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

Abstract

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 STAREFinland radar velocity was larger than the EISCAT convection 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 "overspeed" 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 maximum 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).

A first comparison of irregularity and ion drift velocity measurements in the E-region

“…The above argument would be valid, however, only for the relatively small range of flow angle near perpendicularity to V d if perfect aspect angle conditions (α=0) are assumed. Uspensky et al (2003) proposed that non-orthogonality of the scatter coupled with the ion motions could play a crucial role in modifying the phase velocity of E-region irregularities as it essentially widens the range of the flow angles for which the ion motions dominate since grows rapidly with the aspect angle α, Eq. (2), thus reducing the first term in Eq.…”

“…Kofman and Nielsen, 1990;Kustov and Haldoupis, 1992;Uspensky et al, 2003Uspensky et al, , 2004 we compared the electron drift velocity component V e0 cos θ and the l-o-s velocity measured with the VHF radar, V VHF los , Figs. 3 and 4a.…”

“…More observations casting doubt on the validity of the cosine law have been presented in a series of recent studies by and Uspensky et al (2003) at VHF and by Makarevitch et al (2002Makarevitch et al ( , 2004a and Milan et al (2004) at HF. It has been suggested that the observations could be explained if the ion motion contribution to the phase velocity of E-region irregularities is taken into account as prescribed by generalized formulas of the linear theory in the frame of reference of the neutrals.…”

“…In all of the above studies, however, no ion drift velocity measurements in the Eregion were available. Instead, the studies have used generic, "model" estimates for V i0 using simultaneous F-region measurements of V e0 by EISCAT (Uspensky et al, 2003) or from further ranges of the SuperDARN radar (Makarevitch et al, 2004a), which allowed the most salient features of the observed backscatter to be largely explained.…”

The STARE/SABRE story

“…This procedure especially improved the velocity estimates for large electric fields -work on this is still ongoing. Uspensky et al (2003) examined the importance of off-orthogonality and ion motion on the STARE velocities, and Uspensky et al (2004) studied westward electron flows. The analysis showed that the double-pulse velocities are consistently smaller than the multipulse velocities in the evening sector at large flow angles.…”