Aspect sensitivity of 2.66‐MHz radar returns from the mesosphere

We have measured radar returns from the mesosphere by operating a 10‐antenna phase‐coherent 2.66‐MHz radar as an imaging Doppler interferometer. We use the resulting three‐dimensional description of the scattering to characterize the aspect sensitivity of the returns for 2 hours of data taken one spring equinox afternoon. The returned power distribution is generally circularly symmetric, so that it can be described at each altitude in terms of its variation with zenith angle, or, more accurately, with the angle relative to the center of power of the distribution. The horizontal displacement from zenith of the center of power versus altitude suggests a gravity wave‐like variation with a 13‐km vertical wavelength and may provide an alternative to wind profiles as a way to observe propagating gravity and tidal waves. We correct the received power for antenna gain and distance and display the dependence of scattered power on the angle measured from the center of power to describe the angular dependence of the scattering processes. These results show that the scattering generally consists of two components : an anisotropic (preferentially normal) component and an isotropic component. The anisotropic component dominates within about 6° of the center of power, while the isotropic component is seen beyond 6°. The angular width of the scattering increases with altitude; above 85 km the scattering becomes almost completely isotropic.