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Structures responsible for rapid fading of medium frequency radio reflections from the day-time E-layer
J~~ndofA~morphericad Printed in Great Britain.
TcrrestridPhysicr.
Vol. 46, No. 12. pp. 1179-I 191. 1984. 0
0021-9169,%453.00+ .JO 1984 Pergamon Press Ltd.
Structures responsible for rapid fading of medium frequency radio reflections from the day-time E-layer K. L.
JONES
Physics Department, University of Queensland, St. Lucia, Queensland 4067, Australia (Receioed
infinal form
14 June 1984)
A~rrct-ThesteerablebcamBribiclslandradar(152”E,27”S)operatingat afrcquencyof 1.98.MHzwasuscd toobtaindatarelevant toreflcctionconditionsnear 1OOkmaltitudeon7daysduringJunc-Qctobcr 1982.The rapid signal fading commonly observed is primarily due to transient reflectorswith lifetimesof a fewseconds, often seen up to angles of 20” from the zenith. Longer lived moving reflectors (presumed to bc sporadic-E clouds)alsoplayapart.Certainpropcrtiesofthe transient rcficctorsarcconsistcntwitha turbulentgeneration mechanism. However, any theory of their origin must explain why, for about a third of the time, they tend to occur preferentiallyto the north and cast of the observingsite. A direct comparison of velocities using Doppler and spaced antenna drifts methods shows reasonable agreement when the data is averaged over quarter hour periods. However, conclusions by previous workers, on the basis of observations of motions of diffraction patterns, that the ionospheric structure responsible for the diffraction pattern observed on the ground is undulations of the isoionic contours by gravity waves, is not supported by a detailed analysis of the data.
1. INTRODUCTION
Fading of medium frequency radio signals reflected from the lower E-layer near vertical incidence is observed on time scales ofseveral minutes (slow fading) down to several seconds (rapid fading). Rapid fading is observed most of the time. In this range of altitudes, the ionosphere is usually thought of as a rough reflector. References and discussion of early work on fading have been given by RATCLI~ (1956) in a classic paper on d&action theory and its application to the ionosphere. More recently, elforts have been made to investigate the structures responsible for fading of lower E-layer echoes by measuring angles of arrival or separating echoes by their dilfering Doppler shifts. These have been reviewed by JONES (1981). However, previous measurements of angles of arrival of signals from the lower E-layer have been limited to systems with just a few antennae. While this is sufficient, for example, to measure an off-vertical tilt of the layer if there is just one signal reflected, it is inadequate to investigate the situation if there are at the same time several signals from quite different directions. In principle, simultaneous signals from different directions may be distinguished using a few antennae by Doppler sorting (WHUEHEADand MONRO, 1975; WHITEHEADet al., 1983). However, this is not feasible for the type ofshortlived echoes observed from the lower E-layer (JONES, 1981). A detailed study of the structures responsible for fading requires the application of a large aperture antenna, so that sufficient angular resolution may be obtained at wavelengths of about 150 m. In a previous paper (JONES, 1981), a radio system was described based
on a one kilometre aperture cross array operating at a frequency of 1.98 MHz and steerable to 40” from the zenith. In that paper, some examples of data analysis were given, but were only short examples of several minutes duration. Those examples may not have been typical. The purpose of this paper is to report on the analysis ofdata from several days of observations of the lower E-layer. During the data analysis it became evident that the conclusions being drawn might be at variance with some drawn from the observation of diffraction patterns using the 1.98 MHz Buckland Park array at Adelaide (35’S, 139”E), (FELGATEand GOLLEY,1971; VINCENT,1972). It therefore seemed desirable to modify the Bribie Island experiment so that data could be collected simultaneously using both the Doppler scanning technique and the spaced antenna technique. The results of this comparison are reported in Section 3.10. It should be emphasized that the investigation reported herein refers to the virtual range 95-120 km. Most of the data analysed come from a virtual range of 95-l 10 km. The data all come from total reflections of 1.98 MHz transmission from the day-time E-layer. Conditions when partial reflections from the upper Dlayer could be mixed with the E-layer signals have been avoided.
2. THE DATA
AND
METHOD
OF ANALYSIS
Data have been analysed for a total of 42 h of recording spread over 7 days in the period June-
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1180
K. L. JON=
October 1982. The 1.98 MHz beam was steered in a raster pattern for 11 x 11 directions, covering an angular range to 40” from the zenith. A raster was completed once per second. In each direction the returning signal was sampled at 2 km intervals in the virtual range 88-128 km. For each direction the controlling computer found the amplitude, phase and range ofthestrongest signa1. Only that information was recorded. In this way, observations could be made continuously for about 112 min before the computer disk was full. In this mode of operation the experiment is not adequate to deal with situations where the returned echoes are spread in range for some direction. The data have been analysed by the method described in JONES (X981) as ‘fitting polar diagrams to the data’. There are two stages in the procedure. Firstly, the directions, amplitude, phase and range of various echoes are calculated for each one second raster. Up to five different echoes are allowed. In a second stage of analysis the output of the first stage is processed to find continuous signals. A signal is regarded as continuous from one raster to the next if its direction stays fixed to within two increments ofangie(about 2.5”). We refer to a time series of continuous signals at one second intervals as a sequence. The processed data consist of sets of amplitude, phase, range and direction at one second intervals, labelled as sequences. It is not a priori evident that these sequences are really one second
samples of echoes from one particular physical structure. However, it is almost invariably found that the phase of the signal within a sequence varies in a continuous manner, rather than randomly from one second to the next. Examples are shown in Figs. 2 and 3. Thus it appears that thesequencesdorepresent the time durations of echoes from identifiable structures.
The results thus produced constitute the basic data discussed in this paper. The processing to this stage takes about four times as long as it took to collect the original data, using a computer similar to the one that controls the experiment, The experiment cannot therefore be run inde~nitely with reai time dataanalysis using the current PDP 1l/34 computer. 3. A GENERAL DESCRIPTION
OF THE RESULTS
The results all refer to the day-time E-layer from virtuaf heights between 95 and 120 km. The layer is usually disturbed, i.e. it departs in form from a plane horizontally stratified reflector. There are usually sequences of echoes from off-vertical angles of up to 20”. The sequences usually last for only a few seconds. Only one 3-h recording period (1400-1700 local time on 1October 1982) did not show off-vertical reflections at greater than about 1”. They were thus present for about 909~ of the time for which data were analysed. The form of variation of amplitude and phase of reflected signals during disturbed conditions is very complex. Nevertheless, certain degrees of disturbance can be recognized. 3.1. Least disturbed conditions Figure 1 displays a sample of the amplitude and phase variation of the signal under conditions of minimal disturbance. Only one reflected signal is resolved. It comes from within one degree of the zenith. The disturbance consists of amplitude fading without deep minima on a one minute time scare. These conditions appear similar to those described in VINCEXT(1972 ; Fig. 3).
Fig. 1. Variation oftheamplitudeand phaseofa reflected signal under conditions ofminimal disturbance. One signal only is detected and it comes from within about lo of the zenith.
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Rapid fading of medium frequency radio reflections 3.2. Moderately disturbed conditions
Figure 2 displays a sample of the variations of amplitude, phase and direction of arrival under conditions that may he described as moderately disturbed. The predominant reflection comes from close to the zenith. but thereare also weaker and shorter lived reflections from off-vertical angles. Thiscondition occurred for about SOo/,of the time in the 42 h of data analysed. 3.3. Very d~tur~d conditions Figure 3 displays data under conditions described as very disturbed. There is no longer a predominant signal from near the zenith. Such conditions prevailed for about 40% of the time for which data were analysed. 3.4. Zenith angle distribution ofoccurrence of repections It is of interest to study the distribution of reflected signals as a function of ot%vertical angle. Typical distributions over 2-h recording periods are shown in Fig. 4 for what has been described as moderately disturbed and very disturbed conditions. The open circles represent the relative probability of occurrence as a function of off-vertical angle. The closed circles
TRANSMlTtPR
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represent the relative probability of occurrence per unit solid angle. For the ‘more disturbed’ conditions, the ma~mum in probability of occurrence moves to greater zenith angles. The probability of occurrence per unit solid angle generally decreases monotonically with zenith angle. 3.5. Duration of continuous signals Continuity of a signal can be detected for periods as short as 1 s (reflection seen onjust two consecutive data scans} and for periods as long as about 2 h (the maximum time for which the experiment can run continuously). Figure 5 shows the relative probability of occurrence of continuous signals as a function of their duration for a moderately disturbed (a) and a very disturbed (b) 2-h period. Note that an integrated probability of durations of 40 s and greater is also plotted, as well as the mean probability ofaduration in the range 21-39 s. For moderately disturbed conditions, the probabitity of durations of 40 s and greater is relatively large, corresponding to the existence of relatively long-lived echoes from near the zenith. Under moderately disturbed conditions, a maximum in relative probability occurs for durations
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Fig. 2. Variation oftheamplitudc,phaseandanglcsoCa~valofrefl~tedsignalsund~conditionsofhrodcrate disturbance’. Fromtbeinterruplaibut persistentsignalfromM”N,the prcscnccof a sporadic-l?cloud as well
as ‘turbuicnttype’scattersis suspected.
K. L. JONES
1182
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Fig. 4. (a) Zenith angle variations of redected signal occurrence under ‘moderately disturbed conditions (as in Fig. 2). 0 Zenith angle variation; l normahi by cosecant (zenith angle) to represent the relative probability per unit solid angle. (11 June 1982.12.28-14.20.) (b) As for Fig. 4a, but under ‘very disturbed’ conditions. (16 August 1982, 13.41-15.33.)
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Fig. 5. The relative probability of occurrence of reflections as a function of their duration. (a) ‘Moderately disturbed conditions; (b) ‘very disturbed’ conditions.
