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Observations of radio star scintillations and spread-F echoes over a solar cycle
Journal of Atmospheric and Terrestrial Physics, 1964, Vol. 28, pp. 1 to 23.
Pergamon PressLtd.Printedin Northern Ireland
Observations of radio star scintillations and spread-s echoes over a solar cycle B. H. BRIGQS Department of Physics, University of Adelaide (Received1 July 1963) Abstract-Observations of the scintillations of the radio source Cassiopeia A were made at Cambridge on a frequency of 38 MC/S over the period 1949-1961. The resultsare presentedin the form of curves showing the mean diurnal variation of the scintillationindex for each month. The variations of scintillationindex with solar time, siderealtime and zenith angle are derived and discussed. Seasonal and solar cycle variations are also considered. Ionograms from Slough were analysed to study the occurrenceof spread-F echoes for the period 194Q-1960, and the results are compared with the observations of scintillations. Special considerationis given to the variations with the solar cycle, which are opposite for the two phenomena; the scintillation effect is greatest at sunspot maximum, but spread-F echoes occur more frequently at sunspot minimum. It is concluded that at sunspot maximum the ionospheric irregularities which cause radio star scintillations must be mainly above the level of maximum ionization of the F-region, and are therefore unobservable by ground-based sounders. For the years near sunspot maximum, scintillationsoccur frequently by day as well as by night. Possible explanations of these daytime scintillationsare considered.
1. INTRODUCTION THE phenomenon of radio star scintillation was first observed by HEY et al. (1946), but was at first thought to indicate a variability in the emission of the source. It was later shown that the fluctuations were imposed in the passage of the radio waves through the terrestrial ionosphere (SMITH et al., 1950). Since then, the study of radio star scintillations has been developed as a valuable method for investigating irregularities in the ionosphere. It is only recently that observations over a complete solar cycle have become available, and it is the object of this paper to present and discuss such observations, made at Cambridge over the period October 1949-December 1961. In view of the known close association of radio star scintillations with the occurrence of spread-F echoes on ionograms, it was considered desirable to make a parallel study of the occurrence of spread-F echoes over a solar cycle. Ionograms from Slough were therefore analysed for the period January 1949-December 1960. The general arrangement of the paper is as follows: Section 2 contains the results of the observations of radio star scintillations, and a discussion of the variations with solar time, sidereal time, zenith angle, season and the solar cycle. Section 3 contains the results of the study of spread-F echoes, and a discussion of the variations with solar time, season and the solar cycle. In Section 4 the two sets of results are compared and discussed.
Fig.
1. Typical
records
of radio star scintillations of scintillation
for each \mlut~
index.
The aerial separation was four wavelengths, which produoed fringes with a period of about 1 hr. The irregular fluctuations of intensity usually had a period of 2 min or less so that typical records had the appearance shown in Fig. 1. A scintillation index in the range O-5 was assigned to the records every hour, according to the convention of RYLE and HEWISH (1950). A practised observer can achieve consistency in the allocation of these indices, but there are small personal differences between one observer and another. It was not possible for the whole series of records to be analysed by the same person, but whenever a change was made a period of overlap was arranged, and small adjustments were applied as necessary. The deflection on the chart is proportional to the intensity of the source, and the scintillation index is intended to measure the “depth” of the fluctuations. i.e. the deviation of intensity from the mean intensity, divided by the mean intensity. Observations were first started in Cambridge in October 1949, and continued until April 1951. The early results have been described by HEWISH (1952). There wax then a break in the observations until 1953. Observations were started agltitr
Observations of radio star scintillations and spread-F
echoes over a solar cycle
3
mainly to study spatial characteristics and movements of the ionospheric irregularities by the use of three separated interferometers (SPENCER, 1955; JONES, 1960). For this work it was not necessary to have continuous observations, and the records are incomplete around this time. In 1956 interesting changes began to be recognized, and these were thought to be associated with the increasing values of sunspot number. In order to study these effects continuous observations were made from 1956 until the end of 1961. This period includes the IGY period, 1957-8, of exceptionally high solar activity, and part of the subsequent period of declining activity. 2.2 Q’enerul description of the results; sidereal time and zenith angle
variations with solar time,
For each month the mean values of the scintillation index at each hour of the day were calculated, and the resulting diurnal curves are shown in Fig. 3 (a, b, c, d). In these diagrams, the mean scintillation index is shown by the heavy line, and the appropriate scale is the left-hand one (range O-5). Since the curves, in general, have a maximum during the night, they are plotted with midnight in the centre; the time used is G.M.T., which is the same as local time. For some hours the number of observations was small; these less reliable parts of the curves are shown dotted. In general, observations made during the daytime were fewer in number and less reliable than those made during the night, because local man-made interference was often present in the daytime. The curves of Fig. 3 provide a general summary of the observations. A marked variation with the solar cycle is evident, with larger values of scintillation index at sunspot maximum (1957-8) than at sunspot minimum (1954-5). The variations with sunspot number will be considered in detail in Section 2.4. For the present the results for each year will be treated separately, and it will be assumed that changes due to the solar cycle can be neglected during any one year. It is important to recognize the changing geometry involved in the observation of a single source throughout the twenty-four hours, and the effect this has on the results. The line of sight to the source intersects the ionospheric irregularities at a point some distance from Cambridge; the locus of this point is shown in Fig. 2, for irregularities at three different heights. The figures on the curves are the hour angle of the source (i.e. hours after upper transit). The hour angle, of course, is related to local time in a different way at different times of year. In Fig. 3, the mean time of lower transit for each month is marked by a short vertical line. For a given degree of irregularity of the ionosphere the scintillation index would be expected to increase with increasing zenith angle of the source, and to reach its maximum value at lower transit. Two effects contribute to this increase; the increasing thickness of the irregular medium measured along the line of sight, and the increasing distance of the irregularities from the observer (BRIGGS and PARKIN, 1963). In addition the degree of irregularity of the ionosphere probably increases at high latitudes when the aurora1 zone is approached. All these effects tend to make the scintillation index larger at lower transit than at upper transit. Another effect which is sometimes important near lower transit, and which tends to reduce the scintillation index, is due to the fact that the Cassiopeia source
Fig. 2. The curves show how the region of the ionosphere which is effective in producing scintillations varies with sidereal time, for three different heights. The figures on the curves indicate hours after upper transit of the source. The ionospheric observatories at Slough and Inverness are also shown.
has an appreciable angular diameter. The effect of this has been considered elsewhere (BRIMS, 1961) and will not be considered further here, except to point out that minima in the monthly curves, which occur at a time between lower transit and midnight, especially for the sunspot maximum years, are probably due to this effect. These minima are marked “M” on the curves in Fig. 3. In assessing the general trends shown by the curves, it is necessary to try to visualize them as they would appear if these minima were removed. We shall show later (Section 2.3) that seasonal variations are very small. The mean scintillation index may therefore be regarded as a function of time of day (i.e. solar time) and of the position of the source in the sky (i.e. hour angle, or sidereal time). By using the data for a complete year the variations with solar and sidereal times can be eeparat,ed. For example, if the twelve monthly mean curves are averaged at each solar hour, the sidereal variation is eliminated, since it varies over a complete twenty-four hour cycle during the course of the year. Similarly if the successive monthly curves are averaged for each sidereal hour by introducing successive displacements of two hours, the variation with solar time is eliminated, and only the variation with sidereal time (or hour angle) remains. This procedure has been carried out separately for each year for which complete observations are available, and the results are shown in Fig. 4. Fig. 4(a) shows the mean scintillation index as a function of solar time. These curves all have maxima near midnight. For individual years, such as 1953 and 1960, the maximum is displaced a little from midnight, but consideration of the results for all the years suggests that these are statistical fluctuations, and the evidence seems to be strong that the maximum effect occurs very close to midnight. Also the curves are fairly symmetrical around midnight.
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Fig. 3[cG).Monthly mean sc~ti~6tion
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and spread-P
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12 Monthly
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and upread-P
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1959-61.
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Fig. 4. (a) variation of scintillation index with solar time for each year, (b) variation of scintillation index with sidereal time for each year, (c) variation of scintili&ion index with zenith angle for each year,
40 nngle,
(4
60 dep.
