Flow dependence of COSCAT spectral characteristics

Journal of Atmospheric and Terrestrial Physics, Vol. 58, No. 1-4, pp. 189~03, 1996

~ Pergamon

Copyright © 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0021-9169(95)00029-1

0021 9169/96 $15.00+0.00

Flow dependence of COSCAT spectral characteristics P. Eglitis, ~ I. W. McCrea, 2 T. R. Robinson, 1 T. B. Jones, t K. Schlegel 3 and T. Nygren 4 Ionospheric Physics Group, Department of Physics and Astronomy, University of Leicester, University Road, Leicester LEI 7RH, U.K. 2EISCAT Group, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, U.K. 3Max-Pla,.ack Institut fiir Aeronomie, Max-Planck Strasse, D-3411 Katlenburg-Lindau, Germany 4 Department of Physics, University of Oulu Linanmaa, Oulu, Finland (Received 27 October 1994; accepted 12 December 1994)

Abstract--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 Sodankyl~i radars receive incoherent scatter from the F-region on a field line conjugate with the scattering irregularities. The velocity data from Tromso and Sodankyl~i 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 ~, 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.

INTRODUCTION Plasma irregularities in the E-region auroral ionosphere have been extensively studied by H F (e.g. Hanuise et al., 1991. ; Villain et al., 1987) and V H F coherent scatter radars (e.g. Greenwald et aL, 1978; W a t e r m a n n et al., 1989; Providakes et al., 1985; Schlegel et al., 1986). The observations have been discussed in terms of the two-stream and gradientdrift plasma instabilities that give rise to type I and type II irregularities respectively (Fejer and Kelley, 1980). Further irregularity types have been identified by Fejer et al. (1984) and Haldoupis and Sofko (1979) and are referred to as types III and IV respectively. The generating mechanisms for these irregularity types have not been. unequivocally established, possible mechanisms are reviewed by Haldoupis (1989). There are fewer studies at shorter U H F wavelengths. Those so far undertaken include observations conducted at the Homer radar (Moorcroft and Tsunoda, 1978) and more recently with the Millstone Hill radar (St-Maurice et al., 1989; Foster and Tetenbaum, 1992; Del Pozo et al., 1993) and the European In-

coherent Scatter (EISCAT) facility (e.g. Schlegel and Moorcroft, 1989). The C O S C A T project is a development of coherent scatter studies using EISCAT. It is a joint undertaking by the University of Leicester, U,K., the Max-PlanckInstitut f~ir Aeronomie, Lindau, Germany, and the University of Oulu, Finland. The experiment is designed to investigate 16 cm wavelength plasma irregularities. In particular, it is important to establish the conditions for their excitation and to study the behaviour of the irregularities with respect to the background electron drift. Short wavelength, i.e. 16 cm, irregularities are of particular interest since these scale sizes are of the same order or smaller than typical E-region ion mean free paths and hence only kinetic theory can be applied to the waves, e.g. Schlegel (1983), as opposed to fluid theory, e.g. Sudan et al. (1973). Theory predicts differences between the properties of irregularities in the fluid and kinetic regimes as detailed by Robinson and H o n a r y (1990). C O S C A T comprises a low power (500W) transmitter linked to a phased array antenna and has been deployed in Oulu since 1989. The transmitter and 189

190

F-REGION

P. Eglitis eta/.

, ~ , / , / ;

SODANKYI~

Fig. 1. A schematic diagram of the geometry employed for the active COSCAT experiments reported in this paper. The transmitter at Oulu illuminates a volume of the E-region observed by the Kiruna remote receiver, which detects coherent scatter. The Tromso and Sodankyl~i EISCAT sites are engaged in incoherent scatter observations in the F-region.

different to the E × B drift velocity by typically less than 50 m s ~owing to non-zero neutral wind velocities. Thus the measurements give an indication of the E-region Vo to which the irregularity characteristics are sensitive. In this paper, the observed relationships between the measured plasma drift and the characteristics of the 16 cm irregularities will be reported. Measurements of the full coherent scatter spectra of the irregularities are made from which the power, mean Doppler shift and the full width at half maximum are calculated. A full discussion of the calculation of these parameters, hereafter known as the power, phase speed and spectral width of the irregularity, was included in McCrea et al. (1991).

