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Ionospheric probing using continuous waves at oblique incidence
Journalof Atmospheric andTerrestrial Physics, 1970, Vol.32,pp. 530-549. Pergamon Preaa.Printed in Northern Ireland
Ionospheric probing using continuous waves at oblique incidence * D. ECCLES, J. W. KING and P. A. BRADLEY Radio and Space Research Station, Ditton Park, Slough, England (Received 5 November 1969) Abstr&---Three different ionospheric probing techniques which all use continuous waves at oblique incidence are discussed. These are (a) the high-frequency Doppler techniquein which the Doppler frequency shifts caused either by vertical ionospheric movements or by ionization changes are studied, (b) the forward-scatter of VHF waves from the ionosphere and (c) the study of radio aurorae using bistatic systems in which the transmitter and receiver are situated several hundred kilometres apart, but both on the equator side of the aurora1 irregularities. The type of ionospheric information which can be obtained using each of the three techniques, the limitations of the techniques and the future research which needs to be done are all discussed. 1. THE HIGH-FREQUENCY 1.1
DOPPLER TECHNIQUE
Outline of the technique and the type of data obtained
The high-frequency Doppler technique involves recording, on slowly moving magnetic tape, the beat frequency obtained by mixing radio waves which have been reflected from the ionosphere with a local, slightly offset, reference signal (DAVIES and BAKER, 1966). When changes occur in the phase height of the sky wave, due either to vertical movements of the ionosphere or to ionization changes, corresponding changes of the beat frequency are produced. The frequency shift depends on the rate of change of phase experienced by the ionospheric waves as the height of the reflection point moves. When the tape recordings are played back rapidly, the changes of the recorded beat frequency can be determined by either using conventional audio frequency analysers or by using a computer to perform an analysis of the data in digital form. Many standard frequency transmissions exist which are quite suitable for this type of experiment and therefore it is only necessary to provide recording equipment which means that the technique is relatively inexpensive. 1.2
Information which can be obtained using the technique
The technique is primarily suitable for investigating phenomena which are associated with short-term changes in the ionosphere. These phenomena can be broadly divided into (a) those associated with enhancements of ionization and (b) those associated with vertical movements of the ionosphere. 1.2(a) Phenomena associated with enhancements of ionization. Rapid increases in the solar EUV radiation cause enhancements of ionization which occur predominantly at E- or P- layer heights; these in turn produce sudden frequency deviations, SFD’s, of the recorded beat frequency (DONNELLY, 1969). SFD’s can be divided * Important
contributions to this paper were made by K. Davies and G. Lange-Hesse. 639
540
D. ECCLES, J. VI’. KINGand P. A. BRADLEY
into two types, one being due to large impulsive changes of electron concentration in the upper E- and P-regions and the second being caused by small, slow ionization changes which tend to occur in the D- and lower E-regions. The flare radiation associated with the impulsive changes is primarily in the 20-1030J%range which has not yet been studied adequately by means of satellites. This radiation is, however, absorbed in a region of the ionosphere which is reasonably well understood and, therefore, impulsive SFD’s can provide useful information on the EUV radiation in this particular part of the spectrum. On the other hand, SFD’s of the second type are associated with radiation in the 0~5-8A range which is absorbed in the D- and lower E-regions. The radiation in this part of the spectrum is currently being monitored by many satellites, but the region of the ionosphere in which it is absorbed is not well understood. The slow SFD’s can therefore provide useful additional information on the behaviour of the D-region. In some cases the variations are too slow to be detected by frequency shift measurements and phase measurements have to be studied instead. SFD data may also be used to estimate the effective recombination coefficient in the E- and El-layers (BAKE%and DAVIES, 1966). 1.2(b) Phenomena associated with vertical movements of the ionosphere. Vertical movements of the ionosphere associated with various phenomena have been observed using the high-frequency Doppler technique. In particular, it has been found useful for detecting atmospheric waves resulting from particle precipitation in the aurora1 zones (GEOROES,1968), and for ~vestigating rapid ionospheric changes which occur during geomagnetic sudden commencements (DAVIES and BAKER, 1966) or during ionospheric storms. In addition, the technique has provided important information about infrasouic waves in the P-region originating from nuclear explosions (BAKERand DAVIES,1968), earthquakes (DAVIESand BAKER, 1965; YUEN et al.,1969),severe thunderstorms (GEORQES,1968; BAKER and DAVIES, 1969), travelling ionospheric disturbances and the movement of irregularities associated with spread-P echoes (CALVERTet al., 1962). I.3 Limitatiolzs of the technique and the accuracy with which the measurements can be made The existing theory is basically sound for those easesin which ray theory is applicable. Cases in which the simplest form of ray theory may not apply include, for example, very large SFD’s in which the ionization enhancement is such that the propagation path switches from the F-region to the E-region; near the time of the switch both paths can exist for a short period. The frequency resolution of the measurements has, until recently, been about 0.1 Hz. The frequency stability of the E-region echoes is generally better than this and the Doppler technique has, therefore, been mainly limited to the study of P-layer echoes. The interpretation of the data is complicated by the fact that the Doppler shift depends on effects integrated over a range of heights up to the reflection level. The ionization changes at different heights can, however, be distinguished by using multifrequency Doppler recordings.
Ionospheric probing using continuous waves at oblique incidence
541
The standards which have been found to be minimal for any high frequency Doppler installation are as follows: Transmitter power: Oscillator stability: Time resolution: Tape speed: Dynamic range: 1.4 C~~~u~~~o~~
50 TN 1 part in 108 2 see 5 . 1O-2 cm see-l 50 dB.
waft other te~h~~q~es
Very good agreement has been reported between the occurrence or duration of SFD’s and various other solar or ionospheric phenomena as follows: (a) (b) (c) (d) (e) (f)
The explosive stages of H-alpha bursts (DAVIESand DONNELLY; 1966), bursts of hard X-rays and the ra,pid rise of soft X-ray bursts, EUV bursts (DONNELLY,1969), enhancements of total electron content (GARRIOTTet al., 1967), impulsive bursts of cm radio noise and geomagnetic crochets and sudden commencements.
