Some properties of ionospheric irregularities as deduced from recordings of the San Marco II and BE-B satellites

Journalof Atmospheric andTerrestrial Physics,

1970, Vol. 32, pp. 1259-1271.

PergamonPress. Printedin NorthernIreland

Some prope~es of ion~phe~c booties as deduced from recordings of the San Marco II and BE-B satellites R. I?. KELLEHER and J. SINCLAIR Physios Department, University College, Nairobi (~e~e~~e~ 3 D~c~~~~~ 1969) Abstract-Recordings of scintillations on signals received at Nairobi from satellites San Marco II and BE-B have been analysed by correlation and dispersion methods, the correlation method equations being derived in a simpler way. The dispersion technique, which gives more detailed results, has been used to derive the height variation of irregularity size and occurrence. The peak of oecurrenee is found to be near h&Vi’. The average size is somewhat greater than 100 m and does not vary si~ific&ntly with height. On ~dividu~ passes, the height of ~reg~arities increases tows&s the magnetic equator. Irregularity patches are found to move with a velocity of about 20 msec-r towards the east before 0600 LMT and to the west after this time. There is evidence of some vertical structure. It is shown that there is considerable polarisation scintillation at 20 Ml&. 1. INTRODUCTION RADIO signals from satellites have been widely used in the investigation of ionospheric irregularities, and in particular in the determination of the height at which they occur. In the usual approach, the movement of the ground diffraction pattern is observed at spaced aerials as a travelling satellite passes overhead. The height and thickness of the irregular layer and the size of the irregularities can then be determined (KENT, 1959; JAMES, 1962; JESPERSON and KANZAS, 1964). Recently, Fourier analysis of fading records has been shown to be a useful additional technique (JONES and MAUDE, 1965 ; BRIGGS, 1969). Dispersion curves are obtained, giving the velocities of the different components in the pattern. This approach would seem to be particularly valuable when applied to height-determination experiments where a particular velocity relates directly to a particular height. This paper presents results of ~regularity height and size determinations made principally on 26 MHz scintillation records obtained from the equatorial satellite San Marco II. Two passes of another satellite (BE-B) are also included. The fading records have been analysed by both correlation and dispersion methods. For the correlation analysis, a modified approach has been used ; this is described in Section 3, where a comparison of the different techniques is given. Section 4 deals with the phenomenon of ‘polarisation scintillation’ and its effects on the experiment. The results of the height and size determinations are presented in Sections 5 and 6. Irregularities are observed to occur in patches of considerable vertical and horizontal extent (see HERMAN, 1966). In Section 7 we give some results on the structure and movement of these patches.

2. EXPER~~~TA~

DETAILS

The San Msroo II satellite (perigee 4?00 km, apogee ~650 km, inclination 2.9”) was launched in April, 1967. On the 26 June, the 20 MIIz beacon was commanded on and continued to transmit for about 3 weeks. A number of horizontal receiving dipoles was set up at Nairobi (1_3?S, 36@E geographic) along an East-West line, i.e. approximately in the direction of the 1259

1260

R. F. KELLEHER

and J. SINCLAIR

satellite motion. For the purposes of the present study, the recordings from aerials situated at points a distance of 114 m apart have been used. At each point, there were in fact two dipoles, one oriented North-South, and one oriented East-West. This was to enable the polarization of the down-coming wave to be investigated. The outputs from four receivers were recorded on pen charts and also on a magnetic tape system. The tapes were subsequently replayed on to a highspeed recorder (200 mm/set). Sections of the final playback which seemed to be statistically stationary and which had a sticient number of fades were then measured in amplitude at suitable intervals of time. The standard correlation analysis in one dimension was carried out and the height and thickness of the irregular layer and the size of the pattern in the ionosphere were calculated as described below. The dispersion analysis consisted of taking the Fourier transformation of the cross-correlation curve (GOODMAN et al., 1961) to give the amplitudes, sizes and velocities of the different components. A total of about 170 passes of the San Marco II satellite was recorded at Nairobi. Most of these did not show any scintillation activity. The results discussed in the next two sections were

taken from 80 fading records derived from 13 different passes. In addition, two passes of the BE-B satellite (height 900 km, orbital inclination 79O)have also been analysed. The satellite signals were recorded at spaced aerials placed along the direction of motion. On one occasion both 40 MHz and 20 MHz recordings were made (at aerial separations of 423 m and 300 m respectively).