The \-ariatiolls 1)it11 11our angle for thck sanu: ~‘~;trs arc sllo\v~~ill I’ig. 40) j. ‘!‘lrr~, CI1rT’C’S.i1S (‘X~K’CtCY!. S!lCl\i :I maximum ill, t 11th tinit, 01 IO\\~f?l’ t,l’itllSit~ c~\c*c’lJtI’o! i.ll(. sunspot nxaxiiniun j’tv\rs l!I;iti-- I !)5!1. wlikh ~Iiow tbvo maxima sli,ghtl~~ rlisl)lac~c~tloti t~ith(~r sicle Of lo\\.Pl'tt.ictIsit. ‘1‘11(~ slll~siC!ii\i~~ millhim \\~liivli o(*(*iil’;; ilt lt~~\vcbI. transit is, Iiowt~vc~r. almost. cbertninly tluc~ t’o tilts finite size ol’thc~ dour(2‘. and sliould he ignored in asscssili~ the general trends 01’ the ~urvt~s. It n,itl 1~ s;(l(‘tI that t~he cur\T(3 ilr(‘ fair]? s\~mmetrical arouuct Ion-er t,ralisit,. The curves for some iutlivitlual years (c.g. l!).i:i aiitl l!l.X) a~qxw ~~mm\hat iuisymmetrica.1. but, it, 2;eems likeI), that these :tr(h only stat,isticat fluctuatioits. an(l the results for all t,hc yea,rs t.uken toget’her sugg& that, t tie mrrximum effc~t o(~curs esac*tl> at thus ii1nrb of lo\vc?r kansit. ( 'HIVEILS (1960) rnutk a similar analysis of scintillations of the ( ‘ussiopein ROUI’W observed at Jodrell Bank (Wr\‘, 2”12’) d uring 1!)5;,dPS; and found that the masimum value of the scintillation index occurred one hour before the time of lower traIlsit. It is therefore of interest to examine t,he present results for these particular J’ears. The curves for 19.55~ 1956 a,nd 195X do appear to be displaced slightly to the left. relative to the vertical liuc \vhich marks the time of the lomc~ transit. This is irl the right sense t#oagree xvith (‘hivc~s’ result,, but, thrb displa,cement is ver!. small. and is of doubtful significa,nce. The curves of Fig. l(b) can also be interpreted as showing the variation of scintillation index with zenith angle, since t’he zenith angle of the source is a A given zenith angle occurs for two different hour function of the hour angle. angles. Since, however. the curves of Fig. 4(b) are approximately symmetrical, it is permissible to avera,pe the two values of scintillation index, which are very nearly equal. in order to obtain a mean “zenith angle curve”. Such a curve is shown for index each year in Fig. 4(c). The curves show a continuous increase of scintillation with increasing zenith angle, except for thr? Fears 1056--S which show a small decrease at the largest, values of zenith angle. This, aga,in2 is almost certainly due to the finite size of the source, and it seems reasonable to eliminate it by extrnpolating the earlier sections of the curves as shown by the dotted lines in Fig. 3(c). This can be done with little uncertaint8y. ,4 quantity of some interest is the “zenith angle ratio”. defined as the ratio of the mean scintillation index at lower transit to t,he mean scintillation index at upper transit. This has been determined for each year from the curves of Fig. 4(c), and the results are given in Table 1. The “corrected” values are those obtained from the est,rapolated (‘urves. ‘I’&lc I. T~itluw of zenith angle rat’io Ytm
1950
1953
19.54
1955
1956
1967
1958
1959
1960
1961
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2.3
2.1
2.X _
3.3
3. 5
The significance of these values has been considered elsewhere (BRIGGS and 1%X3), and it has been shown that they are larger than can be explained theoretically on the basis of the changing zenith angle alone. It appears that, in PARKIN,
Observations
of radio star scintillations
and spread-F
echoes over a solar cycle
11
addition, the degree of irregularity of the ionosphere must increase with increasing latitude. The monthly mean diurnal curves of Fig. 3, from which all the above results have been derived, give no idea of the shorter term variations of the scintillation index. In order to illustrate this variability, the individual hourly values have been used to construct the typical histograms shown in Fig. 5(a). These show the number of hours during the night (defined as 2100-0300) for which the scintillation index had the values stated, and are for September 1954 (sunspot minimum) and Sep. -
1957
Sep. 1954
LLhL (a)
~
012345
012345
Scintillation
Sep.
Scsntlllation
index
1954
Sep.
1957
-F
index
Index
r-r-n
Spread-F
Index
Spread
Fig. 5. Histograms to show the number of hours during the night (2100-0300 hours) for which the indices took the stated values. (a) scintillation index, (b) spread-F index.
September 1957 (sunspot maximum). It is interesting to note that at sunspot minimum there were 31 hr for which no observable scintillations were present, while for the same month at sunspot maximum there were no such hours. Usually the variation in scintillation index from one hour to the next is small, but there are often great variations between one night and another in the same month. 2.3 Seasonal variations of scintillation index The monthly diurnal curves (Fig. 3) indicate a tendency for low values of scintillation index to occur in September and for high values to occur around March. This is almost certainly not a real seasonal effect, but is a result of the combination of the variations with solar and sidereal times discussed in Section 2.2.
Since scintillations occur mainly at, night. it!ld RIB0 t’Pll(t to incroascb at I~~\\‘r~l’ transit, the greatest value of’ scintillation index \vill occur whc~rrlower trwsit oc~~tn~s near midnight. This happens in March. III Scptembcr, when upper transit owws near midnight, small values of scintillation index are to be expected. It is important to d&ermine whether there is a true seasonal effect or not. This is of some interest in connection with theories of the origin of the irregularities and their relation to spread-F echoes (see Section 3.2). Also the procedure used f’ol separating the solar and sidereal variations used in Section 2.2 is based on the assumption that all the monthly curves havr rqual weight,, i.e. t,hat t’here is NO
-
June
1960
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12
14
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20
22
2
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4
6
9
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12
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Fig. 6. The monthly mean diurnal curves for June and December 1960, superimposed for comparison. The approximate symmetry with respect to midnight shows that there is no seasonal variation, other than produced by the change in the time of lower transit.
seasonal variation. Unfortunately, it is not possible to determine solar, sidereal and seasonal variations separately. However, some useful evidence can be obtained by studying the behaviour during the months of June and December. In June, lower transit occurs 6.1 hr before midnight, and in December it occurs 59 hr after midnight. Due to this approximate symmetry around midnight the diurnal curves for these months should be mirror images with respect to midnight, provided there is no seasonal effect, since the variation with solar time is symmetrical around midnight (Fig. 4(a)). To illustrate this, the curves for June and December 1960 are drawn superimposed in Fig. 6. It will be seen that they are approximately mirror images with respect to midnight, and the amplitudes are closely similar. This shows that, for this year, there was no seasonal difference between June and December. The results for other years (Fig. 3) are not quite so clear cut, but in most years the curve for June has its maximum before midnight, the curve for December has its maximum after midnight, and the values of these maxima are approximately
Observations of radio star scintillations and spread-B’ echoes over a solar cycle
13
equal. It is concluded that any seasonal variation as between summer and winter must be very small. CHNERS (1960) reported maxima of the mean scintillation index at the equinoxes, and minima at the solstices, for the years 1956-8, near sunspot maximum. Such an effect would not, of course, be detected by the above method, in which only the months of June and December were examined. It is difficult to see how it can be established with certainty in the presence of the other variables though a An examination of the present results large effect would be evident by inspection. (Fig. 3) gives no support to the suggestion that the scintillation effect is a maximum at the equinoxes. 2.4 The variation of scintillation index with the solar cycle The variation of the scintillation index with sunspot number is at once evident from an inspection of Figs. 3 and 4. The mean values are larger at sunspot maximum. There are also changes in the form of the diurnal curves. The minima (M) attributed to the effect of the finite size of the source become more pronounced at sunspot maximum (BRIWS, 1961). This to some extent obscures the true magnitude of the variation of scintillation index from sunspot minimum to sunspot maximum, which would be larger if the minima were removed. It is clear that the degree of irregularity of the ionosphere must be greater at sunspot maximum or, alternatively, the irregular region must be thicker. At sunspot minimum, scintillations occur mainly by night. At sunspot maximum, they occur by day as well as by night, and for some months (e.g. September 1957) there is hardly any diurnal variation. The daytime scintillations will be discussed in Section 2.5. Further maxima and minima appear in some of the diurnal curves at sunspot maximum, in addition to those attributed to the size of of the source, and some of these features seem to show systematic displacements in time from month to month (e.g. June, July, August, September 1958 and 1959). The origin or significance of these variations is not understood. The variation of the scintillation index with the solar cycle can be shown most clearly by considering annual mean values. However, it must be remembered that the latitude of the effective region of the ionosphere varies with the hour angle of the source (Fig. 2), and it is possible that the degree of the irregularity of the ionosphere may vary differently with sunspot number at different latitudes. The use of overall annual means would obscure any such variations with latitude. Therefore we shall consider separately the annual means for observations made at upper transit and for observations made at lower transit. These can be obtained immediately from the curves in Fig. 4(c), and are plotted in Fig. 7. The “corrected” values at lower transit are those read from the dotted extrapolated curves in Fig. 4(c). Fig. 7 also shows the variation of the 12-monthly running-mean values of sunspot number for the period concerned (1950-1961). It is clear from Fig. 7 that there is no significant difference between the behaviour at upper transit and at lower transit. In both cases the variation of the scintillation index appears to follow that of sunspot number. The mean scintillation index is, of course, higher at lower transit, but the ratio of the value at sunspot maximum to that at sunspot minimum is about the same as for upper
transit,. The actual vulnc~sof this “solar cycle r&o” ill‘e tr’i~ll
._‘**...Sunspot number 5
A-A
Sci~tilloti~~
rndex, upper
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Scintillation
index,
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Lower transit,
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1953
1954
1955
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is57
1958
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1959
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1960
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1961
Fig. 7. The variation of the annual mean scintillation index at upper transit and at lower transit, for the period 1950-1961. The dotted curve shows the 12-month13 running mean values of the sunspot number.
