PHASE-VELOCITY STUDIES OF 34-CM E-REGION IRREGULARITIES OBSERVED AT MILLSTONE HILL

Phase velocity observations at E-region heights made with the Millstone Hill 440 MHz radar find no evidence of an ion acoustic limiting speed for phase speeds observed near 0-degrees magnetic aspect angle. Under most circumstances the phase speed increases steadily with increasing backscattered power amplitude. For a 34 cm volume backscatter cross-section, sigma(v), less than approximately 5 x 10(13) m-1, the phase speed is at or below the usual ion acoustic speed in the E-region (350 m/s), and increases only slowly with the observed backscattered power amplitude (approximately 50 m/s per 10 dB). At higher power levels, the phase speed exceeds 350 m/s, reaching values in excess of 750 m/sat times, and increases more rapidly with backscattered power (approximately 200 m/s per 10 dB). Phase velocity/time maps observed over a 3-degrees span of latitude suggest that many features of the phase speeds observed are directly related to changes in the ambient convection electric field in the E-region due to changing activity conditions or the effects of superimposed magnetospheric pulsations.