Radio wave scattering from the southern hemisphere D-region

Radio wave scattering from the southern hemisphere D-region H. CUAWRA*

and R. A. VINCENT

Department of Physics, University of Adelaide, Australia Abstract---Observations of HF radio wave scattering from the D-region made at a mid-latitude, southern hemisphere station (Adelaide) are compared with observations of VHF scattering made at a low-latitude location (Jic~ar~). It is shown that, at heights below 80 km, the observations show many similar features and a common mechanism may be responsible for many of the irregularities which cause the scattering observed at the two sites.

1. 1NTRODUCFlON have been few observations of radio wave scattering from the D-region of the ionosphere at low latitudes. Much of what information has been available has come from recent observations made using the VHF incoherent scatter facility situated at Jicamarca (12“s) and which has been reported by FLOCK and BAL~LEY(1967~. WOODMANand GUELEN (1974) and RASFOGIand WOODMAN(1974); their observations have been summarized by CUNNOLD (1975). Further observations made over a wide range of height intervals have been reported by HARPERand W~~DMAN (1977). The scattering appears to come from layers of limited vertical extent which are located at heights between 60 and 85 km; the layer thicknesses have not been well determined because of the finite radar pulse lengths used but they may be smaller than 1OOm; the layers persist for several minutes or longer and the scattered power can change by a factor of 20dB in a short time; at night the scatter signals are absent. W~ORMAN and GUILLEN (1974) and CUNNOLD(1975) point out that there are similarities between these VHF scatter observations and the observations of partial reflections from the D-region made at frequencies of the order of 2 to 6 MHz at mid-latitudes. They suggest that both techniques may be observing the same typ of irregularities. In this paper we discuss observations of partially reflecting irregularities made at another southern hemisphere location, Adelaide (35”S, 138”E). The aim of the paper is to compare these observations with the Ji~rnar~ ob~rvations in order to bring out any and differences. The measurements simiiarities To date. there

*On leave from Physical Research Laboratory, ragnpura, Ahmedabad, India.

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reported here were made using a 178 dipole antenna array located just north of Adelaide {BR~GGSet al., 1969). The array was used for reception, the elements being combined in phase to produce a beam of width +4.5” (cf. the kO.5” beam at Jicamarca) at the operating frequency of 1.98 MHz. Transmissions were made with a 4 dipole array phased to give either the ordinary or extraordinary polarizations; the peak powers were about 20 kW and the pulse used was Gaussian shaped with a half-power width of 25~s.

2. OBSERVATIONS One of the main features of the observations at Adelaide is the apparent difference in the nature of the reflections from heights above and below 80 km. In general there are differences in the echo fading rates, echo phase coherence and the angular spectra of the backscattered radiation. These points are discussed in more detail below. Figure 1 shows how the fading periods and phase variations of the echoes often vary as a function of height. There is an overall decrease in the fading periods with height, contrasted with the general increase in the phase ~uctuation of the echoes (the phase variations have been detrended in order to remove the effects of slow variations with periods greater than one minute or longer). The figure also shows the presence of at least one discrete layer located at a height of about 68 km. At this height the mean fading periods are larger and the phase deviations smaller than these quantities at other heights. A measure of the type of reflection process responsible for the partial echoes can be gained by studying the amplitude probability distributions. Scattering from many independent irregularities will produce the

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Fig. I. Profiles of r.m.s. phase (dotted line) and fading period (solid line) of partial reflections. 19 June 1975. well known Rayleigh distribution whereas if an appreciable specular component is also present then the distributions will be of the Rice form. Figure 2 shows a height profile of the so called ‘Rice parameter’ which is a measure of the specular to the scattered or random amplitude ratio. The diagram is a composite made from a number of independent 5 min observations of the amplitude probability distributions; the Rice parameter for each distribution was found by least-squares fitting of theoretical probability distributions of the Rice type and selecting the ones that gave the best fit using a chi-square test as a measure of the goodness of fit. This procedure used was similar to that used by VON BIEL (1970). It is apparent that during the whole 2 h observing period the echoes at all heights appeared to have an appreciable specular component although random scattering becomes more important above 80 km. Usually these amplitude measurements, and measurements made by LINDNER (1975) using a different technique, show that the random components tend to dominate above 80 km. One of the principal features of Fig. 2 is the highly specular nature of the echoes from a height interval centred on 68 km. This is a characteristic of many other echoes made in the height range near 70 km. Another useful way in which the irregularities can be studied is by measuring the angular spectrum of the returned echoes. Observations made at Adelaide have been reported by LINDNER (1975). He shows that at all seasons the angular spectra are narrow (the order of a few degrees) at heights below 80 km, but the spectra broaden rapidly at heights above 80 km. The narrower angular spectra, the slower fading rates and the greater phase coherence of the reflections from below about 80 km are consistent with a

