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The scintillation of radio signals from the Discoverer 36 satellite
Journalof Atmospheric andTerrestrial Physics,1964,Vol. 26,pp.1175-1185. Pergamon PressLtd. Printedin NorthernIreland
The scintillation of radio signals from the Discoverer 56 satellite W. J. G. BEYNON
and E. S. 0.
JONES
Department of Physics, University College of Wales, Aberystwyth (Received 27 July 1964) &&r&--The results of measurements on “scintillation” observed on 20 MC/S radio signals from satellite Discoverer 36 are presented. The observations cover geomagnetic latitudes 44”-67” N and show a steady increase in the scintillation intensity with latitude. A sharp increase in scintillation intensity occurs at a latitude a few degrees to the equatorward side of the aurora1 zone. There is a close correspondence between the incidence of scintillation and spread-F and it is concluded that the same irregularities in the P-layer (above about 250 km) are responsible for the two phenomena. Data are presented on the diurnal variation in the lower latitude limit of scintillation and its variation with magnetic activity. 1. INTRODUCTION
scintillation of radio waves during transmission through the ionosphere has been studied using recordings of 20 MC/S signals from the satellite Discoverer 36 (1961 a Kl) received at Aberystwyth (52’22’ N, 4’ 03’ W). The records were made on a fast response pen recorder running at 15 cm/min and the period of observations THE
4
I
Fig. 1. Sample record showing Faraday fading and scintillation. 14 December 1961-23 February 1962 included all but three days of the satellite’s active life. In this period records were obtained for a total of 261 satellite passes. The received signal usually showed the well-known regular variations associated with rotation of the plane of polarisation (Faraday fading) and, in addition, there was often clear “scintillation” of the signal. A typical sample record showing both the Faraday fading and scintillation is shown in Fig. 1. In assessing the intensity
s
1176
of the scintillation \vc }IiLVC used tlrc usual scirit~illat~ioii index 1 dcliricd as I (.-I,. ~- .+l,7)i:(A,r ! A,,) uhew <-l, anti A, respectively represent the mean maximum and minimum amplitude of tlic signal. 1n general this index could only be evaluated reliably from the pen records at the peaks of the Faraday fading. The index I was evaluated for each 15 set of the record. this bimr interval corresponding t,o a movcment of the satellite over about 1” of’ latitude. To indicst,e the flu&uation rate, three indices L. M and l!! were adopted corresponding to fluctuation rates of less than S/SW (L), from 3 to 6 fluctuations/set (&1) and greater than 6~s~~. (11). The limit in fluctuation rate which could be reliably resolved on the pen records was about O;sec. The scintillation data have been examined in relation to the well-known “spreadE”’ phenomenon found in vertical incidence soundings of the ionosphere. For this purpose, tabulated data for four other stations, Slough, Inverness, de Bilt and Lindau have been used. These tabulations provided two qualitative indices of the intensity of spread-F. In addition to these tabulated data a certain number of ionograms taken at Aberystwyth a,t the time of the satellite recordings was also available. The orbital data were obtained from an ephemeris provided by the Radio Research Station, Slough, and it is estimated that the times given are accurate to within a few seconds. The principal orbital elements of the satellite were : inclination 81.2”, apogee 486 km, perigee 239 km. 2. EXPERIMENTAL RBX~XXS 2.1.
Incidence
of~cintiElatio7~
The analysis of the satellite data shows clearly that, scintillation is mainly a night-time phenomena and occurs predominantly between 1800 and 0600 hours local time; in the two-month period of observation, scintillation was only evident in some 25 per cent of the day-time passes whereas it appeared in 81 per cent of the night-time passes. Not only does scintillation occur much more frequently by night than by day, but the intensity of the scintillation is also greatest near or just after local midnight. Thus from the present data the intensity index I averages O-3 and 0.27 for the periods 1800-2200 and 0200-0600 hours L.M.T. respectively, whereas for the midnight period 2200-0200 hours L.M.T. it is 0.43. 2.2.
htituae
variation
in scintillation index
The latitude variation of the scintillation index for night-time transits calculated for each degree of latitude is shown by the solid line curve in Fig. 2. Each point in this diagram is the mean of about 160 observations and has an estimated uncertainty of less than 5 per cent. It will be seen that apart from some irregularity between geomagnetic latitudes 50°-62”, there is a steady decrease in the scintillation index with geomagnetic latitude and “cut off” appears to have occurred near geomagnetic latitude 44”. The records obtained from Sputnik I by KENT (1959) at Cambridge, which has the same geomagnetic latitude as Aberystwyth, are shown by the dotted curve in Fig. 2 (the magnetic ordinate used by KENT has here been converted to geomagnetic latitude).
