VHF double-peaked spectra at negative Doppler shifts in the morning sector of radio aurora

Planet. ScmccSci.,Vol. 2% PP.233-241 Q Pergamm PressLtd., 1979.Printed in Northern Ireland

00324633/79/03014233$02.00/0

VHF DOUBLE-PEAKED SPECTRA AT NEGATIVE DOPPLER SHIFTS IN THE MORNING SECTOR OF RADIO AURORA CWSTOS

Institute of Space and Atmospheric

HALDOUPIS

and GEORGE

SOFRO

Studies, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OWO

(Receiued 29 June 1978) Abstract-This

paper presents a new Doppler spectral type of VHF (42 MHz) radio aurora1 backscatter. This spectrum, which has a double-peaked structure, was observed repeatedly during the morning sector of an exceptionally strong event (A,, = 48) and is due to irregularities moving northwards with quite different velocities. The stronger spectral component, which has a smaller Doppler shift, is centred in frequency at - -130 Hz, corresponding to the ion-acoustic velocity range in the medium; the weak component, which has a greater frequency shift, usually is centred at about -300 Hz (- 1050 m s-l). Evidence based on spectral analysis of sequential short time sequences shows that the spectral power alternates in time between the two distinct frequency bands where the peaks are located, suggesting that the double-peaked spectrum may result from two competing processes which cannot operate simultaneously. The possibility exists that the theoretical model proposed by Sato (1977), which predicts two different quasi-linear stabilization mechanisms for the two-stream instability, could explain the observed double-peaked spectral type.

INTRODUCllON

of backscatter from the equatorial electrojet (Bowles et al., 1960, 1963) supported strongly the notion of plasma wave scatterers moving near the ionacoustic velocity, Doppler spectrum measurement of radio backscatter from both the equatorial and aurora1 ionospheres has proven to be a useful diagnostic tool in investigating the nature of the scattering irregularities. In the equatorial electrojet region, at least two irregularity types have been identified by their spectra (Cohen and Bowles, 1967; Balsley, 1969). As a result of these observations, substantial theoretical work, including linear and nonlinear plasma instability mechanisms (for references see Sato, 1976), has been developed for the understanding of the observed spectral characteristics. The spectra1 investigations of radio aurora (e.g. Balsley and Ecklund, 1972; Ecklund et al., 1973; Greenwald and Ecklund, 1975; Haldoupis and Sofko, 1976; Moorcroft and Tsunoda, 1978) indicate that the aurora1 situation is more complicated than the equatorial electrojet case. This is expected because the radio aurora is a larger scale, highly astable and violent phenomenon associated closely with polar magnetic substorms characterized by enhanced particle precipitation and strong electrojet current systems. Because of the complexity of the radio aurora1 process, the existing theoretical models (mainly the two-stream and gradient-drift

Ever

2

since

the first VHF spectral observations

233

instability theories) are not as successful in explaining the aurora1 spectral characteristics (e.g. Greenwald et al., 1975; Tsunoda, 1976; Haldoupis and Sofko, 1978) as they have been in the equatorial case. There is little doubt of the need for both experimental studies, especially on a microscale basis, and more theoretical work for a better understanding of the radio aurora1 phenomenon. The purpose of this paper is to describe an interesting type of radio aurora1 Doppler spectrum which is observed during the morning time sector. Although this spectral type has been observedand classified as unusual-in a previous recorded event (Haldoupis and Sofko, 1976; Fig. 6b), it was the data obtained from the exceptionally strong event of April 18-19, 1977, which identified this as a distinct spectra1 type. EXPERlMJ%NTAL DETAILS The observing system is a VHF bistatic radio system located near Saskatoon, Canada (geographic coordinates, 52”N, 106S”W; geomagnetic coordinates 60”N, 49%‘). The transmitter, which generates a continuous wave (CW) signal at 42.1 MHz, is separated by 35 km from the receiving site, roughly along the east-west direction. The transmitting and receiving 5-element Yagi antennas are fixed in position pointing northward along the geomagnetic meridian. The data are recorded in analogue form on cassette tapes and contain frequencies in the range 1562* 500 Hz. with 1562 I-Lzreoresenting zero DouDler shift. In addition to analogue tab recordings, conihuous chart recordings of signal strength and mean Doppler shift are made using a linear envelope detector and a frequency demodulator respectively. It should be mentioned that the

