Observations of 50-and 12-MHz auroral coherent echoes at the Antarctic Syowa station

Data on 50- and 12-MHz E region coherent echoes from approximately the same directions and in a broad range of azimuths are available for 1995-1997 ra dar observations at the Antarctic Syowa station, One event of such observat ions is considered to compare power and Doppler velocity of echoes at these significantly different frequencies. For the considered event, Doppler vel ocities of more than 600 ms(-1) (as recorded at both radar frequencies) wer e observed. We show that even though the 50-MHz echoes exhibit strong flow angle variation of the power, the 12-MHz echoes do not, Apparent aspect ang le power attenuation is found to be 10 and 2.5 dBdeg(-1) at 50 and 12 MHz, respectively. Measured Doppler velocities along specific radar directions a re found to be comparable in spite of the significant difference in radar f requencies, with the velocity ratio V-50MHz/V-12MHz being in between 1.6 an d 0.9 for high velocities of more than 400 ms(-1). The velocity ratio varie s with slant range and azimuth of observations. Ionospheric propagation eff ects for radio waves and plasma physical effects for irregularities are con sidered in an attempt to explain several observed features, for example, th e echo power and velocity ratio variations with slant range.