1177
The scintillation of radio signals from the Discoverer 36 satellite
2.3. Horizontal extent of scintillation regions Records for consecutive transits were available on sixty-four occasions. These transit pairs were examined for the incidence or otherwise of scintillation and the results are summarized in Table 1. It will be seen that if scintillation was observed on any given transit, there was
0. 20
\ \ \ \ \ 0.04
Geomagnetic ttitude Fig. 2. Variation of mean scintillation index with geomagnetic latitude. Table 1. Analysis of records on consecutive transits
Scintillation not present on either transit Scintillation on one transit only Scintillation on both transits
Number of pairs
Percentage of total
10 16 38
16 25 59
a high probability that it would also be observed on the succeeding transit. Furthermore, it was found that when scintillation was observed on two successive transits, both the intensity of the scintillation (I) and its fluctuation rate (L, bl and H) were also very closely similar on the two transits. On only three of the thirty-eight cases given in Table 1 could the scintillation observed on the successive transits be described as dissimilar.
At the latitude of these scintillations the scparatJion of successive passes for this satellite was 1500 km. The average height of t,he satellite was about 360 km and if’ it be assumed that the scintillation irregularities are located above about 250 km. the results indicate that these irregularities frequently extended in an 3%’ direction over horizontal distances of at least, the distance b&veen successive passes. The high inclination of the satellite orbit permitt,ed some study of the extent of the scintillation patches in the NS direction. In general it was found that the NS (or Iatit.ude)
1
4
1
I
250
270
280
I 310
I
330
I 350
Hdght km@,
Fig. 3. Average time occurrenceof scintillation m a function of satellite altitude.
dimension of the scintillation patch was in some degree dependent on the latitude at which it occurred, the size of the patch increasing with latitude. On all occasions on which the lower latitude limit of scintillation area WM 55” N or more, then the area was continuous up to at least 67”N (this being the higher& latitude for which reliable observationa could be obtained from the Aberyatwyth satellite records). Subject again to an aaeumption of the height of irregularities, the present work indicates that the average NS extension wag about 766 km with a minimum value of about 400 km. Scintillation areaa with smaller dimensions were rare. 2.4. Heighd of sciddlation irregdarities With the present data from one observing station only it is not possible to fix the preoise height of the ionospheric irregularities which produoe the scintillation but certain points can be eatabliehed. Figure 3 shows the percentage time on which
The scintillation of radio signals from the Discoverer 36 s&Jlite
1179
scintillation is present as a function of satellite altitude. The fact that scintillation is obtained more frequently at higher satellite altitudes may simply mean that the irregularities occur at all altitudes above about 250 km or, alternatively, it could indicate a greater degree, or a greater incidence, of irregularity with altitude. Additional information on this point is provided by some studies of spread-F made in the course of the present work. It is well established that there is a close correlation between spread-P and scintillation and it seems likely that both phenomena arise from the same ionospheric irregularities. On a number of occasions
1
1
73O_ 71° ; .,o
69’ 0
tEfl& :
65’ 63O 61° -
.
16
I
I
I
1
1
I
I
I
18
20
22
60
2
4
6
8
I 10
UT Fig. 4. Diurnal variation of lower latitude limit of scintillation zone8.
we have observed very strong spread-F although the satellite records showed no scintillation at all. In such cases the satellite altitude at the times concerned was always less than the lower height limit of the spread-F (about 250 kms as deduced from the ionograms at Aberystwyth) so that the absence of scintillation was not unexpected. The present results indicate a lower height limit of about 250 km for the irregularities producing scintillation. A similar figure has been deduced by YEH and SWENSON(1959) and by KENT (1959).
2.5. Latitude limit of scintillation It is well-known that the lower latitude limit of the aurora1 zone shows a diurnal variation, the lowest latitude occurring on the average near local midnight. If such a diurnal variation also occurs in the lower latitude limit of scintillation, then the limit observed on successive transits might be expected to show a systematic variation with time of day. We have examined some thirty pairs of successive transits and the results are plotted in Fig. 4. In this Figure, for each pair of successive transits we have plotted the magnetic dip angles corresponding to the lower
latitude limit of scintillatioll. 1t is COl)C il~~~~l’~‘Cii~tl31tll;tt tllcl (l;Lt,it rt+l~ to clifli~wut days spread ovel a l)crLod of t,\\o mon181~satltl a day-to-da?. A-ariattility (~a11I)(, expect8cld. However there is clear cvidencc iilr II diarnal variation in thcl Iatitjutic~ limit of the scintillation, t’hc lowest latit,ude t)cing reached just after local rnithtight The present data indicate t)hat this lower latitude limit of scint,illation c~orrespontls to a magnetic dip angle of about 63” (Figs. Z and 1). The magnit’udc of the diurnal shift in the lower latitude limit of scint’illation a,ppears t)o be compara~t~l~~ with tllat of the aurora1 zonk’.
