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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 21, issue 3
Ann. Geophys., 21, 729–743, 2003
https://doi.org/10.5194/angeo-21-729-2003
© Author(s) 2003. This work is distributed under
the Creative Commons Attribution 3.0 License.
Ann. Geophys., 21, 729–743, 2003
https://doi.org/10.5194/angeo-21-729-2003
© Author(s) 2003. This work is distributed under
the Creative Commons Attribution 3.0 License.

  31 Mar 2003

31 Mar 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 M. Uspensky et al.
  • 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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