C. HALWUPIS and G. SOFKO

234

new type of aurora1 echo presented in this uaper was first identi&d by comparison of the simultaneok mean signal strength and Doppler shift pen recordings. Details about the experimental system are given by Haldoupis et al. (1978). - 0;; major advantage of a CW system, as compared to a pulsed radar system, is that it can provide continuous information over short time intervals. This allows a detailed study of short-term characteristics (fine structure) of the scattered signal. In this experiment, to analyze short period time sequences, the analogue signal recorded on the cassette tape recorder is fed into the analog-to-digital converter of a HP-5451A Fourier Analyser System. By sampling ‘2048 points with sampling interval bt = 0.1 ms, individ~1 time sequences of -0.2s duration are obtained. E&h discrete sequence can be plotted out on an X- Y plotter (e.g. Fig. 5a) and then transferred to magnetic tape for further analysis on the IBM 370 system. Individual (one record) and averaged power spectra were computed using the Fourier Analyser System. Also, averaged fading spectra were examined by sampling the envelope of the backscattered signal. This was accomplished by inserting a linear envelope detector between the tape recorder and the A/D converter of the Fourier Analyser. For the study of short-term changes of the Doppler spectrum, very short sequential data segments were analyzed by the Maximum Entropy Method (MEM) (e.g. Ulrych and Bishop, 1975) in order to increase the frequency resolution. The results presented in the following section are from the strong event of April 19, 1977 (average indices of geomagnetic activity were C, = 1.6, A, = 48). EXPEND

REZRJL.TS

In this section the observed characteristics of the new spectral type are described. Figure 1 shows the

Frc.1.

period variations of the echo streneth and - . simultaneo~ mean Doppler shifts recorded during the morning hours of April 19, 1977. Inspection of the mean frequency variations shows that occasionally, and for short periods of time (120 Hz) mean negative Doppler shifts are observed, co~es~nding to northward motions of the scattering medium. Examination of the concurrent signal strength variations shows that the relatively large negative displacements in frequency (marked with Greek letters in Fig. 1) correspond to strong short-lived echoes. These sudden “jumps” are associated with signal amplitude increases of >20 db. The observation that these echoes occur during the morning hours of the event (which is usually occupied by long-lived echoes with well defined growth and decay phase in their intensity), and are characterized by relatively large mean Doppler shifts, suggested the possibility of a new spectral type. This was verified from the results of spectral analysis. The new spectral type is demonstrated by a series of six spectra shown in Fig. 2; each is designated with a Greek letter and corresponds to a separate burst, marked with the same letter in Fig. 1. The spectral shape indicates the existence of two widely separated spectral components of different intensity; both appear at negative Doppler shifts and therefore are due to irregularities moving northwards. The most striking feature is the large value loner

CHARTRECORD~NGSOFMEANDOPPLERSHIFISANDSIGNALSTRENG~OFAURORALBACKSCA~R DU~G~MORNINGHOU~OF~RU 19,1977.

Both receiving and transmitting antennas were pointing toward GMN. The horizontal axis represents local time (CST).

