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In the coherent radar technique, bacsccatter is obtained from plasma irregularities even though the radar frequency can greatly exceed the ionospheric plasma frequency maximum. From the velocity spectrum of the received signals an estimate of the flow velocity can be obtained and hence the electric field determined. Information regarding the irregularity scattering cross section is obtained from the amplitude of the backscatter return. Current radar studies of a range of geophysical phenomena are presented. In addition, attention is drawn to the problems of interpreting the radar observations in terms of the underlying geophysical processes.

During the last three years, a series of EISCAT experiments has been carried out involving the COSCAT UHF transmitter, which enables coherent scatter to be studied at an irregularity wavelength of 16 cm. The most recent experiments have utilised a geometry in which the Kiruna radar is pointed at low elevation to detect E-region coherent scatter, while the Tromso and Sodankyla radars receive incoherent scatter from the F-region on a field line conjugate with the scattering irregularities. The velocity data from Tromso and Sodankyla are then combined to construct a bistatic determination of flow speed and direction, assuming that there is no plasma flow parallel to the magnetic field. This flow velocity is approximately equal to the E-region electron drift. Observations of coherent scatter have been correlated with the flow speed and direction of the derived F-region flow, and some general conditions for the excitation of the coherent scatter have been determined. It appears that there is some evidence of marginal excitation of the scattering irregularities for line-of-sight flow components of order 400 m s−1, although there is appreciable variability between the different days of the study. The phase speed of the E-region irregularities has the same sense as the line-of-sight component of the F-region ion drift. There is a clear tendency toward higher phase speeds as the flow speed increases.

Results are presented from recent ionospheric F-region modification experiments in which the heater induced anomalous absorption and electron temperature changes were measured for a variety of heater frequencies which included the third and fourth harmonics of the electron gyro frequencies. The results indicate that there are strong minima in the responses of both anomalous absorption and electron heating in the vicinity of the two harmonic frequencies. These new observations provide evidence for the view that anomalous electron heating in the presence of small scale field aligned plasma irregularities dominates over collisional heating at high latitudes.

Experimental results concerning European incoherent scatter observations of heater-induced electron temperature enhancements, anomalous absorption of low-power HF probe waves, and the spectrum of stimulated electromagnetic emission (SEE) in the sidebands of a high-power HF electromagnetic wave are presented. For the experiments reported in this paper, an O mode pump wave was transmitted vertically into the F region above Tromsø, Norway, while the injected frequency was varied in small steps around the third harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the data obtained from all three diagnostics. Measurements of anomalous absorption, the downshifted maximum (DM) spectral feature, and heater-induced electron temperature enhancements all exhibited broad minima as the heater frequency approached the third harmonic of the electron gyrofrequency. In addition, the signal strength of the HF probe wave measured during heater off periods is also reduced at these and higher heater frequencies. The experimental findings suggest that at heater frequencies in the vicinity of the third gyroharmonic, small-scale field-aligned irregularities are not excited, whereas very small scale irregularities, of the order of a few electron cyclotron radii, which are responsible for the production of fast electrons, may be generated. The observed reduction in the diagnostic signal strength is then attributed to the ionized patches produced by these energetic electrons.

Results are presented from recent ionospheric modification experiments in which the EISCAT UHF radar measured the E-region temperature and density response to high power RF heating above Tromso. A variety of electrojet conditions were encountered during these experiments. In particular, the electron drift velocity varied considerably allowing the heating efficiency of the RF heater to be investigated as a function of electron flow velocity. These observations constitute the first direct investigation of electrojet temperature modifications by high power radio waves and provide a test of a recent theoretical model in which the combined effects of RF heating and of natural plasma turbulence associated with the Farley-Buneman instability have been considered.

Observations of the anomalous absorption of low-power HF probe waves caused by the action of a high-power HF ordinary mode radio wave are presented. At pump frequencies in the vicinity of harmonics of the electron gyrofrequency the anomalous absorption measured on the low-power probe waves exhibit local minima. Also at these frequencies, large-scale changes deduced from phase measurements of the low-power probe waves and heater self-absorption are reduced, whereas the growth time of the anomalous absorption is increased. The electron gyrofrequency estimated from the frequency of the absorption minimum compares favorably with that derived from the International Geomagnetic Reference Field (IGRF) magnetic field model.