Rapid fading of medium frequency radio reflecrions ofseveralseconds(in thiscase4). Incontrast, under very disturbed conditions, the relative probability of occurrencedecreases monotoni~lIy ~a function ofthe duration of an echo. It should be stated that problems remain in studying the data for duration of continuous signals. These problems arise from imperfect knowledge of the antenna polar diagram. That the polar diagram is imperfectly known (particularly the minor sidelobes at great angles) is made evident by considering the results of data processing for a quiet period of time when there is but one signal from near the zenith. The data processing also produces weaker signals at large angles which have amplitude and phase variations similar to the strong signal from near the zenith. These are evidently sideiobe responses. When the data are further processed to investigate some particular property, a sidelobe rejecting algorithm is included to reject as many of the spurious sidelobe responses as possible. The algorithm compares the total phase change through a sequence with the corresponding phase change through sequences whose amplitudes are greater than it at all times. Ifthe magnitude of the phase changes differ by less than 40”, the weaker sequence is rejected. The principle behind the algorithm is that the spurious sidelobe responses occur predominantly at large angles. from which directions there shoutd be relatively large phase shifts because the atmosphere has a horizontal velocity component of tens ofmetres per second. The 40’ phase change criterion was shown by trial to give a very high rejection of spurious sidelobe responses, judged from the simple case where there is just one signal from near the zenith. The particular point under discussion here is that the algorithm may also reject true sequences which just happen to have phase variations similar to sequences of larger amplitude. The probability of this occurring increases as the sequence duration decreases, because the total phase change becomes less. Another limitation imposed on the data analysis by an imperfect knowledge of the antenna polar diagram is a possible failure to detect the continuity of a relatively weak and fading signal in the presence ofa stronger one. This may break up the weaker signal into shorter sequences than would be the case if the antenna polar diagram were perfectly known. 3.6. Fossibie Fhysical models of the distuT~~aJ~ces (a) Possible role ojgracity waves. It is most unlikely that a spectrum of internal gravity waves (NIXES,1960) is directly responsible for the short-lived reflections from of&vertical angles ofseveral degrees, as illustrated in Fig. 3. Gravity waves have a horizontal scale of tens ofkilometres and a time scale ofseveral minutes. While
1183
interference between reflections could produce strong amplitude fading if two reflection points were so close as not to be resolved by the beam, it would be expected that the signal would persist (though appearing and disappearing) from the same direction for some minutes. It should be possible sometimes to track waves for long periods (say 30 min) and watch a reflection point cross the observer’s field of view, Furthermore, if 2O”off-vertical echoes were due to largescale distortion of the isoionic surfaces, there should be observed quasiperiodic variations in the virtual height of the echoes, which would be easily detectable, even with a range sampling interval of 2 km. An amplitude of the order of 5 km would be predicted. No such variations in virtual height are observed. When height (rather than range) of the various echoes is plotted as a function of time it appears that there is no large scale deviation from horizontal stratification. (There must of course be deviations on some scale in order to get off-vertical reflections). On the other hand, all these expected features of gravity waves are observed in the F-layer. They are seen, for example, at night, when 1.98 MHz is reflected from the F-layer. They are also commonly observed in the day-time f-layer using higher frequencies. A study of these is being actively pursued by other workers with this equipment. The slow fading, on a time scale of minutes, of the type illustrated in Fig. 1 may be a direct effect of gravity waves distorting the isoionicsurfaces. Thosedata have been interpolated to find the direction of arrival to one hundredth of a degree in computed accuracy. If the resultinganglesareplottedasisdoneforthedatain Fig. 2, but on a much finer scaie,one observes fluctuations of directions of arrival of amplitude less than one degree, but on a time scale of minutes(i.e. on a time scale similar to the amplitude fluctuations). This is probably a measure of the tilts in isoionic surfaces produced at 100 km altitude by gravity waves. The sameconclusion for similar conditions has previously been reached by VIXSCENT
(1972).
(b) Possible rule of rurbulence. Turbulence in the neutral air, with consequent displacement ofplasma, is a possible mechanism for roughening the isoionic surhces. This possibility has been previously mentioned by VISCEXT(1972).The application ofthe theory of homogeneous turbulence to radio wave scattering has been discussed by RASTXXIand BowHrLr_(197&b). (Rastogi and Bowhill were investigating weak VHF scatter from the mesosphere.) In the theory, the energy dissipation rate per unit mass E,the kinematic viscosity \’ and a characteristic time t are related by Y E-t2 .
K. L.
1184
The shortest spatial scale, below which viscosity damps motion heavily, is expected to be the Kolmogorov microscale
REESet nl. (1972) have studied turbulence in the Eregion by observing the breakup of vapour trails released by rockets. They identify the characteristic time c previously mentioned withthe time for the onset of turbulence to be observed in the trail. They give typical values at 100 km, based on their observations, of s=O.O66Wkg-‘,
t = 21 s,
q= 145 m.
Thus there may be some irregularities smaller than a Fresnel zone in size, and indeed spatial scales right down to the radio wavelength. (See also RIND,1977, for a summary of turbulence parameters.) The scattering properties of irregularities are strongly influenced by their shape. in the (linear) theory of waves in neutral air (HINES, 1960) it is found that propagation of acoustic waves of frequency much greater than the Brunt-VCiilii frequency is little affected by gravity, while fo! the large scale gravity
-
OBSERVED POLAR DIAGRAM
oo-0
CAVERN
JONES
wgves the propagation is very anisotropic, with horizontal scales b@M much greater than vertical scales. ~though turbulence is a more compli~t~ phenomenon, nevertheless, it is to be expected that the very smallest scale eddies of dimensions of a few hundred metres will be roughly isotropic, while larger scale eddies of several kilometres in size will be much more horizontally stratified. Indeed, REESet al. (1972) quote other vapour trail observations to support the view that eddies of less than 1 km dimension are isotropic. Snudlscale isotropicscatterers. A model ofsmall scale (compared to the Fresnel zone width), and therefore isotropic, scatterers may be used to explain why, under disturbed conditions, the data consists of short-tived sequences appearing randomly at quite large offvertical angles. The isotropic nature of the scatterers allows backscatter of radio waves coming from a wide range of angles. If the angular scale of the scatterers as viewed from the radar is small compared to the radio beamwidth, they wili not be individually resolved. The apparent angular spectrum of the signal at any time is the true spectrum convolved with the polar diagram of the antenna (as discussed by BRICGS,1980). Figure 6
OF FGtAR DIIIGRAM WITM A RANGGY ANGULAR SPECTRUM (16 PER FRESNEL ZONE!
ZENITH ANGLE RXGREES)
Fig. 6. l The receiving antenna polar diagram; 0 the convolution of this polar diagram with an angular spectrum random in amplitude and phase on a scale of l/l&h of a Fresnel zone.
1185
Rapid fading of medium frequency radio reflections
illustrates the results of a computer simulation (in one dimension) of the polar diagram of the receiving antenna convoIved with an anguIar spectrum which varies randomly in amplitude and phase on a scale of one-sixteenth of a Fresnel zone. The result looks like two discrete plane reflectors. This is the kind of angular spectrum observed. Can this model explain the short life of the reflections? The lifetime should depend on how long the echoes from a number of small scatterers each movingindependently will remain in phase. It thus depends on their relative velocities. COLE (1962) discusses a dimensional argument by Kolmogorov and Obukhov to the effect that for isotropic turbulence, the velocity variation over a small distance d is given by u = (&ri)~~3. Putting E = 0.066 W kg- ’ and da few hundred metres gives u z 4 m s-l. A relative motion of a quarter wavelength or %40 m is required to pass from constructive to destructive interference between reflections. This suggests a ‘lifetime of the order of 10 s, in reasonable agreement with the present observations. It may be noted that the data OfREEser al. (1972) on the rate of expansion of vapour traits also suggests velocities of a few metres per second. .. The small scale isotropic reflector model encounters dimculties, however, in explaining the decrease in the probability of occurrence of reflections with increasing zenith angle, as discussed in Section 3.4. Why would such reflectors not produce detectableechoes from. say, 30” zenith angle? Possible reasons are distance attenuation and absorption. (if Distance ~r~e~~urjo~. If, as seems to be the case. the isoionic surfaces remain approximately horizontally stratified at height /I, say, then the range r to the scatterers at zenith angle 8 is
reflected at 30’ from the zenith may be computed to be orie-third of that reflected along a vertical path. Such a signal is unlikely to be missed in the data analysis. Anisotropic scatterers. The foregoing discussion implies that it is unlikely that the scatterers are completely isotropic. This implies that they have a scale greater than a few hundred metres. These scales of turbulence will be cpnstrained by gravity to be approximately horizontally stratified. We therefore consider a different model, in which the scatterers are anisotropic and the duration of the signals is determined not by interference of many scatterers, but by the time for which single scatterers remain correctly oriented to produce strong reflection. The probability of receiving a reflection will thus be a decreasing function of zenith angle. Consider now the limit in which the reflection is dominated by just one turbulent eddy. What might the ‘iifetime’of a reflection from such an eddy be? As a very simple model, consider a plane reflecting surface rotating about a fixed axis like a rigid body at an angular rate de -=dr
u t’
where u is the turbuldnt velocity discussed aboveand 1is the scale size. ifthe radio wavelength is i. and 12 3A,the range of angles over which the anisotropic scatterer reflects radio waves is
Thus the time to rotate the range of angles A@past the observer is
r = h see 8. The received scattered amplitude from an individual reflector will vary as r-’ (BRICOSand VINCENT.1973). Thus, the relative amplitude from 30’, as compared to the zenith, may be ZZXOS-*30”, or 0.75. This wiI1 not explain the absence ofsignals from 30” zenith angle. (ii) Absorption. Absorption along the extra path langth at off-vertical angies will decrease the amplitude as the zenith angle increases. Absorption measurements have been made by calibrating the equipment at night using first and second hop reflections. The day-time absorption at 1.98 MHz tends to be in the range 20-60 db, depending on solar zenith angle and solar disturbance. (Much higher values of absorption are occasionally observed during strong solar flares.) Consider an approximately horizontally stratified ionosphere in which the total absorption at vertical incidence is 60 db. The amplitude of a signal totally
Using i. = ljOmandwintherange4-8ms-‘(for/in the range 1-8 km) gives r 55 20-10 s. These times are greater than those often observed. but the discrepancy is not too serious considering the approximations involved. In the foregoing discussion it has been implicitly assumed that the only factor modifying electron concentrations is transport arising from collisions between ionized particles and the neutral air and so the isoionic contour also moves with the neutral air. The effects of chemitai r~ombination and diffusion have been neglected. These assumptions can be justified at altitudes near 100 km by consideration of the time scales involved. The characteristic recombination time in the E-layer is (2&J)- ‘. where z is the recombination coefficient and N the electron concentration. Times of
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K. L.