For some years, particularly during the period 1957-1960, there is appreciable scintillation in the daytime. Fig. 8 shows the annual variation of the monthly mean scintillation index at noon for each of these years. Smooth curves have been drawn through the points to assist the eye in following the general trends. The less certain parts of the curves are drawn dotted. It should be emphasized again (see
Fig. 8. The monthly mean scintillation index at noon, plotted for each month from 1957 to 1900. The times of year at which upper and lower transits occur at noon am indicated.
Observations of radio star scintillations and spread-P echoes over a solar cycle
15
Section 2.2) that the daytime values of scintillation index are less reliable than those obtained at night; nevertheless the general form of the annual variation is seen to be very similar from year to year, and is almost certainly significant,. It is impossible to decide with certainty whether this annual variation is a seasonal effect, or an effect, involving sidereal time, since, at noon, the times of upper and lower transit undergo a cyclic variation with a period of one year. The months in which upper and lower transit occur at noon are shown in Fig. 8. It will be seen that the maxima of the curves occur near to lower transit, and the minima are near upper transit (actually displaced by one month). Thus it seems most likely that the effect involves sidereal time, though the possibility of a seasonal effect cannot be ruled out. If the variations shown in Fig. 8 are interpreted as an effect of sidereal time, the question arises as to whether the whole variation can be explained by the changing zenith angle of the source. It appears that it cannot be so explained, since the “zenith angle ratio” is too large, even for irregularities at E-region heights. A possible explanation is that during the daytime there is a pronounced increase in the degree of irregularity of the ionosphere at the higher latitudes involved near lower transit. Consideration of Fig. 2 suggests that the variation can be explained if it is supposed that the irregularities are in the P-region and increase in intensity We have already shown that a north of a latitude of about 56’N (SO’ geomagnetic). variation of this type is needed in order to explain the variation with sidereal time for the results as a whole; the present considerations show that the latitude variation must be particularly pronounced in the daytime at sunspot maximum. An alternative explanation is to suppose that the daytime scintillations are due to irregularities in the E-region, possibly associated with E,. WILD and ROBERTS (1956)have found such an association for daytime scintillations observed in Australia. An attempt was made to correlate the scintillation index with the occurrence of E, at Slough and Inverness (see Fig. 2). Unfortunately there were so many gaps in the data, both for scintillations and E,, that no conclusion could be reached. If the daytime scintillations are due to irregularities in the E-region, the range of latitudes involved will be small, due to the lower height, and the variations shown in Fig. 8 must be interpreted as a seasonal effect. Daytime scintillations would repay further study, but it may be necessary to wait, until the maximum of the next solar cycle before this will be possible at, Cambridge. 3.
THE OCCURRENCE OB SPREAD-FECHOES
3.1Description
AT SLOUUH OVER A SOLAR CYCLE
of the method used
The closest station to Cambridge for which regular ionospheric soundings are available is at Slough (51*PN, low), and all the results to be described are for this station. The occurrence of spread-p echoes is indicated in the published “Bulletins of Ionospheric Characteristics”, but these have been found unsatisfactory for a detailed study of the occurrence and were not used. Instead the actual ionograms (recorded once per hour) were obtained and a “spread-F index” was allocated to each, according to a convention described in an earlier paper (BRIGGS,1958a,Fig.1). At Slough, the spread-F echoes are mainly of the “frequency-spreading” type, Z
and the index, with a scale 0~3, is intended to be roughly proportional to the spread An index of 2 corresponds t,o a spread of about 0.5 Xcjs. so that the ordinary and extraordinary traces art’ both considerably slu~ad, but stili appear separate. An index of 1 corresponds t-0 a spread of about, 0.25 Xc/s. .4n index of 3 was only used when the spreading \vas so great that the ordinary and extraordinary traces merged together into a single diffuse mass: this indicatchs a spread of critical frequency of more than 0.7 MC/S. The hourly values of spread-F index were averaged for each month, in the same way as the values of sci~~tillatioll index, and the resulting diurnal curves are shown in Fig. 3 (a, It, c, d). In these diagrams the mean spread-P index is shown by the dotted curves, and the appropriate scale is the right-hand one (range O-3). From the curves in Fig. 3 a general comparison can be made between the two sets of data.
in critical frequency.
The curves of Fig. 3 show some of the well-known properties of spread-P echoes; a maximum intensity at or just after midnight, a seasonal variation with a maximum in winter, and a variation with the solar cycle with a maximum intensity at sunpot minimum. The seasonal and solar cycle variations can be shown more clearly by plotting the mean values of the spread-F index at midnight for each month as in Fig. 9. IO
;2 .c tL +' L -0 1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
Fig. 9. Monthly mean values of the spread-F index at midnight plotted for each month from 1949 to 1960. The upper curve shows the corresponding values of the quantity P (the product of the spread-p index and the critical frequency in I&/s). The points for *January of each year fall on the vertical lines.
The midnight value, in general, gives a good measure of the mean spread-F index for most of the night. Fig. 9 shows clearly the seasonal variation; the index is about three times as large in December as in July. The variation with the solar cycle is also shown. At sunspot minimum the seasonal oscillation takes place around a mean value of about l-8 and at sunspot maximum around a mean value of about 0.9. Thus the solar cycle produces a change of about a factor of two in the spread-F index. If summer and winter are considered separately, the solar cycle effect is seen to be larger in summer than in winter. Very low values of spread-F
Observations of radio star scintillationsand spread-P echoes over a solar cycle
17
index occur in summer at sunspot maximum, e.g. in the summer months of 1958. The electron density at the maximum of the F-layer, N, is proportional to fo2, where fO the critical frequency. Hence the deviation of electron density from the mean, AN, due to a spread of critical frequency Af,,, is proportional to f,, Af,,. This suggests that f,, Af,, may be a more significant parameter than Af,, itself. If the data are to be compared with radio star scintillations, AN is the important quantity, since it can be shown that the scintillation index is proportional to AN (or, more precisely, to the root-mean-square deviation of N from the mean; see, for example BRIWS and PARKIN (1963)). The spread-F index used here is proportional to AfO. To obtain the variation of AN, the index must therefore be multiplied by fo. The monthly mean values of the critical frequency of the F-layer at midnight were obtained for Slough from the published Bulletins, and the upper curve in Fig. 9 shows the variation of the product P =
spread-F index x critical frequency [m;:;nm;) [ m”;:My;an ]*
It will be noticed that the effect of multiplying by the critical frequency is to remove the variations with season and with the solar cycle. The quantity P shows considerable random fluctuations from month to month, but there are no obvious trends. From the definition of the spread-F index it can easily be shown that AN w 6 x 103P electrons/cm3 where P is in units of (spread-F index x MC/S). The mean value of P is seen from Fig. 9 to be about 6; hence AN M 3.6 x lo4 electrons/cm3. This result shows that near the level of maximum ionization of the F-layer, at midnight, the irregular part of the ionization is characterized by this constant value of AN, which is independent of season, and independent of sunspot number . This suggests that the variations of the spread-F index with season and with the solar cycle are secondary effects, and are due to the fact that the critical frequency varies with season and with the solar cycle. This point has been investigated rather differently by SINGLETON (1962)for a number of stations, using hourly values of f,,, and hourly values of Af,. His conclusion was the same; the value of Af,, was found to be large when f. was small, and conversely, so that AN tended to remain constant. The actual values of AN determined by SINGLETON are rather larger than the mean value deduced above for Slough. The remarkable constancy of the monthly mean values of AN should not be allowed to obscure the great variability on different occasions. Spread-F echoes are essentially a sporadic phenomenon; at any particular time AN may have any value from zero to many times the above mean value. This is illustrated in the histograms of Fig. 5(b), which show the number of hours during the night (2100 to 0300) for which the spread-F index had different values, for two typical months near
sunspoC minimum and sunspot maximum. izlso the diurnal variat.ion remains; AN is small or zero during the daytime. 111 this respect is it, hard to understittld the range of values of AX given by SINCLICTOTS, which seems to imply that’ Ai\: is nww less than ii ;C 10” electrons/cm3. 4. %E
Iter,~~xos
BETWEEN R,ADW STAR SCINTILLATIOW AXD SPREEAD-FECHOES
4.1 General ~?iscu~ssio~~ The correlation between the occurrence of radio star scintillations and spread-F echoes is well established for the angles of elevation involved in the present observations (e.g. RYLE and HEWISH, 1950; WRIGHT et al. 1956; BRIGGS, 1958; LAWRENCE, JESPERSON and LAMB, 1961). Some puzzling features of the relationship remain however. Scintillations show no seasonal variation (Section 2.3), whereas spread-F echoes show a marked seasonal effect with a maximum occurrence in winter (Section 3.2). The scintillation index increases with sunspot number (Section 2.4), whereas the spread-F index decreases with sunspot number (Section 3.2). The seasonal discrepancy is easily explained. It has already been shown that the quantity AN shows no seasonal variation. Theory shows that the scintillation index is proportional to AN; hence no seasonal variation would be expected in the scintillation index. This confirms conclusions regarding the seasonal variations which were reached in an earlier paper (BRIGGS, 1958b). The changes with the solar cycle cannot be reconciled in this way, since it has been shown that AN shows no variation with the solar cycle. Explanations of this discrepancy have been suggested by other authors, and these will first be discussed in the light of the present .results. SISGLETON (1961) has suggested that the discrepancy may be explained by the changing latitude of the region of the ionosphere which is effective in producing scintillations (Fig. 2). At high magnetic latitudes spread-F increases with increasing sunspot number; consequently if such latitudes exert the dominant influence, the scintillation index would also be expected to increase. However, in Section 2.4 the results were divided into those obtained when the source was near lower transit and those obtained when the source was near upper transit, and in both cases the scintillation index was found to increase with increasing sunspot number. If SINGLETON’Sexplanation were correct the results obtained near upper transit would have shown an opposite variation, since the effective region of the ionosphere then lies between the ionospheric observatories at Slough and Inverness (Fig. 2), and at both these stations spread-F echoes are known to correlate &versely with sunspot number. KOSTEK.and WRIGHT (1960), in considering observations made at Ibadan, near the equator, noted a similar discrepancy in the behaviour of radio star scintillations and spread-F echoes during the rising part of the solar cycle, 1952-1957. They suggested that the thickness of the irregular region may be greater at sunspot maximum, and in support pointed out that the value of ym (the semi-thickness of the F-layer) in the evening hours at Ibadan is twice as large at sunspot maximum as at sunspot minimum. However, the scintillation index varies only as the
Observations
of radio star scintillations and spread-P
19
echoes over a solar cycle
square root of the thickness of the irregular region, and so it seems unlikely that It would be very difficult to explain the this can provide a complete explanation. observations during the summer of 1958 along these lines. At this time the mean scintillation index is large, yet the mean spread-F index is exceptionally low, and, in fact, as will be shown in the next section, spread-F echoes are completely absent on many nights when strong scintillations are occurring. 4.2
The correlation between scintillation for individual nights
index and spread-F
index
We shall now consider the changes with the solar cycle differently by making use of the original hourly observations rather than the monthly means. We first ask, how does the detailed, short term correlation between the two indices vary with the solar cycle? Table
2. Correlation coefficients between scintillation 1955
1956
1957
1958
indices and spread-p 1959
1960
indices Mean -
Jan.