3. THEORY ground diffraction pattern (E is a distance along the direction of motion and T is time) can be written for small E, T in the usual notation as (BRIQ~S, PHILLIPS and SHINN, 1950) The correlation

function

p([,

P-

T) of a one-dimensional

_ 1 _ 52 + VC’V d”

-

2VL+

(1)

where d is the size of the pattern, V,’ is the fading velocity and V is the steady drift velocity. If we consider a thin layer of ionospheric irregularities at height h, and observe the effects on the ground of a signal from a satellite at height H moving with a velocity U, then (KENT, 1959) V = Uh/(H - h). The size of the pattern on the ground will be greater than the size D in the ionosphere by a factor H/(H - h). The parameter V, (charaoterising the rate of change of the irregularities, V, = size/lifetime) can be taken as zero since the irregularities can be assumed not to change over the short time they are observed; thus V,’ = V. We therefore have

p-l-

Now consider

two irregular layers at heights hi and h, with h = (h2 + h,)/2 and A = (h, - M/2, i.e. h and A are respectively the mean height and the semi-thickness (half-distance between the layers). If the diffraction effects are independent, the correlation function of the resulting pattern on the ground is the sum of the individual correlations. We assume that the sizes in the two layers are the same, take the approximations for small t, T, add the correlation functions and compare the result

Some properties of ionospheric irregularities with (1).

1261

After some simple algebra, we obtain ~ = Uh(H (H ”

= ’

h) -

A2

(2)

h)2 + A2

HA (H _ h)2 + A2

(3)

DH

d=

d2[(H

- h)2 + A21

(4)

where V, V, and d now refer to the values obtained by applying the standard correlation analysis to the composite ground pattern. The first two of these equations were obtained by JAMES (1962), who used a different method. However, he subsequently omits the terms in A2. If we retain these and solve (2) and (3) for F, and A, we obtain h=H

V(UfV7)-tVc2 i (U + VI2 + vo2

(5)

UH V, A = (U + V)Z + vc’,2*

(6)

I

These reduce to the equations given by James (except for a factor of 2 in (6)) when V,< (U + V) and Vc2< V(U + V). J ames omitted the terms in A2 in (2) because during his derivation, he had to take A < (H - h) in order to replace a finite limit of integration by infinity. In practice, for low altitude satellites (such as San Marco) it is possible that (H - h) N A and in this case his equations will not give reasonable results. The approximations involved in the present derivation rely only on the expansion of the correlation function and can be shown to reduce to the condition that the size of the irregularities should not be smaller than about one-half the aerial spacing. Another method of height determination is the use of time-shifts between similar fades on the two aerial records to give the ground velocity of the diffraction pattern. This is equivalent to the use of the apparent velocity V’ in the equation F, = H V’/ (U + V’). It is not difficult to show that the height derived from this method (h’) and the height derived from the James equations (h,) are related to our height h by h’ = Fy+ A2/7b and hJ = h - A2/(H - h). For a high altitude satellite where HN 1000 km, and h - 300 km, with A N 100 km, both of the above estimates are in very close agreement. In such a case, the use of similar fades is preferable as it For the San Marco satellite, where H N 400 km and entails far less computation. h - 300 km, it can be seen that the similar fades method will give better results than the use of James’ equations. In the present work, all three estimates of irregularity height have been calculated, and the values are in good quantitative agreement with the above equations. The dispersion analysis has been carried out as described, for example, in BRIWS (1969). The velocity (v) and the size (d) of the components in the ground pattern are derived, and these are converted into equivalent heights and ionospheric sizes D by