JONES
the order of 600 s are typical (RISHBETHand GARRIOI-~, 1969a). The characteristic time for molecular diffusion is X2/D, where H is thescale height and D the ambipolar diffusion coemcient D=2kT. mv Taking values for T, the gas temperature, m, the mean molecular mass, and v, the ion neutral collision frequency, relevant to 100 km altitude from FISHBETH and GARRIOT (1969b), the characteristic time for molecular diffusion is typically IO6 s. Thus recombination and molecular diffusion can be neglected on time scales less than 50 s, which are typical of the ‘turbulent type’ data. 3.7. Obserwd echoesfrom long lasting discrete reflectors Another class of disturbance phenomenon, very different from thosedescribed in Sections 3.2. and 3.3.. is illustrated in Fig. ,7. ,For about an hour late in the morning of 1 October 1982, discrete moving reflectors were observed, sometimes lasting long enough for changes in position of the reflectors to become apparent, i.e. many minutes. Another was observed in a quiet period on 31 October 1983,in thedatacollected to compare Doppler and spaced antenna drift observations. This type of disturbance was significantly
40” w
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different from the more irregular types discussed in Sections 3.2. and 3.3. in respect of its height of occurrence. The more irregular types are compatible with the assumption that the isoionic contour corresponding to a plasma frequency of 1.98 MHz is continuous, quasi-horizontally stratified, but somewhat distorted. However, both occurrences of the phenomenon under discussion were at heights about 6 km lower than the height of re+ction of the signal from near the vertical. The form of the data is suggestive of a blob of ionization distinct and separate from the nearly horizontally stratified 1.98 MHz isoionicsurface above it. Thesearepresumably movingpatchesofsporadic-E. These are sometimes suspected of occurring during periods of short time scale disturbances (there is an example in Fig. 2), but the ‘turbulence type’ irregularities obscure them. Properties of sporadic-E have recently been investigated using a higher frequency version of this experiment (FROM, 1983). These moving patches have properties similar to the ones observed by FINDLAY (1953) in phase path experiments. 3.8. Doppler shifts and winds The phase shifts observed on the short-lived 05 vertical reflections may be interpreted as Doppler shifts due to horizontal motion and a corresponding
ANGLE (EAST - WEST)
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10.52 LOCAL
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1982
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Variation of long lived discrete reflections (presumably sporadic-E clouds).
10.53
Rapid fading of medium frequency radio reflections
115 km, but mostly near 100 km. Velocities were in the range30-60msW’. There was no common pattern of velocity variation on the different days. One day (18 August) the direction changed by about 180” (from south-east through south to north-west) in about 6 h. as though a semidiurnal effect predominated. The data slightly overlaps in height range that which can be used by the partial reflection drifts technique (VINCENTand Sruens, 1977). As a technique for observing horizontal motions it suffers in comparison from two defects. Firstly, data at only one height is obtained and this height is liable to change with time during the day. Secondly, since total reflection is used, the apparent height of reflection measured may be markedly different from the true height.
horizontal velocity component calculated L Ad, PC-----4r sin 8 At ’ where a phase shift AQ is observed in a time At at a zenith angle 0, the radio wavelength being i.. If there is a large number of such observations distributed in azimuth, a plot of this horizontal velocity component against azimuth will be sinusoidal in form if all the scatterers are moving with a common horizontal velocity. For this purpose the data were divided into approximately 15 min intervals and plots of the form shown in Fig. 8 were prepared for each. A least square fit of a sinusoid to the points was performed to find the magnitude and azimuth of the velocity. In Fig. 8, the maximum negative Doppler shift gives the direction as receding reflectors give negative Doppler shifts. The scatter of points in Fig. 8 will be partly due to the random velocities associated with turbulent motions, but also to errors in computing the zenith angle B of the echo, particularly when 0 is only a few degrees. The data comes from 7 isolated days in the period June-October 1982 and from virtual heights of 95-
BEST FIT VELOCITY
so-
.
. . ..
.
64
1187
3.9. Azimuthal dependence of irregularity occurrence It is of interest to investigate whether the probability of occurrence of irregular reflectors is uniform across the field of view. The dependence on zenith angle has been discussed already (Section 3.4.). Plots of the form of Fig. 8 are useful for searching for any azimuthal dependence. An examination of about 150 such plots (each covering about 15 min) showed that in two-thirds
AZIMUTH
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11 JUNE 1962
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horizontal
.
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AZIMUTH @EST FIT VELOCITY
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velocity against azimuth.
100
1188
K. L. JONES
of cases there was a uniform distribution of occurrence as a function of azimuth. The remaining third showed marked non-uniformity. The di$ibutions of preferred azimuths of occurrence in 8 points of the compass (as judged subjectively from plots of Fig. 8 type) are shown in Fig. 9. There is a clear preference for the north and east sectors. Turbulence in the neutral air may be generated at these altitudes by a fortuitous wncurrence of gravity waves and wind shear (BRETHERTCJN, 1969; JONESand HOUGHTON,1971; GELLERet al., 1975). If so, it is possible that marked variations in generation of irregularities could occur on a horizontal scale oftens of kilometres. However, the fact that the preferred directions of occurrence group so strongly in the north and east implies that there must be some aspect sensitive property ofthe reflectors, since it can hardly be supposed that sb many would form in that sector by chance. Furthermore, the 15min periods in which there is a preferred azimuth of reflections are not randomly distributed in time. There are periods of up to lf h for which there is a particular preferred range of azimuths. No other properties (such as zenith angle dependence) have been discovered which distinguish between the aspect sensitive and non-aspect sensitive reflectors. This is a puzzling phenomenon for which there is at present no explanation. 3.10. Comparison o/results with those/rom the Buckland Park arrq The Buckland Park array at Adelaide(35”S 139”E) is of similar size to the Bribie Island array and also N
22
12 E
54 cases of preferred
directions
101 cases uniformly distributed
Fig. 9. Number
of occurrences of preferred azimuth of echoes (in I5 min time divisions).
operates on a fr&ptency of 1.98 MHz (BRIGGSet al., 1969). A comparison of results from Bribie Island and Buckland Park is therefore of particular interest. The Buckland Park array has been used principally for measuring ionospheric drifts using the spaced antenna drift method (FELGATEand GOLLEY,1971; VINCENT, 1972). However, these workers have attempted to draw conclusions as to the nature of the irregularities in the E-layer causing the diffraction patterns whose velocities they measure. They have suggested a gravity wave interpretation of their data. This conclusion is at variance with what was decided in Section 3.6.(a)above. It therefore seemed desirable to operate the Bribie Island array in the spaced antennadrift mode, as well as its normal mode, to sec. whether this conflict could be resolved. It is not possible to operate the Bribie Island array in precisely the same mode as the Buckland Park array. Buckland Park has a filled-in receiving array with one wide-angle transmitter, while Bribie Island has one (north-south) linear receiving array and one (eastwest) linear transmitting array. The Bribie Island array was modified to operate as follows. (‘Modifying’ the array means modifying a computer program.) At the end of each directional scan (once per second) the following sequence of data sampling was performed. First, just one of the transmitters was left switched on and the data from the 10 north-south receiving antennae were sampled in turn. Then, one receiving antenna was used to sample the signal when each of the 10 transmitters in theeast-west transmitting array was switched on in turn. This extra data sampling took about 100 ms. Because of extra switching times, it was found that the full 11 x 11 directional raster together with the spaced antenna data sampling could not be carried out once per second. Therefore, the directional raster was reduced to 9 x 9 (steering to about 30” from the zenith). This would make negligible difference to the results, since ionospheric echoes are rarely if ever observed at zenith angles greater than 30’. The spaced antenna data were analysed to measure the apparent drift velocity of the diffraction pattern as defined in BRrc%setal.(1950).Thedetailedvariation ofamplitude and phase signals for each antenna was plotted for detailed examination. The directional raster data were also analysed in the usual manner, to study the directions and durations of echoes. In the discussion that follows, all the velocities quoted are apparent drift velocities of the dilfraction pattern unless otherwise stated. The data analysed was for a 2-h period (14.00-16.00) on 31 October 1983.Asfaras thecharacter ofthedatais concerned, it fortuitously fell into two nearly equal sections. In the first hour the E-layer was moderately
Rapid fading of medium frequency radio reflections
disturbed, with two or three off-vertical echoes, as well as a strong echo from the zenith. The scale of the diffraction pattern on the ground varied from less than 100 m to about 300 m maximum. (The scale of the diffraction pattern here is defined as the distance along the ground for the zero delay cross-correlation of amplitudes to fall to 0.5.) The fading period of the signal amplitude was S-10 s. In the second hour the E-layer was little disturbed, conditions being as described in Section 3.1. In this period of time the scale of the diffraction pattern was much larger than the dimensions of the array (1053 m). The data analysis is concentrated on the disturbed period. Data were taken at 1 s intervals. The analysis of the data for the disturbed period, 14.00-15.00 on 31 October 1983, is now discussed. Consider, first, the diffraction pattern data. As a preliminary analysis, for both the receiving and transmitting array, an antenna at one end of the array was used as a reference and auto- or cross-correlations as relevant were calculated for all 10 antennae at time delays from - 20 s to + 20 s. For each of the 9 antenna spacings (110-1053 m) a maximum in the crosscorrelation as a function of time delay was sought. Three point interpolation in time was performed by fitting a quadratic function to the maximum and the point on either side. This improved the accuracy of the estimation of the delay. It was noted that the crosscorrelation as a function of time delay usually showed maxima spaced several seconds apart. Even the autocorrelation as a function of time delay often displayed maxima several seconds apart. This reflects the quasiperiodic variation ofsignal amplitude during disturbed conditions. When the antenna spacing becomes greater than the scale of the diffraction pattern on the ground, these latter maxima often predominate and the maximum indicating the velocity of the diffraction pattern becomes difficult or impossible to locate. On the other hand, using as great an antenna spacing as possible improves the accuracy of measuring the time delay. Thus, to apply the spaced antenna method in all conditions of ionospheric disturbance one requires a number ofantennae with spacingscovering the range of scales of diffraction patterns. This result reinforces the view expressed in WHITEHEADet al. (1983) that while the Doppler and spaced antenna methods of measuring velocity are equivalent, one cannot achieve as useful a result by using one method with just a few antennae or small aperture as compared to using the other with many antennae spread over a greater aperture. On the basis of the preliminary analysis, pairs of antennae were chosen, straddling the centre of the array, to give the maximum possible antenna spacing that was less than the scale of the diffraction pattern.