-0.01
-0.41
-0.14
-0.12
-0.17 ____
Feb.
-
-
-0.16
-0.43
$0.01
+0*27
-0.08
Mar.
-0.05
-
+0.42
-0.06
+0.20
+0.2s
+0.16
Apr. -_____
-
-
-0.26
-0.33
+0.01
+0.12
-0.11
-
-0.18
$0.05
+0.32
+0*08
+0.34
+0.12
June
$0.15
-
$057
$0.20
$0.53
$0.55 +0.40 __~____
July _______
$0.21
-
$0.48
+0.46
+0.67
$056
$0.48
Aug.
$067
-
$0.74
$0.51
+0.73
+0.76
$0.72
Sep. __~_
-
+0.44
+0.50
+0.76
+0.61
$0.73
$0.60
-
+ 0.48
$0.22
+0.26
-0.48
$0.20
-
May
Oct.
+0.72
+0.70
$0.30
$0.25
+0.41
Nov.
+0.61
+0.30
-0.10
$0.15
$0.39 _____
Dec.
+0.32
-
+0.45
+0.18
$0.52
-
In order to answer this question it was considered sufficient to use mean values of the indices for each night, defined as the period 2100-0300 hours G.M.T. The justification for this procedure is that the indices do not usually change greatly from one hour to the next, but they do vary greatly from one night to another. By evaluating the correlation between these mean night-time values of the indices, the effect of the diurnal variations is eliminated. Any correlation which is then found indicates a tendency for the two phenomena to occur on the same nights.
C’orrelation coe&cients were evaluated for each month for which twenty or more pairs of values of the mean night-time indices were available, ant1 the results are shown in Table 2. It will be seen that high positive values of the correlation coetbcient occur mainly around August, September and October. This result is as expected, since in September upper transit occurs near midnight, and so the effective region of the ionosphere is t,hen close to the ionospheric observatory at Slough from which the spread-F data was obtained. The fall-off in correlation at other times indicates a patchiness in the distribution of the irregularities, with a scale of the order of 500 km (Fig. 2), and implies that the occurrence of spread-F echoes is uncorrelated at points separated by this distance. This is in agreement with the results of an earlier paper (BRIGGS, 11)68a). The importance of the results in Table 2 from the present point of view is that they indicate that the close, night by night, association between the occurrence of spread-F echoes and scintillations observed near upper transit is maintained throughout the whole of the solar cycle, in spite of the inverse trends of the mean values. This is rather surprising in view of the infrequent occurrence of spread-F echoes at sunspot maximum. ln order to investigate this point further, the values of the two indices were plotted against each other to produce scatter diagrams. These diagrams give more information about the form of the relationship between the two quantities than does the correlation coefficient alone. Scatter diagrams were plotted for each month, but as expected only those for August, September and October showed a significant relationship between the two indices. In order to reduce the number of diagrams to be reproduced, the results for these three months have been combined so that a single scatter plot is obtained for each year. These plots are shown in Fig. 10. The results used in constructing these diagrams were the same as those used for the calculations of the correlation coefficients of Table 2, viz., the mean values for 2100-0300 hours G.M.T. for each night. The diagrams of Fig. 10 show that at sunspot maximum there are many individual nights for which there are no spread-F echoes but nevertheless a considFor example, the plot for I958 shows that for 16 erable degree of scintillation. individual nights no spread-F echoes were observed at all in the period 2100-0300 hours G.M.T., and yet the scintillation index for the same period reached values as high as 3. By contrast, the plot for 1965 (sunspot minimum), shows no such occasions. The only possible explanation of these results appears to be that at sunspot maximum the irregularities are often situated at great heights, above the level of maximum ionization density of the F-layer, and are therefore unobservable by ground-based sounders. Even when spread-F echoes are visible it seems likely that the irregularities are more intense at greater heights, in order to account for the high values of scintillation index observed. I>irect confirmation of this hypothesis should be possible by the use of top-side sounders carried in satellites. Spread-F echoes have, in fact, been observed on top-side ionograms obtained by the Alouette satellite (PETRIE, 1963). However these observations were made at a time of low sunspot number, approaching the
Observations of radio star scintillations and spread-F 5-
1956
. . .
4 -
* .
5 .. .
.
. : l. * I .. . -. :; .
: .
21
echoes over E solar cycle
. % r3;.* .. . .* z .. 22:; *. z *. .o m m. I-.. .
4
.
. .
. *..
Spread-F
index
1958
5v . l
4-•
l. .
4-
.. .
%, .g
. I : v '. .z 3 . .:'
G=i
6 5 z2 E GJ
n* 3. ': s ::, 3:. . " 5 1 .', .: Z c2 , ,*. -. . .
...
0
I-
., : .*
1
I
I
Spreod
-F
2 index
.-. . I__ 1960
l
.
.
;:
.
l
.
.
l
0.
.
.. .
.
-
I
.=:
’ .
l
5
l
.E
. .
. .
l*:::.r
l
‘0 E*:.
. :. IE Spread-F
:'
l
s
.**.
0
I.
.
it 0 .E3 t
,
3
index
.
:
l .
-F
4
*
.
.
.
l
: ;:. . .; . 0 *4 :
x
I *.
Spread
.
.:
0
5
.
.*
. . . ... .
, 3
I959
“r
.
1 2 index
I I Spread-F
0
2
3 index
.
0
3
Sp’read
-F:ndex
Fig. 10. Scatter diagrams showing the relation between scintillation index and spread-F index for the months August, September, October, when upper transit occurs at night.
