ANOMALOUS ECHOES OBSERVED WITH THE EISCAT UHF RADAR AT 100-KM ALTITUDE

We have observed a number of strong echoes with the European incoheren t-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5 degrees and 78.6 degrees with the geomagnetic field and at about 100-km altitude north in the auroral zone. The echoes are shortlived and occur in single 2- or 10-s data dumps. They are offset by 125-130 kHz with respect to th e transmitted frequency. In most cases the offset compares well with t he frequency of gyro lines in the incoherent-scatter spectrum, as give n by the standard linear dispersion relation. But sometimes the measur ed offsets deviate significantly from the model calculations, and the interpretation in terms of gyro lines becomes questionable. The discre pancy could possibly be explained by local deviations in the magnetic field from the model (IGRF 1987), which are generated by incoming part icle beams. A more serious problem with the gyro-line theory is how th e line can be excited at altitudes where the collisional damping is su bstantial. The high intensity and short lifetime of the signal point t o a fast-growing plasma instability as the likely excitation mechanism , if the gyro-line interpretation is correct. The cause of the instabi lity could be the same particle beams as those causing the disturbance s in the magnetic field. Alternatively, the observations may be interp reted as meteor head echoes. The large Doppler shifts, the short lifet imes and the altitudes of the Signals support this explanation. The ma in difficulty is that the distribution of measured offsets appears to be different in magnetically active conditions and in less active cond itions. Also, the occurrence of echoes does not seem to follow the exp ected changes in meteor density. More observations in different condit ions are needed to decide between the two interpretations. As it is, w e are inclined to believe in the meteor head echo theory, the objectio ns to the gyro-line theory being more fundamental.