and R. A. VINCENT model in which the irregularities situated below this height are more anisotropic than the irregularities above 80 km. That is their horizontal dimensions are greater than their vertical dimensions. These comments apply particularly to the discrete, stratified regions which are often, although not always, observed at Adelaide at heights below 80 km. Although layer-like structures are seen at heights over 80 km they usually appear to be thick ( a few kilometres) and composed of a number of individual scattering centres (see also GREGORY and VINCENT, 1970). One convenient way in which the stratifications can be studied is by using the height-time (h’(t)) technique where the echo amplitudes are used to intensitymodulate an oscilloscope and the resulting trace is recorded on a slowly moving film. An example is shown in Fig. 3(a) which shows 3 sections, each of 3 min duration, taken from a h’(t) record made on 31 July 1975; this record is notable because of the strong stable reflecting region which was observed at a height of about 66 km. At 0700 h the echo had just appeared, the solar depression angle being about 4’; the only other echoes visible were rapidly fading reflections from between 80 and 100 km. By noon strong echoes were also observed down to 70 km but the layer is still apparent because of its slower fading characteristics. Finally, at 1700 h the layer is still visible even though the only other reflections

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Fig. 2. Height profiles of Rice parameter, 1975.

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Fig. 3. (a) Height-time records, 31 July lY75, showing Height-time record, 15 July 1972, showing sudden

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stable layer at about 66 km altitude. increase in echo strength at 75 km.

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Radio wave scattering from the southern hemisphere occurred at heights greater than 80 km. After this the echo gradually decreased in strength until it disappeared into the noise. What makes this record of even more interest was that the same, or a similar, structure was observed during daylight hours for 3 consecutive days centred on the 31 July. Whatever atmospheric or ionospheric processes were responsible for the echoes, they must have been of a long lasting nature. Not all the stratified regions are as strong or as stable as that discussed above. Other regions have a more ‘patchy’ appearance; intermittent echoes are observed to appear suddenly at a given height before weakening and perhaps even disappearing after a few minutes. At some later time strong echoes from the same height are again observed, the echo powers changing by factors of up to 20dB. This type of behaviour IS very similar to the changes in echo power observed at Jicamarca (RASTOGIand WOODMAN,1974; HARPER and WOODMAN, 1977). Figure 3(b) shows a sample of a h’(t) record exhibiting this type of event. At 75 km a strong echo was observed to appear and disappear within a few minutes; immediately before and after this event weak random scattering only was observed in the 70-80 km region. Phase records taken simultaneously show that the irregularity responsible for the strong echo was moving horizontally with an inferred speed of the order of 100 m s- ‘, giving an estimated scale for the irregularity of the order of 10 km. Irregularities of this nature are similar to those observed by FRASER and VINCENT (1970) at Christchurch (44”s). The inferred scale of 10 km also agrees well with the estimated values for the horizontal scales measured at Jicamarca (WOODMAN and GUILLEN. 1974; RASTCMYand BOWHILL, 1976). One other similarity between the Adelaide and Jicamarca observations concerns the relation between the echo fading rates and the strengths of the scattered signals. In general the stronger echoes are associated with smaller fading rates; at Jicamarca for example the scattered powers are often found to be proportional to the signal correlation times which are a measure of the fading periods. The longest correlation time at 50 MHz are about 2 s whereas at 2 MHz the strongest echoes may have fading periods of the order of tens of seconds.

