Directional characteristics of ionosonde interference patterns from the Filchner ice shelf

J o u r n a l of Atmospheric and Terrestrial Physics, 1968, Vol. 30, pp. 1115-1134. P e r g a m o n Press. Printed in Northern Ireland

Directional characteristics of ionosonde interference patterns f~om the Filchner ice she]/ G. G. Bow~Aw* Research and Technology Laboratories, AVC0 Corporation, Wilmington,

Massachusetts

(Received 3 Auguat 1967) Abstract--Interference effects appear on vertical-incidence ionograms from Ellsworth Station, which is located on the Fflchner ice shelf in Antarctica. These effects are caused by energy which travels through the ice and is reflected upwards at the bottom of the ice shelf. Because the frequencies at which the minima of the interference pattern occur change in a systematic way for off-vertical reflections, it has been possible to calibrate the ionosonde system so that the angle-of-elevation of off-vertical traces can be calculated. This calibration has allowed a calculation of the ice thickness and the dielectric constant of the ice. It also gives information on the behaviour of the ionosphere when off-vertical reflections are present. In particular, it has allowed the detection of large-scale ionospheric disturbances which are observed at Ellsworth in the early evening hours. Tonosondes coupled to antenna systems, which would make them swept-frequency interferometers, have been suggested. These systems would produce interference patterns similar to those produced by the Flichner ice shelf.

I. INTRODUCTION PIGGOTT and B~RCLAY (1961) were first to report interference effects on verticalincidence ionograms, resulting from the location of an ionosonde on an ice shelf. EVANS (1961) subsequently interpreted the interference patterns as being produced when the r a y radiating directly upwards from the antenna interferes with the r a y which initially travels downwards through the ice before being reflected upwards from the b o t t o m of the ice shelf. Using off-vertical reflections from the sporadic-E layer, he was able to calculate the dielectric constant (2.78 4- 0.05) and the thickness of the ice (265 4- 3 m) at Ellsworth, Antarctica. EUsworth is located on the Filchner ice shelf. These calculations were made by assuming t h a t the effective ground level was located at the b o t t o m of the ice shelf at the ice-seawater interface. Here the reflection coefficient is almost unity, whereas at the top of the ice sheff (at the air-ice interface) it is about 0.3 for the frequency range of the ionosonde (VoN HIPPEL,

1954). I n this present investigation the dielectric constant and thickness of the ice at Ellsworth have been calculated using the t heory from physical optics concerning the interference produced by thin films (JENKINS and WHITE, 1957). The thickness of the ice sheff is such t h a t the shelf appears as a 'thin' film for the frequencies used b y the ionosonde (1-20 Mc/s). The geographical configurations at Ellsworth are drawn approximately to scale in Fig. 1. Possible r ay paths are also shown. As in the optical case, the rays emerging into the air from the second (D and D') and subsequent reflections from the b o t t o m of the ice shelf will have considerably less signal strength t h a n the r ay from the first reflection (B and B'). Consequently, these rays can be * On leave from Department of Physics, University of Queensland, Brisbane, Australia. 1115

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considered as making no significant contribution to the interference pattern. However, there are other complications to this simple treatment, which will be discussed in Section 3. Section 2 describes the observed character/stics of the interference patterns, while Section 3 discusses possible ray paths which might produce the observed patterns. Section 4 presents the details of the calculations of the dielectric constant and the thickness of the ice at Ellsworth. Ionospheric behaviour which can be deduced from angle-of-elevation information, is discussed in Section 5. Section 6 considers possible ways of reproducing these ice shelf effects using swept-frequency interferometers. NORTH/SOUTH PROFILE THROUGH ELLSWORTH

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Filchner ice shelf. In this paper the word 'null' will be used to refer to the apparent absence of the reflected signal at certain frequencies on the ionograms. The observations suggest that this is caused by interference between two rays of unequal intensity and therefore represents a minimum in signal strength. The finite sensitivity of the ionosonde is responsible for these minima appearing as nulls on the ionograms. 2. C~A~ACTERISTICS OF THE I N T E R F E R E N C E PATTERNS

Inspection of the Ellsworth ionograms reveals that nulls, evenly spaced in frequency, often appear on the traces for reflections from both the E- and ~Llayers. When the traces are spread in range as a result of the presence of irregularities, the position of each null progressively shifts to a higher frequency as 'spread' echoes are

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considered with increasing displacement in range from the main trace. The existence of nulls on a trace seems to depend on the signal strength of t h a t trace. "For each trace, t h e y appear immediately after the minimum frequency recorded. Usually several nulls are present; however, t h e y disappear at higher frequencies, presumably when the strength of the signals is sufficiently high. The average position of nulls on the ~V-layer traces has been determined from a number of ionograms in J u l y and August 1957, in which t h e y were shown clearly. Ionograms representative of this group are shown in Fig. 2(f) and (g) and Fig. 3(a). The crosses in Fig. 4 show the average frequency position for each null recorded. I n