Heater induced electron temperature and electron density temporal fluctuations have been observed for the first time in EISCAT backscatter data at Tromso. These fluctuations are also accompanied by large scale (bulk) changes in electron temperature and electron density. The maximum heating effect is observed near 190 km where photochemical processes are important, in contrast to most published data where heating is reported to take place at higher altitudes where transport processes are dominant. Data from two separate experiments on 19 and 26 February 1987 are presented. In both experiments these fluctuations are observed throughout the F-region during full power overdense heating, with the maximum fluctuation amplitude being located at the heater reflection height. In an experiment on 19 February 1987, the maximum amplitudes of the fluctuations in the electron temperature and the electron density arc comparable. However, in the experiment on 26 February 1987, there is a considerable natural variation in the ambient electron density which makes it impossible to distinguish between the natural and heater induced changes in electron density and only fluctuations in electron temperature are clearly observable. The data presented in this paper indicate that the coupled electron temperature and electron density oscillation is driven by chemical perturbation rather than the self focusing instability found in transport dominated regions. Spectral analysis of the data indicates that the strongest fluctuations have periods of 2–5 min with scale sizes perpendicular to the magnetic field of between 6 and 10 km.

Equatorward propagating bands of auroral radar backscatter of period 150–600 s are a commonly observed phenomenon on the SABRE VHF coherent radar, but until very recently were rarely observed on other coherent radar systems located at different latitudes. A recent statistical study (Mao Tian et al., 1991, Planet. Space Sci.39, 1239) provided strong evidence that such propagating bands on SABRE were geomagnetic pulsations with resonant structures on the plasmapause. Here a detailed analysis of such pulsations observed by SABRE, together with the SAMNET and EISCAT cross magnetometer arrays, is presented. The pulsations have rapid westward azimuthal phase propagation independent of local time and a latitudinal phase change in both geographic and geomagnetic coordinate systems which, on average, exceeds the 180° phase change predicted by field line resonance theory. Furthermore, the events exhibit a strong attenuation between their ionospheric signature and their ground magnetometer signature. These results are consistent with an excitation mechanism involving wave-particle interactions, similar to that postulated for storm-time Pc5 pulsation events. The azimuthal phase propagation of the pulsations, if used to calculate a drift velocity mapped out to the equatorial plane, is consistent with that of protons of energy 35–70 keV undergoing gradient-curvature drift. It is suggested that these particles drift westwards and interact with the dusk plasmapause region, producing a resonant wave which is detected by the coherent radar as an equatorward propagating pulsation event.

A statistical analysis is presented of Pc 5 pulsations observed by the Wick component of the SABRE auroral radar during a 5-year period (1985–1989). The observations made by SABRE consist of both the usual poleward moving, and equatorward moving bands. The statistical analysis demonstrates that the occurrence of poleward propagating events peaks near midnight whereas for equatorward propagating events the occurrence peaks at about 18:00 U.T. There is a further class of event where the phase propagation changes direction during the event. The occurrence of each event is related to the estimated position of the plasmapause, based upon an empirical relationship between plasmapause L-shell and both local time and KP. Poleward propagating events occur mostly for low plasmapause L-shells, whereas equatorward propagating events occur mostly at higher L-shells, when the plasmapause is expected to lie within the SABRE field of view. By considering the theory of hydromagnetic field line resonance and examining the pulsation dependence on magnetic activity, local time, the background convection pattern and the estimated plasmapause position, we conclude that the equatorward propagating pulsations correspond to resonant structures at the plasmapause, whereas the majority of the poleward propagating pulsations observed by SABRE correspond to field line resonances within the plasmatrough.