Many workers have suggested that radio signal scintillation and spread-y result from the same ionospheric irregularities and some information on this point is provided by the present data. Figures ri(a, h. c) show three examples in which t,ht:
Fig. 5. Horizontal
extent
of scintillation
(shaded areus).
latitude extent of scintillation was observed for successive transits of Discoverer 36. In Fig. 5(a), both the lower and upper latitude limits were well-defined and it could be inferred that, at this time, scintillation irregularities were located in a belt extending over the lower half of the British Isles and Northern France. At this time it was found that spread-F occurred at Slough, de Bilt and Lindau but not at In Fig. 5(b) the lower latitude limit of scintillation was in Southern Inverness. Scotland and, on this occasion, spread-F occurred at Inverness but not at Slough, de Bilt or Lindau. In Fig. 5(c) the scintillation zone covered the whole of the British Isles and spread-F was observed at all four ionospheric stations. In the course of the present work many examples of this correspondence between zones of scintillation and spread-F were noted. We have also found that not only do the geographical limits of the zones correspond but that there &often correlation between the intensities of the two phenomena. Thus in the case of the observations in Fig. 5(c), it was found that at the latitude of Inverness the scintillation intensity index 1 was about 0.2 and spread-F was class&d as “partial”, whereas for the latitude of Slough the scintillation index was In Table 2 we show the results of an 1.0 and the spread-F was classified as “total”. analy&s of the incidence of sointill&tion and spread-F for latitudes 52” N and 58” N. It will be seen that for both latitudes the mean scintillation index shows a
The scintillation of radio signals from the Discoverer 36 satellite
1lSI
Table 2. Intensity of spread-P and of scintillation Degree of spread-a
Mean scintillation index I Inverness Slough Lat. 52” *Y Lat. 58” N
Total Partial
0.72 (22)
Sane
0,27 (17)
0.53 (27)
0.70 (12) 0.45 (14)
0.11 (36)
(The figures in brackets are the number of observations concerned.)
increase with the degree of spread-F. The data also shows that there is measurable scintillation even in the apparent absence of any spread-F. However the tabulation of spread-F is certainly only a first order indication of its intensity and the scintillation index I is probably a far more sensitive indicator of irre~larities in the F-layer. For all three categories of spread-F the value of 1 is larger for the higher latitude station. We may quote one other remarkable example of the simultaneous occurrence of scintillation and spread-F from observations made at Aberystwyth on the satellite Cosmos 5. The total active life of this satellite extended from 28 May 1962 to 19 August 1962 and during this period, our recordings were almost completely free from scintillation apart from a single continuous period of 10 days in June. From
systematic
6 to 15 June the signals showed a well-defined intense scintillation are& extending down to at least 40” N and, during this period, some very close correlation was noted between scintillation and spread-F as observed at a number of European
ionospheric sounding stations.
To investigate a possible correlation between scintillation and magnetic activity we have limited attention to those cases in which scintillation was observed on successive transits. In this way we can be reasonably certain that the scintillation observed had an EW extent of at least 1500 km. In Fig. 6 the mean scint~lation indices for these transits pairs are plotted against the corresponding K figure for the nearest magnetic observatory-Hartland Point. The points plotted represent average values of I for K = O-l, l-2 eto. Although the number of transit pairs is somewhat limited, these results indicate that there is a clear positive correlation between the intensity of scintillation and magnetic activity. It has been shown by SHIMAZA~~ (1959)that between latitudes 20” and 60° there is a strong positive correlation between the occurrence of spread-F and magnetic activity. The results shown in Fig. 6 are thus consistent with the conclusion that the same ionospheric irregularities are responsible for scintillation and spread-F. The present results also indicate that the lower latitude limit of scintillation decreases with increasing magnetic activity. These geomagnetic latitude limits are shown in Fig. 7, where, with the exception of the mean point for K lying between 3 and 4, the data suggest a decrease of about 4” in latitude for an increase of K from 0 to 5. This compares with an average decrease of the latitude of the aurora1 zone of about 7’ for the same range of K (BLESSetal.,1959).
0.8
1
0
2
3
4
K Fig. 6. Variation
of mean scint,illLtt.ion index with geomagnetic
activity.
I
56O -
0
@
$ a 3 3 u 3 B E”
0 54O -
0
52O-
0
0 c”
so0
*
0
1
1
Fig. 7. Mean lower latitude
1
2
I
K
limit of scintil.I&ion activity.
3
I
4
zone aa a function
I of geomagnetic
1183
The scintillation of radio signals from the Discoverer 36 satellite
2.8. Azimuth variation of scintillation It has been suggested by various workers that the ionospheric irregularities responsible for scintillation are aligned along the magnetic field. If this is the case, it might be expected that the degree of scintillation would show a variation with the direction of propagation of the received signal. We have examined our data for evidence of a dependence of scintillation intensity on azimuth and the results are plotted in histogram form in Fig. 8. It will be seen that the histogram is asymmetric with the maximum
towards West of North, i.e. in the direction of magnetic North.
