ASPECT ANGLE VARIATIONS IN INTENSITY, PHASE-VELOCITY, AND ALTITUDE FOR HIGH-LATITUDE 34-CM E-REGION IRREGULARITIES

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-degrees and 20-degrees, aspect angles between 0-degrees and 10-degrees (from perpendicularity with the magnetic field) are viewed at E region heights at invariant latitudes between 61-degrees-LAMBDA and 57-degrees-LAMBDA. During disturbed 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 shift 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-degrees away from perpendicularity. We find that the 440-MHz aspect sensitivity is about -15 dB deg-1 for aspect angles between 0-degrees and 3-degrees, -10 dB deg-1 for aspect angles between 3-degrees and 6-degrees, and -7 dB deg-1 for aspect angles between 6-degrees and 9-degrees. The magnitude of the phase velocity is at an approximate ion acoustic level (350 m/s) for aspect angles <2-degrees and decreases to <200 m/s as the aspect angle increases to >3-degrees. For highly disturbed conditions the magnitude of the velocity can increase to >700 m/s for aspect angles <2-degrees. The tendency for the altitude of the most intense return to decrease by approximately 5 km as the aspect angle increases beyond 2-degrees can be explained as a consequence of the variation of aspect angle with height.