Enhancements in the F-region ion temperature, caused by frictional heating, are a notable manifestation of the interaction between the ionized and neutral atmospheres. Heating of this type arises whenever the ion and neutral populations move with different velocities. A velocity difference of order 1 km s−1 provides sufficient heat input to double the unperturbed ion temperature. This paper presents a synoptic study of ion heating events observed by the EISCAT UHF facility during the 2-year period from January 1985 to December 1986. The data set consists of observations obtained during the ‘Common Programme’ operation of EISCAT in the CP-1 and CP-2 modes. Only ion temperatures measured in the field-aligned direction have been considered, since in this direction the line-of-sight ion temperature can be obtained by assuming that the ion thermal velocity distribution remains Maxwellian. This assumption is known to break down progressively as the difference between the ion and neutral velocities increases, but the resulting discrepancies in the field-parallel ion temperature are smaller than those for other observing directions and do not significantly affect the conclusions presented here. A criterion previously applied to the examination of satellite data has been adopted to identify intervals of F-region frictional heating and employed to derive their diurnal distribution in 1000 h of EISCAT measurements. Almost equal numbers of heating events were encountered in both the dawn and dusk sectors. While this result contrasts with some earlier findings, the present study makes use of a much larger data set than previous investigations and is concerned with somewhat lower latitudes. The magnitudes of the ion temperature enhancements are similar for both the pre-midnight and post-midnight sectors, although the longest intervals of ion heating tended to be concentrated in the early morning hours. Many of the ion heating events occurred in association with intervals of intense and structured particle precipitation, with F-region ion temperatures and velocities being enhanced at the boundaries of auroral forms, but depressed above the regions of precipitation themselves. It is suggested that this behaviour can be explained by the presence of a polarization electric field within the auroral form, whose direction opposes that of the convection electric field. Similar observations of reduced electric fields above regions of particle precipitation have been reported by previous workers, who have classified such events as ‘anti-correlation’ or ‘asymmetric’ auroral arcs.

In this article we describe a computer‐controlled active feedback dielectrophoretic (DEP) levitator and its measurement capabilities for characterizing single particles and particle chains (20–500 μm in length). Edge detection using a linear array of photodiodes is employed and an adaptive controller updates the voltage required for stable levitation at a rate of 4.7 kHz. Rms voltage fluctuations are typically less than 0.2% of the levitation voltage. The most precise data to date for the effective dipole moment of conducting particle chains up to four in length are presented along with experimental measurements of the effective dipole moment of two particle chains of unequal size as tests of accuracy and precision of the apparatus.

Measurements of the effective electric dipole moments of conducting particle chains suspended in insulating dielectric liquid reveal the influence of dielectric breakdown between particles. This breakdown can lead to reversal of the sign of the interparticle force from attractive to repulsive. Other measurements with a vibrating sample magnetometer of the effective magnetic dipole moments of ferromagnetic particle chains provide clear evidence that chaining enhances nonlinear magnetization at a field intensity about a factor of 10 below the value at which saturation is observed for single particles. Both of these physical situations are examples of how nonlinear effects can influence the electromechanical interactions of closely spaced particles. The nonlinearity is caused by the very strong field intensification that occurs in the gaps between particles for chains parallel to an applied electric or magnetic field. A correspondingly strong nonlinear effect is predicted for the forces between particles in chains.

Several interesting features of the propagation of HF signals over high latitude paths have been observed: (a) large Doppler spreads, often in excess of 10 Hz, are frequently observed on all paths indicating the presence of a turbulent 'rough' reflector which produces multiple sub-modes. (b) The Doppler spreading of trans-auroral oval signals is well correlated with the position of the average auroral oval relative to the propagation path. This region is a source of major disturbance. (c) The bit error rate of low speed FSK signals is not influenced by the high level of Doppler spreading observed on trans-auroral oval paths. The error rate is well related to the received SNR and is in good agreement with the theoretical distribution. (d) The distribution of bit errors is markedly affected by changes in the Doppler spreading of the received signals. At high Doppler spreads, the length of the error bursts is significantly less than for the cases where the Doppler spreading is small. However, no significant affect on the length of the guard space between error bursts is evident. The large spread in the frequency spectrum of the received signals is a well known feature of high latitude propagation. However, the present studies show that the Doppler spread is sensitive to auroral activity and to the geometry of the propagation path in relation to the auroral oval. Furthermore, it has proved possible to relate the Doppler spreading imposed on the signal by the ionospheric reflection processes to the error distribution characteristics of low speed FSK signals. >

Simultaneous observations are presented of stimulated electromagnetic emission (SEE) spectra and the anomalous absorption of a diagnostic HF wave, caused by powerful radio waves generated by the Tromsø heating facility. The pump frequency was varied in small steps around the third, fourth, and fifth harmonic of the electron gyrofrequency. Systematic variations with pump frequency were observed in the anomalous absorption data and the entirety of SEE properties. The anomalous absorption exhibits a pronounced minimum at a certain pump frequency near (or at) the gyroharmonic in question. Of the SEE properties the strength and discreteness of the downshifted maximum and the width of the downshifted continuum minimize together with the anomalous absorption. Other SEE properties (namely the broad upshifted maximum, the downshifted peak, and the broad symmetrical structure) exist only in well-defined narrow pump frequency ranges near the absorption minimum. Significant quantitative and qualitative differences are found between the SEE properties around the third gyroharmonic on the one hand, and the fourth and fifth gyroharmonics on the other hand.