1189
The data from these antenna pairs was used to calculate the time delays in the north-south and east-west directions for maximum amplitude correlation. The data were analysed on a time scale of 1 min to search for changes in the apparent drift velocity (as defined by BRICKSet al., 1950) on a time scale less than the gravity wave time scale. The magnitudes and directions so calculated are shown in Fig. 10. Since the directions are often approximately constant over several minutes and the amplitudes at the individual antennae are quasiperiodic, as discussed before, the result appears compatible with the FELGATE and GOLLEY (1971) observation that sets of interference fringes appear to last for a few minutes. The degree of agreement between the calculated Doppler velocity and the apparent drift velocity is shown in Table 1.The l-h disturbed period was divided into four approximately f-h periods and these two velocities calculated. The directions are azimuths measured in degrees from the south toward east. The magnitudes are in metres per second. As expected, the measured velocity of the dilfraction pattern is approximately twice that of the reflecting surfaces as indicated by the Doppler velocity. When the results of data analysis to find the occurrence and direction of signals are plotted for the period of time used to prepare Fig. 10, the result is somewhat similar to Fig. 2. Most of the time, the strongest signal comes from near the vertical. It shows deep amplitude fading and a slow phase variation compared to the (usually two or three) transient echoes which are confined to the north-east quadrant in azimuth. The diffraction pattern on the ground in this case is formed by interference between the signal from the vertical and these two or three transient signals, which come from one quadrant and apparently have similar velocities and thus produce(as is observed much of the time) quasi-periodic amplitude fading. The results discussed above are consistent with the conclusions of FELCATEand GOLLEY(1971) that the diffraction pattern on the ground is due to interference between just a few reflections. Though a complete twodimensional dilfraction pattern was not recorded for this data. it is consistent with Felgate and Galley’s observation that fringes from one distinct set appear to last - 3 min. This could happen even if the lifetime of the individual off-vertical echoes was considerably less than 3 min, since they come from much the same direction and share a common velocity. The velocity of the diffraction pattern measured here is about 100 m s- II which implies an ionospheric velocity of the order of 50 m s-l, in agreement with the results of Golley and Felgate. These investigators further remark (p. 1368) that in
K. L.
JONEs
.
.
SO
I
40
14.20
I
1
,
1
14.30
14.40
1450
1500
LOCAL
EAST
2 . i
I
TIME
Fig. 10.Time variation of the apparent velocity ofdiffraction patterns. l Magnitude in m s-’ ; 0 azimuth in degrees measured from south toward east.
the several minutes that a particular set of fringes appears to last, the reflecting region must move several kilometres. Now what was actually observed in the present work was that the reflecting region appeared to move nowhere,even though it had an apparent velocity of 50 m s-i. For about an hour the transient echoes came from a particular region of the sky (north-east quadrant out to about 20’ from the zenith), with no suggestion of systematic change in direction. From the extensive data analysis discussed in Section 3.9., it is known that this restriction of signal directions to that particular region occurs frequently and since the observing site is in no particularly priviliged position, it is implausible to suppose that the reflectors just happened to be confined to that region on this occasion. Thedatasuggestamotionofthereflectingsurfaceswith
Table 1
Doppler velocity
Apparent drift velocity
Magnitude
Direction
Magnitude
Direction
39 46 65 47
5 24 46 54
95 106 147 169
-8 -2 31 54
a velocityof50m s- ‘.Inaddition, theisoioniccontours must be distorted on a scale considerably greater than the radio wavelength so the distortions can be anisotropic. Although these distortions occur with the same mean probability everywhere, their shape must give rise to an aspect-sensitive property. It will be recalled that in Section 3.6.(b) it was deduced, independently from the zenith angle variation of the probability of echo occurrence, that the scatterers are anisotropic and therefore several radio wavelengths in extent. FELGATEand GOLLEY (1971) (also VINCENT. 1972) further suggest that, since the mean lifetime of a set of fringes appears to be about 3 min, this is long enough to hypothesize a gravity wave origin of the off-vertical reflections. Felgate and Golley also pointed out (p. 1368) that if the phase, as well as amplitude, of the diffraction pattern wasrecorded, thedirection ofarrival of the different downcoming reflected waves could be measured and the validity of various models confirmed or otherwise. This in elfect is just what is done with the Bribie Island steerable beam radar, though not in precisely the way Felgate and Golley envisaged. When this is done. the gravity wave hypothesis is not confirmed. There are three signatures to be expected from reflections caused by undulation of an isoionic surface by gravity waves. They are a continuity for several minutes, a systematic change in the direction of
Rapid fading of medium frequency radio relleclions
the echo as the structure moves and a systematic (undulatory) variation in the apparent height of reelection of the echo. There is no suggestion of any of these signatures in the Bribie Island data. Another aspect of the results 10 be compared between the Bribie Island and Adelaide data is the fraction of the time for which there is very little disturbance (see Section 3.1. and Fig. 1). Figure 1 is to be compared with VINCENT(1972 ; Fig. 3). Vincent’s discussion could be taken to imply that these conditions occur for something approaching 50% of the time, as compared to 10% at Bribie Island. However, FELGATE and GOLLEY(1971; Table 1, p. 1357) report that the scale of the diffraction pattern is greater than the scale of the array for about 127, of the time for normal E-region echoes. This is comparable with the Bribie Island result. It is thus concluded that ther.e is no compelling evidence for a significant diflerence between observations at the two sites.
1191
E-layer in the height range 95-l 15 km. About 10% of the time the reflecting layer departs very little from horizontal stratification, giving rise to just one reflection from close to the zenith. At these times there are occasionally also discrete off-vertical reflections lasting a few minutes, which are identified as isolated sporadic-E clouds. .About 907; ofthe time there appear to be present transient reflectors, with lifetimes ofa few seconds,seen too&vertical angles of20” or more. These do not appear to be a direct manifestation of gravity waves producing undulations in the isoionic surfaces. Nor can they be accounted for by a large number of small isotropic scatterers. A possibility has been discussed that turbulence might produce the required transient departures from horizontal stratification. This hypothesis must remain tentative. Some aspects of the observations still require explanation. In particular. there is the tendency for reflections to be commonest in the north and east, an effect sometimes, but not always, present.
4. SUMhlARY
About 40 h of data taken on 7 days has been used to study the structure responsible for rapid fading of medium frequency radio waves reflected from the lower
Acknowledgements-This work issupported by the Australian Research Grants Scheme. The author thanks Professor J. D. WHITEHEAD for useful comments and Dr B. H. BRIGGS for editorial assistance.
REFERENCES
BRETHERTON F. P. BRIGGSB. H. BRIGGSB. H., PHILLIPSG. J. and SHINN D. H.
BRIGGSB. H., ELFORDW. G., FELGATED. G.. GOLLEYM. G., RO~SITER D. G. and SMITH J. W. BRIGGS B. H. and VIE~CENT R. A. COLE G. H. A.
FELC~ATE D. G. and GOLLEYM. G. FINDLAYJ. W. FROMW. R. GELLERM. A., TANAKAH. and FRITTSD. C. HINESC. 0. JONESK. L. JON= W. L. and HOUGHTOXD. D. RATCLIFFE J. A. RATCLIFFE J. A. REEKD., ROPERR. G., LLOYDK. H. and Low C. H R~~nw;l P. K. and BO~HILL S. A. RAS~~;I P. K. and BOWHILLS. A. RIND D. RISHBETHH. and GARRIOTT0. K.
1969 1980 1950 1969
Q. J1 R. met. Soz. 95, 213. J. atmos. terr. Phys. 42, 823. Proc. phys. Sol. Land. B63, 106. Nature, Lond. 223, 1321.
1973 1962 1971 1953 1983 1975 1960 1981 1971 1948 1956 1972 1976a
Aust. J. Phys. 26. 805. Fluid Dynamic:. p. 200, Methuen. J. atmos. terr. PhJs. 33, 1353. J. atmos. rerr. Phys. 3, 73. Planet. Space SC%31, 1397. J. utmos. Sri. 32. 2125. Can. J. Whys. 28, 1441. 1. atmos. ferr. Phys. 43, 1015. J. atmos. Sci. 28.604. Nature 162,9 Rep. Prog. Ph.s. 19, 188. Phil. Trak. R: Sot. A271, 63 1. J. atmos. terr. Phvs. 38.399. J. atmos. terr. Phrs. 38; 449. J. armos. terr. Phi,s. 39,445. Introducrion I,, fonospheric Physics,
1976b 1977 1969a
p. 166. Academic
Press. RISHBETH H. and GARRIOTT0. K.
1969b
fntroduction
III ionospheric Physics,
Press. VINCENTR. A. VINCENTR. A. and STU~BST. J. WHITEHEAU J. D. and MONROP. E. WHITEHEAD J. D., FROMW. R.. JONESK. L. and MONROP. E.
1972 1977 1975 1983
J. armos. terr. Planet. Space J. utmos. terr. J. atmos. terr.
Phys. 34, 1881. Sci. 25,44 1. Phys. 37, 1427. Phys. 45, 345.
p. 131. Academic
TcrrestridPhysicr.