minimum of the solar cycle. A crucial test of the above hypothesis can only be made at sunspot maximum, when it should be found that spread-J’ echoes are commonly observed on the top-side of the F-region even when none are observable on the bottom side. 4.3 The relation between daytime scintillation
and spread-P
echoes
As explained in Section 2.5, the daytime scintillations, which occur mainly near the maximum of the solar cycle, may be due to irregularities in the E-region. It is of some interest, however, to ask whether an explanation in terms of P-region irregularities is possible. At first sight such an explanation seems unlikely, since spread-P echoes are
usually thought of as a night-time phenomei~on. H o\ve\~t:r.at high latituth~s t,lw occurrence of spread-F echoes is different from that at lower laMrules (SIN~I.F:W13. l!lfiO; 8HIm4zAKr, l!t6'). 11 study of Fig. :! Of’ the tllikt north of 60” geomagnetic latitude spread-E’ clchoes occur by da?. as \\.ellits 1)~ iright. illso, during the day they do not occur south of‘ this latit,udc. There exists. in Incat, just the sudden increase at 60” geomagnrt~ic latitude which was postulated to explain the curves of Fig. S. Thus it seems very likely the day-time scintillations can be explained by irregularities iii the .I’-region. The analysis of SINGLETON applies to the ICY period, i.e. the maximum ot’ the solar cycle. The present results suggest that’ t,he irregularities which occur iii the F-region in the daytime, and the sharp increase at 60” geomagnetic latitude, exist only near the maximum of the solar cycle, since daytime scintillations do not occur at sunspot minimum. pil]Er
by
SINGI,E'I'OY
SllO\Vh
5. C:ONCLTJSlONS In the main, the conclusions reached are in agreement with existing knowledge, and serve to confirm results deduced from more limited samples of data. We shall not attempt to summarize these results here; the Figures throughout the paper However, some new results have emerged and these provide the best summary. should, perhaps, be emphasized. The study of the spread-F index near midnight (Section 3.2) has shown that the degree of irregularity of the ionization, AN, near the level of maximum ionization of it does not vary either with season the F-layer, is remarkably constant; or with the solar cycle. This suggests that many of the observed variations of the occurrence and intensity of spread-F echoes are secondary effects due to changes in the critical frequency of the F layer. Further, the results of Section 4.2 show that there must be irregularities above the maximum of the F layer, which this is particularly the case at are unobservable by ground-based sounders; sunspot maximum. For these reasons, the study of spread-F echoes seems to be an unreliable method for investigating the occurrence of irregularities in the ionization. The study of radio star or satellite scintillations is better, as the probing waves then traverse the whole thickness of the ionosphere. Even here, however, it must be remembered that if amplitude or intensity scintillations are observed, irregularities at different distances from the observing point are not equally effective: there is a “weighting function” which is different for radio stars and for satellites @RIGGS and PARRIN, 1963). The results have shown that the degree of irregularity of the ionosphere must increase with increasing latitude, since the variation of scintillation index with sidereal time is too large to be explained by the variation of zenith angle alone. At sunspot maximum a new phenomenon appears in the form of daytime scintillations. It has been shown that these are probably due to irregularities in the Fregion, and that the variation with latitude must be particularly large in this case. Finally, the very great variation in the scintillation index between sunspot maximum and sunspot minimum should by emphasized. This may be of importance in the planning of experiments in radio-astronomy, particularly those on lower frequencies where scintillations cause difficulties.
Observ&ions of mdio star scintilletions and spread-P echoes over a solar cycle
23
Acknowledgements-The observations of radio star scintillations were carried out when the author was at the Cavendish Laboratory, Cambridge, and would not have been possible without the co-operation of many colleagues who assisted in the running of the apparatus. I am indebted to the Director of Radio Research of the Department of Scientific and Industrial Research for the loan of ionograms for Slough for the period 1949-1960. I am grateful for the assistance received from the computing staffs at the Cavendish Laboratory and at the University of Adelaide in connection with the analysis of the large number of records, and the subsequent reduction of the data. REFERENCES BRINGS B. H. BRIGGS B. H. BRIGGS B. H. BRIGGS B. H. and PARKIN I. A. CHIVERS H. J. A. FORSYTH P. A. and PAULSON K. V. HEWISH A. HEY J. S., PARSONS S. J. and PHILLIPS J. W. JONES I. L. KOSTER J. R. KOSTER J. R. and WRIG~ R. W. PETRIE L. E. RYLE M. RYLE M. and HEWISH A. SHIMAZAKI T. SINGLETON D. G. SINQLETON D. G. SINGLETON D. G. SMITH F. G., LIYLZE C. G. and LOVELL A. C. B. SPENCER M. WILD J. P. and ROBERTS J. A.
1958a 195813 1961 1963 1960 1961 1952
J. Atmo8ph. Terr. Phys. 12, J. Atmosph. Terr. Phys. 12, Ceophya. J. 5, 306. J. Atmosph. Terr. Phys. 25, J. Atmosph. Terr. Phys. 19, Canad. J. Phys. 39,502. Proc. Roy. Sot. A214, 494.
1946 1960 1958 1960 1963 1952 1950 1962 1960 1961 1962
Nature, Lond. lU, 234. J. Atmosph. Terr. Phy8. 19, 26. J. Atmosph. Tern. Phys. 12, 100. J. Qeophys. Res. 65, 2303. Canad. J. Phys. 41, 194. Proc. Roy. Sot. A211, 351. Mon. Not. Roy. Ast. ~Joc. 110,381. J. Beophye. Rea. 67, 4617. J. Beophys. Rea. 65, 3615. J. Atmoaph. Terr. Phys. 22, 219. Au&. J. Phys. 15,242.
1950 1955 1956
Nature, Lond. 165,422. Proc. Phys. Sot. Lond. B68,493. Nature, Lond. 178, 377.
34. 89. 339. 54.
Pergamon PressLtd.Printedin Northern Ireland
Observations of radio star scintillations and spread-s echoes over a solar cycle B. H. BRIGQS Department of Physics, University of Adelaide (Received1 July 1963) Abstract-Observations of the scintillations of the radio source Cassiopeia A were made at Cambridge on a frequency of 38 MC/S over the period 1949-1961. The resultsare presentedin the form of curves showing the mean diurnal variation of the scintillationindex for each month. The variations of scintillationindex with solar time, siderealtime and zenith angle are derived and discussed. Seasonal and solar cycle variations are also considered. Ionograms from Slough were analysed to study the occurrenceof spread-F echoes for the period 194Q-1960, and the results are compared with the observations of scintillations. Special considerationis given to the variations with the solar cycle, which are opposite for the two phenomena; the scintillation effect is greatest at sunspot maximum, but spread-F echoes occur more frequently at sunspot minimum. It is concluded that at sunspot maximum the ionospheric irregularities which cause radio star scintillations must be mainly above the level of maximum ionization of the F-region, and are therefore unobservable by ground-based sounders. For the years near sunspot maximum, scintillationsoccur frequently by day as well as by night. Possible explanations of these daytime scintillationsare considered.
1. INTRODUCTION THE phenomenon of radio star scintillation was first observed by HEY et al. (1946), but was at first thought to indicate a variability in the emission of the source. It was later shown that the fluctuations were imposed in the passage of the radio waves through the terrestrial ionosphere (SMITH et al., 1950). Since then, the study of radio star scintillations has been developed as a valuable method for investigating irregularities in the ionosphere. It is only recently that observations over a complete solar cycle have become available, and it is the object of this paper to present and discuss such observations, made at Cambridge over the period October 1949-December 1961. In view of the known close association of radio star scintillations with the occurrence of spread-F echoes on ionograms, it was considered desirable to make a parallel study of the occurrence of spread-F echoes over a solar cycle. Ionograms from Slough were therefore analysed for the period January 1949-December 1960. The general arrangement of the paper is as follows: Section 2 contains the results of the observations of radio star scintillations, and a discussion of the variations with solar time, sidereal time, zenith angle, season and the solar cycle. Section 3 contains the results of the study of spread-F echoes, and a discussion of the variations with solar time, season and the solar cycle. In Section 4 the two sets of results are compared and discussed.
Fig.
1. Typical
records
of radio star scintillations of scintillation
for each \mlut~
index.
The aerial separation was four wavelengths, which produoed fringes with a period of about 1 hr. The irregular fluctuations of intensity usually had a period of 2 min or less so that typical records had the appearance shown in Fig. 1. A scintillation index in the range O-5 was assigned to the records every hour, according to the convention of RYLE and HEWISH (1950). A practised observer can achieve consistency in the allocation of these indices, but there are small personal differences between one observer and another. It was not possible for the whole series of records to be analysed by the same person, but whenever a change was made a period of overlap was arranged, and small adjustments were applied as necessary. The deflection on the chart is proportional to the intensity of the source, and the scintillation index is intended to measure the “depth” of the fluctuations. i.e. the deviation of intensity from the mean intensity, divided by the mean intensity. Observations were first started in Cambridge in October 1949, and continued until April 1951. The early results have been described by HEWISH (1952). There wax then a break in the observations until 1953. Observations were started agltitr
Observations of radio star scintillations and spread-F
echoes over a solar cycle
3
mainly to study spatial characteristics and movements of the ionospheric irregularities by the use of three separated interferometers (SPENCER, 1955; JONES, 1960). For this work it was not necessary to have continuous observations, and the records are incomplete around this time. In 1956 interesting changes began to be recognized, and these were thought to be associated with the increasing values of sunspot number. In order to study these effects continuous observations were made from 1956 until the end of 1961. This period includes the IGY period, 1957-8, of exceptionally high solar activity, and part of the subsequent period of declining activity. 2.2 Q’enerul description of the results; sidereal time and zenith angle
variations with solar time,
For each month the mean values of the scintillation index at each hour of the day were calculated, and the resulting diurnal curves are shown in Fig. 3 (a, b, c, d). In these diagrams, the mean scintillation index is shown by the heavy line, and the appropriate scale is the left-hand one (range O-5). Since the curves, in general, have a maximum during the night, they are plotted with midnight in the centre; the time used is G.M.T., which is the same as local time. For some hours the number of observations was small; these less reliable parts of the curves are shown dotted. In general, observations made during the daytime were fewer in number and less reliable than those made during the night, because local man-made interference was often present in the daytime. The curves of Fig. 3 provide a general summary of the observations. A marked variation with the solar cycle is evident, with larger values of scintillation index at sunspot maximum (1957-8) than at sunspot minimum (1954-5). The variations with sunspot number will be considered in detail in Section 2.4. For the present the results for each year will be treated separately, and it will be assumed that changes due to the solar cycle can be neglected during any one year. It is important to recognize the changing geometry involved in the observation of a single source throughout the twenty-four hours, and the effect this has on the results. The line of sight to the source intersects the ionospheric irregularities at a point some distance from Cambridge; the locus of this point is shown in Fig. 2, for irregularities at three different heights. The figures on the curves are the hour angle of the source (i.e. hours after upper transit). The hour angle, of course, is related to local time in a different way at different times of year. In Fig. 3, the mean time of lower transit for each month is marked by a short vertical line. For a given degree of irregularity of the ionosphere the scintillation index would be expected to increase with increasing zenith angle of the source, and to reach its maximum value at lower transit. Two effects contribute to this increase; the increasing thickness of the irregular medium measured along the line of sight, and the increasing distance of the irregularities from the observer (BRIGGS and PARKIN, 1963). In addition the degree of irregularity of the ionosphere probably increases at high latitudes when the aurora1 zone is approached. All these effects tend to make the scintillation index larger at lower transit than at upper transit. Another effect which is sometimes important near lower transit, and which tends to reduce the scintillation index, is due to the fact that the Cassiopeia source
Fig. 2. The curves show how the region of the ionosphere which is effective in producing scintillations varies with sidereal time, for three different heights. The figures on the curves indicate hours after upper transit of the source. The ionospheric observatories at Slough and Inverness are also shown.