1
C
0.4 0.3 -
0.2
-
0.1
-
r-r
(81) (83) (84) (79))(63
1 140) (36)
(541(72)
(80) 06)
60’ 0 3o” 3o” East West Fig. 8. Distribution of mean scintilletion index with azimuth. 9o”
60°
(75) 9o”
This result thus suggests that the degree of scintillation does in fact depend on the direction of propagation, with an enhancement of intensity when propag&ion is in the direction of the magnetic meridian. 3. DISCTJSSION OF RESULTS Although in the present recordings scintillation appeared much more frequently by night than by day, the phenomenon was completely absent in some 30 per cent of the night-time records. This result is in marked contrast to the observations of certain other workers. Thus YEH and SWENSON(1969), MUNRO(1963) and KENT (1969) state that they observed scintillation on nearly every night-time transit. For those occasions in the present work on which night-time scintillation wa,s completely absent, the height of the satellite varied between 250 and 400 km and we must conclude that the ionospheric irregularities normally responsible for scintillation must then, if present at all, be at EI,grea.ter altitude than 400 km. Figure 2 shows the variation of the scintillation index I with geomagnetic latitude as given by the present work and by KENT (1969) from observations in
l!Gi on Sputnik 1. The form of the curves for the two sets of tlat’a is similar but the values of I for the Aberystwyth 1962 data are significantly larger than those give11 by the Gmbridge 1957 observations. In seeking an explanation for this differencts. it is t#obe noted that, at the time of both sets of observations, the meail \,alue of tiltmagnetic index A,, was I-1 but the sunspot number R for the two periods differed considerably. In October 1957 R was 180 and in the period December 1961 February 1962 it was only about 40. It is known-~SHr,MszA&xr (1969)-that at middle latitudes the spread-F phenomenon is considerably more marked at sunspot, minimum than sunspot maximum and, if WC assume that these ionospheric: irregularities are also responsible for scintillation. this might well explain the displacement of the curves in Fig. 2. BRIGGS (1964) has suggested that the observed solar cycle change in spread-F results from an alt,itude change in the irregularities. The two satellites concerned in these data were at comparable altitudes in the rauge 200-500 km and it seems reasonable to suppose that a moderate height variation in spread-P irregularit,ies with solar activity could produce the change in the scintillatiou level shown by the curves of Fig. L’. A second feature of the Aberystwyth curve in Fig. 2 is the sharp increase in scintillation near 61” geomagnetic latitude (magnetic dip angle 72’). This latitude is a few degrees lower than that of the aurora1 zone. Each point in Fig. 2 is the mean of about 160 observations, the standard devation being less than 5 per cent. We t,hus consider it likely that this “kink” in the curve is physically significant and support for this conclusion comes from the work of SINGLETON (1!)60) and from recent satellite observations (PAULIKAS and FREDEN, 1964). SIRGLETON: using data from twenty-three ionospheric stations, has examined the variation of spread-F with latitude. The data were grouped according to season and show a marked increase in spread-J’ towards the aurora1 zone. Each of the curves (summer: winter and equinox) also shows a distinct subsidiary maximum near magnetic dip $O”--75’. This experimental evidence for a sharp increase in both spread-F and scintillation near magnetic dip ‘is”, i.e. at a lat,itude several degrees lower than that of the aurora1 zone, may be fortuitous but additional evidence comes from two other sources. I?AULIKASand FREDEN (1964) have published data on electron fluxes obtained from satellite studies made in September 1962. These authors found that peaks in the energetic electron flux (=, 0.9 MeV) occur at magnetic L values (LpiICILWAIN. 1961) I.“. 2.0 and 4--6. The largest of the L values corresponds to a latitude a few degrees lower than that of the aurora1 zone and is in fact very close to that a,t which the sharp increase in the scintillation index I of Fig. 2 occurs. It is also of interest to record that HARTZ (1963), from a study of high latitude ionospheric radio wave absorption, has shown that there is a clear latitude variation in this absorption with a maximum intensity again located a few degrees below the latitude of the aurora1 zone. These different high latitude phenomena arise from various types of solar particle bombardment and it is to be expected that, in respect of their general variation with latitude, with solar and magnetic activity, they will show many similarities. The principal features of radio wave scintillation which appear in the present satellite observations-such as the variation with latitude, the diurnal variation in the
The scintillation of radio signals from the Discoverer 36 satellite
1185
lower latitude limit, the correlation with magnetic activity, the sharp increase in intensity just below the aurora1 zone-are all consistent with the supposition that night time scintillation at least results from irregularities produced in the ionosphere at altitudes above about 250 km by an incident flux of electrons. Acknowledgements -The authors wish to acknowledge the invaluable assistance of Mr. K. J. EDWARDS with the experimental work. One of us (E. S. 0. J.) is indebted to the Department of Scientific and Industrial Research for financial assistance. REFERENCES BLESS R. C., GARTLEIN C. W., KIMBELL D. S. and SPRAGUE G. BRIGGS B. H. HARTZ T. R.
1959
J. Geophys.
1964 1963
KENT G. MCILWAIN C. E. MUNRO G. H. PAULIKAS G. A. and FREDEN S. C. SHIXUAKI T. SINGLETON D. G. YEH K. C. and SWENSON G. W.
1959 1961 1963 1964 1959 1960 1959
J. Atmosph. Terr. Phys. 26, 1. Rad. Ast. and Sat. Stud. of the Atm. Holland, Amsterdam. J. Atmosph. Terr. Phys. 16,10. J. Geophys. Res. 66, 3681. J. Geophys. Res. 68, 1851. J. Geophys. Rec. 69, 1239. J. Rad. Rea. Labs, Tokyo 6, 685. J. Geophys. Res. 65, 3615. J. Geophys. Res. 84, 2281.
