, b). In general, it was found that backscatter was observed adjacent to auroral arcs as opposed Correspondence to: S. E. Milan (steve.milan@ion.le.ac.uk) to the two phenomena being co-located.…”

“…Two mechanisms for the generation of metre-scale ionospheric irregularitiesthe targets from which VHF coherent radars scatter -were suggested to explain this interrelationship. In the first, it was the electron density gradients at the edges of auroral arcs that provided the seed for irregularity formation by the gradient drift instability Tsunoda et al, 1974). The growth rate of this instability becomes positive when the local electron density gradient has a component parallel to the background electric field.…”

“…In the observations of Greenwald et al, the eastward electrojet was found equatorward of the visual aurora. Perhaps the discrepancy between these two generation scenarios can be understood in terms of the distinction made by Tsunoda et al (1974) between diffuse and discrete radar echoes; the former is perhaps more associated with the electrojet and the latter with discrete visual aurora.…”

On auroral dynamics observed by HF radar: 1. Equatorward edge of the afternoon-evening diffuse luminosity belt

“…They showed that the bright aurora and the auroral radar backscatter prefer to be in close spatial contact and/or common motion (Greenwald et al, 1973;Tsunoda et al, 1974. A number of papers that described the pre-substorm, substorm or post-substorm behaviour of the aurora and the auroral radar backscatter supported or even ®tted well within a scheme (Ecklund and Unwin, 1971;Tsunoda and Fremouw, 1976;Uspensky et al, 1982Uspensky et al, , 1983Uspensky et al, , 1992Starkov and Uspensky, 1988).…”