ASPECT ANGLE DEPENDENCE OF THE RADAR AURORA DOPPLER VELOCITY

The aspect angle dependence of the Doppler velocity of radar auroral echoes was studied by means of a comparison between the signals received simultaneously on four 50-MHz CW links. In most of the events, the observed Doppler velocity decreased with increasing aspect angle but the rate of decrease was lower than that predicted by linear theory. The velocity decrease was about a factor of 0.4 between geometrical aspect angles of 1.5 degrees and 4.0 degrees and 0.6 to 0.7 between 3.2 degrees and 5 degrees. A model for the Doppler velocity behavior of the radar auroral signals is proposed, combining both propagation and scattering effects. Allowance is made for ''ray spreading'' arising from processes like refraction and diffraction during propagation. In addition, height integration over the scattering volume is taken into account.