Res. 64, 949.
North-
The scintillation of radio signals from the Discoverer 56 satellite W. J. G. BEYNON
and E. S. 0.
JONES
Department of Physics, University College of Wales, Aberystwyth (Received 27 July 1964) &&r&--The results of measurements on “scintillation” observed on 20 MC/S radio signals from satellite Discoverer 36 are presented. The observations cover geomagnetic latitudes 44”-67” N and show a steady increase in the scintillation intensity with latitude. A sharp increase in scintillation intensity occurs at a latitude a few degrees to the equatorward side of the aurora1 zone. There is a close correspondence between the incidence of scintillation and spread-F and it is concluded that the same irregularities in the P-layer (above about 250 km) are responsible for the two phenomena. Data are presented on the diurnal variation in the lower latitude limit of scintillation and its variation with magnetic activity. 1. INTRODUCTION
scintillation of radio waves during transmission through the ionosphere has been studied using recordings of 20 MC/S signals from the satellite Discoverer 36 (1961 a Kl) received at Aberystwyth (52’22’ N, 4’ 03’ W). The records were made on a fast response pen recorder running at 15 cm/min and the period of observations THE
4
I
Fig. 1. Sample record showing Faraday fading and scintillation. 14 December 1961-23 February 1962 included all but three days of the satellite’s active life. In this period records were obtained for a total of 261 satellite passes. The received signal usually showed the well-known regular variations associated with rotation of the plane of polarisation (Faraday fading) and, in addition, there was often clear “scintillation” of the signal. A typical sample record showing both the Faraday fading and scintillation is shown in Fig. 1. In assessing the intensity
s
1176
of the scintillation \vc }IiLVC used tlrc usual scirit~illat~ioii index 1 dcliricd as I (.-I,. ~- .+l,7)i:(A,r ! A,,) uhew <-l, anti A, respectively represent the mean maximum and minimum amplitude of tlic signal. 1n general this index could only be evaluated reliably from the pen records at the peaks of the Faraday fading. The index I was evaluated for each 15 set of the record. this bimr interval corresponding t,o a movcment of the satellite over about 1” of’ latitude. To indicst,e the flu&uation rate, three indices L. M and l!! were adopted corresponding to fluctuation rates of less than S/SW (L), from 3 to 6 fluctuations/set (&1) and greater than 6~s~~. (11). The limit in fluctuation rate which could be reliably resolved on the pen records was about O;sec. The scintillation data have been examined in relation to the well-known “spreadE”’ phenomenon found in vertical incidence soundings of the ionosphere. For this purpose, tabulated data for four other stations, Slough, Inverness, de Bilt and Lindau have been used. These tabulations provided two qualitative indices of the intensity of spread-F. In addition to these tabulated data a certain number of ionograms taken at Aberystwyth a,t the time of the satellite recordings was also available. The orbital data were obtained from an ephemeris provided by the Radio Research Station, Slough, and it is estimated that the times given are accurate to within a few seconds. The principal orbital elements of the satellite were : inclination 81.2”, apogee 486 km, perigee 239 km. 2. EXPERIMENTAL RBX~XXS 2.1.
Incidence
of~cintiElatio7~
The analysis of the satellite data shows clearly that, scintillation is mainly a night-time phenomena and occurs predominantly between 1800 and 0600 hours local time; in the two-month period of observation, scintillation was only evident in some 25 per cent of the day-time passes whereas it appeared in 81 per cent of the night-time passes. Not only does scintillation occur much more frequently by night than by day, but the intensity of the scintillation is also greatest near or just after local midnight. Thus from the present data the intensity index I averages O-3 and 0.27 for the periods 1800-2200 and 0200-0600 hours L.M.T. respectively, whereas for the midnight period 2200-0200 hours L.M.T. it is 0.43. 2.2.
htituae
variation
in scintillation index
The latitude variation of the scintillation index for night-time transits calculated for each degree of latitude is shown by the solid line curve in Fig. 2. Each point in this diagram is the mean of about 160 observations and has an estimated uncertainty of less than 5 per cent. It will be seen that apart from some irregularity between geomagnetic latitudes 50°-62”, there is a steady decrease in the scintillation index with geomagnetic latitude and “cut off” appears to have occurred near geomagnetic latitude 44”. The records obtained from Sputnik I by KENT (1959) at Cambridge, which has the same geomagnetic latitude as Aberystwyth, are shown by the dotted curve in Fig. 2 (the magnetic ordinate used by KENT has here been converted to geomagnetic latitude).
1177
The scintillation of radio signals from the Discoverer 36 satellite
2.3. Horizontal extent of scintillation regions Records for consecutive transits were available on sixty-four occasions. These transit pairs were examined for the incidence or otherwise of scintillation and the results are summarized in Table 1. It will be seen that if scintillation was observed on any given transit, there was
0. 20
\ \ \ \ \ 0.04
Geomagnetic ttitude Fig. 2. Variation of mean scintillation index with geomagnetic latitude. Table 1. Analysis of records on consecutive transits
Scintillation not present on either transit Scintillation on one transit only Scintillation on both transits
Number of pairs
Percentage of total
10 16 38
16 25 59
a high probability that it would also be observed on the succeeding transit. Furthermore, it was found that when scintillation was observed on two successive transits, both the intensity of the scintillation (I) and its fluctuation rate (L, bl and H) were also very closely similar on the two transits. On only three of the thirty-eight cases given in Table 1 could the scintillation observed on the successive transits be described as dissimilar.