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Fig. 4. Calculated and extrapolated positions of nulls for vertically-incident ~2-layer signals. some frequency ranges the nulls are more prevalent than in others. The positions of 18 nulls were determined, l~ive of them, centered on about 3 l~Ic/s (null numbers 8 through 12), were calculated for an average S.E. of the mean of 0.008 Mc/s. The average number of values used for the determination of each null in this group was 17.0. For the remaining 13 nulls, the average S.E of the mean was 0.02 !~c/s and the average number of values used for each null was 5.7. The positions are therefore accurately determined. All lie reasonably close to a straight line drawn through them, and are evenly spaced in frequency. Using the average separation in frequency between nulls (0.32 Mc/s) determined from this straight line, three extrapolated positions at lower frequencies were calculated and are indicated in Fig. 4. I t should be noted from this figure that the straight line intersects the horizontal axis half-way between the origin and the position of null number 1. This must be considered when

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attempting to explain the precise w a y in which the interference patterns are produced, and will be discussed in the next section. There are some occasions in Antarctica when the sporadic-E layer shows retardation effects. Usually, however, these effects are negligible. I f spread echoes are present from the sporadic-E layer when the retardation is negligible, they must come from off-vertical directions. Furthermore, measurement of their ranges compared with the range of the normal-incident reflections (minimum range), will give (by simple geometry) the angle-of-elevation of their arrival. Therefore, the range at which a particular null is located can be used to determine the zenith angle-ofarrival for signals producing the null. This method can be used to calibrate the system so that radiation diagrams for any particular frequency can be determined. This can be done without any knowledge of how or why the interference patterns are produced. Fig. 2(a) shows the nature of the spread echoes used in this calculation. Unlike the ionogram shown here, only ionograms in which the height of the sporadic-E layer was 100 km at all frequencies, were used. I t was found somewhab easier to read the positions of maxima rather than minima. The crosses in Fig. 5 3.03 MCI,~

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show the average positions of maxim a for ranges between 100 and 200 kin, determined from a number of suitable records in J u l y and August, 1957. The corresponding off-vertical angles are shown on the right-hand side of the diagram. The positions of minima are located b y the dashed lines half-way in frequency between adjacent maxima. The information in Fig. 5 allows the determination, for any frequency, of the radiation diagram resulting from the interference. For example, at a frequency of 3.03 Mc/s minima appear at off-vertical angles of 0 and 50 °. Maxima appear at offvertical angles of 34 and 60 ° . Radiation diagrams for four frequencies, including 3.03 Mc/s are shown in Fig. 6. The maxima have been arbitrarily drawn of equal magnitude, since no information is available on relative amplitudes of maxima. The average number of values which were used to determine each point of Fig. 5 was 13.4; the average standard error of the mean for all points was 0.001 Mc/s.

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Regardless of the height from which any particular signal is reflected, these calibration curves (see Fig. 5) will allow a determination of the off-vertical angle of arrival. This is accomplished by measurement of the null displacement (in units of frequency) f r o m t h e frequency of the relevant null at verticalincidence. Fig. 7 shows an example of additional curves which have been drawn to help with this measurement. I t shows a plot of displacement frequency vs. off-vertical angle for displacements from the frequency of null number 8 at vertical incidence. The width of the trace can be regarded as a measure of signal strength. When nulls are just visible, an estimate of the signal strength variation can be gained by inspection of the trace widths between nulls. The behaviour of the trace widths between nulls on Fig. 2(d), (e), (f), and (g) suggests a simple interference pattern with a monotonic variation between maxima and adjacent minima. This variation was found on all other records of this type. I t suggests that the patterns result from the interference of two rays only. Also, Fig. 2(e) and (f) shows that as the frequency increases, the signal strength at the positions where nulls are expected increases, until the minima at these positions can no longer be detected. The results shown in Fig. 4 are also consistent with those expected from the interference of two rays, one starting upwards from the top of the ice sheff and the other travelling upwards after reflection at the bottom surface of the ice. Because the frequency of the first null (extrapolated value) is just one-half the null separation, nulls can be expected when the path difference between the two interfering rays is

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(n -t- ½)2, where t is the wavelength (in a vacuum) at the frequency of a null and n is an integer. At vertical incidence, if the dielectric constant is regarded as the same over the frequency range being considered, the equivalent path difference (vacuum) is equal to 2tV'e, where 8 is the dielectric constant and t is the ice-shelf thickness. Thus for nulls at vertical incidence (n + ½)2 = 2t~/8 = constant ---- X. As n changes (0, 1, 2, 3 . . . ) 4 will change to satisfy the above equation. The series of values taken b y 1 will be 2X, 2X 2X 2X 3'5'7 --