The value of the effective recombination coefficient in the E- and Pl-layers deduced from SFD data is in good agreement with the values obtained by other methods (BAKER and DAVIES, 1966). 1.5 Observatories using the technique
Table 1 contains a list of the laboratories which are known to make regular high-frequency Doppler observations. The National Physical Laboratory, India, records only on 10 MHz and for part of the day the dominant signal received in Delhi is not JJY but ATA. It should be noted, from a monitoring point of view, that in Europe some of the standard frequency stations (e.g. MSF and HBN) operate on a time-sharing basis, I .6 Future research and ~~ve~o~~ents There is no doubt that the HF Doppler technique will continue as an important auxiliary technique for studying solar EUV radiation bursts until routine satellite data having adequate time resolution become available. In order to obtain continuous coverage the present network should, in fact, be improved by extending the range of observing frequencies and times; it appears likely that the best way in which this could be done would be to bring into operation a few transmitters specifically for Doppler measurements. The multi-frequency Doppler technique with spaced transmitters (or spaced receivers) is a useful technique for measuring F&layer drift velocities in three dimensions. It could also be used to provide further useful information about the effects of infrasonic waves, (produced by sources near the ground) on the ionosphere and also for studying ionospheric transients associated with magnetic storms. SFD data frequently show a marked periodicity and Dr. R. F. Donnelly, ESSA Research Laboratories, Boulder, Colorado (private communication) has
542
D. ECCLES,J. W. KING and P. A. BRADLEY Table 1. Observatoriesmaking regular Doppler technique recordings
Institution University of Hawaii, U.S.A. Kyoto University, Japan National Physical Laboratory, India University of Leicester, England
Principal investigator P. C. Yuen T. Ogawa A. P. Mitra T. B. Jones
ESSA Research Laboratories, K. Davies U.S.A.
Path* WWVH-Honolulu IJT-Kyoto JJY-Delhi MSF-Leicester OMA-Leicester HBN-Leicester Illinois-Boulder Sunset-Boulder
South Pole--Byrd South Pole-McMurdo South Pole-Vostok McMurdo-Byrd McMurdo-South Pole McMurdo-Vostok Illinois-Boulder G. M. Lerfald WWV-Boulder
Path length (kin) 150 360 5900 20 900 900
Carrier frequency (MHz) 2.5, 5, 10, 15 5, 10, 15 10
2.5, 5.0
1100 1200 1200 1500 1200
8.9, 11.1, 13.0 2.1, 3.3, 4.0 4.9, 5.1 6 6 6 7 7
1300 1300 80
7 8.9, 11.1 2.5, 5.0
1300 25
* The locations of the Standard Frequency Stations are as follows: WWV Fort Collins, Colorado, MSF Rugby, England HBN Geneva, Switzerland WWVH Maui, Hawaii, OMA Prague, Czechoslovakia U.S.A. JJY Tokyo, _ Japan U.S.A.
suggested that it would be interesting to investigate the relationship these variations and the fine structure observed in hard X-ray bursts.
between
2. FORWARD SCATTER 2.1
Ih-0&.&07b
Many successful attempts have been made to transmit signals over long oblique propagation paths at frequencies which are higher than the expected maximum usable frequencies for the paths involved. Some of the earliest reports that satisfactory communication could be maintained at such high frequencies were made by radio amateurs and it was soon established that this type of propagation was most common during years of high sunspot activity, and that it occurred particularly in equatorial regions. TILTON (1947) reported that on certain occasions signals at frequencies near 50 MHz were propagated satisfactorily over transequatorial paths of up to 7000 km, It was believed at first that such propagation occurred solely because the ionospheric layers were tilted, but it became evident later that scattering mechanisms were also sometimes involved. BAILEY et al. (1952, 1955) carried out detailed investigations of the characteristics of VHF forward-scattered signals for both temperate and high-latitude paths in North America. Studies relating to lowlatitude paths in the Far East were later reported by BATEMAN et al. (1959).
Ionospheric probing using continuous waves at oblique incidence
643
Comparisons of the signals received over low-latitude propagation paths in the Americas and the Far East respectively indicated that the behaviour was not the same in the two zones (SMITH and FINNEY, 1960). 2.2 Importance
and relevance of the technique
Radio circuits which rely on forward-scatter generally display a weak continuous signal caused by scattering from the D-region, together with fluctuating components resulting from reflection or scattering from meteor trains. Decreases in the signal strength are observed during the day at high latitudes; these occur when the absorption changes as a result of increases in the ionization below the scattering level. The ionization at these lower heights can be affected by solar X-rays emitted during flares, solar protons (observable mainly in the polar cap) or by electron precipitation in regions for which the magnetic L-value lies between approximately 4 and 8 (BAILEY, 1964). Forward-scatter circuits are particularly valuable in unlike other techniques, they are not susceptible to studies of these phenomena; blackout effects, for example, because the scattering levels are below the region where most of the aurora1 absorption takes place. Enhancements of forward-scatter signals can occur both by day or at night; these arise from increases in the ambient electron concentration at the scattering level. Such increases are associated with clouds of sporadic-E ionization or with spread-F irregularities. It is apparent, therefore, that synoptic studies using the forward-scatter technique enable the behaviour of these types of phenomena to be studied; MIYA and SASARI (1966) have carried out investigations of sporadic-E in this way. The occurrence of transequatorial VHF F-layer forward scatter has been found to be correlated to some extent with direct backscatter returns from irregularities, with magnetic activity and with flutter fading observed on HF signals. The heights of the scattering centres may be deduced if the antennae used at both the transmitter and the receiver have narrow beam-widths such that the region investigated extends over a limited height range (COHEN and BOWLES, 1963). Alternatively, some indication of the height may be obtained by using a pulse transmitter and making time-delay measurements; the earliest method was based on measurements of delays with respect to the relatively stable signal scattered from the D-region, but this technique has been superseded by the use of highstability frequency standards and time references at each end of the path. Measurements of changes in the azimuth of the arriving scatter signals may be used to derive information about the drifts experienced by the scattering centres (MIYA et al., 1961). COHEN and BOWLES (1963), using pulse transmissions, have examined the extent of the pulse broadening in order to assess the range of heights over which the scattering irregularities exist. Linearly polarised antennae have been used to investigate the extent to which the irregularities are field-aligned, and estimates of their sizes have been made. The fading rates observed on forward-scatter circuits depend on the ionospheric drifts which occur and signals from highly stable continuous wave transmitters may, therefore, be analysed in order to derive the mean drift velocity of the irregularities and also the spread of velocities involved (NIELSON, 1968).
