STARE velocities: the importance of off-orthogonality and ion motions

• Main Authors: Uspensky, M., Koustov, A., Janhunen, P., Pellinen, R., Danskin, D., Nozawa, S.
• Other Authors: Geophysical Research Division Helsinki, Finnish Meteorological Institute (FMI), Institute of Space and Atmospheric Studies Saskatoon (ISAS), Department of Physics and Engineering Physics Saskatoon, University of Saskatchewan Saskatoon (U of S)-University of Saskatchewan Saskatoon (U of S), Solar-Terrestrial Environment Laboratory Nagoya (STEL), Nagoya University
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2003
• Subjects: [SDU.OCEAN]Sciences of the Universe [physics]/Ocean; Atmosphere; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; EISCAT
• Online Access: https://hal.science/hal-00317018; https://hal.science/hal-00317018/document; https://hal.science/hal-00317018/file/angeo-21-729-2003.pdf

International audience 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 V ph is not collinear with the V E × 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 V E × B velocities, as was found earlier by Nielsen and Schlegel (1983).