Abstract
Abstract. Observations of a pair of auroral arc features by two imagers, one ground- and one space-based, allows the associated field-aligned current (FAC) and electric field structure to be inferred. Simultaneous observations of HF radar echoes provide an insight into the irregularity-generating mechanisms. This is especially interesting for the E-region echoes observed, which form the focus of our analysis, and from which several conclusions can be drawn, summarized as follows. Latitudinal variations in echo characteristics are governed by the FAC and electric field background. Particularly sharp boundaries are found at the edges of auroral arcs. Within regions of auroral luminosity, echoes have Doppler shifts below the ion-acoustic speed and are proportional to the electric field, suggesting scatter from gradient drift waves. Regions of downward FAC are associated with mixed high and low Doppler shift echoes. The high Doppler shift component is greatly in excess of the ion-acoustic speed, but seems to be commensurate with the driving electric field. The low Doppler shift component appears to be much depressed below expectations.Key words. Ionosphere (ionospheric irregularities; electric fields and currents)
Highlights
Previous studies have demonstrated the existence of a number of echo types in HF radar backscatter from the E-region (Villain et al, 1987, 1990; Hanuise et al, 1991; Milan and Lester, 2001; Milan et al, 2003) and have investigated the relationship between E-region backscatter and visual auroral forms (Milan et al, 2000, 2001, 2002)
Coherent HF radar measurements of E and F-region backscatter from the postmidnight sector suggest a close relationship between echo characteristics and a pair of auroral luminosity features
Assumptions regarding the fieldaligned current structure associated with the arc system allow the latitudinal variation in the convection electric field to be predicted, these predictions matching the observed echo Doppler shifts well
Summary
Previous studies have demonstrated the existence of a number of echo types in HF radar backscatter from the E-region (Villain et al, 1987, 1990; Hanuise et al, 1991; Milan and Lester, 2001; Milan et al, 2003) and have investigated the relationship between E-region backscatter and visual auroral forms (Milan et al, 2000, 2001, 2002). In this study we present observations of an auroral feature made from above and below, that is by a ground- and a space-based imager simultaneously, and show the remarkable association with HF radar E-region echoes
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