Vol. 46, No. 12. pp. 1179-I 191. 1984. 0
0021-9169,%453.00+ .JO 1984 Pergamon Press Ltd.
Structures responsible for rapid fading of medium frequency radio reflections from the day-time E-layer K. L.
JONES
Physics Department, University of Queensland, St. Lucia, Queensland 4067, Australia (Receioed
infinal form
14 June 1984)
A~rrct-ThesteerablebcamBribiclslandradar(152”E,27”S)operatingat afrcquencyof 1.98.MHzwasuscd toobtaindatarelevant toreflcctionconditionsnear 1OOkmaltitudeon7daysduringJunc-Qctobcr 1982.The rapid signal fading commonly observed is primarily due to transient reflectorswith lifetimesof a fewseconds, often seen up to angles of 20” from the zenith. Longer lived moving reflectors (presumed to bc sporadic-E clouds)alsoplayapart.Certainpropcrtiesofthe transient rcficctorsarcconsistcntwitha turbulentgeneration mechanism. However, any theory of their origin must explain why, for about a third of the time, they tend to occur preferentiallyto the north and cast of the observingsite. A direct comparison of velocities using Doppler and spaced antenna drifts methods shows reasonable agreement when the data is averaged over quarter hour periods. However, conclusions by previous workers, on the basis of observations of motions of diffraction patterns, that the ionospheric structure responsible for the diffraction pattern observed on the ground is undulations of the isoionic contours by gravity waves, is not supported by a detailed analysis of the data.
1. INTRODUCTION
Fading of medium frequency radio signals reflected from the lower E-layer near vertical incidence is observed on time scales ofseveral minutes (slow fading) down to several seconds (rapid fading). Rapid fading is observed most of the time. In this range of altitudes, the ionosphere is usually thought of as a rough reflector. References and discussion of early work on fading have been given by RATCLI~ (1956) in a classic paper on d&action theory and its application to the ionosphere. More recently, elforts have been made to investigate the structures responsible for fading of lower E-layer echoes by measuring angles of arrival or separating echoes by their dilfering Doppler shifts. These have been reviewed by JONES (1981). However, previous measurements of angles of arrival of signals from the lower E-layer have been limited to systems with just a few antennae. While this is sufficient, for example, to measure an off-vertical tilt of the layer if there is just one signal reflected, it is inadequate to investigate the situation if there are at the same time several signals from quite different directions. In principle, simultaneous signals from different directions may be distinguished using a few antennae by Doppler sorting (WHUEHEADand MONRO, 1975; WHITEHEADet al., 1983). However, this is not feasible for the type ofshortlived echoes observed from the lower E-layer (JONES, 1981). A detailed study of the structures responsible for fading requires the application of a large aperture antenna, so that sufficient angular resolution may be obtained at wavelengths of about 150 m. In a previous paper (JONES, 1981), a radio system was described based
on a one kilometre aperture cross array operating at a frequency of 1.98 MHz and steerable to 40” from the zenith. In that paper, some examples of data analysis were given, but were only short examples of several minutes duration. Those examples may not have been typical. The purpose of this paper is to report on the analysis ofdata from several days of observations of the lower E-layer. During the data analysis it became evident that the conclusions being drawn might be at variance with some drawn from the observation of diffraction patterns using the 1.98 MHz Buckland Park array at Adelaide (35’S, 139”E), (FELGATEand GOLLEY,1971; VINCENT,1972). It therefore seemed desirable to modify the Bribie Island experiment so that data could be collected simultaneously using both the Doppler scanning technique and the spaced antenna technique. The results of this comparison are reported in Section 3.10. It should be emphasized that the investigation reported herein refers to the virtual range 95-120 km. Most of the data analysed come from a virtual range of 95-l 10 km. The data all come from total reflections of 1.98 MHz transmission from the day-time E-layer. Conditions when partial reflections from the upper Dlayer could be mixed with the E-layer signals have been avoided.
2. THE DATA
AND
METHOD
OF ANALYSIS
Data have been analysed for a total of 42 h of recording spread over 7 days in the period June-
1179
1180
K. L. JON=
October 1982. The 1.98 MHz beam was steered in a raster pattern for 11 x 11 directions, covering an angular range to 40” from the zenith. A raster was completed once per second. In each direction the returning signal was sampled at 2 km intervals in the virtual range 88-128 km. For each direction the controlling computer found the amplitude, phase and range ofthestrongest signa1. Only that information was recorded. In this way, observations could be made continuously for about 112 min before the computer disk was full. In this mode of operation the experiment is not adequate to deal with situations where the returned echoes are spread in range for some direction. The data have been analysed by the method described in JONES (X981) as ‘fitting polar diagrams to the data’. There are two stages in the procedure. Firstly, the directions, amplitude, phase and range of various echoes are calculated for each one second raster. Up to five different echoes are allowed. In a second stage of analysis the output of the first stage is processed to find continuous signals. A signal is regarded as continuous from one raster to the next if its direction stays fixed to within two increments ofangie(about 2.5”). We refer to a time series of continuous signals at one second intervals as a sequence. The processed data consist of sets of amplitude, phase, range and direction at one second intervals, labelled as sequences. It is not a priori evident that these sequences are really one second
samples of echoes from one particular physical structure. However, it is almost invariably found that the phase of the signal within a sequence varies in a continuous manner, rather than randomly from one second to the next. Examples are shown in Figs. 2 and 3. Thus it appears that thesequencesdorepresent the time durations of echoes from identifiable structures.
The results thus produced constitute the basic data discussed in this paper. The processing to this stage takes about four times as long as it took to collect the original data, using a computer similar to the one that controls the experiment, The experiment cannot therefore be run inde~nitely with reai time dataanalysis using the current PDP 1l/34 computer. 3. A GENERAL DESCRIPTION
OF THE RESULTS
The results all refer to the day-time E-layer from virtuaf heights between 95 and 120 km. The layer is usually disturbed, i.e. it departs in form from a plane horizontally stratified reflector. There are usually sequences of echoes from off-vertical angles of up to 20”. The sequences usually last for only a few seconds. Only one 3-h recording period (1400-1700 local time on 1October 1982) did not show off-vertical reflections at greater than about 1”. They were thus present for about 909~ of the time for which data were analysed. The form of variation of amplitude and phase of reflected signals during disturbed conditions is very complex. Nevertheless, certain degrees of disturbance can be recognized. 3.1. Least disturbed conditions Figure 1 displays a sample of the amplitude and phase variation of the signal under conditions of minimal disturbance. Only one reflected signal is resolved. It comes from within one degree of the zenith. The disturbance consists of amplitude fading without deep minima on a one minute time scare. These conditions appear similar to those described in VINCEXT(1972 ; Fig. 3).
Fig. 1. Variation oftheamplitudeand phaseofa reflected signal under conditions ofminimal disturbance. One signal only is detected and it comes from within about lo of the zenith.
1181
Rapid fading of medium frequency radio reflections 3.2. Moderately disturbed conditions
Figure 2 displays a sample of the variations of amplitude, phase and direction of arrival under conditions that may he described as moderately disturbed. The predominant reflection comes from close to the zenith. but thereare also weaker and shorter lived reflections from off-vertical angles. Thiscondition occurred for about SOo/,of the time in the 42 h of data analysed. 3.3. Very d~tur~d conditions Figure 3 displays data under conditions described as very disturbed. There is no longer a predominant signal from near the zenith. Such conditions prevailed for about 40% of the time for which data were analysed. 3.4. Zenith angle distribution ofoccurrence of repections It is of interest to study the distribution of reflected signals as a function of ot%vertical angle. Typical distributions over 2-h recording periods are shown in Fig. 4 for what has been described as moderately disturbed and very disturbed conditions. The open circles represent the relative probability of occurrence as a function of off-vertical angle. The closed circles
TRANSMlTtPR
4o*w-
represent the relative probability of occurrence per unit solid angle. For the ‘more disturbed’ conditions, the ma~mum in probability of occurrence moves to greater zenith angles. The probability of occurrence per unit solid angle generally decreases monotonically with zenith angle. 3.5. Duration of continuous signals Continuity of a signal can be detected for periods as short as 1 s (reflection seen onjust two consecutive data scans} and for periods as long as about 2 h (the maximum time for which the experiment can run continuously). Figure 5 shows the relative probability of occurrence of continuous signals as a function of their duration for a moderately disturbed (a) and a very disturbed (b) 2-h period. Note that an integrated probability of durations of 40 s and greater is also plotted, as well as the mean probability ofaduration in the range 21-39 s. For moderately disturbed conditions, the probabitity of durations of 40 s and greater is relatively large, corresponding to the existence of relatively long-lived echoes from near the zenith. Under moderately disturbed conditions, a maximum in relative probability occurs for durations
8EAY ANGLE (EAST-WEST)
S!ENlTH
4oDt400 N-m--d-
I
RECEIVER SEAM ANGLE (SOUTH-NORTHI -2”l.-cc,-d
f
12.2s
11 JUNE 1082 LOCAL
-
A-
8
12.20
-c
,
12.3t
TIME
Fig. 2. Variation oftheamplitudc,phaseandanglcsoCa~valofrefl~tedsignalsund~conditionsofhrodcrate disturbance’. Fromtbeinterruplaibut persistentsignalfromM”N,the prcscnccof a sporadic-l?cloud as well
as ‘turbuicnttype’scattersis suspected.
K. L. JONES
1182
TRAWSYITTER SEAM ANGLE (EAST-kES1)
4WN
RECEIVER SEAM ANGLE (SOUTII-NORTH)
F
L -.s....
ZENIW.
7
--
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r
-
--
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I 14.12
I
14.11 LOCAL
_-
,_ -,
_
A
i.“.-
_
x.(4
_’
I
16AUGUST
TIME
14.13
1902
Fig. 3. Variation of the amplitude, phase and angles of arrival of reflected signals under very disturbed conditions.
0 ,
oc.
0
Id
moo 0
0
.
1lJUNE 1962 11.04-14.20
o o o 26Nlltl ANGLC VARIATIGN
. . . 2ENlTH
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11 JUNE 1982 11.06-14.20
VARIATION
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0
(8) . 266
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.