THE ACTIVE COSCAT

the antenna have been described by Thornhill and Chapman (1989) and Schlegel et al. (1988) respectively. In an initial series of passive experiments begun in October 1989, the signal transmitted from Oulu and scattered from E-region irregularities was received by the EISCAT remote sites at Kiruna and Sodankyl~i. The aim of these observations was to establish the optimum observing geometry for the detection of coherent scatter and to characterise the observed spectra (McCrea et al., 1991). The present phase of the COSCAT study focuses on the question of how the Eregion irregularities are related to background plasma parameters. This requires observations of both coherent and incoherent scatter, utilising the geometry of the tristatic EISCAT U H F system in a novel way. The Kiruna dish is pointed at low elevation to the North to receive passively the coherent scatter. The Tromso and Sodankyla radars meanwhile make bistatic observations of the F-region, with their common volume located on a field line (determined from the International Geomagnetic Reference Field, I G R F ) nominally conjugate with the E-region, volume observed by Kiruna. The observing configuration is represented schematically in Fig. 1. The incoherent scatter part of the experiment enables two line-of-sight components of the F-region ion velocity vector to be measured. A third component is estimated from these using the procedure outlined in Section 2. In the E-region, since the ion drift velocity is small, the plasma drift velocity (or electron ion difference velocity) VD is approximately equal to the E x B drift velocity of the electrons. The F-region measurements made in the experiment described above allow the F-region ion drift velocity to be measured, which is

EXPERIMENT

In an earlier active COSCAT (ACOS) experiment, described in McCrea et al. (1991), both remote sites alternated between E- and F-region observations. The later versions of the experiment have all the U H F antennas fixed, with E- and F-region observations being made continuously by separate radars. The coherent scatter part of the experiment, run at Kiruna, is essentially the same as the passive experiment described by McCrea et al. (1991). The COSCAT transmitter is operated in continuous wave (CW) mode, with short breaks in transmission of thirty seconds every five minutes to enable a regular estimate of the background noise level to be made and to prevent transmitter overheating. The COSCAT signal is sampled at Kiruna on two receiver channels each tuned to a frequency of 929.5 MHz. Four 'signal' gates are recorded on each channel and the eight estimates are averaged together for the purpose of noise reduction to determine a single measurement of the coherent scatter autocorrelation function (ACF). Sixty-four lags of the autocorrelation function are calculated, with the interlag spacing being 20/~s. The total length of the A C F is thus 1.26 ms. The G E N BOXCARACF correlator program (Turunen, 1986) was used to calculate the autocorrelation functions from the measured samples. The data were dumped at a time resolution of 5 s. Rather than being scanned, as in some of the passive observations, the Kiruna radar is held fixed at the optimum pointing direction for the detection of the coherent scatter signal. The look directions of the Kiruna receiver and the Oulu transmitter differ by 15° in azimuth. The range of the scattering region is of the order of 1000 km. The 3 dB (point-to-point) beam-widths of the transmitter and receiver are 4 ° and 1.2 °, respectively. The