D. ECCLES, J. ‘VV.Krrvaand P. A. BRADLEY
544
Comparisons of riometer observations and VHF forward”s~atter observations made at the same time have led to an improved understan~ng of the effects of electron and proton precipitation near the aurora1 zone (BAILEY, 1968). The interpretation of forward-scatter observations is considerably assisted by a knowledge of the midpoint vertical electron distributions (COH[ENand BOWLES, 1963) or by simultaneous oblique sounding data acquired over the same propagation path. 2.3 Observatories
using the technique
Various types of equipment ranging from relatively simple to very complex systems have been used for forward-scatter investigations. Useful information can be obtained by monitoring television transmissions over paths such as Korea to Townsviile, Australia (CARBZAN et al., 1963). On the other hand, sophisticated equipment is involved in the power spectra studies which are being made in the Far East by workers from the Stanford Research Institute (NIELSON,1969). Forward-scatter circuits generally involve transmitters having powers of a few kW and which operate at frequencies near 50 MHz; rhombic or yagi antennae are used for transmission and reception. During the IQSY, the Environmental Science Services Administration operated six forward-scatter links, three in Antarctica and three in the Arctic. These circuits were used specifically to study PCA events and operated at frequencies near 23 MHz at the time of sunspot minimum. 2.4 Future
research
and de~elo~~~~ts
Useful information on the high-latitude ionosphere could be obtained from a forward-scatter installation whioh was used to acquire data at the same time as a riometer and a vertical incidence ionosonde were operating at the midpoint of the path. In particular, if the heights of the scattering centres were determined (using narrow-beam antennae, for example) and two or more frequencies were employed, the system could be used to distinguish between the various effects which enhance or absorb the scattered signals (BAILEY, 1968). Observations made using a combination of the three experimental techniques referred to above would provide information, which would otherwise be difficult to obtain, about turbulence in the mesosphere. The characteristics of the equatorial electrojet, equatorial sporadic-E and the irregularities which give rise to spread-P echoes are not fully understood. Further studies of these phenomena, and also of ionospheric movements at both E- and P-region heights, using the forward-scatter technique are likely to prove valuable. 3. STUDIESOB RADIO AURORAEUSING BISTATICSYSTEMS 3.1 Introduction Radio waves can be reflected from ionization irregularities which appear to be associated with visual aurorae; these ‘radio aurorae’ can be studied either by radar techniques, that is, aurora1 radar (BRADLEYet a&, 1970), or by using continuous wave bistatic systems. The main difference between the two techniques is that in the bistatic system the transmit~r and receiver are not at the same location, as in
Ionospheric probing using continuous waves at oblique incidence
545
the radar case, but can be separatedby distancesof up toseveralhun~ed kilometres. Various authors, including EGELANDet at. (1961), EGELAND(1962a, b) and OKSMAN (1964, 1966), have studied the occasions when signals from various VHF broadcast stations in Scandinavia were received at other sites after being reflected from aurora1 irregularities. Similar studies have been carried out in Canada (COLLINS and FORSYTH,1959 and GREEN, 1961) and in the U.S.A. (DYCE, 1955b). Other bistatic VHF studies covering larger geographical areas have also been made using networks of amateur radio stations in North America (MOORE,1951; DYCE, 1955a; GERSON, 1955a, b), inCheUnitedKingdom (STONE,1960,1965; &WITH-ROSE, 1960; NEWTON,1966) and in Germany (LANGE-HESSE,1962; 1963a, b, 1964a, b; LANCE-HESSEand C~~CHOWSKY,1965, 1966). This Section is primarily concerned with the principles of the continuous wave bistatic technique for studying radio aurorae. Details of the results obtained using the technique have been published in review papers by EGELAND(1962a, b) and by LANGE-HESSE(1968). 3.2 Principle of the technique BOOKER (1956) suggested that aurora1 radar echoes arise when the incident radio waves are scattered by ionospheric irregularities. According to the theory the irregularities are field-aligned columns of ionization which exist in the E- and lower P-regions of the ionosphere. Energy will only be scattered back to the radar site by irregularities situated such that the radar beam intersects the Earth’s magnetic field lines nearly perpendicularly. E~ELAND (1962b) extended the Booker scattering theory to apply to bistatic observations of radio aurorae in which radio waves are received after being scattered in directions other than directly backwards. He showed that, similar to the radar case, there is, for any particular positions of the transmitter and receiver, a limited region in which the irregularities must be situated for energy to be scattered towards the receiving site. Figure 1 illustrates the geometry of the propagation path for bistatic aurora1 communication between a transmitter and a receiver. The optimum condition for energy to be scattered by an irregularity at I towards the receiver is that eos &, -+- cos Q, = 0 (EGELAND, 19623). This means that !& + s;Z, = 180”, that is, the signal transmitted from T is specularly reflected by the field-aligned irregularity towards the receiver. If the sum of the angles !& and Q1 deviates from the ideal backscatter conditions by only a few degrees (similar to the radar ‘offperpendicular’ angle, see BRADLEYet al., 1970) the power of the received signal will be substantially decreased. Figure 2 illustrates the method used to determine the region within which the field aligned columns of ionization must occur in order to obtain bistatic aurora1 communication between T and R. The dashed lines show the loci of the points where radio waves transmitted from T intersect the Earth’s magnetic field lines at a fixed height, usually assumed to be 100 km, at angles of 88”, 90” and 92” respectively. These angles correspond to a, in Fig. 1. The dotted lines relate to the angles between the Earth’s magnetic field lines and the direction of propagation of the radio waves arriving at R(!& in Fig. 1.). The solid curve which joins the points where Q, + .R, = 180’ represents
D. ECCLES,J. W.
546
and P. A.
KINU
BRADLEY
Fig. 1. Illustration of a propagation path between a transmitter, T and reo&er, R.
SE0 %_
-.._
/ _...
99 _=..__
x 2
%_ ;x__
/
/
SE? ,90”
/
/
I
/
,92’
’ I’ _--_ I , r“_s-_/’ / __- 92%.-___ ---___ / ----____ ” -----____ --------<_______- >-6 y _-----__ ---------_________ _-/ _/-/ ------.- -.___ _.._________._._ /-----c----__ / Y-l..__
55’N
$ -I
50°N[
I
0%
I
IO’E
5%
I IPE
LONGITUDE
Fig. 2. Diagram showingthe location of the ‘backscatter curve’ (continuouscurve) for a typical bistatic aurora1 communication link in which the transmitter and receiver are situated at T and R respectively.
the ‘backscatter curve’; provided thlat field-aligned irregularities occur at one point on this curve maximum signal strength will be received at R. It has already been pointed out, however, that bistatio aurora1 communication is possible even if the ideal conditions are not fulfilled. Depending on the power of the transmitter and the gain of the antennae, radio aurorae may be detected in an area bounded by the points at which fz, + Q2, = 180” &- 4”. 3.3 Iuformation
The technique
which can be obtained usiug the technique has been much used for studying the occurrence
of radio aurorae.
EUELAND (1962a), for example, has established from a network of stations in Sweden how the frequency of occurrence of radio aurorae depends on local time,
season, sunspot cycle and magnetic activity. In principle the location of the radio aurorae can be made using a single bistatic system, particularly if the corresponding backscatter curve extends over a limited
Ionosphericprobingusing continuouswavesat obliqueincidence
547
region (OKSMAN,1966). Much more useful information is obtained, however, from observations made simultaneously using a network of transmitters and receivers. Obviously, since the backscatter curves for the various transmit~r to receiver links are known, it is possible to study the geographical extent of the radio aurorae by noting which of the stations within the network obtain signals simultaneously (LANOE-HESSE,1968). If the different backscatter curves occur over a range of latitudes, bulk movements of the radio aurora1 region in the North-South direction can be detected. Observations made using appropriate networks of stations can thus provide useful information on the morphology of substorm efFects and, possibly, about the relationship between plasma acoustic waves and radio aurorae. Measurements of the amplitude distribution and fading rate of the observed signals provide some information on the motions of the scattering centres (EGELAND, 1962a) and also on the mechanism by which the irregularities are produced (SOFKOand KAVADAS, 1969). The bistatic technique can only provide qualitative information about radio aurorae. The most serious limitation arises from the fact that the area within which the radio aurorae occur can only be approximately located; the radar method, on the other hand, provides much better positional information. Although measurements of the amplitude and fading rate of the backscattered C.W. signals do indeed provide information about the motion of the irregularities, their actual velocities can be measured more easily and accurately by means of the Doppler radar technique in which the transmitter and receiver are at the same site. Doppler shift measurements could in principle be made using the VHF continuous wave bistatic technique at locations where it is possible to receive reference signals propagated directly via the troposphere from the transmitter. Alternatively, crystal oscillators could be used to control the transmitted signal and to provide a reference signal at the receiving site. 3.5 Correlation between,visual and radio aurorae It is well known that the regions where visual and radio aurorae occur are not always coincident (see BRADLEYet al., 1970). The qualitative nature of the observations made using the continuous wave bistatic technique limits the usefulness of the technique for determining the extent to which the irregularities responsible for radio aurorae are located in regions displaying visual aurorae. Radio aurorae observations made using a network of stations on occasions when visual aurorae occurred have, however, been reported by LANGE-HESSE(1968); he demonstrated that bistatic aurora1 communications were only possible on those paths within the network for which the backscatter curves passed through the area containing the visual aurorae. Ac~owledge~e~~~This article is published xitfl the permission of the Director of the Radio and Sp~e ResearchStation.