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Thus the first null is expected at a frequency of c/2X and each subsequent null is spaced from its neighbour at intervals of c/X, or c/2t~/s. An examination of ionograms shown in Fig. 3(d) and (e) allows a qualitative estimate of the relative amplitudes of the interfering rays. These two ionograms were taken on successive ionosonde sweeps, 13.5 see apart. Figure 3(d)is with medium gain and Fig. 3(e) is with high gain. I f the location of nulls on both the first- and second-multiple traces are considered, it will be seen that the signal strength is the

Oblique incidence interference observed by ionosonde on ice shelf

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important factor. :For medium sensitivity (Fig. 3(d)), a number of nulls are recorded adjacent to the minimum frequency for both the first- and second-multilSle traces. For high sensitivity (Fig. 3(e)), the signal strength is apparently so high that no nulls are seen on the first-multiple trace, l~or the second-multiple trace, the set of nulls has shifted to a lower frequency range. Similar effects are shown by the set of ionograms given in Fig. 2(g), (h), and (i). These are taken on successive sweeps and are at low, medium, and high gain, respectively. Figure 8 is presented in an attempt to explain the change of interference features with change of gain of the ionosonde. This figure only illustrates the situation qualitatively, since the overall increase in signal strength with increase in frequency for

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PRODUCTION OF THE ISTERrERE~OE PATTERNS

This section will show that the simple mechanism of the interference of two rays is adequate, although not strictly correct. Di~culties which arise from a consideration of phase changes on reflection at the bottom of the ice shelf will also be considered. The energy which is directed upwards from the antenna, combined with the energy

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reflected from the top surface of the ice, can be regarded as one ray (ray A). Because of the 180 ° phase change on reflection (STRATTO~, 1941), the energy of this ray will be reduced b y the reflected component. A second ray (ray B) results from the energy which penetrates the ice, is reflected from the b o t t o m of the ice shelf, and finally propagates in an upward direction (see Fig. 9). However, if these paths are permitted RAY A

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f Fig. 9. Possible ray paths for energy leaving the transmitter and arriving at the receiver at Ellsworth station. for the upgoing energy, they must also be permitted for the energy returning to the transmitter site. Suppose that rays A and B retain their identity in travelling to the ionospheric reflection point and return. The attenuation of a ray travelling in the ice is 0.022 dB/m for 1 Mc/s and 0.026 dB/m for 10 Mc/s. These have been calculated using an equation and ice properties given b y Vo~ H~P~.L (1954). Later in this section it will be explained why the reflection coefficient at the b o t t o m of the ice shelf is probably not unity, b u t some smaller value. Because of this, no precise calculations of intensity for the various components can be made. To allow some calculations, certain values will be suggested. For simplicity it will be assumed that the processes involved in producing ray B are such that, as it emerges from the ice shelf on the w a y up, its intensity is reduced to one-tenth of the intensity of the upgoing ray A. When these rays return from the ionosphere they will have the same relative intensity. I f an arbitrary intensity of the returning ray A is 1.0, the intensity of the returning ray B is 0.1. Electromagnetic theory indicates that for vertical propagation, the resultant radiation intensity at the antenna due to a beam of incident radiation, together with

Oblique incidence interference observed by ionosonde on ice shelf

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radiation reflected from the air-ice boundary, is equal to the intensity of the beam propagating downwards into the ice. Figure 9 illustrates the various routes whereby downcoming radiation reaches the antenna and defines the notation used in Table 1, which shows the relative intensities of the several components. The returning energy can be regarded as made up of three rays, since the path lengths for rays (b) and (c) in Table 1 are the same. The ratios of the intensities of these three rays are 100:20:1. One ray has an intensity five times greater t h a n a second ray and one hundred times greater t h a n a third ray. Table 1. Relative intensities and number of ice shelf transits of various portions of the energy returned to the receiver