Flow dependence of COSCAT spectral characteristics intersection of these two radar beams covers a region of some 250 km in range. However, if a scattering layer of 10 km thickness in altitude is assumed, the Kiruna radar receiver beam looking at an elevation of 4 ° will bisect this scattering volume for a range extent of approximately 150 km. This determines the spatial resolution of the measurements. The scattered signals are sampled at 20 microsecond resolution. This gives a point to point resolution in the coherent scatter power spectra of 390 Hz (or 60 m s -1 in Doppler velocity). In the incoherent scatter part of the experiment, both the Tromso and Sodankyl~i radars were held fixed, pointing to art F-region position which is conjugate with the expected E-region volume observed by the K i r u n a radar. Three 350 #s pulses are transmitted from Tromso, along with two 60/~s pulses used for the determination of a power profile. Seven signal gates are recorded at Tromsg on each of the three channels, the ranges and gate lengths on each channel being synchronised. The fourth gate, at an altitude of 350 km, provides the c o m m o n volume for observations with Sodankyl~t. Thirty-one lags are measured in each incoherent scatter ACF, with an interlag spacing of 10 #s. Three background and one noise injection gate are al,;o measured on each channel, and the channels are combined together in analysis to reduce the r a n d o m noise. At Sodankyl/i, each of the three transmitted pulses is received on a separate channel and three signal .gates are determined such that the central gate contains the signal from the c o m m o n volume. Two background and two noise injection gates are formed. Each A C F is sampled to 30 lags. The incoherent scatter observations are employed to estimate the full vector of the F-region ion velocity by assuming that there is no ion flow parallel to the magnetic field (i.e. VII = 0). The validity of adopting this procedure to estimate the ion velocity had previously been examined by analysing data from similar EISCAT experiments for which tristatic observations were available. The: velocities measured at two sites were combined together, with the assumption that VII was zero and the resultant velocity compared with those derived from tristatic observations. Figure 2 depicts the result of this procedure for CP-2 (common program 2) obserwttions during a period of strongly enhanced ion flow of the kind which might be expected to arise during conditions suitable for the generation of E-region irregularities. There is good agreement between the velocity magnitude calculated from the bistatic estimate (dashed-line) with the tristatic determination of the ion drift speed (full line). Figure 3 is a simulation of the effect Vii has on the error in flow angle for conditions typical of the CP-2 observations

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discussed above. Even when the parallel velocity approaches 500 m s -a, the error in the derived flow angle remains less than 6 °. In the ACOS experiment, the derived flow directions are much less affected by variations in Vii than those measured in the CP-2 data,

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Fig. 3. The error in estimating the F-region ion drift direction as a function of the size of the ion velocity component parallel to the geomagnetic field. The error is calculated for plasma drift conditions that comprised a 40 m s- ~north component of the ion drift and a 650 m s- ~east component of the ion drift relative to the geomagnetic field. These values were taken from the CP-2 data illustrated in Fig. 2 and correspond to flow conditions when the largest difference between the bistatic and tristatic estimate was obtained.

192

P. Eglitis et al.

Table 1. Times (year/month/day) of active COSCAT experiments COSCAT Experiment Start time/UT Finish Time/UT

Times of occurrence of backscatter/UT

91/03/08 2 3 2 5

91/03/090400

91/03/13 0 0 1 5 91/03/17 1 6 0 0 91/06/18 1 5 3 0 91/06/19 1 6 0 0 91/06/24 1 6 0 0 92/03/28 1 6 0 0 92/03/29 1 6 0 0

91/03/130404 91/03/172201 91/06/182100 91/06/192203 91/06/242103 92/03/282233 92/03/292203

2325 0140 (intermittent) 0312-0335 0120-0150 None None None 1610-1620 None None

since in the ACOS geometry the look directions are more perpendicular to the magnetic field. OBSERVATIONS

The revised version of the ACOS experiment was run in three U K EISCAT campaigns from March 1991 to March 1992. The times of the experiments and the intervals during which coherent scatter was observed are recorded in Table 1. A time series of coherent scatter spectra recorded at Kiruna during the early morning of 9 March 1991, the most intense period of backscatter observed during the active experiments, is displayed in Fig. 4. For each spectrum the background noise has been subtracted. There is a very large dynamic range in the spectral power. The strongest scatter near 03:15 UT corresponds to a signal-to-noise ratio (SNR) of 4 dB in contrast the scatter near 03:30 UT was no greater than - 1 0 dB. The SNR values are low owing primarily to the low power of the Oulu transmitter. The spectral shape is typical of the spectra observed in the earlier passive observations, with phase speeds of order 400 m s-J and spectral widths of around 250 m s-~. These are consistent with type I coherent backscatter. In Fig. 5, panel (a) contains the corresponding time series of the irregularity phase speed (each point marked by a square block) derived from the spectra along with the component of the F-region ion velocity measured at 350 km altitude in the Kiruna-Oulu mirror direction, Kma~(full lines), derived by the bistatic method described in Section 2. Panel (b) contains the coherent scatter SNR measured by the Kiruna receiver and panel (c) comprises the F-region electron density (350 km altitude) derived from Tromso incoherent scatter measurements. It should be noted, that in the time series for the irregularity parameters measured by Kiruna there are instances when there are points plotted for coherent scatter power but no cor-