REFERENCES BAILEY BAILEY
D.K. D.K.
1964 1968
Planet. Space Sci. 12, 496. Rev. Lfeophys. 6, 289.
548
D. ECCLES,J. W. KING and P. A. BRADLEY
BAILEY D. K., BATEMANR., BERKNERL. V., BOOKERH. G., MONT~OJ~ERY G. F., PURCELL E. M., SALISBURY W. W. and WEISNER J. B. BAILEY D. K. BATEMAN R. and KIRBY R. C. BAKER D. M. and DAVIES K. BAKER D. M. and DAVIES K. BAKER D. M. and DAVIES K. BATEMAN R., FINNEY J. W., SMITH E. K., TVETEN L. H. and WATTS J. M. BOOKER H. G. BRADLEY P. A., ECCLES D. and KING J. W. CALVERT W., DAVIES K., STILTNER E.
1952
Phys. Rev. 86, 141.
1955
Proc.
1966 1968 1969 1959
J. J. J. J.
1956 1970 1962
J. Atmosph. Terr. Phys. 8, 204. J. Atmosph. Terr. Phys. 32, 499. Proceedings of the International Conference on the Ionosphere (Edited by A. C. STICKLAND), p. 316. The Iustitute of Physics and the Physical Society, London. Aust. J. Phys. 16,171.
and BROWN J. T.
1963
IRE
43, 1181.
geophys. Res. 71,2840. geophys. Res. 73, 448. Atmosph. Terr. Phys. 31, 1345. geophys Res. 64, 403.
CARMAN E. H., GIBSON-WILDE B. C. and CONWAY R. J. COHEN R. and BOWLES K. L. COLLINS C. and FORSYTH P. A. DAVIES K. and BAKER D. M. DAVIES K. and BAKER D. M. DAVIES K. and DONNELLY R. F. DONNELLY R. F. DYCE R. DYCE R. EGELAND A. EGELAND A. GARRIOTT 0. K., DA ROSA A. V. and VILLA~D 0. G. GEORGES T. M.
1963 1959 1965 1966 1966 1969 1955s 195513 1962s 1962b 1967
J. Res. natn. Bur. Stand. 67,459. J. Atmosph. Terr. Phys. 13, 315. J. geophys. Res. 70, 2251. Radio Sci. 1,545. J. geophys. Res. 71, 2843. J. geophys. Res. 74,1873.
1968
GERSON N. C. GERSON N. C. LANGE-B&SE G. LANGE-HESSE G. LANGE-HESSE G.
1955a 195513 1962 1963a 1963b
LANGE-HESSE G. LANGE-HESSE G.
1964a 196413
LANGE-HESSE G. and CZECHOWSKY P. LANGE-HESSE G. and CZECHOWSKY P. LANGE-HESSE G.
1965 1966 1968
Symposium Proceedings on AcousticGravity Waves in the Atmosphere (Edited by T. M. GEORGES), p. 171. U.S. Government Printing Office, Washington D.C. Proc. phys. Sot. 68, 408. J. Atmosph. Terr. Phys. 6, 263. Arch. elekt. &ertr. 16, 251. 2. Geophys. 29, 35. Abhandlungen der Akademie der Wissenschaften. in GGttingen, Math-Phys. Klasse, Beitriige zum Internationalen Geophysikalischen Juhr, Heft 10. Verlag Vandenhoec k and Ruprecht, GGttingen, Germany. Arch. elekt. Ubertr. 18,430. Arctic Communications, AGARDograph 78 (Edited by B. LANDMARK), p. 253. Pergamon Press, Oxford. Arch. elekt. iibertr. 19,511. Arch. elekt. Ubertr. 20, 365. Ionospheric Radio Communicationx, p. 174. Plenum Press, New York
&ST 39, 11. IRE Trans. Antennas Propag. Ark. Geofys. 4,103. Ark. Geofys. 4, 171. J. geophys. Res. 72, 6099.
A&3,76.
Ionospheric probing using continuous waves at oblique incidence MIYA K. and SASAKI T. MIYA K., SASAKI T. and ISHIKAWAM. MOORER. K. NEWTON C. NIELSOND. L.
1966 1961 1951 1966 1968
OKSMANJ. OKSMANJ. SETH E. K. and FINNEY J. W. SMITH-ROSER. L. SO~KO G. J. and KAVADAS A. STOXE G. M. C. STONE G. M. C. TILTON E. P. YUEN P. C., WEAVER P. F., SUZAKAR. K. and FURUMOTOA. S.
1964 1966 1960 1960 1969 1960 1965 1947 1969
Radio Sci. 1,99. J. Res. natn. BUT. Stand. 65, 93. J. geophys. Res. 56, 97. RSGB Bull. 42, 289 and 785. AGARD Conference Proceedings 37, NATO, 7 Rue Ancelle Neuilly-sur-Seine, France. An& Acad. Sci. 169,1. Geophylsia 9, 235. J. geophys Res. 65, 885. RSGB Bull. 35, 392. J. geophys. Res. 74, 3651. RSGB Bull. 35, 395. Int. Radio Jl 20, 24. &ST 31, 56. J. geophys. Res. 74, 2256.
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No. 92
Reference is also mude to the following unpublished material: EQELANDA., ORTNERJ. and HULTQVISTB.
1961
GREEN F. D.
1961
hTn%soN D. L.
1969
A study of the statistics of VHF oblique amoral reflections, Scientifi Rep. No. 7. Kiruna Geophysical Observatory, Sweden. P&tern in the beheviour of VHF bistatic radio reflectionsin the aurora1 zone, Radio Studies, Rep. No. RS-9. Institute of Upper Atmospheric Physics, Univ. of Saskatchewan, Saskatoon, Canada. Long-rangeVHF propagation across the geomagnetic equator, Research Rep., Stanford Research Institute, Menlo Park. California.