The effects of ionospheric heating on low-power diagnostic radio waves at Arecibo are reported. During O mode heating it was found that the signal strengths of the diagnostic waves were reduced by up to approximately 7 dB. The diagnostic signals are only significantly attenuated during heating at full power (effective radiated power of 80 MW). During X mode heating, no amplitude effects could be detected. The maximum attenuation of the diagnostic signals measured at Arecibo is lower than those typically observed at Tromso, which is consistent with the predictions of theory. These results are discussed in the context of theoretical calculations of the anomalous absorption due to mode coupling caused by small-scale field-aligned irregularities. From observations of the effects of heating on the amplitudes of several low-power diagnostics, the length of these irregularities along the geomagnetic field is estimated.

Copyright © 1991 IEE. This paper was published as: Microwaves, Antennas and Propagation, IEE Proceedings H, 1991, 138 (2), pp. 121-130. It is available from http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=68343

Recurrent variation of geomagnetic activity at the {approximately}27-day solar rotation period and higher harmonics is a well-documented phenomenon. Auroral radar backscatter data from the Sweden and Britain Radar-Auroral Experiment (SABRE) radar provide a continuous time series from 1981 to the present which is a highly sensitive monitor of geomagnetic activity. In this study, Maximum Entropy Method (MEM) dynamic power spectra of SABRE backscatter data from 1981 to 1989, concurrent interplanetary magnetic field (IMF) and solar wind parameters from 1981 to 1987, and the Kp index since 1932 are examined. Data since 1977 are compared with previously published heliospheric current sheet measurements mapped out from the solar photosphere. Stong periodic behavior is observed in the radar backscatter during the declining phase of solar cycle 21, but this periodicity disappears at the start of solar cycle 22. Similar behavior is observed in earlier solar cycles in the Kp spectra. Details of the radar backscatter, IMF, and solar wind spectra indicate that the solar wind momentum density is the dominant parameter in determining the backscatter periodicity. The temporal evolution of two- and four-sector structures, as predicted by SABRE backscatter spectra, throughout solar cycle 21 generally still agree well with heliospheric current sheet measurements.more » For one interval, however, there is evidence that evolution of the current sheet has occurred between the photospheric source surface and the Earth.« less

Copyright © 1990 IEE. This paper was published as: Microwaves, Antennas and Propagation, IEE Proceedings H, 1990, 137 (1), pp. 25-30. It is available from http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=52817

Observation of the convection flows associated with the morning discontinuity indicate that under highly disturbed conditions ( K p = 8 +, Dst = −300 nT, B ty > 0), the polar cusp may be shifted equatorwards to the latitudes covered by the SABRE radar (61–65° N geomagnetic). The convection reversal occurred over a narrow range of local time ( B y > 0.

Observations of a Pc 5 micropulsation event, using the Wick half of the new SABRE auroral radar, are presented. During a 3-hour period in the early morning of September 18, 1981, a train of pulsations were recorded. Analysis revealed that there were three main events, rather than one continuous disturbance. The first event appeared as a perturbation of only one or two cycles, extending throughout the latitude of the viewing area. It was also visible in the STARE region, but the perturbation drift velocity was in the north-south direction as opposed to the east-west perturbation drift of a more conventional, toroidal mode field line resonance. The second disturbance was an isolated, one-cycle perturbation, also visible throughout both SABRE and STARE viewing areas and also having a dominant north-south velocity component. Finally, a very localized monochromatic event, lasting five cycles or more, with a period of about 5 min, was recorded. The characteristics of the third event were found to be consistent with those predicted by field line resonance theory. 18 references.

In this paper the effective dipole moment of two intersecting, electrically conducting spheres is calculated by using the method of electrical inversion. By invoking the constraint that the volume of the body formed by the two intersecting spheres must be fixed and equal to the original volume of the two spheres, a simple model for fusing biological cells, coalescing liquid droplets, etc., is developed. With this volume‐constrained effective dipole moment, the net dielectrophoretic force on merging particles may be calculated as a function of their center‐to‐center spacing.