responding phase speed measurement. These are noisy returns in which no coherent scatter spectrum could be resolved. The minimum power at which spectra could be resolved was - 2 0 dB. A number of points which are general to the comparison of the coherent and incoherent scatter data are illustrated in Fig. 5. There is a clear correlation between the irregularity phase velocity and Kmac, Fig. 5(a), including evidence for the existence of a threshold velocity for the generation of the E-region irregularities. The onset of backscatter occurs at around 03:12 UT when Kmac is elevated to near 600 m s- ~. Prior to this Kmac w a s consistently less than 400 m s- ~. Thereafter, the component is always higher than this value and coherent backscatter is observed continuously except for the short breaks when the COSCAT transmitter is turned off. The interval of apparently high velocity between 02:40 and 02:55 UT does not correspond to an interval of coherent backscatter. At the same time the F-region electron density is low. These conditions make accurate determination of velocities difficult, indicating that the data may be unreliable during this interval. A decrease in the Fregion electron density, Fig. 5(c), and hence SNR in the incoherent scatter part of the experiment is unfortunately also an inherent feature of the intervals during which coherent backscatter is observed. Figure 5(d) illustrates the variation in ion temperature for the same interval. There are increases in ion temperature during the depletion in electron density (e.g. between 03:12 and 03:20 UT). Hence, the depletion in electron density can be attributed to enhanced ion recombination that occurs during high ion temperatures. The increased ion temperatures arise through frictional heating during intervals of strong electric fields, i.e. conditions necessary for the generation of twostream plasma irregularities. Data from the experiment run on 13 March 1991 is depicted in Fig. 6. There is one long interval of

Flow dependence of COSCAT spectral characteristics

193

COSCAT passive spectra recorded at Kiruna From 03:(3,7:20 on 09•03 1991 To 03:47:;~.0 on 09/03 1991 Filter Half Width 25 kHz

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backscatter between 01:20 and 01:47 UT. During the first 5 min of this interval, K.,ac is enhanced to values of around 800 m s -1. An interesting interval of coherent scatter occurs just after 01:40 UT when irregularity phase speeds near zero are observed when Km.~ is less than 400 m s -j. Spectra from this interval are illustrated in Fig. 6(d). The spectra are noisy and so 12 5-s dumps have been integrated together to give spectra at 1 min resolution for 01:41:20 and 01:42:20 UT. In comparison, the observed spectrum from 5 min later is also plotted with the same post integration. The spectrum at 01:47:20 is characteristic of type I irregularities already discussed. The spectra at 01:41:20 and 01:42:20 UT have smaller powers and lower Doppler shifts. The spectrum at 01:42:20 UT possibly contains two spectral peaks at opposite Doppler shifts. Moment analysis of these two spectra gives low phase speeds near zero Doppler shift as indicated in the time series of Fig. 6(a). The ]Doppler shifts of the peaks in each

spectrum are all of the order of 250 m s -1 in magnitude. It is uncertain whether these low power spectra arise from irregularities propagating in opposite senses along the coherent scatter direction or are type II irregularities (low phase speed and broad spectrum) that exhibit multiple peaks as a result of noise. A possibility is that they are secondary irregularities generated perpendicular to the main electrojet flow (Sudan et el., 1973) due to currents and gradients set up by primary irregularities. Similar observations have been reported by Moorcroft and Tsunoda (1978). The type I spectrum at 01:47:20 has a broadened base that exhibits similarities to the previous two spectra. The shape of the COSCAT spectra is too large a topic to discuss here and will require future study of the more abundant COSCAT passive data set for it to be resolved. The experiment conducted on 24 June 24th, 1991 (Fig. 7) contains the highest electron densities observed and, therefore, gives well-determined vel-

