Dependence of auroral power spectra variations upon electron drift velocity in the eastward electrojet

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Abstract
Power spectral observations of auroral electron density fluctuations with a scale length of nearly 1 m have been made with the STARE system (Scandinavian twin auroral radar experiment). Simultaneous measurements of the mean radial Doppler velocities were used to derive estimates of the ionospheric electron drift velocity. The data were analyzed to determine the spectral dependence on the magnitude and direction of the electron drift velocity. The two spectral types, characterized as “narrow” and “broad,” were observed simultaneously from the same scattering volume. The width of the broad spectrum (up to about 1200 Hz) can be at least 3 times that of the narrow spectrum. Narrow spectra were observed for both small and large flow angles, while broad spectra were observed only for larger flow angles (typically larger than 70°). Narrow spectra have a width which is insensitive to variations of the flow angle and to variations in the magnitude of the electron drift velocity. The width of the broad spectra, on the other hand, increases rapidly with increasing flow angle and with increasing electron drift velocity. The spectra tend to the asymmetric when the mean line‐of‐sight Doppler velocity exceeds 350 to 400 m s−1. These observations are discussed in terms of the combined effects of the two‐stream and gradient drift instabilities.