Strong interactions occur between the solar wind and the Earth’s magnetic field which result in the convection of ionospheric plasma over the polar cap regions. This generally forms a two-cell pattern with westward and eastward flows in the pre- and post-midnight sectors respectively. The flow pattern is sensitive to the flux of the solar wind and the direction of the interplanetary magnetic field. Observations of the flow pattern are thus of considerable value in the interpretation of the magnetosphere-ionosphere coupling processes and in identifying the influence of the solar wind on the Earth’s environment. The plasma convection can be observed by ground-based coherent and incoherent scatter radars and the flow vectors determined. Measurements for a range of flow conditions are presented. These are interpreted in terms of the interactions of the solar wind with the magnetosphere and the resulting electric fields which drive the plasma flows in the ionosphere.

The anomalous self-absorption of a high-power radio wave measured during recent ionospheric modification experiments exhibits a hysteresis effect. These results are consistent with the assumption that the growth of field-aligned irregularities due to the interaction of a high-power pump wave and the ionospheric plasma occurs in two stages. During the first stage, striation growth is due to an instability which has a threshold independent of striation amplitude. When the striation level is sufficiently high, the threshold of the second stage is reached. The second-stage threshold is dependent on the striation amplitude. The state of background ionosphere is also found to be an important factor influencing the observed effects.

The heating facility at Ramfjordmoen near Tromso, Norway, is briefly described, and a survey is given of the experiments performed with this facility until now. These experiments comprise D -region modification, polar electrojet modulation at VLF, ELF and ULF, HF absorption and backscatter due to short-scale field-aligned irregularities, stimulated radio wave emission of the modified ionospheric plasma, short-time scale HF absorption due to the parametric decay instability, airglow modification, excitation of large-scale irregularities, and F -region cross modulation.

Some results of recent high-latitude ionospheric heating experiments are presented. Phase changes were induced in a diagnostic radio wave reflected from the lower F region of the ionosphere as it passed through a region of the ionosphere which was being illuminated by a high-power electromagnetic wave. From the results obtained there are indications that anomalous absorption contributes much more than deviative absorption to ionospheric heating at high latitudes.

Data from the Sweden and Britain Radar Auroral Experiment have been utilized in a statistical study of auroral backscatter due to 1-m-scale plasma irregularities and of plasma convection in the auroral E layer. An examination of approximately 15,000 hours of backscatter amplitude data indicates that (1) the strongest backscatter is invariably associated with the main electrojets, between 1300-1900 UT and 2300-0300 UT, (2) only weak backscatter occurs in the vicinity of the convection reversals, particularly in the morning sector, and (3) the backscatter intensity can exhibit an aspect angle attenuation of between 0 and 10 dB/deg depending on the absolute intensity measured. Average convection patterns obtained from approximately 1400 hours of two-station measurements reveal a well-defined two-cell structure for all magnetic conditions. As magnetic activity increases the Harang discontinuity moves toward earlier local times, the flow speed increases and the morning convection cell expands relative to the evening cell. 32 references.

Measurements are presented of interference phenomena in amplitude and phase of VLF and LF signals along propagation paths from central England to the Norwegian Sea. The data are interpreted by means of the ‘wave-hop’ propagation theory, incorporating full wave evaluation of ionospheric reflection coefficients with realistic D-region models. No published electron density profiles are found which completely satisfy the experimental data, but modified profiles are presented which provide a better fit to the observations.

When a time−periodic magnetic field with a strong gradient is imposed tangentially at the interface of a ferromagnetic liquid, parametric surface instability may result. Under typical conditions this instability grows to a certain amplitude, where nonlinear dynamic effects become dominant, resulting in a standing−wave pattern. In the linearized small−amplitude limit, the surface dynamics are described by Hill’s equation. The nonlinear magnetization characteristic of the ferrofluid is successfully modeled by the Langevin superparamagnetic theory, using a single−sized particle approximation. The wavelength observed in the nonlinear amplitude limited state is successfully correlated with theory by determining the ’’most unstable wavelength’’, that is, the wavelength with the maximum growth rate for given conditions of applied magnetic field strength and frequency.