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ocities at b o t h T r o m s o a n d Sodankyl~i. There is only a very brief interval o f c o h e r e n t scatter at a r o u n d 16:10 UT. This period is interesting because Kmac varied very s m o o t h l y d u r i n g the time for which backscatter was observed, the d u r a t i o n o f the backscatter coinciding closely with the time for which the Kmae

exceeded 450 m s - 1 . This would seem to be a very good indicator o f a threshold velocity for irregularity generation on this day. There are, however, a n u m b e r o f intervals o n the other days o f the study for which Km,c exceeded 450 m s-l, b u t n o c o h e r e n t scatter is observed. N o t all o f these intervals c a n be explained

Flow dependence of COSCAT spectral characteristics

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by unreliability of the velocity data. There are additional indications that the picture is more complicated than a simple comparison with Kma¢ would suggest. Figure 8 illustrates the data obtained during the experiment of 8 March 1991, the beginning of which (after 23:30 UT) is characterised by an interval of intense back scatter. The derived ion flow for this interval, panel (a), is extremely scattered, with rapid variations in Kmac. Despite these variations, backscatter is present almost continuously until 00:00 UT on 9 March, and is stronger than any subsequent backscatter in this figure, although substantially larger flow velocities occurred after midnight. C O M P A R I S O N OF T H E IRREGULARITY P H A S E SPEED W I T H THE PLASMA DRIFI"

Coherent scatter observations from all the experiments are combined in order to investigate the general behaviour of the 16 cm irregularities with respect to the plasma drift velocity. All coherent scatter observations illustrated in the time series of Figs 5 to 8 are included, except for data where the measured irregularity phase speed was in the opposite sense to the derived Kmac of the F-region ion velocity. Such data is unphysical and indicates a poor estimate of the drift velocity (perhaps due to low SNRs) or incorrect determination of the irregularity phase speed due to noisy coherent spectra. This selection criterion removed only 5% of the experimental data. The excitation of the irregularities in a given direction is dependent on the size of the flow component in that direction (eg. see Fejer and Kelley, 1980). The variation of the irregularity phase speed as a function o f Kmac is depicted in Fig. 9. In general, most coherent scatter is observed when Kma~is greater than 400 m sin magnitude (marked on Fig. 9 with vertical dotted

lines) and this is indicative of an excitation threshold for type I 16 cm waves. Subsequently, the phase speed increases from some 250 m s ~to approach 600 m s as the Kmacvaries from around 400 m s ~ to 1 km s - ' . The irregularities excited for K~ac less than 400 m shave characteristics of the low phase speed irregularities discussed above with respect to Fig. 6(d). Once excited, the properties of the irregularities are controlled by the magnitude of the E x B drift velocity. Figure 10 illustrates the variation of the irregularity phase speed with respect to the bistatic determination of the E × B drift speed. Some flow direction information is also included. If the component of the flow is directed towards the Kiruna radar the plasma drift speed is given a positive sign, if the component is away from the Kiruna radar the plasma drift speed is given a negative drift speed. Most data are clustered in two regions on the graph. There is a slight increase in the phase speed as the electron drift speed varies from 500 to 2000 m s -1. Robinson and Eglitis (1993) have compared these data with a kinetic calculation of the dependence of the irregularity phase speed on the Eregion electron drift speed, Robinson and Honary (1990). This comparison is illustrated in Fig. 11, for data taken from the COSCAT run on 9 March 1991, where the average irregularity phase speed in 300 m s drift speed bins has been plotted. The data compare well with the theory and suggests the backscatter altitude is just below 100 km. The altitude estimate is lower than typical irregularity observations consistent with the previous EISCAT observation of 16 cm irregularities by Schlegel et al. (1988). DISCUSSION

The component of the electron drift velocity required for irregularity generation was found to vary

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between each experiment. This is an indication that the generation threshold is not dependent on the velocity component alone (or the background electric field in which all the drift speeds have their origin). Kustov et al. (1993) has made the most recent review of the ionospheric conditions necessary for the generation of coherent scatter. Furthermore, their studies at 83 MHz radar frequency indicate that the echo occurrence is most dependent on the ambient electric field and the mean electron density. Depletions in the electron density may account for the non-occurrence

of coherent scatter during the observation of large electron drift speed components in the COSCAT observations. Kustov et aL (1993) found that even when electric fields were large ( ~ 100 mV m - i ) echoes were not always observed, implying a saturation of the electron density fluctuation amplitude and electron densities insufficient for irregularity excitation. Tsunoda and Presnell (1976) working at 398 MHz, nearer to the frequency regime of COSCAT, found no echoes when the E-region electron density was less than 5 × 10l° m -3. A drawback with the data presented