has an appreciable angular diameter. The effect of this has been considered elsewhere (BRIMS, 1961) and will not be considered further here, except to point out that minima in the monthly curves, which occur at a time between lower transit and midnight, especially for the sunspot maximum years, are probably due to this effect. These minima are marked “M” on the curves in Fig. 3. In assessing the general trends shown by the curves, it is necessary to try to visualize them as they would appear if these minima were removed. We shall show later (Section 2.3) that seasonal variations are very small. The mean scintillation index may therefore be regarded as a function of time of day (i.e. solar time) and of the position of the source in the sky (i.e. hour angle, or sidereal time). By using the data for a complete year the variations with solar and sidereal times can be eeparat,ed. For example, if the twelve monthly mean curves are averaged at each solar hour, the sidereal variation is eliminated, since it varies over a complete twenty-four hour cycle during the course of the year. Similarly if the successive monthly curves are averaged for each sidereal hour by introducing successive displacements of two hours, the variation with solar time is eliminated, and only the variation with sidereal time (or hour angle) remains. This procedure has been carried out separately for each year for which complete observations are available, and the results are shown in Fig. 4. Fig. 4(a) shows the mean scintillation index as a function of solar time. These curves all have maxima near midnight. For individual years, such as 1953 and 1960, the maximum is displaced a little from midnight, but consideration of the results for all the years suggests that these are statistical fluctuations, and the evidence seems to be strong that the maximum effect occurs very close to midnight. Also the curves are fairly symmetrical around midnight.
I-. . . . . .
.
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:
1953
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‘.
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Feb. 1954
. .,_
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.,,,_
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May
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t
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I954 ._,.......
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1955
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&.T.
Fig. 3[cG).Monthly mean sc~ti~6tion
4
r’
_
8
12
and spread-P
indices, 1956458.
i
i
I
June 1960
I
1 July
1960
Aug.
1960
i
I
/
I
4
iii
t-3
‘6
20
0 G.M.T.
Fig.
4
3(d).
a
Aug. 1961
Dec.i961
12 Monthly
mean scintillation
and upread-P
indices,
1959-61.
I
4
2 2 ~ .//
01
3 it
16
20
0 G.&T. @I
4
8
12
0
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8 Xours
after
I2
16
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20
24
0
20 Zenith
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Fig. 4. (a) variation of scintillation index with solar time for each year, (b) variation of scintillation index with sidereal time for each year, (c) variation of scintili&ion index with zenith angle for each year,
40 nngle,
(4
60 dep.
The \-ariatiolls 1)it11 11our angle for thck sanu: ~‘~;trs arc sllo\v~~ill I’ig. 40) j. ‘!‘lrr~, CI1rT’C’S.i1S (‘X~K’CtCY!. S!lCl\i :I maximum ill, t 11th tinit, 01 IO\\~f?l’ t,l’itllSit~ c~\c*c’lJtI’o! i.ll(. sunspot nxaxiiniun j’tv\rs l!I;iti-- I !)5!1. wlikh ~Iiow tbvo maxima sli,ghtl~~ rlisl)lac~c~tloti t~ith(~r sicle Of lo\\.Pl'tt.ictIsit. ‘1‘11(~ slll~siC!ii\i~~ millhim \\~liivli o(*(*iil’;; ilt lt~~\vcbI. transit is, Iiowt~vc~r. almost. cbertninly tluc~ t’o tilts finite size ol’thc~ dour(2‘. and sliould he ignored in asscssili~ the general trends 01’ the ~urvt~s. It n,itl 1~ s;(l(‘tI that t~he cur\T(3 ilr(‘ fair]? s\~mmetrical arouuct Ion-er t,ralisit,. The curves for some iutlivitlual years (c.g. l!).i:i aiitl l!l.X) a~qxw ~~mm\hat iuisymmetrica.1. but, it, 2;eems likeI), that these :tr(h only stat,isticat fluctuatioits. an(l the results for all t,hc yea,rs t.uken toget’her sugg& that, t tie mrrximum effc~t o(~curs esac*tl> at thus ii1nrb of lo\vc?r kansit. ( 'HIVEILS (1960) rnutk a similar analysis of scintillations of the ( ‘ussiopein ROUI’W observed at Jodrell Bank (Wr\‘, 2”12’) d uring 1!)5;,dPS; and found that the masimum value of the scintillation index occurred one hour before the time of lower traIlsit. It is therefore of interest to examine t,he present results for these particular J’ears. The curves for 19.55~ 1956 a,nd 195X do appear to be displaced slightly to the left. relative to the vertical liuc \vhich marks the time of the lomc~ transit. This is irl the right sense t#oagree xvith (‘hivc~s’ result,, but, thrb displa,cement is ver!. small. and is of doubtful significa,nce. The curves of Fig. l(b) can also be interpreted as showing the variation of scintillation index with zenith angle, since t’he zenith angle of the source is a A given zenith angle occurs for two different hour function of the hour angle. angles. Since, however. the curves of Fig. 4(b) are approximately symmetrical, it is permissible to avera,pe the two values of scintillation index, which are very nearly equal. in order to obtain a mean “zenith angle curve”. Such a curve is shown for index each year in Fig. 4(c). The curves show a continuous increase of scintillation with increasing zenith angle, except for thr? Fears 1056--S which show a small decrease at the largest, values of zenith angle. This, aga,in2 is almost certainly due to the finite size of the source, and it seems reasonable to eliminate it by extrnpolating the earlier sections of the curves as shown by the dotted lines in Fig. 3(c). This can be done with little uncertaint8y. ,4 quantity of some interest is the “zenith angle ratio”. defined as the ratio of the mean scintillation index at lower transit to t,he mean scintillation index at upper transit. This has been determined for each year from the curves of Fig. 4(c), and the results are given in Table 1. The “corrected” values are those obtained from the est,rapolated (‘urves. ‘I’&lc I. T~itluw of zenith angle rat’io Ytm
1950
1953
19.54
1955
1956
1967
1958
1959
1960
1961
Uncorrecttxl
3
8
2.6
2.3
2.;
2.2
1.X
1.7
2.2
3.3
3 *5
Corroctetl
3.x
2.G
” .3
1.7
2.;
2.3
2.1
2.X _
3.3
3. 5
The significance of these values has been considered elsewhere (BRIGGS and 1%X3), and it has been shown that they are larger than can be explained theoretically on the basis of the changing zenith angle alone. It appears that, in PARKIN,
Observations
of radio star scintillations
and spread-F
echoes over a solar cycle
11
addition, the degree of irregularity of the ionosphere must increase with increasing latitude. The monthly mean diurnal curves of Fig. 3, from which all the above results have been derived, give no idea of the shorter term variations of the scintillation index. In order to illustrate this variability, the individual hourly values have been used to construct the typical histograms shown in Fig. 5(a). These show the number of hours during the night (defined as 2100-0300) for which the scintillation index had the values stated, and are for September 1954 (sunspot minimum) and Sep. -
1957
Sep. 1954
LLhL (a)
~
012345
012345
Scintillation
Sep.
Scsntlllation
index
1954
Sep.
1957
-F
index
Index
r-r-n
Spread-F
Index
Spread
Fig. 5. Histograms to show the number of hours during the night (2100-0300 hours) for which the indices took the stated values. (a) scintillation index, (b) spread-F index.
September 1957 (sunspot maximum). It is interesting to note that at sunspot minimum there were 31 hr for which no observable scintillations were present, while for the same month at sunspot maximum there were no such hours. Usually the variation in scintillation index from one hour to the next is small, but there are often great variations between one night and another in the same month. 2.3 Seasonal variations of scintillation index The monthly diurnal curves (Fig. 3) indicate a tendency for low values of scintillation index to occur in September and for high values to occur around March. This is almost certainly not a real seasonal effect, but is a result of the combination of the variations with solar and sidereal times discussed in Section 2.2.
