Phase and amplitude scintillations due to electrojet irregularities

Joumal

ofAlmospheric and Terrestrial

Physics, Vol. 53, No. 5, pp. 379-387,

C!OZl-9169191 $3.00+

1991.

.OO

Pergamon Press plc

Printed in Great Britain.

Phase and amp~~de ~~~~Uatio~ due to eltxtrojet irregularities A. BHA~ACHARYYA Indian Institute of Geomagnetism, Colaba, Bombay 400 005, India

Abstract-Features of the power spectra of weak amplitude and phase. scintillations on a VHF signal, transmitted from a geostationary satellite and recorded at an equatorial station, are found to be in good agreement with theory. Irregularity drift speeds transverse to the signal path were extracted from the first few pronounced Fresnel minima in the power spectra. For some data intervals, a low frequency peak, associated with an irregularity wavelength greater than the Fresnel dimension, could be identified in the phase spectrum. The dominant wavelength of the large scale waves is found to be L I .5 km.

1. INTRODUCTION Plasma instabilities occur in the E-region of the equatorial ionosphere, due to the electrojet current which flows in this region whenever conditions are favourable (FARLEY, 1985). Radar and rocket experiments have yielded much information about properties of the density fluctuations caused by these plasma instabilities (FEIER and KELLEY,1980; PRAKASHand PAL, 1985; KUDEKIet al., 1987; PFAFFet al., 1987). Before 1980, VHF radars were used to study the backscatter from density irregularities with wavelengths of a few me&es. In the early I98Os, a radar interferometer technique was developed at the Jicamarca Radio Observatory, whereby kilometre scale electrojet waves could also be monitored. This led to the detection of large scale plasma waves, with wavelengths of a few kilometres in the electrojet region (KUDEKI et al., 1982). Subsequently, these large scale waves, which arise due to the gradient-drift instability, were also studied using rocket data (PRAKASHand PAL, 1985 ; PFAFF et al., 1985). The Condor equatorial electrojet campaign was specially designed to carry out simultaneous observations using VHF radar interferometer, HF radar backscatter, and rockets equipped with plasma density and electric field probes (KUDEKI, et al., 1987 ; PFAFFet al., 1987). These measurements, carried out during the presence of a strongly driven electrojet, have clearly delineated three different height regions within the unstable daytime equatorial electrojet layer over Peru. On the topside, the region between 103 and 111 km, where the current was considered to be strongest, was identified as a two-stream region. In this region, the electron drift speed is sometimes large enough to generate small scale irregularities, with wavelengths of the order of a few metres or less,

through the two-stream instability. On the bottomside, between 90 and 106.5 km, where the background, zero-order, electron density has an upward gradient, the gradient-drift instability can grow when the electrons drift westward. Kilometre scale waves were observed in this region. In the height range of 103106.5 km, both the two-stream and gradient-drift instabilities can operate. Density irregularities in the ionosphere give rise to spatial variations in the amplitude and phase of a radio-wave which traverses them. In the case of a radio-wave transmitted from a geostationa~ satellite and received at a station on the ground, movement of the irregularities across the path of the radio-wave causes the spatial variations to be translated into temporal fluctuations or scintillations of amp~tude and phase recorded by a receiver. Thus, info~ation about the irregularities can be obtained from observations of ionospheric scintillations. Such studies are mostly based on the theory of weak scintillations, which is described in detail in a review by YEH and LRJ (1982). The large conductivity of the E-region during daytime prevents the growth of plasma instabilities in the equatorial F-region by short-circuiting any perturbation electric field associated with a plasma wave. Hence, daytime equatorial scintillations are attributed to irregularities in the electrojet region (BASU et al., 1977; ~TCYX and MULLEN, 1981). Fluctuations in phase reflect all the scale sizes present in the irregularity spectrum, with the largest scale sizes dominating as a result of the nature of the irregularity spectrum. On the other hand, due to diffraction effects, weak amplitude fluctuations have maximum contribution from irregularities with scale size equal to the Fresnel dimension. The Fresnel dimension is given by m, where a is the signal wavelength, and Z, is the effective distance of the irregularity layer from the receiver.

379

Hence. earlier studies of daytime amplitude scinlillations on VHF/UHF signals. in the equatorial region, did not reveal the signature of kilometre scale electrojet irregularities (BHATTACHARYYA and RASTOGI, 1986a). In the present paper, daytime phase scintillations recorded at an equatorial station arc analysed in conjunction with simultaneously recorded amplitude scintillations on a VHF signal to determine some of the electrojet irregularity characteristics. The results obtained are compared with radar and rocket observations.