1262

R. F. KELLEEER and J. SINCLAIR

the equations h = vH/(U + w), D = d(H - h)/h, which should hold for single components. Also calculated is the weighted height 5 = Xp,h,/Zpi, where the pi are the amplitudes of the different components. It has been found that the values of 5 are usually in very close agreement with the heights derived from the correlation approach. There remains the problem of whether the correlation technique or the dispersion analysis are valid when the scattering is strong, i.e. when the irregular phase deviation (4,) imposed on the wave through the ionosphere is greater than 1 rad., as is probably the case at 20 MHz (see Section 6 on page 1264). The principal justification would seem to be the very good agreement that has been found between the results obtained on two different frequencies and which is discussed below. Corrections are sometimes made for the fact that the direction of motion of the pattern on the ground is not exactly opposite to that of the satellite if the irregularities are elongated in a non-horizontal direction (i.e. along the magnetic field lines). An experiment carried out with a geometric optics scale model showed that in the present case there was no need to introduce any correction to the measured velocities. Moreover, it is shown below that there is little evidence of any particular elongation of the irregularities over Nairobi. 4. POLARISATION SCINTILLATION It has been mentioned above that recordings of the San Marco satellite were made on two pairs of aerials. The spaced aerial results discussed in later sections were in fact derived from the East-West oriented dipoles only. The recording system on the North-South oriented dipoles proved to be unreliable and on only a few occasions was it possible to make use of the fading records from them and thus to derive information on the polarisation of the down-coming wave. The existence of polarisation scintillation can most easily be detected by correlating the fading records from orthogonal dipoles situated at one point. If the direction of polarisation were constant, the instantaneous cross-correlation would be +1-O. An examination of some ten crossed-aerial records showed that the correlation was between -0.2 and +0*5. It is clear therefore that polarisation scintillation does occur at 20 MHz and that its effect is comparable with the amplitude variation. This is consistent with the theoretical prediction of YEH and LIU (1967) and with the observations of MCCLURE(1964). KOSTER(1966) found on one occasion that polarisation scintillation was negligible at 136 MHz. It should be noted that the records analysed in our study were usually much shorter than the period of a normal Faraday fade. A number of records from all four aerials on one San Marco pass was analysed to try to separate the amplitude and polarisation effects. The fading amplitudes from the two crossed aerials at one point, a, and u2, were converted to ‘total’ amplitude, a, and polarisation angle (in the range O-90”), 8, using the equations a = d/al2 + u22 and 8 = tan-l a1/a2. Thus, altogether, instantaneous total (unresolved) amplitude and angle values were available at two spaced points. Analysis showed that (1) The average r.m.s. deviation in the direction of the polarisation vector was about 20”. (There is some uncertainty in this figure because the angle can only be

Some properties of ionospheric irregularities

1263

determined within the range of O-90”). This may be compared with a total Faraday rotation through the ionosphere of about 26 rev at the time of the recordings. (2) The polarisation direction and the amplitude of the down-coming signal were uncorrelated. (3) When correlation analysis was applied to spaced aerial values of (a) polarisation direction, (b) resolved amplitude, (c) unresolved amplitude, the results were in good agreement (although the polar&&ion ‘size’ was sometimes found to be somewhat smaller than the amplitude size). Thus although the polarisation sc~tillation at 20 MHz is considerable, it is not necessary to consider its effects separately in spaced-aerial experiments. Further evidence of this conclusion is provided by the fact that spaced-aerial height measurements at 40 MHz (where the r.m.s. angle would be only a few degrees and thus almost negligible) lead to results almost exactly the same as those obtained on 20 MHz. 5. HEIGHT

OF OCCURRENCE OF IRREGWARITIES

The height of occurrence of irregularities has been derived from the results of the dispersion analysis. Fourier components with amplitudes above a certain minimum level have been taken to indicate the presence of significant irregularities (over each 10 km interval). Figure 1 shows the results for all San Marco records. Although the 1’

I

1

*

#

I

,

I

I

1

,

I

I

*

v

I

i 400-

100

I T

5

OCCURRENCE Fig. 1. Height of occurrence of i~e~l~itiea

I

.

.