Since scintillations occur mainly at, night. it!ld RIB0 t’Pll(t to incroascb at I~~\\‘r~l’ transit, the greatest value of’ scintillation index \vill occur whc~rrlower trwsit oc~~tn~s near midnight. This happens in March. III Scptembcr, when upper transit owws near midnight, small values of scintillation index are to be expected. It is important to d&ermine whether there is a true seasonal effect or not. This is of some interest in connection with theories of the origin of the irregularities and their relation to spread-F echoes (see Section 3.2). Also the procedure used f’ol separating the solar and sidereal variations used in Section 2.2 is based on the assumption that all the monthly curves havr rqual weight,, i.e. t,hat t’here is NO
-
June
1960
: :: *.*. . ..*
12
14
I6
I8
20
22
2
,.... *.** **.* *. .:.. ...*
4
6
9
IO
12
G. MOT.
Fig. 6. The monthly mean diurnal curves for June and December 1960, superimposed for comparison. The approximate symmetry with respect to midnight shows that there is no seasonal variation, other than produced by the change in the time of lower transit.
seasonal variation. Unfortunately, it is not possible to determine solar, sidereal and seasonal variations separately. However, some useful evidence can be obtained by studying the behaviour during the months of June and December. In June, lower transit occurs 6.1 hr before midnight, and in December it occurs 59 hr after midnight. Due to this approximate symmetry around midnight the diurnal curves for these months should be mirror images with respect to midnight, provided there is no seasonal effect, since the variation with solar time is symmetrical around midnight (Fig. 4(a)). To illustrate this, the curves for June and December 1960 are drawn superimposed in Fig. 6. It will be seen that they are approximately mirror images with respect to midnight, and the amplitudes are closely similar. This shows that, for this year, there was no seasonal difference between June and December. The results for other years (Fig. 3) are not quite so clear cut, but in most years the curve for June has its maximum before midnight, the curve for December has its maximum after midnight, and the values of these maxima are approximately
Observations of radio star scintillations and spread-B’ echoes over a solar cycle
13
equal. It is concluded that any seasonal variation as between summer and winter must be very small. CHNERS (1960) reported maxima of the mean scintillation index at the equinoxes, and minima at the solstices, for the years 1956-8, near sunspot maximum. Such an effect would not, of course, be detected by the above method, in which only the months of June and December were examined. It is difficult to see how it can be established with certainty in the presence of the other variables though a An examination of the present results large effect would be evident by inspection. (Fig. 3) gives no support to the suggestion that the scintillation effect is a maximum at the equinoxes. 2.4 The variation of scintillation index with the solar cycle The variation of the scintillation index with sunspot number is at once evident from an inspection of Figs. 3 and 4. The mean values are larger at sunspot maximum. There are also changes in the form of the diurnal curves. The minima (M) attributed to the effect of the finite size of the source become more pronounced at sunspot maximum (BRIWS, 1961). This to some extent obscures the true magnitude of the variation of scintillation index from sunspot minimum to sunspot maximum, which would be larger if the minima were removed. It is clear that the degree of irregularity of the ionosphere must be greater at sunspot maximum or, alternatively, the irregular region must be thicker. At sunspot minimum, scintillations occur mainly by night. At sunspot maximum, they occur by day as well as by night, and for some months (e.g. September 1957) there is hardly any diurnal variation. The daytime scintillations will be discussed in Section 2.5. Further maxima and minima appear in some of the diurnal curves at sunspot maximum, in addition to those attributed to the size of of the source, and some of these features seem to show systematic displacements in time from month to month (e.g. June, July, August, September 1958 and 1959). The origin or significance of these variations is not understood. The variation of the scintillation index with the solar cycle can be shown most clearly by considering annual mean values. However, it must be remembered that the latitude of the effective region of the ionosphere varies with the hour angle of the source (Fig. 2), and it is possible that the degree of the irregularity of the ionosphere may vary differently with sunspot number at different latitudes. The use of overall annual means would obscure any such variations with latitude. Therefore we shall consider separately the annual means for observations made at upper transit and for observations made at lower transit. These can be obtained immediately from the curves in Fig. 4(c), and are plotted in Fig. 7. The “corrected” values at lower transit are those read from the dotted extrapolated curves in Fig. 4(c). Fig. 7 also shows the variation of the 12-monthly running-mean values of sunspot number for the period concerned (1950-1961). It is clear from Fig. 7 that there is no significant difference between the behaviour at upper transit and at lower transit. In both cases the variation of the scintillation index appears to follow that of sunspot number. The mean scintillation index is, of course, higher at lower transit, but the ratio of the value at sunspot maximum to that at sunspot minimum is about the same as for upper
transit,. The actual vulnc~sof this “solar cycle r&o” ill‘e tr’i~ll
._‘**...Sunspot number 5
A-A
Sci~tilloti~~
rndex, upper
O-O
Scintillation
index,
QD-Qp
Lower transit,
corrected
..’
..I.
,:.
,:’
J 200 ‘..._*..
..
. .._
....
.:’
..
.f
ll)b/H'r
/.. . ..f...
fronsit
lower tronsii
r a... I 4 - ._
_g
fOllO\VS:
1 ,si; f
I... ‘...
/
0 + 3
.. .**.
1 3 z 1 ; :2s i? I A--------___-_______.A_A-A 0
t
,950 195:
,
:IIII ! 950
1953
1954
1955
/ 956
is57
1958
I
1959
1
1960
i
1961
Fig. 7. The variation of the annual mean scintillation index at upper transit and at lower transit, for the period 1950-1961. The dotted curve shows the 12-month13 running mean values of the sunspot number.
For some years, particularly during the period 1957-1960, there is appreciable scintillation in the daytime. Fig. 8 shows the annual variation of the monthly mean scintillation index at noon for each of these years. Smooth curves have been drawn through the points to assist the eye in following the general trends. The less certain parts of the curves are drawn dotted. It should be emphasized again (see
Fig. 8. The monthly mean scintillation index at noon, plotted for each month from 1957 to 1900. The times of year at which upper and lower transits occur at noon am indicated.
Observations of radio star scintillations and spread-P echoes over a solar cycle
15
Section 2.2) that the daytime values of scintillation index are less reliable than those obtained at night; nevertheless the general form of the annual variation is seen to be very similar from year to year, and is almost certainly significant,. It is impossible to decide with certainty whether this annual variation is a seasonal effect, or an effect, involving sidereal time, since, at noon, the times of upper and lower transit undergo a cyclic variation with a period of one year. The months in which upper and lower transit occur at noon are shown in Fig. 8. It will be seen that the maxima of the curves occur near to lower transit, and the minima are near upper transit (actually displaced by one month). Thus it seems most likely that the effect involves sidereal time, though the possibility of a seasonal effect cannot be ruled out. If the variations shown in Fig. 8 are interpreted as an effect of sidereal time, the question arises as to whether the whole variation can be explained by the changing zenith angle of the source. It appears that it cannot be so explained, since the “zenith angle ratio” is too large, even for irregularities at E-region heights. A possible explanation is that during the daytime there is a pronounced increase in the degree of irregularity of the ionosphere at the higher latitudes involved near lower transit. Consideration of Fig. 2 suggests that the variation can be explained if it is supposed that the irregularities are in the P-region and increase in intensity We have already shown that a north of a latitude of about 56’N (SO’ geomagnetic). variation of this type is needed in order to explain the variation with sidereal time for the results as a whole; the present considerations show that the latitude variation must be particularly pronounced in the daytime at sunspot maximum. An alternative explanation is to suppose that the daytime scintillations are due to irregularities in the E-region, possibly associated with E,. WILD and ROBERTS (1956)have found such an association for daytime scintillations observed in Australia. An attempt was made to correlate the scintillation index with the occurrence of E, at Slough and Inverness (see Fig. 2). Unfortunately there were so many gaps in the data, both for scintillations and E,, that no conclusion could be reached. If the daytime scintillations are due to irregularities in the E-region, the range of latitudes involved will be small, due to the lower height, and the variations shown in Fig. 8 must be interpreted as a seasonal effect. Daytime scintillations would repay further study, but it may be necessary to wait, until the maximum of the next solar cycle before this will be possible at, Cambridge. 3.
THE OCCURRENCE OB SPREAD-FECHOES
3.1Description
AT SLOUUH OVER A SOLAR CYCLE
of the method used
The closest station to Cambridge for which regular ionospheric soundings are available is at Slough (51*PN, low), and all the results to be described are for this station. The occurrence of spread-p echoes is indicated in the published “Bulletins of Ionospheric Characteristics”, but these have been found unsatisfactory for a detailed study of the occurrence and were not used. Instead the actual ionograms (recorded once per hour) were obtained and a “spread-F index” was allocated to each, according to a convention described in an earlier paper (BRIGGS,1958a,Fig.1). At Slough, the spread-F echoes are mainly of the “frequency-spreading” type, Z
and the index, with a scale 0~3, is intended to be roughly proportional to the spread An index of 2 corresponds t,o a spread of about 0.5 Xcjs. so that the ordinary and extraordinary traces art’ both considerably slu~ad, but stili appear separate. An index of 1 corresponds t-0 a spread of about, 0.25 Xc/s. .4n index of 3 was only used when the spreading \vas so great that the ordinary and extraordinary traces merged together into a single diffuse mass: this indicatchs a spread of critical frequency of more than 0.7 MC/S. The hourly values of spread-F index were averaged for each month, in the same way as the values of sci~~tillatioll index, and the resulting diurnal curves are shown in Fig. 3 (a, It, c, d). In these diagrams the mean spread-P index is shown by the dotted curves, and the appropriate scale is the right-hand one (range O-3). From the curves in Fig. 3 a general comparison can be made between the two sets of data.
in critical frequency.