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In view of the evidence discussed in Section 2 indicating t h a t one of the interfering rays is considerably weaker than the other, and the evidence for there being only two rays, the proposed reduction in energy for transit through the ice seems reasonable. The third ray can therefore be neglected, as its effect on the interference pattern would seem to be negligible. The system can then be regarded as essentially the same as the simple system proposed in Section 2. The fact t h a t extrapolation of the null locations does not give a null at zero frequency leads to difficulties in interpreting precisely how the interference occurs. At the ice-seawater interface, a phase change of 180 ° can be expected (STRATTON, 1941). At vertical incidence, if this phase change does occur, nulls should be present when n2 ---- 2tv/e and not when (n + ½)~l = 2t~c/e as is observed. Depth sounding of ice shelves at frequencies near 35 Mc/s has shown t h a t reflections from the bottom surface are sporadic, strong in some regions and nonexistent in others (EvANs and ROBIN, 1966). There is also evidence t h a t at times the reflection comes from sloping or perhaps rough surfaces (W~FoRD, 1964). Some recordings have shown t h a t it is possible to obtain reflections from horizontal strata in the interior of the ice shelf (BArr.EY, EVANS and ROBIN, 1964), suggesting discontinuities in the dielectric constant at these levels. To explain the observations from the interference effects observed at Ellsworth, it is here suggested t h a t there is a horizontal discontinuity associated with the contact of seawater with the base of the ice shelf, where the dielectric constant decreases on the way down just before the seawater level is reached. For the iceeap in Antarctica, where ice thicknesses are usually several thousands of meters, evidence from seismic studies indicates a velocity boundary several hundred meters above the base of the ice (BENTr.~.r, 1964). In view of this, it is possible t h a t a thin layer of some kind could exist at the base of an ice shelf. EVANS (1961) has found two sets of interference patterns on the ionograms produced by the Halley Bay station, which is located on an ice shelf. The frequency

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spacing for the nulls of each set is the same, namely 0.60 Mc/s. This spacing is consistent with results expected from the ice shelf thickness estimated by M~cDowAH. (1960) (see also PIGGOTT and B~RCLAY, 1961). For these sets, the nulls (at vertical incidence) are consistent with the relationship n2 ---- 2tv/e for one set and (n ~- ½)4 2t~/e for the other. Thus, the energy which travels through the ice seems to be partly reflected without change of phase near the bottom surface. The rest of the energy will be reflected at the ice-seawater interface, since the reflection coefficient here is unity. There will be a 180 ° phase change for this reflection. I f similar effects occur at Ellsworth, the ice-seawater interface is probably rough, preventing phase-coherent reflection at this boundary from taking place. A delta antenna was used at Ellsworth (the dimensions are given by EVANS, 1961). Although there are small differences in the amount of energy propagated in upward and downward directions from a delta antenna (BA~.EY, 1951), for the purposes of discussion here they can be regarded as equal. Since the attenuation in travelling through the ice is small, and the energy for the ray path through the ice must be doubled (see Table 1), a reflection coefficient of unity at the bottom of the ice would result in two interfering rays comparable in magnitude. Because one interfering ray seems to be considerably weaker than the other (see Section 2), a small reflection coefficient such as would be expected from an ice sheff discontinuity seems likely at the reflection level. 4. CALCULATIONS OF DIELECTRIC CONSTANT AND THICHNESS OF THE FILCHNER ICE SHELF AT ELLSWORTH

I t is possible to calculate both the dielectric constant and the thickness of ice by using the interference patterns from the sporadic-E off-vertical reflections (Fig. 5). The dielectric constant determined here is an average value, since there is a small variation over the frequency range used (2-5 Mc/s) and also a variation at different ice depths, particularly near the surface. As in the case of thin films in physical optics (JENKn~S and WKITE, 1957), it can be shown, for off-vertical directions, t h a t 2t%/e cos 4 ---- (n ~- ½)4 for a minimum, and 2t~/s cos 4 ~-- ni~ for a maximum, if phase changes on reflection are neglected. Also, SNELL'S law applies for each refraction at the ice surface (~/e sin 4 ----sin 0). Angles 4 and 0 are illustrated in Fig. 1. For a particular frequency, two maxima, two mlnlma or a maximum and a minimum can be regarded as a set. I f a maximum and a m ~ i m u m are used, the following equations will apply: 2t~/e cos 41 = (nl + ½)4 (minimum) ~/8 sin 41 = sin 01 2t%/e cos 43 = n~t ~'e sin 42 = sin 02

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As the parameters 2, n 1, n~, 01 and 03 are all known, equations (5) and (6) can be solved for e and t. Sets were determined b y drawing vertical lines on Fig. 5 a~ 0.1Me/s intervals over a range of frequencies (2-5 lVIc/s). From the positions of maxima and minima at particular frequencies, off-vertical angles were determlued. The order of interference could also be specified for each maximum or minimum. In all, 90 sets were used to calculate different values of e and t. The average of these values was 281 + 3 m for t, and 2.86 ± 0.01 for e. The errors quoted here are the standard errors of the mean. These values are comparable with the 265 4- 3 m for thickness and 2.78 4- 0.05 for dielectric constant found b y EVANS (1961), b u t the thickness is somewhat larger than the 230 m found b y seismic measurements (BEHRENDT, 1962). 5. APPLICATIONS OF ANGLE-OF-ELEVATION INFORMATION

