On ion-acoustic plasma waves in radio aurora and the two-stream instability

Experimental results based on 42.1 MHz CW radio auroral backscatter and ground magnetograms are used to test the validity of the two-stream instability as a causal scattering mechanism in radio aurora. Spectral studies yield considerable evidence that the two-stream instability mechanism may operate to a limited extent (~ 10–15% of the echoes) in the auroral plasma. For these echoes, which last short periods of time (approximately a few minutes), averaged power spectra show that the Doppler spectrum is dominated by a strong narrow peak due to irregularities moving with a mean radial speed in the 380–510 m/s range. The position of the peak is found to be unchanged for different observation times and azimuth angles. Occasionally, the narrow peak can coexist with a broad long-lived spectral component centered closer to zero shift position. The vast majority of the ion-acoustic echoes occur around local midnight and are associated with strong scatterers moving southward. Ground magnetometer data indicates that (1) there are sudden current intensifications during the time the ion-acoustic echoes are observed, (2) the direction of the current is appropriate for generation of plasma waves propagating along the radio signal line-of-sight, (3) the echoes seem to originate from the vicinity of the Harang discontinuity. Finally, the experimental evidence suggests strongly that the linearized theory is inadequate in explaining the behaviour of the unstable plasma waves.