10 (San M~rcof.

aititude of the satellite varied between 200 and 600 km, it is believed that the effect of this on the curve is not too serious, as in most cases the greatest height at which irregularities were detected was well below the satellite. It may be noted in connection with Fig. 1 that irregularities below 120 km and above 390 km were only found after ionospheric sunrise, i.e. in the time range 0530-

1100 LMT. The majority of records were taken between midnight and 0500. The principal features of the occurrence curve are a rather broad maximum between about 220 and 300 km and a secondary maximum around 180 km. The mean value of hh,F2 for the times concerned was about 300 km. Though the fairly

R. F.

1264

end J. SINCLAIR

KELLEHER

sharp fall above this height may be accentuated by the low altitude of the satellite, it does seem that the irregularities are principally below h,, and in fact spread-P was seen to occur on most of the ground-based ionograms taken at times near the satellite passes. As mentioned before, two separate passes of the BE-B satellite have also been studied. The first of these was at 0130 on the 4 July, during the period of San Marco observations. Figure 2 shows the heights of occurrence of irregularities for this pass as derived independently for 20 and 40 MHz. It can be seen that there is good agreement between the results although the range is greater at the lower frequency. 800-

2 600Y

__-- t <---_.D ___---6 -- -- --- “_o__ - -_o OZZZ----e------_0__

-- 0

b-3 I

o--‘_o 200

I

I

I 5

I

I

I IO

OCCURRENCE

Fig. 2. Height of occurrenceof irregularities(BE-B pass 13,696): xx 40 MHz; @- - -0 20 MHz.

The value of Fv, at the time of this pass was about 300 km, i.e. the irregularities were almost entirely above the maximum of the F&layer. It will be shown later (Section 7) that there was a significant change of the height of irregularities with latitude, but this has not been taken into account in Fig. 2. A further pass at 0530 on the 13 February, 1969, has been analysed for 40 MHz. The value of h, was about 260 km and the irregularities were mainly below the maximum. 6. IRREGULARITY SIZES The pattern sizes in the direction of motion of the satellite can be obtained from either the correlation or the dispersion analysis. The former gives only one size which may be defined as that over which the correlation falls to 0.5. In the dispersion approach, the wavelength of each component in the pattern is determined. It is these wavelengths that have been used below. For the purposes of comparison, the ‘wavelengths’ have been divided by a factor of 6 to give equivalent ‘0.5 correlation’ sizes. It should be noted that there was very good agreement between the weighted Fourier size for each record and the correlation size. In order to allow for the finite altitude of the satellites, all ground sizes have been multiplied by the ratio (H - h)/lt.

Some properties of ionospheric irregularities

1265

Figure 3 shows the average size (East-West) against height for the San Marco records, pre- and post-ionospheric sunrise being taken separately. The principal features are the minima at E, heights and around 300 km, and the maximum about 180 km. Since the irregular phase deviation +nz at 20 MHz in the ionosphere is probably greater than 1 radian, the sizes presented here are smaller than the actual sizes of the irregularities by a factor of l/4, (BRAMLEY, 1955). The value of 4, will depend on 6N, the increase or decrease in the electron density inside the irregularities. Some information about 6N at different heights may be gained from the amplitudes

200

100 SIZE,

M

Fig. 3. Average East-West size* of irregularities: xX pre-sunrise; a-- - -0 post-sunrise.

of the components in the dispersion analysis. A study of the values obtained has shown that they are usually greatest at E-region heights, around 180 km, and near the maximum of the FL?-layer. This may be taken as indicating that the values of 6N and hence of vrn are largest in these places. It thus seems probable that the actual East-West sizes of the ionospheric irregularities do not vary by more than a small factor from 100-400 km, although they may show an increase at about 180 km (i.e. just below the boundary of the night-time F-layer). The average sizes shown in Fig. 3 vary over a rather narrow range (40-210 m). There is some indication that the post-sunrise sizes are slightly larger in the F-region, though this again may simply mean that the irregularities are weaker. The greatest number of individual Fourier components occurs around 300 km and there is a secondary maximum at 180 km. This may indicate that 6N is highest at these places and that the angular spectrum contains a large number of sidebands (RATCLIFFE,