Figure 2(d) is an example of a well-defined off-vertical trace from a sporadic-E layer. Measurements of frequency displacements from null locations numbers 7 through 11 give off-vertical angles of 40, 39, 38, 40 and 42 °. Using an average value of 40 ° and the recorded range of the trace of 127.5 km, simple geometry will give the height of the reflecting layer as 97.5 kin. A vertical-incidence trace observed 30 rain previously had a range of 98 kin. I t seems likely that ionization at this level (98 km) is moving horizontally. I f the off-vertical trace onFig. 2(d) was observed without the interference pattern, it could easily be mistaken for a vertically-incident trace. Although retardation effects are usually negligible and height changes are small for the sporadic-E layer, this is not true for the F2-1ayer at night. I f the off-vertical angle is greater than 20 ° (for example), the position of reflection will be more than 100 km from the zenith position and the shape and height of the F2-1ayer m a y be somewhat different at this displaced location. In frequency regions close to the critical frequency, retardation effects can also contribute considerably to the observed range. When the extra traces on 'spread-F' ionograms are resolved so that an individual trace can be identified over a range of frequencies, it is found that the off-vertical angle usually stays reasonably constant (within 5 degrees) over the range of frequencies where nulls are recorded. In Fig. 10, which is a tracing of the ionogram illustrated in Fig. 2(b), the off-vertical angles measured from null locations numbers 9, 10 and 11 are 36, 36 and 40 °, respectively. Figure 10 illustrates the method of measuring the displacement frequency to determine the off-vertical angles. I f it is assumed that the off-vertical angle is constant at all frequencies of the satellite trace, an N(h) profile can be determined which would give some indication of the electron density distribution in the measured off-vertical direction. Figure l l(a) gives the electron density distribution deduced b y using N(h) profiles (VENTRICE and SCH~ERLn~G,

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Oblique incidence interference observed by ionosonde on ice shelf

1127

1958) for the main trace and the satellite trace of Fig. 10. Thus, the angle-of-elevation information allows a reasonable estimate of the ionization distribution, despite the assumptions which have been made. Figure ll(b) gives the shape of the base of the F2-1ayer at 18.00 (L.T.) on 20 April 1958, and was obtained using the interference pattern information. The relevant ionogram is shown in Fig. 3(g). The minimum range of each trace has been plotted as the true range (as an approximation). The second-multiple of the lowest trace shows well-defined nulls, indicating an off-vertical angle of 26 ° . Spacings between the first-, and second-, and third-multiple traces suggest associated paths as illustrated in Fig. 11 (b). The nulls on the first multiple trace, which has a minimum range of 305 kin, show t h a t the ray path for this trace is near vertical. Additional ray paths have been drawn on Fig. ll(b) to correspond to the traces with ranges of 600 and 590 kin, respectively. These traces occur just below the second multiple of the verticallyincident trace at 305 km. These reflection positions, determined from the information available, give a plausible shape for the base of the layer. The layer level changes from 305 km above the ionosonde to 259 km at a horizontal distance of 118 kin. The critical frequency of the _~2-1ayer vertically above the ionosonde is 5.5 Mc/s, and not 6.5 Mc/s as suggested b y the lower trace. This lower trace apparently gives the critical frequency of the F2-1ayer 118 km away. Figure 11 (c) has been constructed using a method similar to the one used to derive Fig. ll(b). I t shows the layer shape at 18.00 (L.T.) on 27 April 1958. The relevant ionogram is shown b y Fig. 3(h). Here the well-defined nulls on the lower trace indicate a direction-of-arrival for these signals from an off-vertical direction of 40 °. The reflection points of the second- and third-multiple traces of this trace have also been estimated using range information. Unexpectedly, the vertically-incident trace (as determined by the location of nulls) is the first-multiple trace, somewhat spread in character, which has the greatest range. This is reflected from a height of 500 kin. Its second-multiple r a y path is also shown in Fig. ll(c). This trace is visible only over a small frequency range. The well-defined trace appearing just above the lower trace records an off-vertical direction of 24 ° . R a y paths for this trace and its second-multiple (small portions visible) are also drawn. The shape of the layer has then been determined by joining the reflection points. I t is imagined, however, t h a t additional irregularities in the form of extra kinks in the layer shape are present and allow specular reflection at the various reflection points. Errors of 10 km or more can be expected in the positioning of any surface in Fig. 11 (b) and (c) because of the use of approximations. However, these errors are relatively unimportant when compared with the gross structure changes from a horizontal layer shape which are revealed. The changes are particularly large in Fig. ll(c). Since ionograms taken prior to 1800 on 27 April 1958 show normal traces with critical frequencies around 9 Mc/s, Fig. 1 l(c) represents a giant-sized irregularity. Without the information from the interference pattern, this would have remained undetected. Here the difference between the t r u e / o F 2 for the vertically-incident trace (2.3 Mc/s) and the value suggested by the lower trace (8.5 Mc/s) is indeed great. The events illustrated by Fig. ll(b) and (c) are relatively common at Ellsworth in the early evening hours. During the equinoctial months, ionograms showing the lowest trace as an off-vertical trace are seen (for each event) for periods of an hour or