Ionospheric probing using continuous waves at oblique incidence * D. ECCLES, J. W. KING and P. A. BRADLEY Radio and Space Research Station, Ditton Park, Slough, England (Received 5 November 1969) Abstr&---Three different ionospheric probing techniques which all use continuous waves at oblique incidence are discussed. These are (a) the high-frequency Doppler techniquein which the Doppler frequency shifts caused either by vertical ionospheric movements or by ionization changes are studied, (b) the forward-scatter of VHF waves from the ionosphere and (c) the study of radio aurorae using bistatic systems in which the transmitter and receiver are situated several hundred kilometres apart, but both on the equator side of the aurora1 irregularities. The type of ionospheric information which can be obtained using each of the three techniques, the limitations of the techniques and the future research which needs to be done are all discussed. 1. THE HIGH-FREQUENCY 1.1
DOPPLER TECHNIQUE
Outline of the technique and the type of data obtained
The high-frequency Doppler technique involves recording, on slowly moving magnetic tape, the beat frequency obtained by mixing radio waves which have been reflected from the ionosphere with a local, slightly offset, reference signal (DAVIES and BAKER, 1966). When changes occur in the phase height of the sky wave, due either to vertical movements of the ionosphere or to ionization changes, corresponding changes of the beat frequency are produced. The frequency shift depends on the rate of change of phase experienced by the ionospheric waves as the height of the reflection point moves. When the tape recordings are played back rapidly, the changes of the recorded beat frequency can be determined by either using conventional audio frequency analysers or by using a computer to perform an analysis of the data in digital form. Many standard frequency transmissions exist which are quite suitable for this type of experiment and therefore it is only necessary to provide recording equipment which means that the technique is relatively inexpensive. 1.2
Information which can be obtained using the technique
The technique is primarily suitable for investigating phenomena which are associated with short-term changes in the ionosphere. These phenomena can be broadly divided into (a) those associated with enhancements of ionization and (b) those associated with vertical movements of the ionosphere. 1.2(a) Phenomena associated with enhancements of ionization. Rapid increases in the solar EUV radiation cause enhancements of ionization which occur predominantly at E- or P- layer heights; these in turn produce sudden frequency deviations, SFD’s, of the recorded beat frequency (DONNELLY, 1969). SFD’s can be divided * Important
contributions to this paper were made by K. Davies and G. Lange-Hesse. 639
540
D. ECCLES, J. VI’. KINGand P. A. BRADLEY
into two types, one being due to large impulsive changes of electron concentration in the upper E- and P-regions and the second being caused by small, slow ionization changes which tend to occur in the D- and lower E-regions. The flare radiation associated with the impulsive changes is primarily in the 20-1030J%range which has not yet been studied adequately by means of satellites. This radiation is, however, absorbed in a region of the ionosphere which is reasonably well understood and, therefore, impulsive SFD’s can provide useful information on the EUV radiation in this particular part of the spectrum. On the other hand, SFD’s of the second type are associated with radiation in the 0~5-8A range which is absorbed in the D- and lower E-regions. The radiation in this part of the spectrum is currently being monitored by many satellites, but the region of the ionosphere in which it is absorbed is not well understood. The slow SFD’s can therefore provide useful additional information on the behaviour of the D-region. In some cases the variations are too slow to be detected by frequency shift measurements and phase measurements have to be studied instead. SFD data may also be used to estimate the effective recombination coefficient in the E- and El-layers (BAKE%and DAVIES, 1966). 1.2(b) Phenomena associated with vertical movements of the ionosphere. Vertical movements of the ionosphere associated with various phenomena have been observed using the high-frequency Doppler technique. In particular, it has been found useful for detecting atmospheric waves resulting from particle precipitation in the aurora1 zones (GEOROES,1968), and for ~vestigating rapid ionospheric changes which occur during geomagnetic sudden commencements (DAVIES and BAKER, 1966) or during ionospheric storms. In addition, the technique has provided important information about infrasouic waves in the P-region originating from nuclear explosions (BAKERand DAVIES,1968), earthquakes (DAVIESand BAKER, 1965; YUEN et al.,1969),severe thunderstorms (GEORQES,1968; BAKER and DAVIES, 1969), travelling ionospheric disturbances and the movement of irregularities associated with spread-P echoes (CALVERTet al., 1962). I.3 Limitatiolzs of the technique and the accuracy with which the measurements can be made The existing theory is basically sound for those easesin which ray theory is applicable. Cases in which the simplest form of ray theory may not apply include, for example, very large SFD’s in which the ionization enhancement is such that the propagation path switches from the F-region to the E-region; near the time of the switch both paths can exist for a short period. The frequency resolution of the measurements has, until recently, been about 0.1 Hz. The frequency stability of the E-region echoes is generally better than this and the Doppler technique has, therefore, been mainly limited to the study of P-layer echoes. The interpretation of the data is complicated by the fact that the Doppler shift depends on effects integrated over a range of heights up to the reflection level. The ionization changes at different heights can, however, be distinguished by using multifrequency Doppler recordings.
Ionospheric probing using continuous waves at oblique incidence
541
The standards which have been found to be minimal for any high frequency Doppler installation are as follows: Transmitter power: Oscillator stability: Time resolution: Tape speed: Dynamic range: 1.4 C~~~u~~~o~~
50 TN 1 part in 108 2 see 5 . 1O-2 cm see-l 50 dB.
waft other te~h~~q~es
Very good agreement has been reported between the occurrence or duration of SFD’s and various other solar or ionospheric phenomena as follows: (a) (b) (c) (d) (e) (f)
The explosive stages of H-alpha bursts (DAVIESand DONNELLY; 1966), bursts of hard X-rays and the ra,pid rise of soft X-ray bursts, EUV bursts (DONNELLY,1969), enhancements of total electron content (GARRIOTTet al., 1967), impulsive bursts of cm radio noise and geomagnetic crochets and sudden commencements.
The value of the effective recombination coefficient in the E- and Pl-layers deduced from SFD data is in good agreement with the values obtained by other methods (BAKER and DAVIES, 1966). 1.5 Observatories using the technique
Table 1 contains a list of the laboratories which are known to make regular high-frequency Doppler observations. The National Physical Laboratory, India, records only on 10 MHz and for part of the day the dominant signal received in Delhi is not JJY but ATA. It should be noted, from a monitoring point of view, that in Europe some of the standard frequency stations (e.g. MSF and HBN) operate on a time-sharing basis, I .6 Future research and ~~ve~o~~ents There is no doubt that the HF Doppler technique will continue as an important auxiliary technique for studying solar EUV radiation bursts until routine satellite data having adequate time resolution become available. In order to obtain continuous coverage the present network should, in fact, be improved by extending the range of observing frequencies and times; it appears likely that the best way in which this could be done would be to bring into operation a few transmitters specifically for Doppler measurements. The multi-frequency Doppler technique with spaced transmitters (or spaced receivers) is a useful technique for measuring F&layer drift velocities in three dimensions. It could also be used to provide further useful information about the effects of infrasonic waves, (produced by sources near the ground) on the ionosphere and also for studying ionospheric transients associated with magnetic storms. SFD data frequently show a marked periodicity and Dr. R. F. Donnelly, ESSA Research Laboratories, Boulder, Colorado (private communication) has
542
D. ECCLES,J. W. KING and P. A. BRADLEY Table 1. Observatoriesmaking regular Doppler technique recordings
Institution University of Hawaii, U.S.A. Kyoto University, Japan National Physical Laboratory, India University of Leicester, England
Principal investigator P. C. Yuen T. Ogawa A. P. Mitra T. B. Jones
ESSA Research Laboratories, K. Davies U.S.A.