2. IRREGULARITY

CHARACTERISTICS

SCINTILLATION 2.1.

FROM

DATA

Data anulysis

Simultaneous measurements of amplitude and phase of a 140 MHz signal transmitted from the geostationary satellite ATS-6, when it was positioned at 35”E, and recorded at the equatorial station Ootacamund (11.4 ‘N, 76.7”E, subionospheric dip 4.4”N) are used in the analysis. The signal path had an elevation angle of 40.5” at an azimuth of 257.5” east of north (DAVIES et al., 1979). Along with the 140MHz signal, the amplitudes of 40 and 360 MHz signals were also recorded. Carrier phases for 40 and 140 MHz carriers with respect to the 360 MHz channel were measured at 10 and 1 MHz common sub-multiple frequencies. The digitally recorded data consist of the phase quadrature components and amplitude values sampled at a rate of 10 Hz. Time scales associated with weak scintillations monitored by the receiver hear a direct relationship to the spatial scale sizes of density irregularities which cause the scintillations (YEH and LIU, 1982). This is no longer the case for strong scintillations, which involve refractive scattering, whereby irregularities of scale sizes larger than the Fresnel dimension give rise to appreciable small scale fluctuations in the amplitude recorded on the ground (BOOKERand MAJDI ABE, 1981). Thus, interpretation of observations in terms of irregularity scale sizes is not possible for strong scintillations. For this reason, the present analysis is restricted to intervals of weak scintillations. In order to obtain meaningful estimates of power spectra, it is essential that the data should exhibit stationarity. it is possible to select segments of amplitude scintillation data which satisfy the stationarity criterion reasonably well, since there are no long period fluctuations in amplitude, as discussed earlier. However, phase scintillation data contain arbitrarily long period fluctuations due to large scale variations in the ambient density, which are present even in the

10 ahscncc of plasma instabilities. It is nccessq remove the long period v,ariations as trends in the dam so that the detrended data are reasonably stationary, A criterion used by FREMOUWc/ al. ( 1978) to scpalatc the data into relatively last phssc scintillations and ii slowly varying trend is adopted hcrc. According to this criterion, variations that arc as fast or faster than the slowest amplitude scintillations arc considered :I\ phase scintillations. and the sloucr variations arc included in the phase trend. It was found that the amplitude scintillation data used in the present analysis essentially consist of fluctuations with time scales shorter than 200s. Hence, a low-pass six-pole Buttcrworth filter with a 3 dB cut-otf at 0.005 Hz wai; applied to the raw phase and amplitude data. The resultant outputs arc the respective trends. Detrended amplitude is obtained by dividing the raw amplitudes by the corresponding values of amplitude trend. whereas for obtaining detrended phase. the phase trend is subtracted from the raw phase data at each point. The S,-index, which is the normalized second central moment of intensity. is used as a measure of the strength of amplitude scintillations for the duration of the data set. For each data interval, the standard deviation, CT,),of detrended phase scintillations is also computed. For the computation of power spectra, 4096 data points are used from each interval. with a sampling period of 0.2s. Raw power spectra arc obtained by using a fast Fourier transform algorithm after the application of a 10% cosine hell window to each data set. Further, for stability, the raw powc~ spectra arc smoothed by I l-point, equally weighted averaging.

2.2. Applicution

cf weak scintillation

them-J

Interpretation of the features seen in the smoothed power spectra of amplitude and phase scintillations is carried out on the basis of weak scintillation theory which is applicable for the data segments analyzed here. The following assumptions are made in the theoretical interpretation : (I) irregularities are ‘frozen-in’, that is, they convect across the signal path without undergoing any changes; (2) density fluctuations can he described by a homogeneous random field with an autocorrelation function which is independent of location within the irregularity layer; (3) drift speed of the irregularities does not depend on the irregularity scale size. The validity of these assumptions is examined below in the light of theoretical arguments and radar and rocket observations of the electrojet irregularities. It has been demonstrated by WERNIK et al. (1983) that while ‘non-frozen’ behaviour of the irregularities.