1128

G.G. Bow~A~

more. The events are spaced b y an average interval of several days. On these occasions, the values OffoF2, which are read from the ionograms b y using the lower trace, can be considerably in error. Instructions in the Annals of the I G Y (WRIGHT, KNECttT and DAWES, 1957) concerning oblique echoes near auroral regions advise the use of the lower trace as the vertically-incident trace in the absence of evidence to the contrary. The features of this large-scale phenomenon have been treated only briefly here. I t is very probable that these irregularities are the same as the ionization troughs detected in polar regions b y MULDgEW (1965) USing data from the topside ionosonde, Alouette I. The phenomenon is being investigated in more detail. On most occasions when spread-iV traces occur, whether they be discrete or diffuse, the frequency of each null will be displaced towards a higher frequency as the range of the spread-iV trace gets further from the main trace (Fig. 3(c), (f) and (i)). This is not true when large-scale disturbances, similar to the example discussed above, are present. This regular displacement towards higher frequencies suggests a simple geometry similar to that used in the calibration method for the sporadic-E interference patterns. There will be occasions when the retardation effects for the lower frequencies recorded are small enough to be neglected. Results shown in Fig. 12 suggest that this was the case when the ionograms shown in Fig. 3(c), (f), and (i) were recorded. Frequency displacements from nulls for these ionograms have been measured for arbitrarily-chosen ranges in the diffuse range-spreading of the traces. The plotting has been made assuming that the layer is fiat at the time, and that observed ranges at the low-frequency end of the ionograms are true ranges. In Fig. 12(b) and (c), the off-vertical angles (calculated from the interference patterns) are in all instances within two degrees of the off-vertical angles obtained b y using the observed ranges and a flat ionosphere. Differences as high as 5 ° are indicated in Fig. 12(a), b u t here the shape of the traces suggests that retardation effects are not negligible. This means that, for the low frequencies at least, the satellite traces are being returned from the same level of the ionosphere as the vertically-incident traces. Thus, the behaviour of the interference patterns for the spread-_~ traces seems to give important information concerning the nature of the spread-F irregularities. Measurements b y KLEMPERER (1963) at a high-latitude location indicated that the returning signals, which constitute the spread-F traces, were confined to a narrow cone of about 13 ° semiangle. His measurements cannot be regarded as representative of all high-latitude spread-F traces; in this analysis traces have been seen on m a n y occasions representing rays from directions as far as 60 ° off-vertical, i.e., t h e y are being reflected or scattered from the ionosphere some 500 km from the zenith position. I f such echoes are possible at the same time, from azimuths 180 ° apart, it means that signals are being returned to a 'vertical-incidence' ionosonde from a region 1000 km wide, centred on the zenith position of the ionosonde. 6. SWEPT-FREQUENCY INTERFEROMETERS AND IONOSONDES A few cases have been described in the previous section where the direction-ofarrival information has been useful in the interpretation of ionograms. I t is possible, with certain restrictions, to reproduce these effects for any ionosonde. The system to be proposed will be essentially the same as the swept-frequency interferometers

Oblique incidence interference observed by ionosonde on ice shelf

1129

OFF-VERTICAL ANGLES CALCULATED FROM DISPLACEMENT; (2)GEOMETRY

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(1958). It hasbeen proposedfor Brisbane,Australia (a mid-latitude station) that relatively large-scale nighttime disturbances associated with spread-F signals are wave-like in nature. The disturbances have fronts which are approximately linear and extend horizontally for hundreds of kilometers. During the years of sunspot-cycle maximum, it was found that the orientation of these fronts was particularly constant (BOWl~AN, 1960). A more extensive study, using the same technique, was carried out by CT,~-~KE (1965) during the years of sunspot minimum. He found remarkable constancy with time of night and time of year for the orientation of these irregularities. For the sunspot-maximum years, the average direction perpendicular to the fronts was about 30 ° west of magnetic north. In sunspot minimum years, the average direction perpendicular to the fronts was about 50 ° west of magnetic north.

1130

G.G. Bow~A~

Thus, at Brisbane these night-time irregularities have orientations which do not change very much, even over a sunspot cycle. Similar recordings do not appear to have been made at any other location. However, it is probable in polar regions that the large-scale irregularities, of the type discussed in the previous section, are frontal in nature. Also, the direction perpendicular to these fronts is probably close to the direction of the magnetic field. The topside sounder results (MUnDREW, 1965) would seem to suggest this.