VHF double-peaked

Doppler spectra of morning radii aurora

shift position. The above Doppler shift range corresponds to a radial velocity range of +500* 100 m s-’ which is somewhat higher than, but close to, the typical ion-acoustic velocity range for auroral heights. 3) The intensity ratio of the strong to the weak spectral component can be found anywhere between 2 and 10 but more frequently is in the 3 to 5 range. 4) This spectral type is related to strong echoes with an approximate duration of -1 to 4 min occurring always after local magnetic midnight (-0200 CST) during the morning hours of the event. 5) Both spectral peaks appear at negative shifts (northward motions); the strongly shifted weak component has never been seen at positive shifts although the rest of the spectral configuration has been’ observed frequently with southward motions. In Fig. 3 a time sequence of five spectra is plotted to illustrate the development of a doublepeaked (D-P) spectrum. The spectrum shown in Fig. 3a is taken at 0527 CST and corresponds to the usual diffuse spectral type observed during the morning hours; it is centred close to zero shift position and is related to long-lived echoes. At

Radio1 Velocity,m/s

Radial

I .300

.200

.I00

Doppler

0

Shift,

-100

235

-200

-mcl

’

-300

I

-600

1

-300

I

Velocfty. 0

nrfr .300 ,600

!

I’

i

.I

I

&co

I

.wxJ

f

-300

Hz

FIG.~. DOUBLE-P~KEDSPE~~ATNEGA~VESHI~.

Each corresponds to a separate echo marked with the same Greek letter in Fig. 1. The circled numbers indicate the number of spectra averaged. (> 1000 m s-l) of the poleward velocity corresponding to the weak component of the spectrum. This is far beyond the observed limits found for the rest of the aurora1 spectra (Haldoupis and Sofko, 1976). Examination of a large number of averaged spectra, obtained from at least seven separate short-lived echoes during the morning hours of April 19, 1977, has shown some very stable pronounced features, as follows. 1) The weak component is well separated from the rest of the spectrum and has a mean Doppler shift in the range of -30O~t 20 Hz, which corresponds to a line-of-sight velocity range of about +1050*70ms-‘; as compared to the strong component, the weak component is narrow, with a half power spectral width of (60 Hz. 2) Enhanced power appears always in the -14Ok 30 Hz Doppler shift range where usually the strongest group of peaks is located; these strong spectral peaks are always superimposed upon a broad spectral background centred closer to zero

I

0528CST

Y t

2

.3x

.zoo

430

0

Doppler Shift, FOG.

3.

400

-200

-300

Hz

SEQUEN~ALDOPPLERSPECTRA DEMONSTRATETHE TIMEDEVELOPMENTOFA D-P SPECXRUM.

Each represents an average of 10 individual spectra over 0.7 min of data.

236

C. HALDOUPIS and G. SOFKO

0528 CST, a minute later, the backscatter intensity increases drastically, followed by well defined changes in the spectral shape as shown in Fig. 3b, where two well separated groups of peaks of different intensity appear, centred at about -130 and -290 Hz. The D-P spectrum retains its main characteristics for about 3 min, as shown in Fig. 3c and d, and then disappears following a decrease in the echo intensity. Figure 3e shows the spectral type present after the D-P spectrum has disappeared; this is the usual broad (diffuse) type similar to the one existing before the appearance of the D-P spectrum. From the plotted spectral sequence, it can be seen that within less than a minute, two separate groups of irregularities developed and moved polewards with mean speeds of about 450 and 1000 m s-‘. The fact that the weak spectral component has never been seen without the strong one (which is located at a Doppler shift band corresponding to the ion-acoustic velocity range), may suggest that there is a kind of relationship between these two spectral peaks. At fir$ sight, the spectral position and the relative intensity of the two components could suggest the possibility that the frequencies associated with the weak peak are the second harmonics of the frequencies of the strong spectral component. The assumption of harmonic relationship implies that both components are present simultaneously, therefore interfering with each other and causing low frequency amplitude variations at their beat frequency (the difference frequency of the two peaks). The above possibility of the simultaneous existence of the two peaks has been investigated by spectral analysis of the fading. Surprisingly, all the fading power spectra decay exponentially as the frequency increases, without showing any enhanced power in a frequency band centred at the beat frequency, so the effect of interference is not evident. In this model a time sequence is synthesized by superposition of a large number (N= 200) of sinusoidal components of known frequency and amplitude but random phase (e.g. a typical element of this sequence is X(r,,) = c Ai cos (2&,, + A). i=,.N After digital Ah4 detection of the synthesized sequence, the envelope power spectrum is computed. Examples of spectra based on the interference model are demonstrated in Fig. 4 where the real spectra (designated by solid lines) also are plotted for comparison purposes. Figure 4a shows the Dop-

