Abstract
Abstract. Data collected by the CUTLASS Finland HF radar are used to illustrate the significant difference between the cosine component of the plasma convection in the F-region and the Doppler velocity of the E-region coherent echoes observed at large flow angles. We show that the E-region velocity is ~5 times smaller in magnitude and rotated by ~30° clockwise with respect to convection in the F-region. Also, measurements at flow angles larger than 90° exhibit a completely new feature: Doppler velocity increase with the expected aspect angle and spatial anticorrelation with the backscatter power. By considering DMSP drift-meter measurements we argue that the difference between F- and E-region velocities cannot be interpreted in terms of the convection change with latitude. The observed features in the velocity of the E-region echoes can be explained by taking into account the ion drift contribution to the irregularity phase velocity as predicted by the linear fluid theory. Key words. Ionosphere (auroral ionosphere; ionospheric irregularities; plasma convection)
Highlights
The auroral ionosphere is filled with the magnetic-fieldaligned irregularities that can be detected by coherent radars (Fejer and Kelley, 1980; Haldoupis, 1989; Sahr and Fejer, 1996; Schlegel, 1996)
In the VHF band, a comparison between F-region drifts observed by the European Incoherent Scatter (EISCAT) radar and the Doppler velocities measured by the Scandinavian Twin Auroral Radar Experiment (STARE) VHF radars (140 MHz) showed that the irregularity phase velocity for directions close to V e0 (θ = 0◦−60◦) is significantly smaller than the projection of V e0 onto the line-of-sight (l-o-s), Vlos=Ve0 cos θ, while for larger flow angles θ = 60◦–90◦, the Doppler velocity generally agrees with the electron drift velocity projection, implying the cosine rule for the Doppler velocity (Nielsen and Schlegel, 1985; Nielsen et al, 2002)
In the HF band, Davies et al (1999) reported smaller velocities of E-region decameter irregularities observed by the Co-ordinated UK Twin Located Auroral Sounding System (CUTLASS) Finland radar as compared to the F-region plasma drifts measured by the EISCAT radar
Summary
The auroral ionosphere is filled with the magnetic-fieldaligned irregularities that can be detected by coherent radars (Fejer and Kelley, 1980; Haldoupis, 1989; Sahr and Fejer, 1996; Schlegel, 1996). Makarevitch et al.: Simultaneous HF measurements of E- and F-region Doppler velocities at large flow angles 1179 indicated in Fig. 1 by black curves. The change in the Doppler velocity sign occurs at beams 9–10, corresponding to the direction nearly perpendicular to magnetic L shells, which is indicative of the L-shellaligned nature of the flow at far ranges. One can estimate that the obtained ratio of ∼5 is very typical for the entire period under study
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