At the latitude of these scintillations the scparatJion of successive passes for this satellite was 1500 km. The average height of t,he satellite was about 360 km and if’ it be assumed that the scintillation irregularities are located above about 250 km. the results indicate that these irregularities frequently extended in an 3%’ direction over horizontal distances of at least, the distance b&veen successive passes. The high inclination of the satellite orbit permitt,ed some study of the extent of the scintillation patches in the NS direction. In general it was found that the NS (or Iatit.ude)
1
4
1
I
250
270
280
I 310
I
330
I 350
Hdght km@,
Fig. 3. Average time occurrenceof scintillation m a function of satellite altitude.
dimension of the scintillation patch was in some degree dependent on the latitude at which it occurred, the size of the patch increasing with latitude. On all occasions on which the lower latitude limit of scintillation area WM 55” N or more, then the area was continuous up to at least 67”N (this being the higher& latitude for which reliable observationa could be obtained from the Aberyatwyth satellite records). Subject again to an aaeumption of the height of irregularities, the present work indicates that the average NS extension wag about 766 km with a minimum value of about 400 km. Scintillation areaa with smaller dimensions were rare. 2.4. Heighd of sciddlation irregdarities With the present data from one observing station only it is not possible to fix the preoise height of the ionospheric irregularities which produoe the scintillation but certain points can be eatabliehed. Figure 3 shows the percentage time on which
The scintillation of radio signals from the Discoverer 36 s&Jlite
1179
scintillation is present as a function of satellite altitude. The fact that scintillation is obtained more frequently at higher satellite altitudes may simply mean that the irregularities occur at all altitudes above about 250 km or, alternatively, it could indicate a greater degree, or a greater incidence, of irregularity with altitude. Additional information on this point is provided by some studies of spread-F made in the course of the present work. It is well established that there is a close correlation between spread-P and scintillation and it seems likely that both phenomena arise from the same ionospheric irregularities. On a number of occasions
1
1
73O_ 71° ; .,o
69’ 0
tEfl& :
65’ 63O 61° -
.
16
I
I
I
1
1
I
I
I
18
20
22
60
2
4
6
8
I 10
UT Fig. 4. Diurnal variation of lower latitude limit of scintillation zone8.
we have observed very strong spread-F although the satellite records showed no scintillation at all. In such cases the satellite altitude at the times concerned was always less than the lower height limit of the spread-F (about 250 kms as deduced from the ionograms at Aberystwyth) so that the absence of scintillation was not unexpected. The present results indicate a lower height limit of about 250 km for the irregularities producing scintillation. A similar figure has been deduced by YEH and SWENSON(1959) and by KENT (1959).
2.5. Latitude limit of scintillation It is well-known that the lower latitude limit of the aurora1 zone shows a diurnal variation, the lowest latitude occurring on the average near local midnight. If such a diurnal variation also occurs in the lower latitude limit of scintillation, then the limit observed on successive transits might be expected to show a systematic variation with time of day. We have examined some thirty pairs of successive transits and the results are plotted in Fig. 4. In this Figure, for each pair of successive transits we have plotted the magnetic dip angles corresponding to the lower
latitude limit of scintillatioll. 1t is COl)C il~~~~l’~‘Cii~tl31tll;tt tllcl (l;Lt,it rt+l~ to clifli~wut days spread ovel a l)crLod of t,\\o mon181~satltl a day-to-da?. A-ariattility (~a11I)(, expect8cld. However there is clear cvidencc iilr II diarnal variation in thcl Iatitjutic~ limit of the scintillation, t’hc lowest latit,ude t)cing reached just after local rnithtight The present data indicate t)hat this lower latitude limit of scint,illation c~orrespontls to a magnetic dip angle of about 63” (Figs. Z and 1). The magnit’udc of the diurnal shift in the lower latitude limit of scint’illation a,ppears t)o be compara~t~l~~ with tllat of the aurora1 zonk’.
