Doppler spectrum anisotropy observed in 1-m irregularity auroral backscatter


  • C. Haldoupis,

  • P. Hoeg,

  • E. Nielsen


We present some atypical Doppler spectral observations made with STARE (≈140 MHz) during very disturbed magnetic conditions. The most striking feature is an anisotropy in spectrum width which is especially pronounced at times when the drift velocity is directed nearly along the bisector of the angle formed by the two radar lines of sight. Instead of both spectra being identical during these times, they were found to be quite different with one being systematically wider than the other even though their corresponding echo power is the same. The observing geometry in conjunction with spectral and drift velocity evidence support the notion that both radars observe secondary irregularities associated with longer wavelength two-stream-generated primaries propagating perpendicular to the observing directions. Even though the variations in spectrum width are closely correlated to drift velocity changes, it is clear that a parameter other than the ambient electric field must play the key role in the observed anisotropy. Following the linearized fluid approach proposed by Sudan et al. (1973), we can explain the observations if we assume an ambient destabilizing electron density gradient to exist in the plane perpendicular to the earth's magnetic field. By using the observed variations in drift velocity direction and assuming a power law dependence of spectrum width on the characteristic gradient length, we obtain a remarkably good agreement between the computed and the observed variations in spectrum width ratio. A consequence of our interpretation is that at times the secondary waves propagating at large angles relative to the main current flow can be highly structured in space resulting in narrow Doppler spectra.