Electrojet irregularities as exemplified by random temporal variations, has an observable effect on the cross-correlation of amplitude scintillations monitored by spaced receivers, there is no such noticeable effect on the autocorrelation function. Thus, in the analysis of data from a single receiver, the first assumption does not impose any serious restrictions on the validity of results obtained. In view of the height dependence of irregularity characteristics derived from radar and rocket experiments, the second assumption appears to be questionable. However, it must be borne in mind that for the ATS-6-Ootacamund path, the Fresnel dimension associated with a 140 MHz signal scattered by Eregion irregularities is approximately 800 m. According to an unified theory of type I and type II irregularities in the equatorial electrojet (SUDAN, 1983), the irregularity power spectrum falls off as a power-law for wavelengths shorter than this value. Hence, irregularities which contribute the most to scintillations are the long wavelength (2 100 m) ones generated by the gradient-drift instability in the unstable bottomside of the E-region, where an upward directed zero-order density gradient exists. The short wavelength (-few metres) irregularities generated by the two-stream instability at greater heights or by a secondary process do not play any role in the observed scintillations. Therefore, as far as scintillations are concerned, the second assumption may be considered to be valid. The phase velocity of equatorial electrojet plasma waves obtained from a local linear fluid theory, under the assumption that the growth rate yk for the plasma wave is much lower than its oscillation frequency wk, is independent of wavelength. But for large scale waves, the condition yk <
381

been found in the power spectra of weak daytime amplitude scintillations recorded at an equatorial station (BHATTACHARYYA and RASTOGI,1986a). Since oscillations of the Fresnel filter functions for amplitude and phase are almost 90” out of phase for a thin irregularity layer, the minima in the power spectrum of phase are expected to coincide with the corresponding maxima in the power spectrum of amplitude, and vice versa. This is clearly discernible for the first few minima in two of the examples shown here. Detrended amplitude and phase scintillations for three data sets are shown together with their power spectra in Figs l-3. All these intervals have weak scintillations. Daytime equatorial amplitude and phase power spectra are seen to be quite distinct from those obtained for nighttime equatorial scintillations (BHATTACHARYYA and RASTOGI,1986b). Firstly, the nighttime spectra do not usually display such marked Fresnel oscillations because of much greater thickness of the F-region irregularity layer, which is mainly responsible for the nighttime scintillations. Secondly, characteristics of phase spectra at low frequencies, which are related to long wavelength (>Fresnel dimension) irregularities for weak scintillations are also quite different for daytime and nighttime scintillations. The nighttime phase spectra show a powerlaw behaviour at the low frequency end as well, whereas the daytime spectra sometimes show more structure, as in Fig. 3. In Table 1, theoretical expressions are given for the frequencies corresponding to the first two minima of amplitude spectrum, denoted by v,, and v,* ; and the first two minima of phase spectrum denoted by vP, and vPb2.These expressions are based on the phase screen theory of weak scintillations, where the irregularity layer is replaced by a single phase-changing screen located at a distance Z, from the receiver. The frequencies v,, , V u2, v,,, and vP2, depend on two unknown quantities, namely V,, the drift speed of the irregularities transverse to the signal path, and Z,. However, the ratios V,,IVpl, v,Jv,,, vPl/vP, and v,,/v,~ are independent of these quantities. Hence, from the values of v,, , vaz,vp, and vP2 derived from data, which are also given in Table 1, the above ratios have been computed. Comparison of the computed and theoretical values indicates very good agreement. For the third data set, it was not possible to determine vP, and vP2accurately from the phase spectra shown in Fig. 3. This is the reason why only values of v,, and v,~ are listed for this case. Thus, according to the contents of Table 1, the assumptions made in the application of weak scintillation theory to observations are valid to a large extent.

A. ATS

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Fig. 1. (Top) Phase trend and detrended phase for data set 1 (a, = 0.33rad). Filter cut-off frequency = 0.005Hz. Values assigned to phase trend are not absolute. (Middle) Amplitude trend and detrended amplitude for this data set (S, = 0.23). Units for amplitude trend are arbitrary. (Bottom) Smoothed power spectra of detrended phase and amplitude scintillations. First few Fresnel minima are marked by arrows.

2.3. Drift speed V, and large scale irregularities V,, the drift speed of the irregularities transverse to the signal path, may now be estimated with some confidence provided a reasonable value is chosen for

ZR. As remarked earlier, irregularities which give rise to daytime scintillations are essentially confined to the bottomside of the E-region. Assuming a constant effective height of 100 km for the irregularity layer, a slant range Z, of approximately 154 km is obtained

383

Electrojet irregularities A T’S- 6

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Fig. 2. Same as Fig. 1 for data set II ((r9 = 0.15 rad ; S4 = 0.14).