Path* WWVH-Honolulu IJT-Kyoto JJY-Delhi MSF-Leicester OMA-Leicester HBN-Leicester Illinois-Boulder Sunset-Boulder
South Pole--Byrd South Pole-McMurdo South Pole-Vostok McMurdo-Byrd McMurdo-South Pole McMurdo-Vostok Illinois-Boulder G. M. Lerfald WWV-Boulder
Path length (kin) 150 360 5900 20 900 900
Carrier frequency (MHz) 2.5, 5, 10, 15 5, 10, 15 10
2.5, 5.0
1100 1200 1200 1500 1200
8.9, 11.1, 13.0 2.1, 3.3, 4.0 4.9, 5.1 6 6 6 7 7
1300 1300 80
7 8.9, 11.1 2.5, 5.0
1300 25
* The locations of the Standard Frequency Stations are as follows: WWV Fort Collins, Colorado, MSF Rugby, England HBN Geneva, Switzerland WWVH Maui, Hawaii, OMA Prague, Czechoslovakia U.S.A. JJY Tokyo, _ Japan U.S.A.
suggested that it would be interesting to investigate the relationship these variations and the fine structure observed in hard X-ray bursts.
between
2. FORWARD SCATTER 2.1
Ih-0&.&07b
Many successful attempts have been made to transmit signals over long oblique propagation paths at frequencies which are higher than the expected maximum usable frequencies for the paths involved. Some of the earliest reports that satisfactory communication could be maintained at such high frequencies were made by radio amateurs and it was soon established that this type of propagation was most common during years of high sunspot activity, and that it occurred particularly in equatorial regions. TILTON (1947) reported that on certain occasions signals at frequencies near 50 MHz were propagated satisfactorily over transequatorial paths of up to 7000 km, It was believed at first that such propagation occurred solely because the ionospheric layers were tilted, but it became evident later that scattering mechanisms were also sometimes involved. BAILEY et al. (1952, 1955) carried out detailed investigations of the characteristics of VHF forward-scattered signals for both temperate and high-latitude paths in North America. Studies relating to lowlatitude paths in the Far East were later reported by BATEMAN et al. (1959).
Ionospheric probing using continuous waves at oblique incidence
643
Comparisons of the signals received over low-latitude propagation paths in the Americas and the Far East respectively indicated that the behaviour was not the same in the two zones (SMITH and FINNEY, 1960). 2.2 Importance
and relevance of the technique
Radio circuits which rely on forward-scatter generally display a weak continuous signal caused by scattering from the D-region, together with fluctuating components resulting from reflection or scattering from meteor trains. Decreases in the signal strength are observed during the day at high latitudes; these occur when the absorption changes as a result of increases in the ionization below the scattering level. The ionization at these lower heights can be affected by solar X-rays emitted during flares, solar protons (observable mainly in the polar cap) or by electron precipitation in regions for which the magnetic L-value lies between approximately 4 and 8 (BAILEY, 1964). Forward-scatter circuits are particularly valuable in unlike other techniques, they are not susceptible to studies of these phenomena; blackout effects, for example, because the scattering levels are below the region where most of the aurora1 absorption takes place. Enhancements of forward-scatter signals can occur both by day or at night; these arise from increases in the ambient electron concentration at the scattering level. Such increases are associated with clouds of sporadic-E ionization or with spread-F irregularities. It is apparent, therefore, that synoptic studies using the forward-scatter technique enable the behaviour of these types of phenomena to be studied; MIYA and SASARI (1966) have carried out investigations of sporadic-E in this way. The occurrence of transequatorial VHF F-layer forward scatter has been found to be correlated to some extent with direct backscatter returns from irregularities, with magnetic activity and with flutter fading observed on HF signals. The heights of the scattering centres may be deduced if the antennae used at both the transmitter and the receiver have narrow beam-widths such that the region investigated extends over a limited height range (COHEN and BOWLES, 1963). Alternatively, some indication of the height may be obtained by using a pulse transmitter and making time-delay measurements; the earliest method was based on measurements of delays with respect to the relatively stable signal scattered from the D-region, but this technique has been superseded by the use of highstability frequency standards and time references at each end of the path. Measurements of changes in the azimuth of the arriving scatter signals may be used to derive information about the drifts experienced by the scattering centres (MIYA et al., 1961). COHEN and BOWLES (1963), using pulse transmissions, have examined the extent of the pulse broadening in order to assess the range of heights over which the scattering irregularities exist. Linearly polarised antennae have been used to investigate the extent to which the irregularities are field-aligned, and estimates of their sizes have been made. The fading rates observed on forward-scatter circuits depend on the ionospheric drifts which occur and signals from highly stable continuous wave transmitters may, therefore, be analysed in order to derive the mean drift velocity of the irregularities and also the spread of velocities involved (NIELSON, 1968).
