The sensitivity of the Millstone 440-MHz radar system is such that coherent echoes from E region irregularities can be observed over a 90-dB dynamic range above the incoherent scatter background. At antenna elevation angles between 4¿ and 20¿, aspect angles between 0¿ and 10¿ (from perpendicularity with the magnetic field) are viewed at E region heights at invariant latitudes between 61¿ &Lgr; and 57¿&Lgr;. During distributed conditions, when convection electric fields in excess of 15 mV/m and E region irregularities span this range of latitudes, antenna scanning experiments have been performed to determine the aspect angle sensitivity with high precision. Our measurements are unique in that they provide a clear high-frequency description of the variation in both power and Doppler shifts as functions of aspect angle, all the way from a region where the waves are known to be linearly unstable, in a direction perpendicular to the geomagnetic field, to as much as 10¿ away from perpendicularity.

We find that the 440-MHz aspect sensitivity is about -15 dB deg-1 for aspect angles between 0¿ and 3¿, -10 dB deg-1 for aspect angles between 3¿ and 6¿, and -7 dB deg-1 for aspect angles between 6¿ and 9¿. The magnitude of the phase velocity is at an approximate ion acoustic level (350 m/s) for aspect angle 3¿. For highly disturbed conditions the magnitude of the velocity can increase to >700 m/s for aspect angles <2¿. The tendency for the altitude of the most intense return to decrease by ~5 km as the aspect angle increases beyond 2¿ can be explained as a consequence of the variation of aspect angle with height. ¿American Geophysical Union 1992