for the ATS-6-Ootacamund signal path. With this value of Z,, estimates of I’, are derived from the various minima of the amplitude and phase power spectra listed in Table 1. The averages of these values yield I’, E 92, 77 and 80 m s- ‘, respectively, for data sets I, II and III. The power spectrum of phase ~intillations shown in Fig. 3 exhibits a broad peak in the low frequency region just before the first Fresnel minimum is encountered. The corresponding peak in the amplitude spectrum is masked by the Fresnel filter effect. The maximum of the phase spectrum peak occurs at a frequency v, = O.&t9 Hz,- which corresponds to an irregularity wavelength im z 1.6 km on using the com-

puted value of 80ms-’ for V,. For the first data set the first peak can also be identified in the phase spectrum at v, = 0.046 Hz, although this peak is not as pronounced as for the third set. Using the value of V, ,N 92 m s- ‘, determined earlier for this interval, the co~esponding irregularity wavelength & z 2.0 km. No such low frequency peak is discernible in the phase spectrum of the second data set. 3. COMPARISON

WITH RADAR AND ROCKET ORSERVATIONS

According to a local, linear fluid theory of the gradient-drift instability, the phase velocity for long

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Fig. 3. Same as Fig. I for data set III (grn= 0.30 rad ; S, = 0.19)

wavelength, small amplitude waves propagating parallel to the electron drift velocity and normal to the geomagnetic field is given by

-I’,~ =co,/k = V,/{(l

+$/,)I1 + Wk)*li

(1)

in the frame of the neutral wind. Here Vd is the drift speed of electrons in the neutral wind frame in which

ions are considered to be practically stationary. tjO is equal to v~v&& where v,(~)and f&,,, are the electron (ion) collision frequency and electron (ion) cyclotron frequency, respectively. In the region of interest, tie has a value typically between 0.1 and 0.3. k, = (v,/QL,J( 1 + tj,) ’ is a characteristic wave number associated with the instability, for a zero-order

Electrojet irre~~ties

385

density gradient scale length LN. If it is assumed that there is no vertical drift of the i~eguIa~ties, and the o~entation of the ATS-~Oota~amund signal path is taken into consideration, it is seen that V, estimated from the data is a component of the horizontal drift speed of the irregula~ties. Therefore, in the frame of the receiver, the horizontal drift speed of the irregularities is given by approximately 1.5 Te, which yieids values between I I5 and 138 m s- ’ for the three data sets presented here. These values are of the same order as the westward drift velocities obtained from Jieamarca radar inte~erometer data in the radar frame of reference. In order to relate the horizontal drift speed to the phase velocity given in (I), it is necessary to transfo~ it to the frame of the neutral wind. The neutral wind velocity prevalent in the E-region over ~ota~amund at the time of observations is not known. Also, the direction of drift of the irregularities cannot be dete~ined from a single site observation of scintillations. However, since electrons in the daytime electrojet usually drift westward, it may be assumed that the irregula~ty drift is also westward. On the basis of rocket measurements made at Thumba, an equatorial station in the Indian region, the eastward neutral wind speed may be anywhere between about - 80 and 80 m s- ’ at an altitude of 100 km (I&s et al., 1976). Therefore, in the frame of the neutral wind, the westward irregularity drift speed, which is usually identified with the phase velocity I’, of the plasma waves, wouId have some value between about 35 and 218 m s- ‘. For irregula~ties of wavelengths much less than 2x/k,, V, = Y+(l +$,) from (1). Thus, in the frame of the neutral wind, the drift speed of the electrons is between 42 and 262 m s-l. This is also the drift speed of electrons relative to the ions, which have been assumed to be almost stationary in the neutral frame. The value of Vd is seen to be well below the ion-acoustic speed so that irregularities which give rise to the observed scintillations are generated by the gradient-drift instability. For the ATS-6-Oota~mund path, the magnetic field strength at a height of 100 km 2: 0.38 x 10w4T. Westward E x B drift velocities of magnitude 42 and 262m s- ’ correspond to upward directed polarization electric fields of magnitude $ c 1.6 and IOmV m- I, respectively. The electrojet over the Indian zone is generally much weaker than that over the American zone, and this is reflected in the smaller values of E’ mentioned here in comparison with the pofarization electric field value reported over Peru from the Project Condor radar/rocket observations (PFAFF et al., 1987). The electrojet was not particularIy strong during any of the three intervals considered in the present paper. The difference between AH, the deviation of