1956). For the BE-B pass 13,696, the sizes derived from the 20 and 40 MHz recordings are shown in Fig. 4. If the irregular phase deviation is greater than one radian at both frequencies, the sizes should be in the ratio of one to two. The 20 MHz results 7

R. F. KELLJDXER

1266

300

I

and J. SRWLAIR

-

2oc II I 2 5 IOC,-

L

I

200

300

400 HEIGHT,

500

KM

Fig. 4. Average North-South sizes of irregularities (BE-B 13,696): xx 40 MHz; O- - -0 20 MHz (sizes multiplied by 2).

have therefore been doubled before plotting. It can be seen that there is very good agreement between the two sets of measurements, especially in view of the fact that the records on the two frequencies did not necessarily coincide in time. The 40 MHz sizes are in fact slightly less than twice the 20 MHz sizes and therefore the phase deviation at the higher frequency is probably somewhat less than a radian. The height of the PZ-peak for this pass was about 300 km over Nairobi, so that the sizes here show a decrease above the maximum. The orbit of the BE-B satellite has an inclination of 79”, so that the sizes in Fig. 4 should correspond roughly to the North-South dimensions of irregularities. It can be seen that they are of the same magnitude as those derived from San Marco recordings, where it is the East-West size that is determined. There is therefore no evidence of any particular elongation of the irregularities along field lines. This is rather strange in view of the high axial ratios reported by other workers but may be connected with the fact that most of the irregularities observed here were above the P-layer maximum. 7. PATCH STRUCTURE A significant property of irregularities is that they occur in patches which extend for a few hundred kilometers in all directions with a probable elongation along field lines (HIRUN, 1966). In the present work, it has been possible to gain some information on the movement and decay of patches, and on their horizontal and vertical structure. (a) _Latitw.$estmcture The two passes of the BE-B satel~te were analysed over a considerable range of latitudes. The results for the ‘thickness’ of the irregular region along the ray paths to Nairobi as measured at 20 and 40 MHz for one pass are shown in Fig. 5. The thickness here is de&red by the upper and lower limits derived from the dispersion analysis. It should be noted that although only the data from individual records are

1267

Some properties of ionospheric irregularities

----

\

IooKu----~-

-------_

N

LATITUDE

SOUTH

Fig. 5. Distribution of irregularities (BE-B 13,696): 20 MHz; - - - 40 MHz.

shown, the entire pass was scintillated and so the irregularities must be thought of as more or less continuous between the outermost lines, so that the North-South extent of the patch is greater than about 1000 km. It can be seen that there is good agreement between the two frequencies except that the thickness is generally greater at the lower frequency. It is clear that the average height and the thickness of the irregularities increases to the north of Nairobi (i.e. towards the magnetic equator which lies at about 9”N). For comparison, the approximate direction of a magnetic field line is given (calculated from tables kindly supplied by NASA). The average irregularity height tends roughly to follow this line and it is interesting to note that the height would decrease to 200 km (i.e. to the base of the F-layer) at a geographic latitude of about 6’S, which is just at the edge of the equatorial belt for northern summer after midnight (SINCLAIR and KELLEHER, 1969). For the other BE-B pass, which was in northern winter, the corresponding 200 km latitude was nearer to 2%. The equatorial belt has been found (Zoc.cit.) to be considerably narrower in this season. (It is also possible that the decrease in height is connected with the fall in h, between the magnetic equator and the latitude of Nairobi-of the order of 50-100 km.) (b) Longitudinal

and temporal eflects

The direct, relatively slow-speed, pen-recordings of the San Marco signals have been studied to derive information on the longitudinal and temporal behaviour of irregularity patches. Most of the data refer to post-midnight and morning passes since severe broadcast interference occurred in the evening and early night-time. The information has been supplemented, where possible, by the results of height and thickness determinations. Figure 6 shows the occurrence of scintillations against sub-ionospheric longitude for a series of passes on a particular night. The height of the irregularities has been taken as equal to 300 km or the satellite height, whichever is the lower. The ends of