Flow dependence of COSCAT spectral characteristics

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here is that no E-region electron densities could be measured. However, during some intervals when Kmac was large, e.g. 02:40 to 02:50 UT on 9 March 1991 and near 01:30 UT for the observations commencing on 8 March 1991, simultaneous depletions in the Fregion electron density were observed. It must also be taken into account, that the problem is one also of echo detection as well as echo occurrence. The irregularity backscatter cross-section is dependent on the radar look direction to the electron drift direction and the magnetic field as well as the mean density and electron density fluctuation amplitude. The variation in the generation conditions observed with COSCAT is similar to that recently observed by Del Pozo et al. (19'93) in coherent scatter observations with the Millstone Hill radar of 34 cm wavelength irregularities. Del Pozo et al. (1993) measured threshold electric fields thai varied between 20 mV m -~ and 25 mV m - 1, these values approximately correspond to electron drifts of 400 m s-~ and 500 m s-1 respectively. They accounted for the variation due to the presence of a non-zero neutral wind in the E-region which imparts motion to the ions with respect to the electron drift, hence modifying the plasma drift velocity. Such a neutral wind could account for the variations in threshold velocity observed for COSCAT. However, to explain the lack: of scatter during large drift speeds, depletion of the ambient electron density is a more plausible cause. These influences can only be investigated at such time as it becomes possible to perform experiments in which the COSCAT scattering volume is probed simukaneously from different sites to measure both coherent and incoherent scatter in

the same volume, so that plasma parameters can be estimated. Most spectra observed seem typical of type I irregularities. This is different to those observed by Schlegel and Moorcroft (1989) with EISCAT, where the spectra were narrow and single peaked but at Doppler shifts near 260 m s -~. They were not able to classify these as either type I or II, but rather as secondary irregularities that fell in an intermediate regime. The earlier EISCAT measurements could only be made at large magnetic aspect angles owing to the local relief near the Tromso EISCAT site obscuring the optimum pointing direction. COSCAT can look close to perpendicular to the magnetic field and the observations suggest it is possible to observe irregularities generated by the primary instability process, as also observed by Millstone Hill (e.g. St-Maurice et al., 1989). The current data set also contains a number of low power spectra possibly composed of two components with opposite Doppler shift, see Fig. 6(d). The problem of the low SNRs in the incoherent scatter measurements, which are inherent during most intervals of backscatter, seems to be largely unavoidable, and can only lead to uncertainties in the magnitude and direction of the derived ion velocity for the active experiment. These problems can never be overcome entirely, but could be ameliorated by designing the active experiments so that the maximum EISCAT transmitter duty cycle is put into long pulse transmission, and by observing at times and seasons when the unperturbed electron density is high. In addition, the bistatic technique of measuring the flow velocity is somewhat limited by the need to assume

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that the field-parallel velocity is negligible, whereas Winser et al. (1988), Jones et al. (1988) among others have show that field-parallel velocities of above 500 m s- ] can exist in a small number of very intense frictional heating events. Even large parallel velocities, however, have a progressively smaller effect on the derived velocity for observing volumes far North of EISCAT, when components, such as K~ao, become increasingly insensitive to flows in the field-parallel direction.

The question of magnetic conjugacy must also be considered when attempting to interpret these observations. Without being certain about the magnetic conjugacy, it is not possible to be certain of whether the F-region velocity measurements we make are really magnetically conjugate to the observations of E-region coherent scatter. The difficulty lies not in the accuracy of the I G R F model used for calculating the geometry of the magnetic field, but in the uncertainty in the location of the scattering region for CW trans-

Flow dependence of C O S C A T spectral characteristics

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02:00

200

P. Eglitis et

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COSCAT Comparison of Velocity and Spectral Moments Combining all 1991 data

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Fig. 9. The irregularity phase speed plotted against the component of the ion velocity in the direction of propagation of the observed irregularities. The two vertical, dotted lines are at velocities of -400 and 400 ms ~.