pler spectra and Fig. 4b the corresponding envelope (fading) spectra, all averages of 10 individual spectra. Notice that the fading spectrum of “one-peak model” (marked with dotted lines) is very similar to the real fading spectrum. On the other hand, the “two-peak model” fading spectrum (dashed lines) demonstrates the effect of interference due to the simultaneous existence of two spectral components (Fig. 4a), similar to those in the real D-P spectrum. Since the fading spectrum based on the two-peak model does not agree with the real fading spectrum, it is likely that the two peaks do not exist simultaneously and that there is no harmonic relationship between them. The two spectral components probably appear together in the spectrum because the latter is the average spectral estimate over the whole duration of the examined record (0.2 s). The temporal relationship between the two components was next investigated by spectral analysis of very short time sequence (~0.1 s). A single amplitude record of 0.2 s duration associated with a strong D-P spectrum type echo is plotted in Fig. Sa. Before sampling this time sequence, a high pass filter was used to attenuate the lower frequencies to make the intensity of the two spectral components comparable. Closer examination of Fig. 5a suggests that the sampled waveform is composed of sequential “signal bursts” of different duration (generally <20 ms) and strength. The normalized power Doppler spectrum, corresponding to the above 0.2s time sequence is shown in Fig. 5b where two distinct groups of peaks appear, centred at about -140 and -300 Hz. Obviously, from this spectrum it cannot be determined whether these two spectral components occur simultaneously or not. This question is answered by computing separately the power spectra for the lst, 2nd, 3rd and 4th quarters (marked by A, B, C and D respectively in Fig. 5a) of the sequence shown in Fig. 5a. The corresponding spectra of the above short (0.05 s) time sequences are shown in Fig. 6; they are relatively smoothed, as compared to the spectrum shown in Fig. 5b, due to the shorter time window used. The sequential spectra in Fig. 6 reveal a rather interesting feature, namely that the power shifts back and forth between two distinct frequency bands. This result resembles a frequency modulated signal and suggests strongly that the two spectral components are not present simultaneously. The detailed time development of the FFT spectrum (e.g. Fig. 5b) has been investigated by using the Maximum Entropy Method of spectral analysis

237

VHF double-peaked Doppler spectra of morning radio aurora

. i”!.

Frequency,

1.0

1460 I Real

1560 I Po*sr

Hz

1660 I ,?A

i

. . . . . One-ps.aL. Model P. sp.

h .;-_/yffy ::

,.~ -100

Doppler

1660 I

a

Spectrum

Two-peak Model P. Sp.

c

1760 I

Shift,

-200

-300

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b -

Rc(II

--

Two-peak Model F. Spectrum

Fadlnp

Spectrum

. . . . . One-pecrk Model F. Spectrum

200 Frequency,

300 Hz

F1c.4. AVERAGEDDOPPLERANDFADINGPOWERSPECIXA OFREALANDSYNIHESIZEDDATATO STRATE THAT 'IWEREISNOEF?E~OFINTERFERENCEBETWEENTHETWOSPECTRALCOMPONENTSOFTHE D-P SPECTRUM.

