We present experimental evidence on a new category of meter wavelength radio auroral irregularities characterized by a unique Doppler spectrum signature. The new echo type has been identified with the Scandinavian Twin Auroral Radar Experiment radars which are capable of observing simultaneously full Doppler spectra from the same scattering volume along two widely different azimuths. The new echoes were seen only by the Finalnd radar which observes at large angles to the electrojet flow and at times when the Norway radar observatory very stable type 1 echoes with phase velocities near the ion acoustic speed. The new spectrum, named type 5, is composed of three main frequency bands, two being symmetric about zero Doppler shift near ¿450 m/s and a third always at very large phase velocities near 1000 m/s, this resulting to a characteristic asymmetry in the spectrum.

The Doppler polarity of the large phase velocity component is always at negative shifts in the eastward electrojet and at positive shifts in the westward electrojet. These echoes, which are not strong and last from a few minutes to tens of minutes, were observed during strong but also moderate geomagnetic conditions but, apparently, always in the presence of large and stable electric fields. The evidence suggested that the new echoes cannot be due to direct generation by the two-stream instability and, most likely, a secondary generation mechanism is involved. Although there are some similarities with 50 MHz type 4 spectral observations, type 5 echoes have unique characteristics which we believe are sufficient to justify their classification into a different echo category. We have no satisfactory theoretical explanation for the new echoes. That the fast moving irregularities have frequencies about two times the frequency of the simultaneously observed ion acoustic waves led us to suggest that a nonlinear resonant wave-wave interaction process might be in operation under some appropriate conditions in the plasma. ¿ American Geophysical Union 1989