3.

DISCUSSION

In this paper we have placed some emphasis on discussing the echo stratification which is often, but not always, observed at Adelaide at heights below 80 km. This is because the scattering observed at VHF at Jicamarca also appears to come from thin

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stratified regions. It is pointed out that the stratified nature of the lower D-region in the Australian-New Zealand sector has been reported before (e.g. GARDNER and PAWSEY, 1953; GREGORY, 1956, 1961; SMITH et al., 1965; FRASER and VINCENT, 1970). In particular it is the intermittent or ‘patchy’ type of stratifications observed with the partial reflection system which appear to show most similarity to the Jicamarca observations. Analysis of the Jicamarca observations shows that they can be explained by sudden increases in the dissipation of turbulent energy, the thickness of the turbulent regions being only of the order of 50-1OOm and the inferred horizontal scales being of the order of 10 km upwards (WOODMANand GUILLEN, 1974; RAST~GI and BOWHILL, 1976). CUNNOLD (1975) estimates that the eddy diffusion coefficients in the layers are of the order of 400 m2 s- ’ and the associated wind shears to be as high as 0.04 s- 1. The mechanism which produces the sudden increases in the turbulence is not yet known. RASTOGI and BOWHILL (1976) point out that thin turbulent layers can be produced in the vicinity of gravity wave critical layers (GELLER et ul., 1975). Another mechanism might be one similar to that in which transient bursts of turbulence are produced in the microstructure of the oceanic thermocline (WOODS. 1969). Here thin stable layers or sheets are sometimes rendered unstable by the passage of isolated, coherent wave packets. The Kelvin-Helmholtz instabilities generated at the wave crests in turn become unstable and a cascade of turbulent eddies is generated. These thin patches of turbulence are observed to have lifetimes of several minutes. After the turbulence has decayed the layers are observed to be re-established. Similar processes have been observed in the lower atmosphere (e.g. GOSSARD and HOOKE, 1975). If thin stable strata do exist in the mesosphere at both low and mid-latitudes the passage of gravity wave packets might explain why the transient bursts of scattered power appear to occur repeatedly at the same height (HARPER and WOODMAN, 1977). While a turbulence hypothesis might explain the intermittent type of layered structure we do not believe that it can explain the long-lived slowly fading reflecting regions such as that shown in Fig. 3(a). It does not seem possible that an atmospheric process could supply energy for periods lasting several hours or longer. Some other mechanism must be responsible for maintaining the sharp vertical gradients in the electron density profile that give rise to the partial reflections. We are investigating these layers further by improving the height resolution of the system and by tilting

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H. CHANDRAand R. A. VINCENT

the beam of the receiving array from the vertical in order to study their horizontal structure.

BRIGGSB. H., ELFORDW. G., FELGATED. G., GOLLEYM. G., R~SSITERD. E. and SMITHJ. W. CUNNOLDD. M. FLOCKW. and BALSLEYB. FRASERG. J. and VINCENTR. A. GARDNERF. and PAWSEYJ. GELLERM. A., TANAKAH. and F~n-rs D. C. GOSSARDE. E. and HOOKEW. H. GREGORYJ. B. GREGORYJ. B. GREGORYJ. B. and VINCENTR. A. HARPERR. M. and WOODMANR. F. LINDNER8. RASTO~IP. K. and BOWHILLS. A. RASTM~IP. K. and WOODMANR. F. VON BIFZLH. A. WOODMANR. F. and GUILLENA. WOODSJ. D.

Acknowledgements-The work described in this paper was supported by the Australian Research Grants Committee.

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Nature, Lond. 223, 1321.

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Reference is also made to the following unpublished material:

HARPERR. and WOODMANR. F.

1976

Radar Observations of waves and winds in the Mesosphere made at Jicamarca. Paper presented 5th

SMITH

1965

Science Rept. No. 3 and Final Rept., Physics Depart-

ISEA, Townsville, Australia. R. A., BOURNEI. A., LOCH R. G. SEETYC. S. G. K., COYNET. N. R., BARRATTD. H. and PRASADB. S. N.

ment, University of New England, Armidale, Australia.