R. F. KELLEHER

1268

and J. SINCLAIR

I

I

1800-

I-

I

I--

1 I

t + 0200-c

5

c-------I

I I

I

-

t

’

I

-4

+-

1000

I I

i I

-

I

I I

I

32

I I

I

I

I

40 LONGITUDE

I

I

48

56

Fig. 6. Time and position of observed irregularities on a particular night.

the records are indicated by short vertical bars. Heavy scintillation is shown as a continuous line. Up to about 0100 there is a number of gaps in the scintillation, but it is difficult to see any definite pattern. From 0200 to 0600, the irregularities are continuous across the observable pass. Just after this time a small gap appears at about 42”E. This moves towards the west and grows wider. A separate small patch is then observed and can be seen in its entirety on the pass near 1000. This moves westwards, growing smaller, until at about 1300 the record is free from scintillation. The westward drift of the patch (or gap) is about 40 msec- l. The results of height determinations for the same records showed that the patch grew thicker at 0800 and then moved upwards well above the PB-peak by 1000. On the ground based ionograms taken at Nairobi, spread-B’ occurred from midnight until 0900. In Fig. 7, the movement of a small patch in the morning of the 5 July can be observed. Irregularities first occurred around midnight and were almost continuous

300

KM--

32

34

36

38

LONGITUDE Fig. 7. Movement

of an irregularity patch.

properties of ionospheric irregularities

Some

1269

at 0200. At 0400 a separate patch was observed, which first moved eastwards and then reversed direction. Its average height was found first to decrease and then to increase. If the positions of the patch in Fig. 7 are taken as limiting, then before 0530 the patch is moving eastwards with a velocity of about 50 msec-l and downwards with a velocity of about 5 msec- l. Between 0530 and 0700, its velocities are of the order of 20 msec-l westwards and 3 msec-l upwards. Figure 8 summarises the estimates of horizontal drift velocity obtained on a total of five different nights. It is obvious that the velocity reverses from East to West at

60t

60

I

I

2400

I

I

I

0400

I

0800

I

I

1200

LMT Fig. 8. Drift velocities of patches.

The dashed lines indicate doubtful values.

about 0600. This reversal agrees with the normal F-region drift pattern observed at low latitudes for small-scale irregularities. The magnitude of the velocities (~20 msec-l) is rather small, perhaps a factor of about 3 lower than the speed of irregularities measured from ground-based drift experiments (SKINNER et al., 1963). There is some indication of another reversal about midnight. The patches observed were often greater than about 500 km in East-West extent. However it must be realised that if the vertical thickness is ~100 km, then small gaps between separate patches would not be observed unless the satellite was almost overhead. Where complete individual patches are seen, it is interesting to note that they are remarkably stable, lasting sometimes as long as 3 hr. On several occasions after the disappearance of scintillations, irregular Faraday fading was observed, indicating the existence of large scale irregular structures. Although the irregularities have been shown above as continuous over a definite height range, it must be pointed out that in fact the dispersion analysis for individual records often indicated the existence of ‘gaps’ of up to 50 km over which no irregularities occur. There is strong evidence that these are real since they were found on both the 20 and 40 MHz BE-B records taken at the same time. The height and extent of the vertical gaps have been found to vary with latitude and longitude and

1270

R. F. ?XELLEHER and J. SINCLATR

there is some indication they follow a kind of wave pattern. This possibility needs to be examined by means of a detailed analysis of a number of satellite passes. 8. SW~~Y

