On auroral electrojet curvature and the interpretation of azimuth‐scan, radar Doppler data

Abstract

In this paper we show that the geometrical configuration of the auroral electrojet is of great importance in the interpretation of Doppler velocity data obtained by auroral backscatter radars operated close to the auroral oval at low elevation angles. We show that some of the characteristic features in the azimuthal dependence of the radar Doppler velocity can be explained by accounting for the auroral electrojet curvature produced by (1) the shape of the auroral oval, (2) the altitude variation of the Hall and Pedersen currents, and (3) other local variations in current flow. An important application of the geometrical considerations described in this paper is the estimation of the ionospheric electric field from Doppler-velocity variations observed in the diffuse radar aurora. 141

Nearly simultaneous measurements of radar auroral heights and doppler velocities at 398 MHz

“…They found that these observations could be accounted for qualitatively by considering the rotation of the current direction with height (due to the height dependence of the Hall and Pedersen conductivities). Brekke et al [1977] have noted that the curvature of the auroral electrojet must be considered when treating this effect qualitatively. Abel and Newell [1969] also examined how the magnitude of the Doppler velocity depended on height.…”

The determination of time‐stationary two‐dimensional convection patterns with single‐station radars

“…For a sufficiently large area we may then take independent 1-o-s velocity measurements along two or more different directions from a single radar site and combine them to yield the two-dimensional velocity vector, provided that the velocity is constant over the common ionospheric volume. This arrangement is illustrated in The radar analysis techniques assuming L shell invariance that axe described below axe entirely consistent with the analysis approach advocated by Brekke et al [1977] and differ from the constant range azimuth scan analysis which they criticized. In the present analyses, a flow along an L shell does not exhibit any curvature relative to the locus of radar measurements.…”

“…Our analysis shows that velocity estimates made using single-station radars under the assumption of any fixed form of spatial uniformity (e.g., L shell invariance) axe subject to substantial errors due to a violation of this uniformity by natural spatial variations that must be present in any realistic global convection pattern, as well as in smaller-scale flow structures associated with magnetospheric phenomena of high scientific interest, such as the ionospheric manifestations of flux transfer events [Lee, 1986;Southwood, 1987] and pressure pulses [Southwood and Kivelson, 1990]. In fact, the bidirectional data sampling modes of single-station incoherent scatter radars, such as the ElSCAT CP-4 or "Polar" modes and the Sondrestrom 13-position scan, cannot test for the spatial uniformity of the convection, and, in contrast to the expectations of Brekke et al [1977], even the multidirectional scan of the Goose Bay and Halley PACE raaars is insensitive to large-scale spatial variations in the convection which can compromise the velocity determination. Most importantly, it is not possible to determine from the data alone those vector determinations that are compromised and those that axe not.…”

“…With singlestation radar facilities such as those at Goose Bay, Halley Bay, Millstone Hill, and Sondre Stromfjord (formerly located at Chatanika, Alaska), and during single-station operations of the EISCAT radar, vector velocity determinations axe derived from l-os velocity measurements obtained along two or more different viewing directions under the assumption that the flow pattern is uniform over time and over the region of space separating the radar beams. Surprisingly, whilst this approach has been widely utilized since 1972, the implications of invalid underlying assumptions have rarely been subjected to critical evaluation [Brekke et al, 1977;Hanuise et al, 1985;Lockwood et al, 1988]. Brekke et al [1977] have previously pointed out that singlestation radar analysis techniques which combine measurements under some assumption of spatial uniformity of the ionospheric flow axe susceptible to potentially large systematic errors if this assumption is violated.…”

“…Surprisingly, whilst this approach has been widely utilized since 1972, the implications of invalid underlying assumptions have rarely been subjected to critical evaluation [Brekke et al, 1977;Hanuise et al, 1985;Lockwood et al, 1988]. Brekke et al [1977] have previously pointed out that singlestation radar analysis techniques which combine measurements under some assumption of spatial uniformity of the ionospheric flow axe susceptible to potentially large systematic errors if this assumption is violated. In their study they considered an auroral electrojet velocity estimation derived from the local azimuthal gradient in the 1-o-s velocity measurements (effectively a twobeam radar analysis technique).…”