(MEM) on very short sequential segments of the original 0.2ms time sequence (e.g. Fig. Sa). By employing the MEM, the frequency resolution is increased and the positions of the existing peaks are located rather accurately. Ulrych and Bishop (1975) have reviewed the theory and applications of MEM, and provided guidance concerning a number of practical problems in its use. Also, Moorcroft (in press), using a variety of tests, has shown that the MEM provides reliable spectral estimates with superior frequency resolution when applied to very short sequences of radio aurora1 data. In the present analysis, each data segment was chosen to represent an individual short-lived signal burst. The results of the MEM spectral analysis can be summarized as follows: (1) each signal burst, especially the short-lived ones, is usually associated

DEMON-

with a single spectral peak whose position is very well defined; (2) these peaks tend to appear in one of two separate Doppler shift bands at -1.50* 50 Hz and -300 f 20 Hz; (3) the signal bursts with spectral peaks at higher Doppler shifts are weaker and shorter in duration than spectral peaks at lower Doppler shifts and (4) usually a group of sequential signal bursts has spectral peaks falling in the same frequency band. The results of MEM spectral analysis for the first quarter of the sequence in Fig. 5a are shown in Fig. 7. It should be mentioned that the spectrum of each data segment has been computed repeatedly for different orders of the autoregressive process ranging from N/3 to N/2, where N is number of points of the data sequence. By changing the order, it has been found that the position of the peak remains fairly constant (maximum relative variation t2%),

C. HALDOUPIS and G. .%FKO

238

0

I

I

1

.05

.I

.,,

.?

Time,,

Doppler

Shift

,

letters: A, B, C . . .) which were analyzed using the MEM. The positions of the main spectral peaks related to each data-segment or burst are shown in Fig. 7b. Figure 7c is a 3-dimensional plot illustrating the time development of the spectrum corresponding to the time segments shown in Fig. 7a; the height of each peak represents the integrated power under the peak, rather than the peak intensity. Overall inspection of Fig. 7 indicates that each signal burst is due principally to an individual irregularity lasting for a short period of time (<20 ms) and moving with constant poleward velocity in either the +500* 150ms-’ or +1050* 100 ms-’ velocity range. Another conclusion is that the deep fast fading observed in the signal amplitude is mainly due to appearance and disappearance (growth and decay) of individual scatterers rather than interference between independent coexisting scatterers. DlSCU!SSlON

Hz

FIG. 5. A SINGLE BACKSCATTER TIME SEQUENCE DURATION AND ITS CORRESPONDING DOPPLER SPECTRUM.

OF 0.2 S POWER

but the peak intensity changes drastically although the integrated power under the peak remains approximately constant. Figure 7a shows eleven sequential segments of data corresponding to separate sequential signal bursts (marked by capital

It appears that no

previous observation of D-P spectra, type found in this experiment, has been

of the same reported in the aurora1 literature. Greenwald et al. (1975) have found that the power spectra for some of the echoes of 50MHz diffuse radar aurora have a double-peaked structure. However, because the two peaks are equally intense and are usually symmetrical about the zero-shift position with a peak-separation in the 30-60Hz range, these spectra are of a different type than the ones described here. They have explained that the two peaks result from the two most unstable regions of secondary gradient drift irregularities. Spectra with double-peaked or multi-peaked structure appear to occur more frequently in the UHF aurora1 scatter (e.g. Hagfors, 1972; Tsunoda, 1976; Moorcroft and Tsunoda, 1978) than in the VHF case. Moorcroft and Tsunoda (1978) describe a double-peaked spectral type (observed with the Homer, Alaska radar during the morning sector when the antenna points northward) with widely spaced peaks and relative peak amplitudes having a broad range of possible ratios. According to them this spectrum is a consequence of an irregular or turbulent electrojet. Again, since the peaks are almost symmetrical about the zero-shift position, these spectra differ from the ones presented here. Perhaps the most important result of this study is that the two components do not exist simultaneously and the D-P spectral shape is the result of time averaging.* This suggests that both spectral components are due to irregularities generated and moving in the same scattering region. Therefore, it

Doppler

Shift

,

Hz

FIG. 6. A SEQUENCEOFFOURDOPPLERSPECTRA(MARKED WITW A, B, c AND D) CORRESPONDINGTO FOUR EQUAL SEQUENTIALDATASEGMENTSSHOWNIN FIG.~~.