.
.
.
.
.
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26 12 l6 o o 02ENilH (DEQREES)
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00
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(b)
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10 AWUST 1082 12.41-11.59
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.
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Fig. 4. (a) Zenith angle variations of redected signal occurrence under ‘moderately disturbed conditions (as in Fig. 2). 0 Zenith angle variation; l normahi by cosecant (zenith angle) to represent the relative probability per unit solid angle. (11 June 1982.12.28-14.20.) (b) As for Fig. 4a, but under ‘very disturbed’ conditions. (16 August 1982, 13.41-15.33.)
. .
200
0. .
t
I
I
8
4
6
l
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.*
0 0, 16
.,
.
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Fig. 5. The relative probability of occurrence of reflections as a function of their duration. (a) ‘Moderately disturbed conditions; (b) ‘very disturbed’ conditions.
Rapid fading of medium frequency radio reflecrions ofseveralseconds(in thiscase4). Incontrast, under very disturbed conditions, the relative probability of occurrencedecreases monotoni~lIy ~a function ofthe duration of an echo. It should be stated that problems remain in studying the data for duration of continuous signals. These problems arise from imperfect knowledge of the antenna polar diagram. That the polar diagram is imperfectly known (particularly the minor sidelobes at great angles) is made evident by considering the results of data processing for a quiet period of time when there is but one signal from near the zenith. The data processing also produces weaker signals at large angles which have amplitude and phase variations similar to the strong signal from near the zenith. These are evidently sideiobe responses. When the data are further processed to investigate some particular property, a sidelobe rejecting algorithm is included to reject as many of the spurious sidelobe responses as possible. The algorithm compares the total phase change through a sequence with the corresponding phase change through sequences whose amplitudes are greater than it at all times. Ifthe magnitude of the phase changes differ by less than 40”, the weaker sequence is rejected. The principle behind the algorithm is that the spurious sidelobe responses occur predominantly at large angles. from which directions there shoutd be relatively large phase shifts because the atmosphere has a horizontal velocity component of tens ofmetres per second. The 40’ phase change criterion was shown by trial to give a very high rejection of spurious sidelobe responses, judged from the simple case where there is just one signal from near the zenith. The particular point under discussion here is that the algorithm may also reject true sequences which just happen to have phase variations similar to sequences of larger amplitude. The probability of this occurring increases as the sequence duration decreases, because the total phase change becomes less. Another limitation imposed on the data analysis by an imperfect knowledge of the antenna polar diagram is a possible failure to detect the continuity of a relatively weak and fading signal in the presence ofa stronger one. This may break up the weaker signal into shorter sequences than would be the case if the antenna polar diagram were perfectly known. 3.6. Fossibie Fhysical models of the distuT~~aJ~ces (a) Possible role ojgracity waves. It is most unlikely that a spectrum of internal gravity waves (NIXES,1960) is directly responsible for the short-lived reflections from of&vertical angles ofseveral degrees, as illustrated in Fig. 3. Gravity waves have a horizontal scale of tens ofkilometres and a time scale ofseveral minutes. While
1183
interference between reflections could produce strong amplitude fading if two reflection points were so close as not to be resolved by the beam, it would be expected that the signal would persist (though appearing and disappearing) from the same direction for some minutes. It should be possible sometimes to track waves for long periods (say 30 min) and watch a reflection point cross the observer’s field of view, Furthermore, if 2O”off-vertical echoes were due to largescale distortion of the isoionic surfaces, there should be observed quasiperiodic variations in the virtual height of the echoes, which would be easily detectable, even with a range sampling interval of 2 km. An amplitude of the order of 5 km would be predicted. No such variations in virtual height are observed. When height (rather than range) of the various echoes is plotted as a function of time it appears that there is no large scale deviation from horizontal stratification. (There must of course be deviations on some scale in order to get off-vertical reflections). On the other hand, all these expected features of gravity waves are observed in the F-layer. They are seen, for example, at night, when 1.98 MHz is reflected from the F-layer. They are also commonly observed in the day-time f-layer using higher frequencies. A study of these is being actively pursued by other workers with this equipment. The slow fading, on a time scale of minutes, of the type illustrated in Fig. 1 may be a direct effect of gravity waves distorting the isoionicsurfaces. Thosedata have been interpolated to find the direction of arrival to one hundredth of a degree in computed accuracy. If the resultinganglesareplottedasisdoneforthedatain Fig. 2, but on a much finer scaie,one observes fluctuations of directions of arrival of amplitude less than one degree, but on a time scale of minutes(i.e. on a time scale similar to the amplitude fluctuations). This is probably a measure of the tilts in isoionic surfaces produced at 100 km altitude by gravity waves. The sameconclusion for similar conditions has previously been reached by VIXSCENT
(1972).
(b) Possible rule of rurbulence. Turbulence in the neutral air, with consequent displacement ofplasma, is a possible mechanism for roughening the isoionic surhces. This possibility has been previously mentioned by VISCEXT(1972).The application ofthe theory of homogeneous turbulence to radio wave scattering has been discussed by RASTXXIand BowHrLr_(197&b). (Rastogi and Bowhill were investigating weak VHF scatter from the mesosphere.) In the theory, the energy dissipation rate per unit mass E,the kinematic viscosity \’ and a characteristic time t are related by Y E-t2 .
K. L.
1184
The shortest spatial scale, below which viscosity damps motion heavily, is expected to be the Kolmogorov microscale
REESet nl. (1972) have studied turbulence in the Eregion by observing the breakup of vapour trails released by rockets. They identify the characteristic time c previously mentioned withthe time for the onset of turbulence to be observed in the trail. They give typical values at 100 km, based on their observations, of s=O.O66Wkg-‘,
t = 21 s,
q= 145 m.
Thus there may be some irregularities smaller than a Fresnel zone in size, and indeed spatial scales right down to the radio wavelength. (See also RIND,1977, for a summary of turbulence parameters.) The scattering properties of irregularities are strongly influenced by their shape. in the (linear) theory of waves in neutral air (HINES, 1960) it is found that propagation of acoustic waves of frequency much greater than the Brunt-VCiilii frequency is little affected by gravity, while fo! the large scale gravity
-
OBSERVED POLAR DIAGRAM
oo-0
CAVERN
JONES
wgves the propagation is very anisotropic, with horizontal scales b@M much greater than vertical scales. ~though turbulence is a more compli~t~ phenomenon, nevertheless, it is to be expected that the very smallest scale eddies of dimensions of a few hundred metres will be roughly isotropic, while larger scale eddies of several kilometres in size will be much more horizontally stratified. Indeed, REESet al. (1972) quote other vapour trail observations to support the view that eddies of less than 1 km dimension are isotropic. Snudlscale isotropicscatterers. A model ofsmall scale (compared to the Fresnel zone width), and therefore isotropic, scatterers may be used to explain why, under disturbed conditions, the data consists of short-tived sequences appearing randomly at quite large offvertical angles. The isotropic nature of the scatterers allows backscatter of radio waves coming from a wide range of angles. If the angular scale of the scatterers as viewed from the radar is small compared to the radio beamwidth, they wili not be individually resolved. The apparent angular spectrum of the signal at any time is the true spectrum convolved with the polar diagram of the antenna (as discussed by BRICGS,1980). Figure 6
OF FGtAR DIIIGRAM WITM A RANGGY ANGULAR SPECTRUM (16 PER FRESNEL ZONE!
ZENITH ANGLE RXGREES)
Fig. 6. l The receiving antenna polar diagram; 0 the convolution of this polar diagram with an angular spectrum random in amplitude and phase on a scale of l/l&h of a Fresnel zone.
1185
Rapid fading of medium frequency radio reflections
illustrates the results of a computer simulation (in one dimension) of the polar diagram of the receiving antenna convoIved with an anguIar spectrum which varies randomly in amplitude and phase on a scale of one-sixteenth of a Fresnel zone. The result looks like two discrete plane reflectors. This is the kind of angular spectrum observed. Can this model explain the short life of the reflections? The lifetime should depend on how long the echoes from a number of small scatterers each movingindependently will remain in phase. It thus depends on their relative velocities. COLE (1962) discusses a dimensional argument by Kolmogorov and Obukhov to the effect that for isotropic turbulence, the velocity variation over a small distance d is given by u = (&ri)~~3. Putting E = 0.066 W kg- ’ and da few hundred metres gives u z 4 m s-l. A relative motion of a quarter wavelength or %40 m is required to pass from constructive to destructive interference between reflections. This suggests a ‘lifetime of the order of 10 s, in reasonable agreement with the present observations. It may be noted that the data OfREEser al. (1972) on the rate of expansion of vapour traits also suggests velocities of a few metres per second. .. The small scale isotropic reflector model encounters dimculties, however, in explaining the decrease in the probability of occurrence of reflections with increasing zenith angle, as discussed in Section 3.4. Why would such reflectors not produce detectableechoes from. say, 30” zenith angle? Possible reasons are distance attenuation and absorption. (if Distance ~r~e~~urjo~. If, as seems to be the case. the isoionic surfaces remain approximately horizontally stratified at height /I, say, then the range r to the scatterers at zenith angle 8 is
reflected at 30’ from the zenith may be computed to be orie-third of that reflected along a vertical path. Such a signal is unlikely to be missed in the data analysis. Anisotropic scatterers. The foregoing discussion implies that it is unlikely that the scatterers are completely isotropic. This implies that they have a scale greater than a few hundred metres. These scales of turbulence will be cpnstrained by gravity to be approximately horizontally stratified. We therefore consider a different model, in which the scatterers are anisotropic and the duration of the signals is determined not by interference of many scatterers, but by the time for which single scatterers remain correctly oriented to produce strong reflection. The probability of receiving a reflection will thus be a decreasing function of zenith angle. Consider now the limit in which the reflection is dominated by just one turbulent eddy. What might the ‘iifetime’of a reflection from such an eddy be? As a very simple model, consider a plane reflecting surface rotating about a fixed axis like a rigid body at an angular rate de -=dr
u t’
where u is the turbuldnt velocity discussed aboveand 1is the scale size. ifthe radio wavelength is i. and 12 3A,the range of angles over which the anisotropic scatterer reflects radio waves is
Thus the time to rotate the range of angles A@past the observer is
r = h see 8. The received scattered amplitude from an individual reflector will vary as r-’ (BRICOSand VINCENT.1973). Thus, the relative amplitude from 30’, as compared to the zenith, may be ZZXOS-*30”, or 0.75. This wiI1 not explain the absence ofsignals from 30” zenith angle. (ii) Absorption. Absorption along the extra path langth at off-vertical angies will decrease the amplitude as the zenith angle increases. Absorption measurements have been made by calibrating the equipment at night using first and second hop reflections. The day-time absorption at 1.98 MHz tends to be in the range 20-60 db, depending on solar zenith angle and solar disturbance. (Much higher values of absorption are occasionally observed during strong solar flares.) Consider an approximately horizontally stratified ionosphere in which the total absorption at vertical incidence is 60 db. The amplitude of a signal totally
Using i. = ljOmandwintherange4-8ms-‘(for/in the range 1-8 km) gives r 55 20-10 s. These times are greater than those often observed. but the discrepancy is not too serious considering the approximations involved. In the foregoing discussion it has been implicitly assumed that the only factor modifying electron concentrations is transport arising from collisions between ionized particles and the neutral air and so the isoionic contour also moves with the neutral air. The effects of chemitai r~ombination and diffusion have been neglected. These assumptions can be justified at altitudes near 100 km by consideration of the time scales involved. The characteristic recombination time in the E-layer is (2&J)- ‘. where z is the recombination coefficient and N the electron concentration. Times of
