Printed in Great Britain. Pergamon Press plc Scattered power from non-thermal, F-region plasma observed by EISCAT-evidence for coherent echoes?

Abstract-Three rapid, poleward bursts of plasma flow, observed by the U.K.-POLAR EISCAT experi-ment, are studied in detail. In all three cases the large ion velocities (> 1 km s-l) are shown to drive the ion velocity distribution into a non-Maxwellian form, identified by the characteristic shape of the observed spectra and the fact that analysis of the spectra with the assumption of a Maxwellian distribution leads to excessive rises in apparent ion temperature, and an anticorrelation of apparent electron and ion tempera-tures. For all three periods the total scattered power is shown to rise with apparent ion temperature by up to 6dB more than is expected for an isotropic Maxwel~an plasma of constant density and by an even larger factor than that expected for non-the~al plasma. The anomalous increases in power are only observed at the lower altitudes f< 300 km). At greater altitudes the rise in power is roughly consistent with that simulated numerically for homogeneous, anisotropic, non-Maxwellian plasma of constant density, viewed using the U.K.-POLAR aspect angle. The spectra at times of anomalously high power are found to be asymmetric, showing an enhancement near the downward Doppler-shifted ion-acoustic frequency. Although it is not possible to eliminate completely rapid plasma density fluctuations as a cause of these power increases, uch effects cannot explain the observed spectra and the correlation of power and apparent ion temperature without an unlikely set of coincidences. The observations are made along a beam directio