* By “two components” we mean the two distinct groups of frequencies centred at about -140 and -3OOHz respectively (e.g. Fig. 3); of course, the broad spectral background centred close to zero shift position is always present and likely is due to a different type of scattering irregularity.

VHF double-peakedDoppler spectra of morning radio aurora

Doppler

FIG. 7.

&ZXJLTs

Shift

* t-12

MEM SPECTRAL ANALYSIS FIG. 7a) EACH REPRESENTING

OF

239

FOR

11 SEQUENTIAL

A SHORT-LIVED

appears that this spectrum results from two competing processes which cannot operate simultaneously. According to Moorcroft (private communication),‘if this were happening it would likely have to occur in a relatively limited volume. The observation that the fast moving irregularities are weaker and last for shorter periods of time (as compared to the ones moving with velocities close to the ionacoustic velocity range) indicates that the conditions in the aurora1 plasma required for their generation and growth are met only for short time intervals (~40 ms) and probably occur in a small scattering volume. The above observations, as well as the limited overall duration (~4 min) of these echoes and the fact that they do not occur with southward motions, support the notion of unusual plasma conditions in the aurora1 medium. The observation that a distinct group of irregularities (the ones associated with the weak component of the D-P spectrum) has a mean northward speed in the range 105Oh 100 m SC’ is rather unusual for VHF radio aurora1 observations. This is well above the limit of -600 m s-’ found for the mean radial speed along the north-south direction with the present bistatic experimental system (Hal doupis, 1978). The only other 50 MHz radio auroral observations indicating large radial motions (in the range 600 to 2000 ms-‘) are the ones

620

SHORT

DATA

ms) SIGNAL

SEGMENTS

(SHOWN

IN

BURST.

reported by Ecklund et al. (1973), Greenwald and Ecklund (1975), and Ecklund et al. (1977). However, these highly shifted spectra are single-peaked with broad spectral widths and were observed with the antenna pointing westward or eastward of magnetic north more along the main electrojet flow. Nevertheless, all these spectra show that irregularities in the aurora1 plasma can propagate with velocities well above the supposed ionacoustic velocity in the medium (this is contrary to the equatorial electrojet case where no echoes are associated with velocities higher than -500 m s-l). A theoretical explanation of irregularity drifts in excess of 600 m SC’ in the aurora1 plasma has been proposed recently by Sato (1977). This is based on the quasi-linear mechanism which controls the phase velocity of the generated two-stream plasma wave irregularities. The general quasi-linear theory of Sato (1976) indicates that the collisional twostream instability would be stabilized predominantly by one of two different mechanisms, namely velocity retardation and density modification of the electrojet. The first mechanism involves a secondary Hall current which reduces the original velocity of the primary electrojet to the critical ionacoustic velocity. This mechanism can account for the constant velocity of the ion-acoustic plasma wave irregularities observed in the equatorial and