AND

CONCLUSIONS

Travelling satellite spaoed-aerial recordings have often been used to derive the height and size of irregularities. If the correlation analysis equations of Section 3 of this paper are used, weighted values of these parameters are obtained. It has been shown that with high altitude satellites (B N 1000 km), the quicker similar-fades analysis gives perfectly satisfactory results. If correlation coefficients are calculated, it is clearly worthwhile to continue the analysis by means of the dispersion technique and so separate the effects occurring at different heights. Although polarisation scintillation occurs at frequencies as high as 20 MHz, it has been shown that its effects can be neglected. Some theories of the origin of ionospheric irregularities are discussed in HERMAN (1966) and REID (1968). It is relevant to note that in the present work we have found irregularities between 85 km and 800 km with the peaks of occurrence near or below 7t,F2 and at E, heights. This is in general agreement with other work (see, for example, Herman, lot. cit.). It has been shown that on individual passes, the mean height of the irregular region increases towards the magnetic equator, possibly following either a magnetic field line or the latitude increase in A,. The sizes of the irregularities do not change very much with height throughout the whole range from 100-500 km, though there is some evidence of an increase near 180 km. The strength 6N may show two maxima- at 100 and 300 km. There is no evidence of elongation over Nairobi, and both East-West and North-South sizes are of the order of 200 m. Slow-speed scintillation recordings can be used to study patches of irregularities. The size of the patches in an East-West direction ranges from less than 100 km to over 1000 km, but it is possible that small gaps remain undetected and the upper limit may be somewhat lower. CLEMESIU (1964), working near the equator, found sizes of up to 400 km. The time variation of patches has been studied and there is evidence of an eastward drift lasting from midnight until about 0600 LBIT. The drift t*hen reverses direction. This is in accordance with ground-based reflection experiments, though the velocities here are rather smaller. Using a different technique, CLEMESHA (1964) found an eastward drift of patches before mi~ight, though it is ~teresting to note that he did not observe patches after this time. There is some evidence of small (-5 msec-l) vertical motions. In the early hours of the morning, the irregularities usually occur both above and below A,. There is a suggestion of a vertical structure with gaps up to 50 km in height, and this may show a Iongitude and latitude variation. The irregular patches begin to break up some time in the morning and small individual patches may then persist for 3 hr or more. AclcnowEedgeme&s--The authors are grateful to Dr. J. A. RATCLIF~TEand Dr. K. G. BUDDEN for someve~useful comments. Theyw~htoth~k Professor &IECcACCI adhis co-workersfrom the Centro Ricerche Aerospaziati, who co-operated in setting up the receiving aerial system at Nairobi. They are grateful for the work done by members of the Ionospheric Group of University College Nairobi during the period of the San Marco experiment. This work has been sponsored in part by the Air Force Cambridge Research Laboratories through the European Office of Aerospace Research (OAR), United States Air Force Contract No. AF 01(052)-909.

Some properties of ionosphericirregularities

1271

REFERENCES BRAM.LEY E. N. BRIGGS B. H. BRIGGS B. H. and PARKIN I. A. BRIGGS B. H., PHILLIPS G. J. and SHINN D. H. CLEMESJXAB. R. GOODMAN N. R., KATZ S., KRAMER B. H. and KUO M. T. HERMAN J. R. JAMES P. W. JESPERSEN J. L. and KAIUS G. JONES D. and MAUDE A. D. KENT G. S. KOSTER J. R. MCCLURE J. P. RATCLIFFE J. A. REID G. C. SINCLAIR J. and KELLEEER R. F. SKINNER N. J., LYON A. J. and WRIQHT R. W. YEH K. C. and CHAO-HAN LIU

1968 1963 1950

Proc. In&n elect. Engrs B102, 533. J. Atmosph. Tew. Phya. 30, 1789. J. Atmosph. Tew. Phys. 25, 339. Proc. phys. Sot. B63, 106.

1964 1961

J. Atmosph. Terr. Phys. Technometrics 3, 245.

1966 1962 1963 1965 1959 1966 1964 1956 1968 1969 1963

Rev. Geophys. 4, 255. J. Atmosph. Terr. Phys. 24, 237. J. Atmosph. Terr. Phys. 26, 457. Nature, Lond. 206, 177. J. Atmesph. Terr. Phys. 16, 10. An& Gkophys. 22, 103. J. geophys. Res. 69, 1445. Rep. Prog. Phys. 19, 188. J. geophys. Res. 73, 1627. J. Atmosph. Terr. Phys. 31,201. The Ionosphere, p. 301. I.P.P.S., London

1967

I.E.E.E.

1955

26, 91.

Trans. AP15, 4, 539.