Many workers have suggested that radio signal scintillation and spread-y result from the same ionospheric irregularities and some information on this point is provided by the present data. Figures ri(a, h. c) show three examples in which t,ht:
Fig. 5. Horizontal
extent
of scintillation
(shaded areus).
latitude extent of scintillation was observed for successive transits of Discoverer 36. In Fig. 5(a), both the lower and upper latitude limits were well-defined and it could be inferred that, at this time, scintillation irregularities were located in a belt extending over the lower half of the British Isles and Northern France. At this time it was found that spread-F occurred at Slough, de Bilt and Lindau but not at In Fig. 5(b) the lower latitude limit of scintillation was in Southern Inverness. Scotland and, on this occasion, spread-F occurred at Inverness but not at Slough, de Bilt or Lindau. In Fig. 5(c) the scintillation zone covered the whole of the British Isles and spread-F was observed at all four ionospheric stations. In the course of the present work many examples of this correspondence between zones of scintillation and spread-F were noted. We have also found that not only do the geographical limits of the zones correspond but that there &often correlation between the intensities of the two phenomena. Thus in the case of the observations in Fig. 5(c), it was found that at the latitude of Inverness the scintillation intensity index 1 was about 0.2 and spread-F was class&d as “partial”, whereas for the latitude of Slough the scintillation index was In Table 2 we show the results of an 1.0 and the spread-F was classified as “total”. analy&s of the incidence of sointill&tion and spread-F for latitudes 52” N and 58” N. It will be seen that for both latitudes the mean scintillation index shows a
The scintillation of radio signals from the Discoverer 36 satellite
1lSI
Table 2. Intensity of spread-P and of scintillation Degree of spread-a
Mean scintillation index I Inverness Slough Lat. 52” *Y Lat. 58” N
Total Partial
0.72 (22)
Sane
0,27 (17)
0.53 (27)
0.70 (12) 0.45 (14)
0.11 (36)
(The figures in brackets are the number of observations concerned.)
increase with the degree of spread-F. The data also shows that there is measurable scintillation even in the apparent absence of any spread-F. However the tabulation of spread-F is certainly only a first order indication of its intensity and the scintillation index I is probably a far more sensitive indicator of irre~larities in the F-layer. For all three categories of spread-F the value of 1 is larger for the higher latitude station. We may quote one other remarkable example of the simultaneous occurrence of scintillation and spread-F from observations made at Aberystwyth on the satellite Cosmos 5. The total active life of this satellite extended from 28 May 1962 to 19 August 1962 and during this period, our recordings were almost completely free from scintillation apart from a single continuous period of 10 days in June. From
systematic
6 to 15 June the signals showed a well-defined intense scintillation are& extending down to at least 40” N and, during this period, some very close correlation was noted between scintillation and spread-F as observed at a number of European
ionospheric sounding stations.
To investigate a possible correlation between scintillation and magnetic activity we have limited attention to those cases in which scintillation was observed on successive transits. In this way we can be reasonably certain that the scintillation observed had an EW extent of at least 1500 km. In Fig. 6 the mean scint~lation indices for these transits pairs are plotted against the corresponding K figure for the nearest magnetic observatory-Hartland Point. The points plotted represent average values of I for K = O-l, l-2 eto. Although the number of transit pairs is somewhat limited, these results indicate that there is a clear positive correlation between the intensity of scintillation and magnetic activity. It has been shown by SHIMAZA~~ (1959)that between latitudes 20” and 60° there is a strong positive correlation between the occurrence of spread-F and magnetic activity. The results shown in Fig. 6 are thus consistent with the conclusion that the same ionospheric irregularities are responsible for scintillation and spread-F. The present results also indicate that the lower latitude limit of scintillation decreases with increasing magnetic activity. These geomagnetic latitude limits are shown in Fig. 7, where, with the exception of the mean point for K lying between 3 and 4, the data suggest a decrease of about 4” in latitude for an increase of K from 0 to 5. This compares with an average decrease of the latitude of the aurora1 zone of about 7’ for the same range of K (BLESSetal.,1959).
0.8
1
0
2
3
4
K Fig. 6. Variation
of mean scint,illLtt.ion index with geomagnetic
activity.
I
56O -
0
@
$ a 3 3 u 3 B E”
0 54O -
0
52O-
0
0 c”
so0
*
0
1
1
Fig. 7. Mean lower latitude
1
2
I
K
limit of scintil.I&ion activity.
3
I
4
zone aa a function
I of geomagnetic
1183
The scintillation of radio signals from the Discoverer 36 satellite
2.8. Azimuth variation of scintillation It has been suggested by various workers that the ionospheric irregularities responsible for scintillation are aligned along the magnetic field. If this is the case, it might be expected that the degree of scintillation would show a variation with the direction of propagation of the received signal. We have examined our data for evidence of a dependence of scintillation intensity on azimuth and the results are plotted in histogram form in Fig. 8. It will be seen that the histogram is asymmetric with the maximum
towards West of North, i.e. in the direction of magnetic North.