D. ECCLES, J. ‘VV.Krrvaand P. A. BRADLEY
544
Comparisons of riometer observations and VHF forward”s~atter observations made at the same time have led to an improved understan~ng of the effects of electron and proton precipitation near the aurora1 zone (BAILEY, 1968). The interpretation of forward-scatter observations is considerably assisted by a knowledge of the midpoint vertical electron distributions (COH[ENand BOWLES, 1963) or by simultaneous oblique sounding data acquired over the same propagation path. 2.3 Observatories
using the technique
Various types of equipment ranging from relatively simple to very complex systems have been used for forward-scatter investigations. Useful information can be obtained by monitoring television transmissions over paths such as Korea to Townsviile, Australia (CARBZAN et al., 1963). On the other hand, sophisticated equipment is involved in the power spectra studies which are being made in the Far East by workers from the Stanford Research Institute (NIELSON,1969). Forward-scatter circuits generally involve transmitters having powers of a few kW and which operate at frequencies near 50 MHz; rhombic or yagi antennae are used for transmission and reception. During the IQSY, the Environmental Science Services Administration operated six forward-scatter links, three in Antarctica and three in the Arctic. These circuits were used specifically to study PCA events and operated at frequencies near 23 MHz at the time of sunspot minimum. 2.4 Future
research
and de~elo~~~~ts
Useful information on the high-latitude ionosphere could be obtained from a forward-scatter installation whioh was used to acquire data at the same time as a riometer and a vertical incidence ionosonde were operating at the midpoint of the path. In particular, if the heights of the scattering centres were determined (using narrow-beam antennae, for example) and two or more frequencies were employed, the system could be used to distinguish between the various effects which enhance or absorb the scattered signals (BAILEY, 1968). Observations made using a combination of the three experimental techniques referred to above would provide information, which would otherwise be difficult to obtain, about turbulence in the mesosphere. The characteristics of the equatorial electrojet, equatorial sporadic-E and the irregularities which give rise to spread-P echoes are not fully understood. Further studies of these phenomena, and also of ionospheric movements at both E- and P-region heights, using the forward-scatter technique are likely to prove valuable. 3. STUDIESOB RADIO AURORAEUSING BISTATICSYSTEMS 3.1 Introduction Radio waves can be reflected from ionization irregularities which appear to be associated with visual aurorae; these ‘radio aurorae’ can be studied either by radar techniques, that is, aurora1 radar (BRADLEYet a&, 1970), or by using continuous wave bistatic systems. The main difference between the two techniques is that in the bistatic system the transmit~r and receiver are not at the same location, as in
Ionospheric probing using continuous waves at oblique incidence
545
the radar case, but can be separatedby distancesof up toseveralhun~ed kilometres. Various authors, including EGELANDet at. (1961), EGELAND(1962a, b) and OKSMAN (1964, 1966), have studied the occasions when signals from various VHF broadcast stations in Scandinavia were received at other sites after being reflected from aurora1 irregularities. Similar studies have been carried out in Canada (COLLINS and FORSYTH,1959 and GREEN, 1961) and in the U.S.A. (DYCE, 1955b). Other bistatic VHF studies covering larger geographical areas have also been made using networks of amateur radio stations in North America (MOORE,1951; DYCE, 1955a; GERSON, 1955a, b), inCheUnitedKingdom (STONE,1960,1965; &WITH-ROSE, 1960; NEWTON,1966) and in Germany (LANGE-HESSE,1962; 1963a, b, 1964a, b; LANCE-HESSEand C~~CHOWSKY,1965, 1966). This Section is primarily concerned with the principles of the continuous wave bistatic technique for studying radio aurorae. Details of the results obtained using the technique have been published in review papers by EGELAND(1962a, b) and by LANGE-HESSE(1968). 3.2 Principle of the technique BOOKER (1956) suggested that aurora1 radar echoes arise when the incident radio waves are scattered by ionospheric irregularities. According to the theory the irregularities are field-aligned columns of ionization which exist in the E- and lower P-regions of the ionosphere. Energy will only be scattered back to the radar site by irregularities situated such that the radar beam intersects the Earth’s magnetic field lines nearly perpendicularly. E~ELAND (1962b) extended the Booker scattering theory to apply to bistatic observations of radio aurorae in which radio waves are received after being scattered in directions other than directly backwards. He showed that, similar to the radar case, there is, for any particular positions of the transmitter and receiver, a limited region in which the irregularities must be situated for energy to be scattered towards the receiving site. Figure 1 illustrates the geometry of the propagation path for bistatic aurora1 communication between a transmitter and a receiver. The optimum condition for energy to be scattered by an irregularity at I towards the receiver is that eos &, -+- cos Q, = 0 (EGELAND, 19623). This means that !& + s;Z, = 180”, that is, the signal transmitted from T is specularly reflected by the field-aligned irregularity towards the receiver. If the sum of the angles !& and Q1 deviates from the ideal backscatter conditions by only a few degrees (similar to the radar ‘offperpendicular’ angle, see BRADLEYet al., 1970) the power of the received signal will be substantially decreased. Figure 2 illustrates the method used to determine the region within which the field aligned columns of ionization must occur in order to obtain bistatic aurora1 communication between T and R. The dashed lines show the loci of the points where radio waves transmitted from T intersect the Earth’s magnetic field lines at a fixed height, usually assumed to be 100 km, at angles of 88”, 90” and 92” respectively. These angles correspond to a, in Fig. 1. The dotted lines relate to the angles between the Earth’s magnetic field lines and the direction of propagation of the radio waves arriving at R(!& in Fig. 1.). The solid curve which joins the points where Q, + .R, = 180’ represents
D. ECCLES,J. W.
546
and P. A.
KINU
BRADLEY
Fig. 1. Illustration of a propagation path between a transmitter, T and reo&er, R.
SE0 %_
-.._
/ _...
99 _=..__
x 2
%_ ;x__
/
/
SE? ,90”
/
/
I
/
,92’
’ I’ _--_ I , r“_s-_/’ / __- 92%.-___ ---___ / ----____ ” -----____ --------<_______- >-6 y _-----__ ---------_________ _-/ _/-/ ------.- -.___ _.._________._._ /-----c----__ / Y-l..__
55’N
$ -I
50°N[
I
0%
I
IO’E
5%
I IPE
LONGITUDE
Fig. 2. Diagram showingthe location of the ‘backscatter curve’ (continuouscurve) for a typical bistatic aurora1 communication link in which the transmitter and receiver are situated at T and R respectively.
the ‘backscatter curve’; provided thlat field-aligned irregularities occur at one point on this curve maximum signal strength will be received at R. It has already been pointed out, however, that bistatio aurora1 communication is possible even if the ideal conditions are not fulfilled. Depending on the power of the transmitter and the gain of the antennae, radio aurorae may be detected in an area bounded by the points at which fz, + Q2, = 180” &- 4”. 3.3 Iuformation
The technique
which can be obtained usiug the technique has been much used for studying the occurrence
of radio aurorae.
EUELAND (1962a), for example, has established from a network of stations in Sweden how the frequency of occurrence of radio aurorae depends on local time,
season, sunspot cycle and magnetic activity. In principle the location of the radio aurorae can be made using a single bistatic system, particularly if the corresponding backscatter curve extends over a limited
Ionosphericprobingusing continuouswavesat obliqueincidence
547
region (OKSMAN,1966). Much more useful information is obtained, however, from observations made simultaneously using a network of transmitters and receivers. Obviously, since the backscatter curves for the various transmit~r to receiver links are known, it is possible to study the geographical extent of the radio aurorae by noting which of the stations within the network obtain signals simultaneously (LANOE-HESSE,1968). If the different backscatter curves occur over a range of latitudes, bulk movements of the radio aurora1 region in the North-South direction can be detected. Observations made using appropriate networks of stations can thus provide useful information on the morphology of substorm efFects and, possibly, about the relationship between plasma acoustic waves and radio aurorae. Measurements of the amplitude distribution and fading rate of the observed signals provide some information on the motions of the scattering centres (EGELAND, 1962a) and also on the mechanism by which the irregularities are produced (SOFKOand KAVADAS, 1969). The bistatic technique can only provide qualitative information about radio aurorae. The most serious limitation arises from the fact that the area within which the radio aurorae occur can only be approximately located; the radar method, on the other hand, provides much better positional information. Although measurements of the amplitude and fading rate of the backscattered C.W. signals do indeed provide information about the motion of the irregularities, their actual velocities can be measured more easily and accurately by means of the Doppler radar technique in which the transmitter and receiver are at the same site. Doppler shift measurements could in principle be made using the VHF continuous wave bistatic technique at locations where it is possible to receive reference signals propagated directly via the troposphere from the transmitter. Alternatively, crystal oscillators could be used to control the transmitted signal and to provide a reference signal at the receiving site. 3.5 Correlation between,visual and radio aurorae It is well known that the regions where visual and radio aurorae occur are not always coincident (see BRADLEYet al., 1970). The qualitative nature of the observations made using the continuous wave bistatic technique limits the usefulness of the technique for determining the extent to which the irregularities responsible for radio aurorae are located in regions displaying visual aurorae. Radio aurorae observations made using a network of stations on occasions when visual aurorae occurred have, however, been reported by LANGE-HESSE(1968); he demonstrated that bistatic aurora1 communications were only possible on those paths within the network for which the backscatter curves passed through the area containing the visual aurorae. Ac~owledge~e~~~This article is published xitfl the permission of the Director of the Radio and Sp~e ResearchStation.
