[1] The Scandinavian Twin Auroral Radar Experiment (STARE) coherent radar system measures the Doppler shifts caused by $1 m plasma waves in the high-latitude E region ionosphere. These Doppler velocities are here related to the electron drift velocity and ion acoustic velocity derived from measurements with the incoherent radar system European Incoherent Scatter (EISCAT). The Doppler velocity is limited in magnitude to near the ion acoustic velocity in the plasma. For large flow angles q, i.e., the angle between the radar line of sight and the electron drift velocity, the Doppler shifts are equal to the component of the electron drift velocity on the line of sight. For q $ 40°the Doppler velocity is equal to the ion acoustic velocity at 105-km altitude, and for decreasing flow angle the Doppler velocity increases. For 0°< q < 60°the variation with flow angle can be described as cos a q, where the a decreases from 0.8 to 0.2 with an increase in drift speed from $400 to 1600 ms À1 . The ratio of the line-of-sight velocity for q $ 0°to the ion acoustic velocity decreases from 1.2 at low velocities to 1.05 at large velocities. The systematic variations of the Doppler shifts with drift speed and flow angle make it possible, in principle, to recover the electron velocity from the coherent radar measurements. The observations are used to illustrate how well the recovery is possible in practice.

Abstract. For the first time we produce high-resolution maps of the characteristic energy of precipitating electrons from ground-based instrumentation in the auroral zone over northern Scandinavia. This is done by combining intensity-calibrated optical data at 557.7 nm from the Digital All-Sky Imager (DASI) with auroral absorption images from the Imaging Riometer for Ionospheric Studies (IRIS). Energy maps are produced with high temporal (10 s) and spatial (10 km) resolution within a common geographic area of 240 x 240 km. Both IRIS and DASI have the European Incoherent Scatter (EISCAT) radar within their common field of view. EISCAT is capable of making accurate measurements of the electron density height profile which, with the assistance of an atmospheric model, are inverted into equivalent energy spectra of the flux of precipitating electrons. However, incoherent scatter radars generally have a very small field of view (<1ø), making studies of the energy spectrum of the precipitating particles over a wide field of view impractical. Since IRIS and DASI are sensitive to high-and medium-energy electrons, respectively, EISCAT data are used to calibrate the characteristic energy of the precipitating particles for an assumed energy spectrum against a combination of IRIS and DASI data. This empirical calibration is then used throughout the common field of view of IRIS and DASI. An initial study illustrates the spatial relationship between the different energy ranges during a substorm onset and illustrates a new way to interpret auroral phenomena.

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...

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