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Ann. Geophys., 21, 729-743, 2003
www.ann-geophys.net/21/729/2003/
© European Geosciences Union 2003
STARE velocities: the importance of off-orthogonality and ion motions
M. Uspensky1,4, A. Koustov2, P. Janhunen1, R. Pellinen1, D. Danskin2, and S. Nozawa3
1Finnish Meteorological Institute, Geophisycal Research, P.O. Box 503, FIN-00101, Helsinki, Finland
2Institute of Space and Atmospheric Studies, Universtity of Saskatchewan, Saskatoon, S7N 5E2, Canada
3Solar-Terrestrial Environment Laboratory, Nagoya University, 3-13 Honohara, Toyokawa, Aichi 442, Japan
4on leave from Murmansk State Technical University, Sportivnaya 13, Murmansk, 183010, Russia
Abstract. A 3.5-h morning event of
joint EISCAT/STARE observations is considered and the differences between the
observed STARE velocities and the electron drift components (EISCAT) are
studied. We find that the STARE-Finland radar velocity was larger than the
EISCAT convec-tion component for a prolonged period of time. In addition, a
moderate 5–20° offset between the EISCAT convection azimuth and the
corresponding STARE estimate was observed. We show that both the STARE-Finland
radar velocity "over-speed" and the offset in the azimuth can be
explained by fluid plasma theory, if the ion drift contribution to the
irregularity phase velocity is taken into account under the condition of a
moderate backscatter off-orthogonality. We call such an explanation the
off-orthogonal fluid approach (OOFA). In general terms, we found that the
azimuth of the maxi-mum irregularity phase velocity Vph
is not collinear with the VE × B electron flow
direction, but differs by 5–15°. Such an azimuth offset is the key factor,
not only for the explanation of the Finland velocity overspeed, but also for
the revisions of the velocity cosine rule, traditionally accepted in the STARE
method at large flow angles. We argue that such a rule is only a rough
approximation. The application of the OOFA to the STARE l-o-s velocities gives
a reasonable agreement with the EISCAT convection data, implying that ion
motions and the non-orthogonality of backscatter are important to consider for
VHF auroral echoes. The data set discussed had the STARE velocity magnitudes,
which were 1.5–2 times smaller than the electron VE × B
velocities, as was found earlier by Nielsen and Schlegel (1983).
Key words. Ionospheric
irregularities; plasma waves and instabilities; auroral ionosphere
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