The variation of backscatter intensity with slant range for the Wick radar of the SABRE system consistently exhibits a minimum at approximately 800 km. Theoretical modelling of the antenna characteristics indicates that this is due to a null in the vertical polar diagram due to interference from ground reflections (Lloyd's mirror effect). By means of this “null” feature, the vertical velocity distribution within the scattering layer can be examined. A sharp discontinuity often exists in the variation of the Doppler velocity with range across the backscatter minimum. These observations can be accounted for in terms of an increase in the drift velocity of about 200 m s−1 with height across the 10 km range of the scattering layer. These results are consistent with the theoretical predictions of St.-Maurice and Schlegel based on the assumption of a two-stream instability.

An inversion technique is outlined for calculating the resolution and gross error associated with D-region electron density profiles obtained by modelling from a given set of VLF radiowave propagation parameters. Two profiles, representing quiet and disturbed ionospheric conditions, are examined in detail and the height resolution and accuracy obtainable from the amount of observational data used in constructing these profiles is determined.

The Sweden and Britain Radar aurora Experiment (SABRE) measures the phase velocity and backscatter amplitude of 1 m wavelength irregularities in the auroral zone E -region. These irregularities are produced by two types of plasma instabilities and theory predicts that the variation in backscatter amplitude with the angle between the electron drift velocity V d and the radar vector k , the so called propagation angle, is different for the two cases. An isotropic angular dependence is expected for gradient drift instabilities, but for the alternative two stream instability process a minimum is produced in the backscatter amplitude for directions orthogonal to the flow. The SABRE data have been averaged to remove small scale fluctuations and for drift velocities less than ~200 ms −1 the backscatter dependence on the propagation angle is isotropic, strongly suggesting that gradient drift processes are dominant. At higher drift velocities anisotropies develop in the angular dependence with deep minima at approximately 90° to the flow and a subsidiary minimum at 0° to this direction. Although these characteristics suggest a two stream instability mechanism, there are features in the experimental data that are not predicted by this simple theory. It is suggested that ion drifts may in part be responsible for these additional features.

The propagation of internal gravity waves in the upper atmosphere can be influenced by the presence of neutral winds. Previous studies have considered the effects of the horizontal wind, but the smaller vertical wind component has generally been neglected. The influence of the vertical wind is investigated by means of ray tracing for a range of vertical wind shears. The effect is found to be small for long wavelengths (~500 km). For shorter wavelengths (

High power radio waves can modify the ionospheric electron density distribution to produce field aligned plasma irregularities which give rise to the anomalous absorption of HF radio waves. The coefficient of anomalous absorption of a vertically propagated radio wave due to scattering from field aligned irregularities has been calculated, taking into account the effects of the geomagnetic field on electron motions. These results are compared with those of other theoretical models. Furthermore, the scale lengths of field aligned irregularities produced by a high power radio wave during recent high latitude modification experiments have been determined from measurements of the anomalous absorption by means of this theory.

The phenomenon of wide band attenuation, i.e., anomalous absorption of an HF wave in the F region subjected to modification by a powerful HF wave, has been measured using the Tromso heating facility. Two types of results were found. In a quiet F region situation, the attenuation amounted to approximately 15 dB and did not change when the effective radiated heating power was lowered from 160 to 20 MW. In a variable F region, indicated by a high fading rate of the diagnostic wave, the absorption was considerably less than 15 dB and showed a strong dependence on the heating power. An explanation for both types of results is offered by extending a recent theory of Das and Fejer.

Theory indicates that following the release of an ion cloud at F -region heights, the electron density of the E -region is disturbed due to electron transfer between the cloud and its electrostatic image. Attempts to detect these disturbances experimentally from their associated optical emissions have however been rather inconclusive. By means of a sensitive radiowave technique, marked localized E -region disturbances have been detected following the release of ion clouds. The effect occurs for both barium and caesium clouds and is present at both temperate and high latitudes. The time of onset and the duration of the measured events is consistent with theoretical predictions for image-like disturbances.

Three coherent radars have been used to obtain simultaneous radial Doppler velocity measurements in three directions from a common volume in the E layer of the high-latitude ionosphere. The cosine relationship, which was derived in the fluid approximation of the plasma instabilities associated with radar aurora, provides a consistent framework in which to interpret radial Doppler velocity observations. However, a systematic discrepancy between the observed radial velocities and the cosine relationship is found and discussed. For observations in the eastward electrojet this discrepancy is qualitatively in line with expectations based on the kinetic theory of the instabilities. It appears that the possible phase velocities in the plasma are limited to lower values than previously thought. This implies that an estimate of the electron drift velocity obtained using the fluid approximation at times may represent an underestimate of the actual electron drift velocity.