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Fig. 13. Ray paths from transmitter and to two receiving antennas for a sweptfrequency interferometer used with an ionosonde. I f a consistent azimuth-of-arrival is found at other locations on the Earth for these night-time irregularities, a simple system for ionosondes, which would give information on the direction-of-arrival of signals from the irregularities, would consist of two broad-band receiving antennas spaced 250 m apart. The line joining them would be perpendicular to the average orientation of the fronts. Cables jolnlng the antennas to the receiver would have different lengths, providing an effective delay line for one of the antennas equal to a free-space distance of 750 m. I t will be assumed that this distance is constant over the frequency range being considered. Transmission would be from an adjacent single broad-band antenna. The system is represented diagrammatically b y Fig. 13. For reflections from a vertical direction, nulls will occur at particular frequencies when (n + ½ ) 2 - 750 m, where 2 is the wavelength given in meters, and n is an integer. For off-vertical reflections, consider for the time being, only energy returned in the vertical plane which passes through the line jolnlng the receiving antennas. Two cases must be considered. Figure 13(a)

Oblique incidence interference observed by ionosonde on ice shelf

1131

illustrates the case where the energy comes from the right of the antennas, and Fig. 13(b) considers energy returning from the left of the antennas. For Fig. 13(a), nulls occur when (n -k ½4 = 750 -- A B sin 0, while for Fig. 13(b) the nulls occur when (n -b ½)4 = 750 -k A B sin 0. The distance separating the antennas is A B , and 0 is the off-vertical angle-of-arrival of the signals. Calculations of the displacement of FREQUENCY,MC/S 15 2.0 5.0 4.0 5.0

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7 9 12 NULLNUMBER Fig. 14. Interference patterns showing the displacement of frequency with offvertical angle for nulls produced by a swept-frequency interferometer. Delay line from one antenna 750 m. Antenna spacing 250 m. the null frequency as the off-vertical angle changes have been made for this system of antennas. The results are shown in Fig. 14(a) and (b). The same frequency range and off-vertical-angle range used for the calibration of the ice shelf interference (see Fig. 5) has been used here. Figure 14(a) gives the displacements corresponding to the case illustrated b y Fig. 13(a). Here the nulls move to higher frequencies as the off-vertical angle increases. I t will be noted that Fig. 14(a) is very similar to Fig. 5, where the ice shelf null behaviour is recorded. Figure 14(b) gives the displacements

1132

G.G.

Bow~A~

for the case illustrated by Fig. 13(b). Here the nulls move to lower frequencies with increasing off-vertical-angle. The geometry is such that the null systems shown in Fig. 14 are not very sensitive to small changes in the azimuth-of-arrival of the signals which would put them outside the vertical plane passing through the line j oining the antennas. If the azimuthsof-arrival of these off-vertical reflections are within ± 15 degrees of the line joining FREQUENCY, MC/s 1.5 200

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Fig. 15. Interference patterns showing the displacement of frequency with offvertical angle for nulls produced by a swept-frequency interferometer. Delay line from one antenna 750 m. Antenna spacing 500 m. t h e a n t e n n a s , t h e error i n v o l v e d in r e a d i n g t h e off-vertical angle f r o m t h e s e calibrat i o n curves (Fig. 14) w o u l d n o t be g r e a t e r t h a n 3 ° . I f m o r e a c c u r a t e i n f o r m a t i o n is required, a z i m u t h s - o f - a r r i v a l of t h e signals a t one or m o r e fixed frequencies could b e

obtained by using a rotating-loop system similar to that used by Bow~_AX (1960). Suitable calibration curves for different a z i m u t h s - o f - a r r i v a l w o u l d t h e n b e necessary. A n e v e n m o r e c o m p l e t e s y s t e m which would ideally give t h e a z i m u t h s a n d elevations of all signals a t all frequencies would consist of t w o i n t e r f e r o m e t e r s similar to t h e one