REFERENCES BAILEY BAILEY
D.K. D.K.
1964 1968
Planet. Space Sci. 12, 496. Rev. Lfeophys. 6, 289.
548
D. ECCLES,J. W. KING and P. A. BRADLEY
BAILEY D. K., BATEMANR., BERKNERL. V., BOOKERH. G., MONT~OJ~ERY G. F., PURCELL E. M., SALISBURY W. W. and WEISNER J. B. BAILEY D. K. BATEMAN R. and KIRBY R. C. BAKER D. M. and DAVIES K. BAKER D. M. and DAVIES K. BAKER D. M. and DAVIES K. BATEMAN R., FINNEY J. W., SMITH E. K., TVETEN L. H. and WATTS J. M. BOOKER H. G. BRADLEY P. A., ECCLES D. and KING J. W. CALVERT W., DAVIES K., STILTNER E.
1952
Phys. Rev. 86, 141.
1955
Proc.
1966 1968 1969 1959
J. J. J. J.
1956 1970 1962
J. Atmosph. Terr. Phys. 8, 204. J. Atmosph. Terr. Phys. 32, 499. Proceedings of the International Conference on the Ionosphere (Edited by A. C. STICKLAND), p. 316. The Iustitute of Physics and the Physical Society, London. Aust. J. Phys. 16,171.
and BROWN J. T.
1963
IRE
43, 1181.
geophys. Res. 71,2840. geophys. Res. 73, 448. Atmosph. Terr. Phys. 31, 1345. geophys Res. 64, 403.
CARMAN E. H., GIBSON-WILDE B. C. and CONWAY R. J. COHEN R. and BOWLES K. L. COLLINS C. and FORSYTH P. A. DAVIES K. and BAKER D. M. DAVIES K. and BAKER D. M. DAVIES K. and DONNELLY R. F. DONNELLY R. F. DYCE R. DYCE R. EGELAND A. EGELAND A. GARRIOTT 0. K., DA ROSA A. V. and VILLA~D 0. G. GEORGES T. M.
1963 1959 1965 1966 1966 1969 1955s 195513 1962s 1962b 1967
J. Res. natn. Bur. Stand. 67,459. J. Atmosph. Terr. Phys. 13, 315. J. geophys. Res. 70, 2251. Radio Sci. 1,545. J. geophys. Res. 71, 2843. J. geophys. Res. 74,1873.
1968
GERSON N. C. GERSON N. C. LANGE-B&SE G. LANGE-HESSE G. LANGE-HESSE G.
1955a 195513 1962 1963a 1963b
LANGE-HESSE G. LANGE-HESSE G.
1964a 196413
LANGE-HESSE G. and CZECHOWSKY P. LANGE-HESSE G. and CZECHOWSKY P. LANGE-HESSE G.
1965 1966 1968
Symposium Proceedings on AcousticGravity Waves in the Atmosphere (Edited by T. M. GEORGES), p. 171. U.S. Government Printing Office, Washington D.C. Proc. phys. Sot. 68, 408. J. Atmosph. Terr. Phys. 6, 263. Arch. elekt. &ertr. 16, 251. 2. Geophys. 29, 35. Abhandlungen der Akademie der Wissenschaften. in GGttingen, Math-Phys. Klasse, Beitriige zum Internationalen Geophysikalischen Juhr, Heft 10. Verlag Vandenhoec k and Ruprecht, GGttingen, Germany. Arch. elekt. Ubertr. 18,430. Arctic Communications, AGARDograph 78 (Edited by B. LANDMARK), p. 253. Pergamon Press, Oxford. Arch. elekt. iibertr. 19,511. Arch. elekt. Ubertr. 20, 365. Ionospheric Radio Communicationx, p. 174. Plenum Press, New York
&ST 39, 11. IRE Trans. Antennas Propag. Ark. Geofys. 4,103. Ark. Geofys. 4, 171. J. geophys. Res. 72, 6099.
A&3,76.
Ionospheric probing using continuous waves at oblique incidence MIYA K. and SASAKI T. MIYA K., SASAKI T. and ISHIKAWAM. MOORER. K. NEWTON C. NIELSOND. L.
1966 1961 1951 1966 1968
OKSMANJ. OKSMANJ. SETH E. K. and FINNEY J. W. SMITH-ROSER. L. SO~KO G. J. and KAVADAS A. STOXE G. M. C. STONE G. M. C. TILTON E. P. YUEN P. C., WEAVER P. F., SUZAKAR. K. and FURUMOTOA. S.
1964 1966 1960 1960 1969 1960 1965 1947 1969
Radio Sci. 1,99. J. Res. natn. BUT. Stand. 65, 93. J. geophys. Res. 56, 97. RSGB Bull. 42, 289 and 785. AGARD Conference Proceedings 37, NATO, 7 Rue Ancelle Neuilly-sur-Seine, France. An& Acad. Sci. 169,1. Geophylsia 9, 235. J. geophys Res. 65, 885. RSGB Bull. 35, 392. J. geophys. Res. 74, 3651. RSGB Bull. 35, 395. Int. Radio Jl 20, 24. &ST 31, 56. J. geophys. Res. 74, 2256.
549
No. 92
Reference is also mude to the following unpublished material: EQELANDA., ORTNERJ. and HULTQVISTB.
1961
GREEN F. D.
1961
hTn%soN D. L.
1969
A study of the statistics of VHF oblique amoral reflections, Scientifi Rep. No. 7. Kiruna Geophysical Observatory, Sweden. P&tern in the beheviour of VHF bistatic radio reflectionsin the aurora1 zone, Radio Studies, Rep. No. RS-9. Institute of Upper Atmospheric Physics, Univ. of Saskatchewan, Saskatoon, Canada. Long-rangeVHF propagation across the geomagnetic equator, Research Rep., Stanford Research Institute, Menlo Park. California.