240

C. HALDOUPEand G. SOFKO

amoral ionosphere (e.g. Farley and Balsley, 1973; Haldoupis and Sofko, 1978). On the other hand, the density modification mechanism causes stabilization without significant alteration of the electroje! velocity. In this case it is natural to expect, according to the two-stream instability theory (e.g. Farley and Balsley, 1973), that large irregularity velocities in the aurora1 regions would result from large electron drift motions. Some understanding of our results can be based on the above theoretical approach. According to Sato (1977), the deciding factor which determines the type of operating stabilization mechanism is the structure of the flowing electrojet. Specifically, there is a critical value of the electrojet width, lC, that determines which of the two stabilization processes acts in producing irregularities of a given size. For 3 m irregularities (the correct size for scattering a 50 MHz radiowave), the velocity retardation mechanism dominates if the electrojet width is larger than a few hundred meters; the electrojet velocity is thus reduced to the ion-acoustic velocity. On the other hand, if the electrojet width is sufficiently narrow (I, 5 100 m), the predominant mechanism is density modification, with the result that the irregularities can move with higher velocities than the ion-acoustic velocity in the medium. Therefore if it happens that the electrojet width, 1, fluctuates about the critical value 1,. then the two competing processes responsible for the D-P spectrum could be the above two stabilization mechanisms of the two-stream instability. However, there is one important experimental feature, which relates to the position of the weak spectral component, still to be explained. The question is why the more strongly shifted peak is confined to the Doppler shift range -300 f 30 Hz corresponding to a northward speed range lOSO* looms-‘. The fact that the irregularities prefer to propagate with velocities in the above range must be closely related to the physical mechanism responsible for the D-P spectral type. If this mechanism is the one based cn Sato’s theory, then in order to answer the above question, we have to postulate a relationship between the electrojet dimensions and the electron drift velocity. More specifically, we must accept that electrojets with a width less than a critical value I, have velocities confined to a well defined range. For example, if I < 100 m, the electrojet velocity (electron drift velocity) must be close to 1000 m SC’ in order to produce agreement with the observed frequency range of the more strongly shifted spectral component. To our knowledge, there is not any known experimental evidence sug-

gesting a possible relationship between the size and the velocity of the electrojet. Hence, there is no way to tell if the above mechanism, based on Sato’s theoretical work, is adequate to account for all the observed characteristics of the D-P spectrum.

SUMMARY AND CONCLUSIONS

From the experimental results and the above discussion, we may conclude that special aurora1 electrojet conditions which are met only rarely and for short periods of time (<4 min) dictate the operation of two competing physical processes in the aurora1 plasma, each generating a group of scattering irregularities which moves with different mean velocity. The observed D-P spectra demonstrate that enhanced power is present in two widely spaced Doppler shift ranges centred at about -140 and -300 Hz (these frequency shifts are due to northward moving irregularities with velocities of -500 and 1100 m s-l, respectively). By computing both the power spectra (using the FIT and MEM techniques) of sequential short time sequences and averaged fading spectra (to check the effect of interference), we have found that for most of the time the two peaks do not exist simultaneously and that the spectral power shifts back and forth from the one to the other. This suggests that two different processes compete and alternate with each other to produce scatters with different dynamic characteristics. Evidence suggests that the more strongly shifted spectral component is weaker because it is composed of weak echoes caused by short-lived fast-moving irregularities with an approximate lifetime of 20 ms. The fact that exactly the same spectral type, associated with sudden, strong and short-lived signal bursts, was observed on a number of occasions during the morning sector of the same event, implies that a distinct plasma mechanism may operate under certain critical conditions in the aurora1 ionosphere. One theoretical model which would explain the alternating spectral character of the overall process responsible for the D-P spectra is that proposed recently by Sato (1977). According to Sato’s model, the density modification and the velocity retardation of the flowing electrojet (the two competing quasi-linear stabilization mechanisms of the two-stream instability) could explain the position in the spectrum of the two separate peaks. However, this is possible only if a specific relationship between the electrojet width and the electron drift velocity exists. Therefore more evidence is needed to ensure the validity of the proposed

VHF double-peaked Doppler spectra of morning radio aurora theoretical model. A better understanding of the mechanism producing the D-P spectra will certainly increase our knowledge about the short-term processes that occur in the aurora1 plasma. Acknowledgements-This work was supported by grants to G. 8ofko by the National Research Council of Canada. C. Haldoupis would like to thank Dr. C. Meek for providing the MEM program; also financial assistance provided by the NRC of Canada in the form of a postgraduate scholarship is gratefully acknowledged.

241

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Hagfors, T. (1972). AGARD Con. Proc. 97, p. 9.1. Haldoupis, C. (1978). Ph. D. thesis. Univ. of Saskatchewan, Saskatoon, Canada. Haldoupis, C. and Sofko, G.

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