HIGH-RESOLUTION BACKSCATTER POWER OBSERVATIONS OF 440-MHZ E REGION COHERENT ECHOES AT MILLSTONE HILL

A 40-mu-s pulse length has been used to provide 10-s temporal and 6-km range resolution observations of E region coherent backscatter from the premidnight eastward electrojet region to the north of Millstone Hill. The sensitivity of the Millstone UHF system is such that coherent returns can be observed over a 80-db dynamic range and at levels down to the incoherent scatter background. Our observations can be divided into two categories: strong events in which the backscattered amplitude nears saturation and weak events in which spatial structure and large-amplitude variations are common. Calibrated observations find a typical volume scattering coefficient of approximately 10(-11)m-1 at 440 MHz during strong events with a maximum level of 9 x 10(-10)m-1 observed for brief intervals. During less intense events the radar backscatter is modulated by approximately 30 dB in amplitude at Pc 5 frequencies (150 - 500 s) by waves with spatial wavelength 50 - 100 km. Our observations support the premise that the weak irregularities grow linearly with electric field strength and reach a saturation amplitude beyond which the oscillating electric field of the Pc pulsation has little effect. The observed variation of backscattered power with range is interpreted using a geometrical model which accounts for the detailed antenna beam pattern, a magnetic aspect angle sensitivity of -10 dB per degree, and a thin layer of irregularities centered at 110 km altitude. For strongly driven conditions a comparison of the range variation of backscattered power with our thin layer model suggests that the signal power becomes increasingly dominated by strong scatterers decreases by a factor of 2 as the amplitude of the backscattered signal increases by a factor of 10.