Abstract
Abstract. A common feature of evening near-range ionospheric backscatter in the CUTLASS Iceland radar field of view is two parallel, approximately L-shell-aligned regions of westward flow which are attributed to irregularities in the auroral eastward electrojet region of the ionosphere. These backscatter channels are separated by approximately 100–200 km in range. The orientation of the CUTLASS Iceland radar beams and the zonally aligned nature of the flow allows an approximate determination of flow angle to be made without the necessity of bistatic measurements. The two flow channels have different azimuthal variations in flow velocity and spectral width. The nearer of the two regions has two distinct spectral signatures. The eastern beams detect spectra with velocities which saturate at or near the ion-acoustic speed, and have low spectral widths (less than 100 m s–1), while the western beams detect lower velocities and higher spectral widths (above 200 m s–1). The more distant of the two channels has only one spectral signature with velocities above the ion-acoustic speed and high spectral widths. The spectral characteristics of the backscatter are consistent with E-region scatter in the nearer channel and upper-E-region or F-region scatter in the further channel. Temporal variations in the characteristics of both channels support current theories of E-region turbulent heating and previous observations of velocity-dependent backscatter cross-section. In future, observations of this nature will provide a powerful tool for the investigation of simultaneous E- and F-region irregularity generation under similar (nearly co-located or magnetically conjugate) electric field conditions.Key words. Auroral ionosphere · Ionospheric irregularities · Plasma convection
Highlights
The SuperDARN coherent HF radars (Greenwald et al., 1995) are designed to employ backscatter from highlatitude ®eld-aligned ionospheric plasma density irregularities as tracers of the bulk plasma motion under the inuence of the convection electric®eld, and as a diagnostic tool for the investigation of large-scale magnetospheric-ionospheric coupling.Studies with VHF radars have shown that the Doppler velocity of backscatter arising from E-region irregularities underestimates the plasma drift velocity (e.g.Nielsen and Schlegel, 1983)
The present study describes a backscatter feature which is often observed within the pre-midnight auroral electrojet region by the CUTLASS Iceland radar
The local time extent, latitude and zonally aligned nature of the ionospheric backscatter is consistent with the expected location of the eastward electrojet, previously observed in VHF radar studies (Greenwald et al, 1973, 1975; Tsunoda et al, 1976) which have shown the co-location of the electrojet region and radar aurora
Summary
The SuperDARN coherent HF radars (Greenwald et al., 1995) are designed to employ backscatter from highlatitude ®eld-aligned ionospheric plasma density irregularities (radar aurora) as tracers of the bulk plasma motion under the inuence of the convection electric. X2i k 2 CS2 can be generated in the upper E region by sub-critical ®eld-perpendicular drifts, as predicted by Chaturvedi et al (1987) These instabilities will produce irregularities with phase velocities higher than the background plasma drift speed, VEB , though which saturate within their cones of instability, and display noow-angle variation. In this case very large electric ®elds are necessary to generate the two-stream instability, and other (not fully understood) instability processes dominate Such high-altitude, orF-region', irregularities are thought to be generated in electron density gradients, in much the same way as by the gradient drift instability (Fejer et al, 1984), with possible contributions from ®eld-aligned currents, by a mechanism known as the current convective instability (Ossakow and Chaturvedi, 1979). To the authors' knowledge this is the ®rst time that simultaneous measurements of irregularities at dierent altitudes in the E and F regions by the same instrument over a large range ofow angles have been possible
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