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Ann. Geophys., 20, 795-805, 2002
www.ann-geophys.net/20/795/2002/
© European Geosciences Union 2002
The spectral characteristics of E-region radar echoes co-located with and adjacent to visual auroral arcs
S. E. Milan1, N. Sato2, M. Lester1, T. K. Yeoman1, Y. Murata3, H. Doi4, and T. Saemundsson5
1Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
2National Institute of Polar Research, Tokyo 173-8515, Japan
3Graduate University for Advanced Studies, Tokyo 173-8515, Japan
4Tokai University, Kanagawa 259-1292, Japan
5Science Institute, University of Iceland, Reykjavik IS-107, Iceland
Correspondence to: S. E. Milan (steve.milan@ion.le.ac.uk)
Abstract. Simultaneous all-sky
camera and HF radar observations of the visual and E-region radar aurora in the
west-ward electrojet suggest a close relationship between a pair of parallel
east-west-aligned auroral arcs, separated by ~ 30 km, and a region of strong
radar backscatter. Poleward of this a broader region of radar backscatter is
observed, though the spectral characteristics of the echoes in these two
regions differ considerably. We suggest that the visual aurorae and their radar
counterparts are produced in a region of upward field-aligned current (FAC),
whereas the backscatter poleward of this is associated with downward FAC.
Relatively low electric fields ( ~ 10 mV m-1) are observed in the
vicinity of the arc system, suggesting that in this case, two-stream waves are
not directly generated through the electrodynamics of the arc. Rather, the
generation of irregularities is most probably associated with the gradient
drift instability operating within horizontal electron density gradients
produced by the filamentary nature of the arc FAC system. The observation of
high Doppler shift echoes superimposed on slow background flow within the
region of backscatter poleward of the visual aurora is argued to be consistent
with previous suggestions that the ion-acoustic instability threshold is
reduced in the presence of upwelling thermal electrons carrying downward FAC.
Key words. Ionosphere (auroral
ionosphere; ionospheric irregularities; particle precipitation)
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