missions. A future series of experiments has been proposed in which the COSCAT transmitter will be used to broadcast a 13 bit Barker code, which can be decoded by the matched filters at the EISCAT receiver sites. The use of these codes for passive bistatic observations will enable some information to be obtained on the structure and range of the scattering region. From the observations reported here, it is clear that there is an appreciable amount of variability between the different events observed, which suggests the need for further observations to extend this data set of 16 cm irregularities. The results reported above, and the possibility to perform further experiments involving phase coding and the simultaneous probing of the Eregion with coherent and incoherent scatter, seem set to guarantee an exciting series of forthcoming experiments using the COSCAT facility.

CONCLUSIONS Results have been reported from a series of COSCAT experiments carried out during 1991 and 1992. In these experiments, coherent backscatter from the auroral E-region was observed with the EISCAT Kiruna radar, while the Tromso U H F and Sodankyl~i radars made bistatic incoherent scatter observations of a magnetically conjugate point in the F-region. The Troms~ and Sodankyl~i data were used to form an estimate of the magnitude and direction of the fieldperpendicular plasma velocity, assuming that there was no field-parallel ion flow. The derived ion velocity was resolved into the Kiruna-Oulu mirror direction, providing an estimate of the component of the Eregion electron velocity in the direction of irregularity propagation. The way in which the spectral charac-

Flow dependence of COSCAT spectral characteristics

201

COSCAT Comparison of Velocity and Spectral Moments Combining all 1991 data °2000 2000

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Fig. 10. The irregularity phase speed plotted against the ion velocity perpendicular to the magnetic field.

teristics of the coherent scatter varied as a function of the electron velocity could then be examined. Although the observed occurrences of backscatter have been few, given the total observing time, enough observations have been made for some general conclusions to be drawn. There was variability in the electron velocity component required to generate the coherent backscatter, with the Kmac as small as 400 m s-~ on some occasions when backscatter was present, and as large as 1 km s-~ on other occasions when it was not. The phase velocity of the irregularities is strongly related to the magnitude of the electron velocity component, with the phase velocity increasing from 2_';0 to 600 m s -~ as the electron velocity component increases from 500 m s -~ to 2 km s- ~. The form of this increase is characteristic of the height of the scattering layer, and is consistent with a low scattering altitude of less than 100 km. The experimental data collected are the most complete set of irregularity measurements in a regime

where only kinetic theory is applicable to their interpretation. This initial paper has presented the experimental observations and investigated the relationship of the irregularity properties to the electron drift. The theoretical implications of some of this work will be dealt with in a future paper. The flexibility of the COSCAT system is such that it should be possible to design a number of more advanced experiments for the study of coherent backscatter at 16 cm wavelength, initially using Barker coded transmission schemes. Ultimately it is hoped to make joint coherent and incoherent scatter observations of the same volume of E-region plasma.

Acknowledyements--Thedata used for this publication were obtained using the EISCAT radars. The authors thank the members of UK campaign teams, and the Director and staff of EISCAT for their assistance during experiments.EISCAT is an international facility supported by the Suomen Akatemia (Finland), the Centre National pour la Recherche Scientifique (France), the Norges Almenvitskaplige For-

P. Eglitis et al.

202 800,,

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,d 200-

0

500

1000

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2000

PLASMA DRIFT SPEED (M/S) Fig. 11. A comparison of the COSCAT data from 9 March 1991 with the kinetic theory of Robinson and Honary (1990). The crosses plotted are the average irregularity phase speed (with standard deviation) plotted for 300 m s-~ bins of bistatically determined plasma drift speed. The full lines superposed, are kinetic calculations of the relationship for altitudes of 95 and 100 km.

skningsr~d (Norway), the Naturvetenskaplige Forksningsr~idet (Sweden), the Max Planck Gesellschaft (Germany) and the Science and Engineering Research Council (SERC, United Kingdom). Thanks are also due to the technical staff at the University of Leicester, University of

Oulu and the Max-Planck Institut, Lindau, for the construction and deployment of the COSCAT system. The work described above was partly funded from SERC ; grant number SGD 10948.