1186
K. L.
JONES
the order of 600 s are typical (RISHBETHand GARRIOI-~, 1969a). The characteristic time for molecular diffusion is X2/D, where H is thescale height and D the ambipolar diffusion coemcient D=2kT. mv Taking values for T, the gas temperature, m, the mean molecular mass, and v, the ion neutral collision frequency, relevant to 100 km altitude from FISHBETH and GARRIOT (1969b), the characteristic time for molecular diffusion is typically IO6 s. Thus recombination and molecular diffusion can be neglected on time scales less than 50 s, which are typical of the ‘turbulent type’ data. 3.7. Obserwd echoesfrom long lasting discrete reflectors Another class of disturbance phenomenon, very different from thosedescribed in Sections 3.2. and 3.3.. is illustrated in Fig. ,7. ,For about an hour late in the morning of 1 October 1982, discrete moving reflectors were observed, sometimes lasting long enough for changes in position of the reflectors to become apparent, i.e. many minutes. Another was observed in a quiet period on 31 October 1983,in thedatacollected to compare Doppler and spaced antenna drift observations. This type of disturbance was significantly
40” w
TRANSMITTER
E
.
-A-
-
-
-..c-
-me_-._
RECEIVER
--
BfAM
different from the more irregular types discussed in Sections 3.2. and 3.3. in respect of its height of occurrence. The more irregular types are compatible with the assumption that the isoionic contour corresponding to a plasma frequency of 1.98 MHz is continuous, quasi-horizontally stratified, but somewhat distorted. However, both occurrences of the phenomenon under discussion were at heights about 6 km lower than the height of re+ction of the signal from near the vertical. The form of the data is suggestive of a blob of ionization distinct and separate from the nearly horizontally stratified 1.98 MHz isoionicsurface above it. Thesearepresumably movingpatchesofsporadic-E. These are sometimes suspected of occurring during periods of short time scale disturbances (there is an example in Fig. 2), but the ‘turbulence type’ irregularities obscure them. Properties of sporadic-E have recently been investigated using a higher frequency version of this experiment (FROM, 1983). These moving patches have properties similar to the ones observed by FINDLAY (1953) in phase path experiments. 3.8. Doppler shifts and winds The phase shifts observed on the short-lived 05 vertical reflections may be interpreted as Doppler shifts due to horizontal motion and a corresponding
ANGLE (EAST - WEST)
^_-._c__
BEAM ANGLE (SOUTH - NORTH)
I
1
10.51
10.52 LOCAL
Fig.
7.
.- /___-___^_-.c-____e_
1
1OCtOBER
1982
YIME
Variation of long lived discrete reflections (presumably sporadic-E clouds).
10.53
Rapid fading of medium frequency radio reflections
115 km, but mostly near 100 km. Velocities were in the range30-60msW’. There was no common pattern of velocity variation on the different days. One day (18 August) the direction changed by about 180” (from south-east through south to north-west) in about 6 h. as though a semidiurnal effect predominated. The data slightly overlaps in height range that which can be used by the partial reflection drifts technique (VINCENTand Sruens, 1977). As a technique for observing horizontal motions it suffers in comparison from two defects. Firstly, data at only one height is obtained and this height is liable to change with time during the day. Secondly, since total reflection is used, the apparent height of reflection measured may be markedly different from the true height.
horizontal velocity component calculated L Ad, PC-----4r sin 8 At ’ where a phase shift AQ is observed in a time At at a zenith angle 0, the radio wavelength being i.. If there is a large number of such observations distributed in azimuth, a plot of this horizontal velocity component against azimuth will be sinusoidal in form if all the scatterers are moving with a common horizontal velocity. For this purpose the data were divided into approximately 15 min intervals and plots of the form shown in Fig. 8 were prepared for each. A least square fit of a sinusoid to the points was performed to find the magnitude and azimuth of the velocity. In Fig. 8, the maximum negative Doppler shift gives the direction as receding reflectors give negative Doppler shifts. The scatter of points in Fig. 8 will be partly due to the random velocities associated with turbulent motions, but also to errors in computing the zenith angle B of the echo, particularly when 0 is only a few degrees. The data comes from 7 isolated days in the period June-October 1982 and from virtual heights of 95-
BEST FIT VELOCITY
so-
.
. . ..
.
64
1187
3.9. Azimuthal dependence of irregularity occurrence It is of interest to investigate whether the probability of occurrence of irregular reflectors is uniform across the field of view. The dependence on zenith angle has been discussed already (Section 3.4.). Plots of the form of Fig. 8 are useful for searching for any azimuthal dependence. An examination of about 150 such plots (each covering about 15 min) showed that in two-thirds
AZIMUTH
23
11 JUNE 1962
16.41-16.55 ‘.
.
‘.
. . .
-1s
-
..
.:.“. .
-150. -100tNORTHl
.:
- .; ..‘.‘.; ,,.::‘.“,...“‘!;:_..:1’, ; :.. ;::... : ‘I ‘! ‘,. .“. ” . ,.:. .;:: _.
-W(WEST)
0 (SO&i)‘
.
83
AZIMUTH
19
.
90 (EAST)
11 JUNE 1962
16.55-17.08
r.
Fig. 8. Scatter plots of apparent
horizontal
.
,:.,,
AZIMUTH @EST FIT VELOCITY
..
velocity against azimuth.
100
1188
K. L. JONES
of cases there was a uniform distribution of occurrence as a function of azimuth. The remaining third showed marked non-uniformity. The di$ibutions of preferred azimuths of occurrence in 8 points of the compass (as judged subjectively from plots of Fig. 8 type) are shown in Fig. 9. There is a clear preference for the north and east sectors. Turbulence in the neutral air may be generated at these altitudes by a fortuitous wncurrence of gravity waves and wind shear (BRETHERTCJN, 1969; JONESand HOUGHTON,1971; GELLERet al., 1975). If so, it is possible that marked variations in generation of irregularities could occur on a horizontal scale oftens of kilometres. However, the fact that the preferred directions of occurrence group so strongly in the north and east implies that there must be some aspect sensitive property ofthe reflectors, since it can hardly be supposed that sb many would form in that sector by chance. Furthermore, the 15min periods in which there is a preferred azimuth of reflections are not randomly distributed in time. There are periods of up to lf h for which there is a particular preferred range of azimuths. No other properties (such as zenith angle dependence) have been discovered which distinguish between the aspect sensitive and non-aspect sensitive reflectors. This is a puzzling phenomenon for which there is at present no explanation. 3.10. Comparison o/results with those/rom the Buckland Park arrq The Buckland Park array at Adelaide(35”S 139”E) is of similar size to the Bribie Island array and also N
22
12 E
54 cases of preferred
directions
101 cases uniformly distributed
Fig. 9. Number
of occurrences of preferred azimuth of echoes (in I5 min time divisions).
