Anomalous echoes observed with the EISCAT UHF radar at 100-km altitude

[摘要] We have observed a number of strong echoeswith the European incoherent-scatter (EISCAT) UHF (930-MHz) radar at angles 83.5°and 78.6° with the geomagnetic field and at about 100-km altitude north in theauroral zone. The echoes are short-lived and occur in single 2- or 10-s datadumps. They are offset by 125–130 kHz with respect to the transmitted frequency.In most cases the offset compares well with the frequency of gyro lines in theincoherent-scatter spectrum, as given by the standard linear dispersionrelation. But sometimes the measured offsets deviate significantly from themodel calculations, and the interpretation in terms of gyro lines becomesquestionable. The discrepancy could possibly be explained by local deviations inthe magnetic field from the model (IGRF 1987), which are generated by incomingparticle beams. A more serious problem with the gyro-line theory is how the linecan be excited at altitudes where the collisional damping is substantial. Thehigh intensity and short lifetime of the signal point to a fast-growing plasmainstability as the likely excitation mechanism, if the gyro-line interpretationis correct. The cause of the instability could be the same particle beams asthose causing the disturbances in the magnetic field. Alternatively, theobservations may be interpreted as meteor head echoes. The large Doppler shifts,the short lifetimes and the altitudes of the signals support this explanation.The main difficulty is that the distribution of measured offsets appears to bedifferent in magnetically active conditions and in less active conditions. Also,the occurrence of echoes does not seem to follow the expected changes in meteordensity. More observations in different conditions are needed to decide betweenthe two interpretations. As it is, we are inclined to believe in the meteor headecho theory, the objections to the gyro-line theory being more fundamental.

[学科分类] 地球科学(综合)