Oblique incidence interference observed by ionosonde on ice shelf

1133

just described. The lines joining each pair would be at right angles to each other. I t would then be necessary to alternate the interferometers used for each ionosonde sweep. HEISLER and WHITEHEAD (1961) have pointed out the dynamic character of the F-region, particularly in the daytime of winter months. This results in the so-called vertically-incident trace often being several degrees off-vertical, even when the ionosphere appears to be a smooth layer. I t would assist considerably in the interpretation of ionospheric behaviour if these deflections of the main trace from the vertical path could be measured, particularly if it was done routinely. The system just described, although suitable for off-vertical angles in excess of 15 °, is not sensitive enough to measure off-vertical angles of a few degrees. However, if the same delay line is kept (750 m) b u t the spacing of the interferometer antennas is increased to 500 m, the system would be quite sensitive for these smaller angles. Two pairs of antennas, each pair at right angles to each other as described previously, would be needed for a complete specification of the direction-of-arrival. For signals arriving in the vertical plane through the line joining the antennas, the frequency displacements are shown b y Fig. 15(a) and (b) (representing the cases illustrated b y Fig. 13(a) and (b), respectively). The nulls produced b y the systems, which have been described in this section, should be observable throughout the frequency range of all the traces, since the interfering rays will be equal in amplitude. The proposed systems would have certain disadvantages. The accuracy with which such parameters as fmin or fbE8 could be read would be reduced on some occasions because of the presence of the nulls. However, errors from this source would probably not be greater than 0.1 Mc/s, which for most purposes would be tolerable. 7. D i s c u s s i o n The details in the previous sections indicate the value of the information which has been added to the ionograms b y the location of Ellsworth Station on the Filchner ice shelf. The information has contributed to an understanding of the nature of the irregularities responsible for the phenomenon of spread-F. I t has also revealed very large-scale irregularities which would have remained undetected because the lower traces with normal foF2 values are many degrees off-vertical, and give the foF2 value for regions hundreds of kilometers away. At these times, the true zenith foF2 value for the zenith position can be several times lower than the false zenith foF2 value of the lower trace. I t should be possible to produce interference effects somewhat similar to those produced b y the ice shelf, b y transmitting from antennas attached to helicopters or tethered balloons at suitable heights. In polar regions, systems similar to those proposed here are almost a necessity if the scaled parameters (e.g., foF2 or h'Es) are to be reliable at all times. Also, since these regions are subject to dramatic changes in the ionization density distributions, any analysis which seeks to describe ionospheric behaviour when these changes occur is made a good deal easier b y direction-of-arrival information.

Acknowledgements--The author

would like to thank Professor H. C. WEBSTER, Dr. R. B. PEI~qDORF and Mr. G. F. Rotm~r~. for helpful assistance in the preparation of this paper. He

1134

G . G . BOWMAN

is also grateful to Mr. W. S. HouGH of the I n s t i t u t e for Telecommunication Sciences a n d Aeronomy, E.S.S.A., Boulder, Colorado, for supplying information on the a n t e n n a and a n t e n n a site at Ellsworth, and to iYLiss E. GISTIS and Mr. S. H. ROSEN, who carried out most of the calculations which were needed. The research was supported by the :National Science F o u n d a t i o n under Contract :NSF-C403. REFERENCES BAILEY J. F., EVANS S. and ROBIN G. DEQ. BAILEY R. BES:RENDT J.C. BENTLEY C . R .

1964

21ature, Lond. 204, 420.

1951 1962 1964

Wireless Engr 28, 208. J. Geophys. l~es. 67, 221. Research in Geophysics, Vol. 2., Chap. 14, p. 379, M.I.T. Press, Cambridge, Mass. Planet. Space Sci. 2, 133. Master of Science Thesis, University of Queensland, Brisbane. J. Geophys. Res. 66, 4137. Nature, JLond. 210, 883. J. Atmosph. Terr. Phys. 22, 186. _~undamentals o] Optics, Chap. 14, MeGraw-l:[ill, New York. J. Geophys. Res. 68, 3191. Pros. R. Soc. $~256, 149. J. Geophys. t~es. 70, 2635. J. Atmosph. Terr. Phys. 20, 289. Electromagnetic Theory, Chap. 9, McGraw-Hill, New York. Scientific Rep. No. 106, Pennsylvania State University.

B O W M A N G. G. CLARKE R. H.

1960 1965

E v e s S. EVANS S. and ROBIN G. DEQ. HEISLER L. H. a n d WHITEHEAD J. D. JEZ~-~rNS F. A. and WHITE H. E.

1961 1966 1961 1957

I~'J.~EMPERERW . K . ~$_.AcDowALL J. MUI~DREW D . B . PIGGOT~ W. R. and BARCLAy L . W . STRATTON J . A .

1963 1960 1965 1961 1941

VENTRICE C. A. and SCHMERLING E. R.

1958

V 0 N S_~PPEL A.

1954

Dielectric Materials and Applications,

1964 1956 1958 1957

pp. 12, 301, M.I.T. Press, Cambridge, Mass. and Wiley, :New York. -TVature,Lond. 204, 317. J. Atmosph. Terr. Phys. 8, 55. Pros. Instn. Radio Engrs 46, 160. Ann.. int. geophys. S, P a r t 1, 90.

W ~ F O R D M. E. R .

WILD J. P. and ROBERTS J. A. WILD J. P. and SHERIDAN K. V. ~VRmHT J. W., K_~rECHTR. W. and DAVIES K.