operates on a fr&ptency of 1.98 MHz (BRIGGSet al., 1969). A comparison of results from Bribie Island and Buckland Park is therefore of particular interest. The Buckland Park array has been used principally for measuring ionospheric drifts using the spaced antenna drift method (FELGATEand GOLLEY,1971; VINCENT, 1972). However, these workers have attempted to draw conclusions as to the nature of the irregularities in the E-layer causing the diffraction patterns whose velocities they measure. They have suggested a gravity wave interpretation of their data. This conclusion is at variance with what was decided in Section 3.6.(a)above. It therefore seemed desirable to operate the Bribie Island array in the spaced antennadrift mode, as well as its normal mode, to sec. whether this conflict could be resolved. It is not possible to operate the Bribie Island array in precisely the same mode as the Buckland Park array. Buckland Park has a filled-in receiving array with one wide-angle transmitter, while Bribie Island has one (north-south) linear receiving array and one (eastwest) linear transmitting array. The Bribie Island array was modified to operate as follows. (‘Modifying’ the array means modifying a computer program.) At the end of each directional scan (once per second) the following sequence of data sampling was performed. First, just one of the transmitters was left switched on and the data from the 10 north-south receiving antennae were sampled in turn. Then, one receiving antenna was used to sample the signal when each of the 10 transmitters in theeast-west transmitting array was switched on in turn. This extra data sampling took about 100 ms. Because of extra switching times, it was found that the full 11 x 11 directional raster together with the spaced antenna data sampling could not be carried out once per second. Therefore, the directional raster was reduced to 9 x 9 (steering to about 30” from the zenith). This would make negligible difference to the results, since ionospheric echoes are rarely if ever observed at zenith angles greater than 30’. The spaced antenna data were analysed to measure the apparent drift velocity of the diffraction pattern as defined in BRrc%setal.(1950).Thedetailedvariation ofamplitude and phase signals for each antenna was plotted for detailed examination. The directional raster data were also analysed in the usual manner, to study the directions and durations of echoes. In the discussion that follows, all the velocities quoted are apparent drift velocities of the dilfraction pattern unless otherwise stated. The data analysed was for a 2-h period (14.00-16.00) on 31 October 1983.Asfaras thecharacter ofthedatais concerned, it fortuitously fell into two nearly equal sections. In the first hour the E-layer was moderately
Rapid fading of medium frequency radio reflections
disturbed, with two or three off-vertical echoes, as well as a strong echo from the zenith. The scale of the diffraction pattern on the ground varied from less than 100 m to about 300 m maximum. (The scale of the diffraction pattern here is defined as the distance along the ground for the zero delay cross-correlation of amplitudes to fall to 0.5.) The fading period of the signal amplitude was S-10 s. In the second hour the E-layer was little disturbed, conditions being as described in Section 3.1. In this period of time the scale of the diffraction pattern was much larger than the dimensions of the array (1053 m). The data analysis is concentrated on the disturbed period. Data were taken at 1 s intervals. The analysis of the data for the disturbed period, 14.00-15.00 on 31 October 1983, is now discussed. Consider, first, the diffraction pattern data. As a preliminary analysis, for both the receiving and transmitting array, an antenna at one end of the array was used as a reference and auto- or cross-correlations as relevant were calculated for all 10 antennae at time delays from - 20 s to + 20 s. For each of the 9 antenna spacings (110-1053 m) a maximum in the crosscorrelation as a function of time delay was sought. Three point interpolation in time was performed by fitting a quadratic function to the maximum and the point on either side. This improved the accuracy of the estimation of the delay. It was noted that the crosscorrelation as a function of time delay usually showed maxima spaced several seconds apart. Even the autocorrelation as a function of time delay often displayed maxima several seconds apart. This reflects the quasiperiodic variation ofsignal amplitude during disturbed conditions. When the antenna spacing becomes greater than the scale of the diffraction pattern on the ground, these latter maxima often predominate and the maximum indicating the velocity of the diffraction pattern becomes difficult or impossible to locate. On the other hand, using as great an antenna spacing as possible improves the accuracy of measuring the time delay. Thus, to apply the spaced antenna method in all conditions of ionospheric disturbance one requires a number ofantennae with spacingscovering the range of scales of diffraction patterns. This result reinforces the view expressed in WHITEHEADet al. (1983) that while the Doppler and spaced antenna methods of measuring velocity are equivalent, one cannot achieve as useful a result by using one method with just a few antennae or small aperture as compared to using the other with many antennae spread over a greater aperture. On the basis of the preliminary analysis, pairs of antennae were chosen, straddling the centre of the array, to give the maximum possible antenna spacing that was less than the scale of the diffraction pattern.
1189
The data from these antenna pairs was used to calculate the time delays in the north-south and east-west directions for maximum amplitude correlation. The data were analysed on a time scale of 1 min to search for changes in the apparent drift velocity (as defined by BRICKSet al., 1950) on a time scale less than the gravity wave time scale. The magnitudes and directions so calculated are shown in Fig. 10. Since the directions are often approximately constant over several minutes and the amplitudes at the individual antennae are quasiperiodic, as discussed before, the result appears compatible with the FELGATE and GOLLEY (1971) observation that sets of interference fringes appear to last for a few minutes. The degree of agreement between the calculated Doppler velocity and the apparent drift velocity is shown in Table 1.The l-h disturbed period was divided into four approximately f-h periods and these two velocities calculated. The directions are azimuths measured in degrees from the south toward east. The magnitudes are in metres per second. As expected, the measured velocity of the dilfraction pattern is approximately twice that of the reflecting surfaces as indicated by the Doppler velocity. When the results of data analysis to find the occurrence and direction of signals are plotted for the period of time used to prepare Fig. 10, the result is somewhat similar to Fig. 2. Most of the time, the strongest signal comes from near the vertical. It shows deep amplitude fading and a slow phase variation compared to the (usually two or three) transient echoes which are confined to the north-east quadrant in azimuth. The diffraction pattern on the ground in this case is formed by interference between the signal from the vertical and these two or three transient signals, which come from one quadrant and apparently have similar velocities and thus produce(as is observed much of the time) quasi-periodic amplitude fading. The results discussed above are consistent with the conclusions of FELCATEand GOLLEY(1971) that the diffraction pattern on the ground is due to interference between just a few reflections. Though a complete twodimensional dilfraction pattern was not recorded for this data. it is consistent with Felgate and Galley’s observation that fringes from one distinct set appear to last - 3 min. This could happen even if the lifetime of the individual off-vertical echoes was considerably less than 3 min, since they come from much the same direction and share a common velocity. The velocity of the diffraction pattern measured here is about 100 m s- II which implies an ionospheric velocity of the order of 50 m s-l, in agreement with the results of Golley and Felgate. These investigators further remark (p. 1368) that in
K. L.
JONEs
.
.
SO
I
40
14.20
I
1
,
1
14.30
14.40
1450
1500
LOCAL
EAST
2 . i
I
TIME
Fig. 10.Time variation of the apparent velocity ofdiffraction patterns. l Magnitude in m s-’ ; 0 azimuth in degrees measured from south toward east.
the several minutes that a particular set of fringes appears to last, the reflecting region must move several kilometres. Now what was actually observed in the present work was that the reflecting region appeared to move nowhere,even though it had an apparent velocity of 50 m s-i. For about an hour the transient echoes came from a particular region of the sky (north-east quadrant out to about 20’ from the zenith), with no suggestion of systematic change in direction. From the extensive data analysis discussed in Section 3.9., it is known that this restriction of signal directions to that particular region occurs frequently and since the observing site is in no particularly priviliged position, it is implausible to suppose that the reflectors just happened to be confined to that region on this occasion. Thedatasuggestamotionofthereflectingsurfaceswith
Table 1
Doppler velocity
Apparent drift velocity
Magnitude
Direction
Magnitude
Direction
39 46 65 47
5 24 46 54
95 106 147 169
-8 -2 31 54
a velocityof50m s- ‘.Inaddition, theisoioniccontours must be distorted on a scale considerably greater than the radio wavelength so the distortions can be anisotropic. Although these distortions occur with the same mean probability everywhere, their shape must give rise to an aspect-sensitive property. It will be recalled that in Section 3.6.(b) it was deduced, independently from the zenith angle variation of the probability of echo occurrence, that the scatterers are anisotropic and therefore several radio wavelengths in extent. FELGATEand GOLLEY (1971) (also VINCENT. 1972) further suggest that, since the mean lifetime of a set of fringes appears to be about 3 min, this is long enough to hypothesize a gravity wave origin of the off-vertical reflections. Felgate and Golley also pointed out (p. 1368) that if the phase, as well as amplitude, of the diffraction pattern wasrecorded, thedirection ofarrival of the different downcoming reflected waves could be measured and the validity of various models confirmed or otherwise. This in elfect is just what is done with the Bribie Island steerable beam radar, though not in precisely the way Felgate and Golley envisaged. When this is done. the gravity wave hypothesis is not confirmed. There are three signatures to be expected from reflections caused by undulation of an isoionic surface by gravity waves. They are a continuity for several minutes, a systematic change in the direction of
Rapid fading of medium frequency radio relleclions
the echo as the structure moves and a systematic (undulatory) variation in the apparent height of reelection of the echo. There is no suggestion of any of these signatures in the Bribie Island data. Another aspect of the results 10 be compared between the Bribie Island and Adelaide data is the fraction of the time for which there is very little disturbance (see Section 3.1. and Fig. 1). Figure 1 is to be compared with VINCENT(1972 ; Fig. 3). Vincent’s discussion could be taken to imply that these conditions occur for something approaching 50% of the time, as compared to 10% at Bribie Island. However, FELGATE and GOLLEY(1971; Table 1, p. 1357) report that the scale of the diffraction pattern is greater than the scale of the array for about 127, of the time for normal E-region echoes. This is comparable with the Bribie Island result. It is thus concluded that ther.e is no compelling evidence for a significant diflerence between observations at the two sites.
1191
E-layer in the height range 95-l 15 km. About 10% of the time the reflecting layer departs very little from horizontal stratification, giving rise to just one reflection from close to the zenith. At these times there are occasionally also discrete off-vertical reflections lasting a few minutes, which are identified as isolated sporadic-E clouds. .About 907; ofthe time there appear to be present transient reflectors, with lifetimes ofa few seconds,seen too&vertical angles of20” or more. These do not appear to be a direct manifestation of gravity waves producing undulations in the isoionic surfaces. Nor can they be accounted for by a large number of small isotropic scatterers. A possibility has been discussed that turbulence might produce the required transient departures from horizontal stratification. This hypothesis must remain tentative. Some aspects of the observations still require explanation. In particular. there is the tendency for reflections to be commonest in the north and east, an effect sometimes, but not always, present.
4. SUMhlARY
About 40 h of data taken on 7 days has been used to study the structure responsible for rapid fading of medium frequency radio waves reflected from the lower
Acknowledgements-This work issupported by the Australian Research Grants Scheme. The author thanks Professor J. D. WHITEHEAD for useful comments and Dr B. H. BRIGGS for editorial assistance.
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FELC~ATE D. G. and GOLLEYM. G. FINDLAYJ. W. FROMW. R. GELLERM. A., TANAKAH. and FRITTSD. C. HINESC. 0. JONESK. L. JON= W. L. and HOUGHTOXD. D. RATCLIFFE J. A. RATCLIFFE J. A. REEKD., ROPERR. G., LLOYDK. H. and Low C. H R~~nw;l P. K. and BO~HILL S. A. RAS~~;I P. K. and BOWHILLS. A. RIND D. RISHBETHH. and GARRIOTT0. K.
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