An alternative explanation of the Doppler spectra of current‐driven plasma instabilities

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Abstract
Radar observations of Doppler spectra from electron density irregularities in the auroral and equatorial electrojets often fall into two categories: narrow spectra with constant Doppler shifts and broad spectra with Doppler shifts dependent on the angle between the radar wave vector and the mean electron drift direction. Until now these spectra have been related to the two‐stream and gradient‐drift instabilities, respectively. In this paper we examine whether effects other than direct association of the instability type with the spectral type could reproduce the spectral observations. In particular, we make three assumptions: (1) both two‐stream and gradient‐drift irregularities have angle‐dependent Doppler shifts, (2) the electron drift direction within the scattering volume is broadened due to plasma turbulence or spatial structure, and (3) the scattering cross section (mean square density fluctuation) is dependent on the angle between the radar wave vector and the local electron drift direction. These assumptions are sufficient to reproduce many of the spectral observations, including Doppler velocity plateaus. The plateau velocities under these assumptions have magnitudes that are slightly less than the electron drift velocity.

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