1
C
0.4 0.3 -
0.2
-
0.1
-
r-r
(81) (83) (84) (79))(63
1 140) (36)
(541(72)
(80) 06)
60’ 0 3o” 3o” East West Fig. 8. Distribution of mean scintilletion index with azimuth. 9o”
60°
(75) 9o”
This result thus suggests that the degree of scintillation does in fact depend on the direction of propagation, with an enhancement of intensity when propag&ion is in the direction of the magnetic meridian. 3. DISCTJSSION OF RESULTS Although in the present recordings scintillation appeared much more frequently by night than by day, the phenomenon was completely absent in some 30 per cent of the night-time records. This result is in marked contrast to the observations of certain other workers. Thus YEH and SWENSON(1969), MUNRO(1963) and KENT (1969) state that they observed scintillation on nearly every night-time transit. For those occasions in the present work on which night-time scintillation wa,s completely absent, the height of the satellite varied between 250 and 400 km and we must conclude that the ionospheric irregularities normally responsible for scintillation must then, if present at all, be at EI,grea.ter altitude than 400 km. Figure 2 shows the variation of the scintillation index I with geomagnetic latitude as given by the present work and by KENT (1969) from observations in
l!Gi on Sputnik 1. The form of the curves for the two sets of tlat’a is similar but the values of I for the Aberystwyth 1962 data are significantly larger than those give11 by the Gmbridge 1957 observations. In seeking an explanation for this differencts. it is t#obe noted that, at the time of both sets of observations, the meail \,alue of tiltmagnetic index A,, was I-1 but the sunspot number R for the two periods differed considerably. In October 1957 R was 180 and in the period December 1961 February 1962 it was only about 40. It is known-~SHr,MszA&xr (1969)-that at middle latitudes the spread-F phenomenon is considerably more marked at sunspot, minimum than sunspot maximum and, if WC assume that these ionospheric: irregularities are also responsible for scintillation. this might well explain the displacement of the curves in Fig. 2. BRIGGS (1964) has suggested that the observed solar cycle change in spread-F results from an alt,itude change in the irregularities. The two satellites concerned in these data were at comparable altitudes in the rauge 200-500 km and it seems reasonable to suppose that a moderate height variation in spread-P irregularit,ies with solar activity could produce the change in the scintillatiou level shown by the curves of Fig. L’. A second feature of the Aberystwyth curve in Fig. 2 is the sharp increase in scintillation near 61” geomagnetic latitude (magnetic dip angle 72’). This latitude is a few degrees lower than that of the aurora1 zone. Each point in Fig. 2 is the mean of about 160 observations, the standard devation being less than 5 per cent. We t,hus consider it likely that this “kink” in the curve is physically significant and support for this conclusion comes from the work of SINGLETON (1!)60) and from recent satellite observations (PAULIKAS and FREDEN, 1964). SIRGLETON: using data from twenty-three ionospheric stations, has examined the variation of spread-F with latitude. The data were grouped according to season and show a marked increase in spread-J’ towards the aurora1 zone. Each of the curves (summer: winter and equinox) also shows a distinct subsidiary maximum near magnetic dip $O”--75’. This experimental evidence for a sharp increase in both spread-F and scintillation near magnetic dip ‘is”, i.e. at a lat,itude several degrees lower than that of the aurora1 zone, may be fortuitous but additional evidence comes from two other sources. I?AULIKASand FREDEN (1964) have published data on electron fluxes obtained from satellite studies made in September 1962. These authors found that peaks in the energetic electron flux (=, 0.9 MeV) occur at magnetic L values (LpiICILWAIN. 1961) I.“. 2.0 and 4--6. The largest of the L values corresponds to a latitude a few degrees lower than that of the aurora1 zone and is in fact very close to that a,t which the sharp increase in the scintillation index I of Fig. 2 occurs. It is also of interest to record that HARTZ (1963), from a study of high latitude ionospheric radio wave absorption, has shown that there is a clear latitude variation in this absorption with a maximum intensity again located a few degrees below the latitude of the aurora1 zone. These different high latitude phenomena arise from various types of solar particle bombardment and it is to be expected that, in respect of their general variation with latitude, with solar and magnetic activity, they will show many similarities. The principal features of radio wave scintillation which appear in the present satellite observations-such as the variation with latitude, the diurnal variation in the
The scintillation of radio signals from the Discoverer 36 satellite
1185
lower latitude limit, the correlation with magnetic activity, the sharp increase in intensity just below the aurora1 zone-are all consistent with the supposition that night time scintillation at least results from irregularities produced in the ionosphere at altitudes above about 250 km by an incident flux of electrons. Acknowledgements -The authors wish to acknowledge the invaluable assistance of Mr. K. J. EDWARDS with the experimental work. One of us (E. S. 0. J.) is indebted to the Department of Scientific and Industrial Research for financial assistance. REFERENCES BLESS R. C., GARTLEIN C. W., KIMBELL D. S. and SPRAGUE G. BRIGGS B. H. HARTZ T. R.
1959
J. Geophys.
1964 1963
KENT G. MCILWAIN C. E. MUNRO G. H. PAULIKAS G. A. and FREDEN S. C. SHIXUAKI T. SINGLETON D. G. YEH K. C. and SWENSON G. W.
1959 1961 1963 1964 1959 1960 1959
J. Atmosph. Terr. Phys. 26, 1. Rad. Ast. and Sat. Stud. of the Atm. Holland, Amsterdam. J. Atmosph. Terr. Phys. 16,10. J. Geophys. Res. 66, 3681. J. Geophys. Res. 68, 1851. J. Geophys. Rec. 69, 1239. J. Rad. Rea. Labs, Tokyo 6, 685. J. Geophys. Res. 65, 3615. J. Geophys. Res. 84, 2281.
Res. 64, 949.
North-