the H-component of the magnetic ticld from the 75 E midnight vaiue, at an equatorial location (Trivandrum) and at a location outside the influence of the clcctrojet (Alibag) was always less than 30 nT. The dominant wavelengths of large scale irregularities found from data sets I and III are similar to those obtained from in-situ and radar data (KUDEKI e/ ul., 1982; PKAKASH and PAL, 1985: KUUEKI et ul., 1987 : PFAFF e’f ul.. 1987). One feature which may be noted in the data sets displayed here is that, whereas short time-scale fluctuations in phase occur with almost uniform amplitude throughout the duration of the first and second events. for the third event they seem to appear in ‘bursts’. These periods of enhanced short time-scale activity in phase are themselves organised in a wave-like pattern on a larger time scale and are associated with greater amplitude scintillation which does not have contributions from large scale irregularities as such. It is conjectured that these are the signatures of short scale waves generated by a twostep gradient drift process (SUDAN c’t u/.. 1973) which involves the large scale wave electric lields and density gradients. Thus, horizontally propagating primary waves can lead to the generation of short scale waves which propagate obliquely in general. Given the orientation of the ATS-6-Ootacamund signal path, these waves also play a role in causing scintillations. The presence of such ‘bursts’ of short scale irregularities has been reported on the basis of rocket data (PKAKASH and PAL. 1985; PPAFt: ?/ (I/., 1987). In the power spectra of amplitude and phase scintillations shown in Figs 1 3. it is seen that the power levels off for scale sizes between 250 m and the Fresnel dimension ( 2 8OOm) in the first two data sets and between 450 and X00 m for the third data set. Fat shorter wavelengths, there is a power-law decrease. Spectral indices computed from the power-law regions of amplitude and phase spectra are found to be in close agreement. The average power-law index from data set I is nr = - 3.6 f 0. I for irregularities of scale sizes between about 70 and 220 m. For data set II. the average value of m computed from amplitude and phase spectra is - 2.4 & 0.1 for scale sizes between 60 and 250 m. and for data set III. average 177 = -3.5 +O. I for irregularity scale sizes between about 80 and 450 m. Power spectral indices derived from in-situ measurements over Thumba in the Indian region (PRAKASH and PAL, 1985) correspond to much shorter wavelengths. Comparison with Project Condor rocket data (PFAM et ul., 1987) shows that only the second data set yields a one-dimensional spectral index (=m - I) close to the value obtained from in.situ data for wavelengths between 30 and 300 m. For the other two data sets, the spectral indices obtained

are stecpcr than the irz-situ values. However. it should be borne in mind that wavelengths obtained from the scintillation data retlect scale sizes in a plane inclined at an angle of about 4X.6 to the horizontal. 4. (‘ONCI,CSlONls The phase screen theory has been applied to weak daytime phase and amplitude scintillations on a VHF signal transmitted from a geostationary satellite and recorded at an equatorial station. The following results have been obtained :

(I) Pronounced Fresnel oscillations have been found in the power spectra of both amplitude and phase scintillations. thereby indicating a thin irregularity layer in the equatorial E-region. Fresnel oscillations in amplitude and phase spectra are in quadrature. as expected from the phase screen theory of weak scintillations. (2) The first few Fresnel minima were used. together with a reasonable value for the effective height of the irregularity layer, to obtain estimates of the irregularity drift speed. I’,,. transverse to the signal path. Considering the thickness of the electrojet irregularity layer, estimated values of I’,, are expected to be accurate to within IO%. (3) During some daytime scintillation events, a low frequency peak can be identified in the power spectrum of phase scintillations. Such a peak has not been seen for nighttime scintillation events. This peak is attributed to dominant kilometre scale irregularities, which have been detected in both VHF radar interferometer data (KUDEKI r’t ~1.. 1982. 1987) and rocket data (PKAKASH and PAL. 1985; PFAFF CI al.. 1985. 1987). Approximate upper bounds for the westward electron drift velocity associated with these electrojet irregularities indicate that the gradient-drift instability operating on the background zero-order density gradient gives rise to the large scale irregularities. (4) Apart from the superimposed Fresnel oscillations, power spectra of daytime scintillations do not show much variation over :I frequency range that corresponds to irregularity wavelengths extending from a few hundred metres to the Fresnel dimension (2800 m). For shorter wavelengths, the power decreases according to a power law. The spectral index associated with this power-law behaviour, which is identical for amplitude and phase spectra, is found to vary between -2.4 and -3.6 for the data presented here. According to weak scintillation theory, this can be identified with the power-law index associated with a two-dimensional power spectrum of the irregularities. ~4cknon,l~~~~lyemrr2I---The author would fessor R. G. Rastogi for the scintillation

like to thank data.

Pro-

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