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Dual-Mode E Region plasma wave observations from Millstone Hill. J. geophys. Res. 98, 6013 6032. Ion cyclotron waves as a possible source of resonant auroral radar echoes. J. geophys. Res. 89, 187-194. Ionospheric irregularities. Rev. Geophys. Sp. Phys. 18, 401-454. Phase velocity studies of 34-cm E-region irregularities observed at Millstone Hill. J. atmos, terr. Phys. 54, 759-768. STARE: a new radar auroral backscatter experiment in northern Scandinavia. Radio Sci. 13, 1021-1039. A review on radio studies of auroral E-region ionospheric irregularities. Ann. Geophys. 7, 239-258. VHF double-peak spectra in the morning sector of radio aurora. Planet. Space Sci. 27, 233-241. Statistical study of high-latitude E-region Doppler spectra obtained from the SHERPA HF radar. Ann. Geophys. 9, 273-285. Large plasma velocities along the magnetic field line in the auroral zone. Nature 336, 231-232. Electric field and electron density thresholds for Coherent auroral echo onset. J. yeophys. Res. 98, 7729-7736. COSCAT, a new auroral radar facility on 930 MHz - system description and first results, Ann. Geophys. 9, 461-469. Rapid scan Doppler velocity maps of the UHF diffuse radar aurora. J. geophys. Res. 83, 1482-1492.

Flow dependence of COSCAT spectral characteristics Providakes J., Farley D.T., Swartz W.E. and Riggin D.

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Robinson T.R. and Honary F.

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Schlegel K.

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Schlegel K. and Moorcroft D.R.

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Schegel K., Thomas E.C. and Ridge D.

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St-Maurice J.-P., Foster J.C., Holt J.M. and Del Pozo C.

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Sudan R.N., Akinrimisi J. and Frey D.T.

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Tsunoda R.T. and Presnell R.I.

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Turunen T.

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Villain J.P., Greenwald R.A., Baker K.B. and Ruohoniemi J.M.

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Watermann J., McNarnara A.G., Sofko G.J. and Koehler J.A.

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Winser K.J., Jones G.O.L. and Williams P.J.S.

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Reference is also made to the following unpublished material : Robinson T.R. and Eg].itis P. 1993

Schlegel K., Jones T.B. and Nygren T.

1988

Thornhill J.D. and Chapman P.J.

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203

Plasma irregularities associated with a morning discrete auroral arc: radar interferometer observations and theory. J. geophys. Res. 90, 7513-7523. A resonance broadening kinetic theory of the modified two-stream instability: Implications for radar auroral backscatter experiments. J. geophys. Res. 95, 1073-1085. Interpretation of auroral radar experiments using a kinetic theory of the two-stream instability. Radio Sci. 18, 108-118. EISCAT as a tristatic auroral radar. J. geophys. Res. 94, 1430-1438. A statistical study of auroral radar spectra obtained with SABRE. J. geophys. Res. 91, 13483-13492. First results on the observation of 440 MHz high-latitude coherent echoes from the E-region with the Millstone Hill radar. J. geophys. Res. 94, 6771-6798. Generation of small-scale irregularities in the equatorial electrojet. J. geophys. Res. 78, 240-248. On a threshold electric field associated with the 398MHz diffuse radar aurora. J. geophys. Res. gl, 8896. GEN-SYSTEM a new experimental philosophy for EISCAT radars. J. amos. terr Phys. 48, 777-785. HF radar observations of E-region plasma irregularities produced by oblique electron streaming. J. geophys. Res. 92, 12327-12342. Distribution of mean Doppler shift, spectral width and skewness of coherent 50-MHz auroral radar backscatter. J. geophys. Res. 94, 6979-6985. Large field-aligned velocities observed by EISCAT. J. amos. terr. Phys. 50, 379 382.

The relationship between the phase velocity of FarleyBuneman irregularities observed by SABRE and COSCAT and the plasma drift speed. Presented at EGS, Wiesbaden. Project COSCAT. Max-Planck-Institut fiar Aeronomie, Katlenburg-Lindau, Bericht, MPAE-W-05-88-34. COSCAT operating and service manual. Ionospheric Physics Group Technical Report, University of Leicester, Leicester, U.K.