A lag‐profile data collection mode has recently become operational at Millstone Hill. A new data analysis technique has been developed in which one or more matrices of ion‐line lagged products are analyzed simultaneously, along with any available a priori information on ionospheric and system parameters such as ionosonde and plasma‐line estimates of f0F2. The analysis yields spline function estimates of height‐varying ionospheric parameter profiles. The effect of the full two‐dimensional (range‐lag) radar ambiguity function is included in this analysis. Millstone Hill single‐pulse lag profile data are presented and analyzed and the new technique is shown to produce ionospheric parameter profile estimates with significantly better height resolution than would be possible if the lag profile matrix were first divided into ACFs by application of a summation rule.

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 ø and 20 ø, aspect angles between 0 ø and 10 ø (from perpendicularity with the magnetic field) are viewed at E region heights at invariant latitudes between 61øA and 57øA. 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 ø away from perpendicularity. We find that the 440-MHz aspect sensitivity is about -15 dB deg -1 for aspect angles between 0 ø and 3 ø, -10 dB deg -1 for aspect angles between 3 ø and 6 ø, and -7 dB deg -1 for aspect angles between 6 ø and 9 ø. The magnitude of the phase velocity is at an approximate ion acoustic level (350 m/s) for aspect angles <2 ø and decreases to <200 m/s as the aspect angle increases to > 3 ø. For highly disturbed conditions the magnitude of the velocity can increase to > 700 m/s for aspect angles <2 ø. The tendency for the altitude of the most intense return to decrease by -5 km as the aspect angle increases beyond 2 ø can be explained as a consequence of the variation of aspect angle with height. INTRODUCTIONDuring periods of strong electric fields, radar backscatter echoes are obtained from heights of 95 to 120 km in the auroral electrojet (radar aurora). These coherent echoes are generally believed to arise from modified two-stream instabilities [e.g., Haldoupis, 1989] and are most intense when the radar beam intersects the scattering volume nearly perpendicular to the Earth's magnetic field. This is in agreement with the linear theory for these instabilities, which predicts that the plasma will be unstable only for directions within 1 ø or 2 ø from perpendicular to the magnetic field (i.e., magnetic aspect angles of 1 ø or 2ø). Nevertheless, as early as 1961 it was known that significant backscatter power could be observed at aspect angles of up to at least 8 ø [Kelly et aL, 1961]. Since that time there have been occasional studies of the dependence on aspect angle of various backscatter properties, and these have increased significantly in number in recent years. Most of that work has focused on the variation of backscattered power with aspect angle, which is referred to as aspect sensitivity, and usually is 'Now at Paper number 91JA03144. 0148-0227/92/91JA-03144505.00 given in units of decibels per degree. The results are quite variable, both within and between experiments, but at UHF, values of the order of -10 dB deg-! have been r...

A 40-/•s pulse length has been used to provide 10-s temporal and 6-kin 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 ---10-1Ira-1 at 440 MHz during strong events with a maximum level of 9x10-10m -1 observed for brief intervals. During less intense events the radar backscatter is modulated by --30 dB in amplitude at Pc 5 frequencies (150 -500 s) by waves with spatial wavelength 50 -100 kin. 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 confined to a narrower altitude range. The apparent altitude extent of the strongest irregularities decreases by a factor of 2 as the amplitude of the backscattered signal increases by a factor of 10. INTRODUCTIONauroral E region, submitted to Journal of Geophysical Research, 1990) (hereinafter referred to as del Pozo et al., 1990), empha-The Millstone Hill 440-MHz UHF radar receives intense sized data taken during synoptic experiments [Foster et aL, 1985] backscatter from 30 cm irregularities when viewing E region which used long pulse lengths and antenna scanning at 1ow-eleheights to the north at aspect angles near perpendicular to the magnetic field [St. Maurice et aL, 1989, del Pozo, 1988]. Early observations from Millstone Hill using an L band (1295 MHz) radar have been reported by Abel and Newell [1969] and Hagfors [1972]. Although the Millstone Hill site is situated at subauroral latitude (288.5 ø E longitude, 42.6 ø N. latitude, 55øA), the appropriate aspect angle for coherent backscatter is attained some 5 ø -8 ø poleward of this using low-elevation angles (4 ø -10ø). When the electric field in the E region exceeds a threshold value of the order of 20 mV/m [Moorcroft, 1979, 1980], electron drifts higher than the local ion acoustic speed lead to the onset of the two-stream instability [Farley, 1963, Bunernan, 1963] and the generation of a spectrum of irregularities which produce vation angle to d...

Comparison of MeanWind and Tidal Fields a t Saskatoon (52"N, 107"W) and Poker Flat (65"N, 147"W) During