The curves of Fig. 3 show some of the well-known properties of spread-P echoes; a maximum intensity at or just after midnight, a seasonal variation with a maximum in winter, and a variation with the solar cycle with a maximum intensity at sunpot minimum. The seasonal and solar cycle variations can be shown more clearly by plotting the mean values of the spread-F index at midnight for each month as in Fig. 9. IO
;2 .c tL +' L -0 1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
Fig. 9. Monthly mean values of the spread-F index at midnight plotted for each month from 1949 to 1960. The upper curve shows the corresponding values of the quantity P (the product of the spread-p index and the critical frequency in I&/s). The points for *January of each year fall on the vertical lines.
The midnight value, in general, gives a good measure of the mean spread-F index for most of the night. Fig. 9 shows clearly the seasonal variation; the index is about three times as large in December as in July. The variation with the solar cycle is also shown. At sunspot minimum the seasonal oscillation takes place around a mean value of about l-8 and at sunspot maximum around a mean value of about 0.9. Thus the solar cycle produces a change of about a factor of two in the spread-F index. If summer and winter are considered separately, the solar cycle effect is seen to be larger in summer than in winter. Very low values of spread-F
Observations of radio star scintillationsand spread-P echoes over a solar cycle
17
index occur in summer at sunspot maximum, e.g. in the summer months of 1958. The electron density at the maximum of the F-layer, N, is proportional to fo2, where fO the critical frequency. Hence the deviation of electron density from the mean, AN, due to a spread of critical frequency Af,,, is proportional to f,, Af,,. This suggests that f,, Af,, may be a more significant parameter than Af,, itself. If the data are to be compared with radio star scintillations, AN is the important quantity, since it can be shown that the scintillation index is proportional to AN (or, more precisely, to the root-mean-square deviation of N from the mean; see, for example BRIWS and PARKIN (1963)). The spread-F index used here is proportional to AfO. To obtain the variation of AN, the index must therefore be multiplied by fo. The monthly mean values of the critical frequency of the F-layer at midnight were obtained for Slough from the published Bulletins, and the upper curve in Fig. 9 shows the variation of the product P =
spread-F index x critical frequency [m;:;nm;) [ m”;:My;an ]*
It will be noticed that the effect of multiplying by the critical frequency is to remove the variations with season and with the solar cycle. The quantity P shows considerable random fluctuations from month to month, but there are no obvious trends. From the definition of the spread-F index it can easily be shown that AN w 6 x 103P electrons/cm3 where P is in units of (spread-F index x MC/S). The mean value of P is seen from Fig. 9 to be about 6; hence AN M 3.6 x lo4 electrons/cm3. This result shows that near the level of maximum ionization of the F-layer, at midnight, the irregular part of the ionization is characterized by this constant value of AN, which is independent of season, and independent of sunspot number . This suggests that the variations of the spread-F index with season and with the solar cycle are secondary effects, and are due to the fact that the critical frequency varies with season and with the solar cycle. This point has been investigated rather differently by SINGLETON (1962)for a number of stations, using hourly values of f,,, and hourly values of Af,. His conclusion was the same; the value of Af,, was found to be large when f. was small, and conversely, so that AN tended to remain constant. The actual values of AN determined by SINGLETON are rather larger than the mean value deduced above for Slough. The remarkable constancy of the monthly mean values of AN should not be allowed to obscure the great variability on different occasions. Spread-F echoes are essentially a sporadic phenomenon; at any particular time AN may have any value from zero to many times the above mean value. This is illustrated in the histograms of Fig. 5(b), which show the number of hours during the night (2100 to 0300) for which the spread-F index had different values, for two typical months near
sunspoC minimum and sunspot maximum. izlso the diurnal variat.ion remains; AN is small or zero during the daytime. 111 this respect is it, hard to understittld the range of values of AX given by SINCLICTOTS, which seems to imply that’ Ai\: is nww less than ii ;C 10” electrons/cm3. 4. %E
Iter,~~xos
BETWEEN R,ADW STAR SCINTILLATIOW AXD SPREEAD-FECHOES
4.1 General ~?iscu~ssio~~ The correlation between the occurrence of radio star scintillations and spread-F echoes is well established for the angles of elevation involved in the present observations (e.g. RYLE and HEWISH, 1950; WRIGHT et al. 1956; BRIGGS, 1958; LAWRENCE, JESPERSON and LAMB, 1961). Some puzzling features of the relationship remain however. Scintillations show no seasonal variation (Section 2.3), whereas spread-F echoes show a marked seasonal effect with a maximum occurrence in winter (Section 3.2). The scintillation index increases with sunspot number (Section 2.4), whereas the spread-F index decreases with sunspot number (Section 3.2). The seasonal discrepancy is easily explained. It has already been shown that the quantity AN shows no seasonal variation. Theory shows that the scintillation index is proportional to AN; hence no seasonal variation would be expected in the scintillation index. This confirms conclusions regarding the seasonal variations which were reached in an earlier paper (BRIGGS, 1958b). The changes with the solar cycle cannot be reconciled in this way, since it has been shown that AN shows no variation with the solar cycle. Explanations of this discrepancy have been suggested by other authors, and these will first be discussed in the light of the present .results. SISGLETON (1961) has suggested that the discrepancy may be explained by the changing latitude of the region of the ionosphere which is effective in producing scintillations (Fig. 2). At high magnetic latitudes spread-F increases with increasing sunspot number; consequently if such latitudes exert the dominant influence, the scintillation index would also be expected to increase. However, in Section 2.4 the results were divided into those obtained when the source was near lower transit and those obtained when the source was near upper transit, and in both cases the scintillation index was found to increase with increasing sunspot number. If SINGLETON’Sexplanation were correct the results obtained near upper transit would have shown an opposite variation, since the effective region of the ionosphere then lies between the ionospheric observatories at Slough and Inverness (Fig. 2), and at both these stations spread-F echoes are known to correlate &versely with sunspot number. KOSTEK.and WRIGHT (1960), in considering observations made at Ibadan, near the equator, noted a similar discrepancy in the behaviour of radio star scintillations and spread-F echoes during the rising part of the solar cycle, 1952-1957. They suggested that the thickness of the irregular region may be greater at sunspot maximum, and in support pointed out that the value of ym (the semi-thickness of the F-layer) in the evening hours at Ibadan is twice as large at sunspot maximum as at sunspot minimum. However, the scintillation index varies only as the
Observations
of radio star scintillations and spread-P
19
echoes over a solar cycle
square root of the thickness of the irregular region, and so it seems unlikely that It would be very difficult to explain the this can provide a complete explanation. observations during the summer of 1958 along these lines. At this time the mean scintillation index is large, yet the mean spread-F index is exceptionally low, and, in fact, as will be shown in the next section, spread-F echoes are completely absent on many nights when strong scintillations are occurring. 4.2
The correlation between scintillation for individual nights
index and spread-F
index
We shall now consider the changes with the solar cycle differently by making use of the original hourly observations rather than the monthly means. We first ask, how does the detailed, short term correlation between the two indices vary with the solar cycle? Table
2. Correlation coefficients between scintillation 1955
1956
1957
1958
indices and spread-p 1959
1960
indices Mean -
Jan.
-0.01
-0.41
-0.14
-0.12
-0.17 ____
Feb.
-
-
-0.16
-0.43
$0.01
+0*27
-0.08
Mar.
-0.05
-
+0.42
-0.06
+0.20
+0.2s
+0.16
Apr. -_____
-
-
-0.26
-0.33
+0.01
+0.12
-0.11
-
-0.18
$0.05
+0.32
+0*08
+0.34
+0.12
June
$0.15
-
$057
$0.20
$0.53
$0.55 +0.40 __~____
July _______
$0.21
-
$0.48
+0.46
+0.67
$056
$0.48
Aug.
$067
-
$0.74
$0.51
+0.73
+0.76
$0.72
Sep. __~_
-
+0.44
+0.50
+0.76
+0.61
$0.73
$0.60
-
+ 0.48
$0.22
+0.26
-0.48
$0.20
-
May
Oct.
+0.72
+0.70
$0.30
$0.25
+0.41
Nov.
+0.61
+0.30
-0.10
$0.15
$0.39 _____
Dec.
+0.32
-
+0.45
+0.18
$0.52
-
In order to answer this question it was considered sufficient to use mean values of the indices for each night, defined as the period 2100-0300 hours G.M.T. The justification for this procedure is that the indices do not usually change greatly from one hour to the next, but they do vary greatly from one night to another. By evaluating the correlation between these mean night-time values of the indices, the effect of the diurnal variations is eliminated. Any correlation which is then found indicates a tendency for the two phenomena to occur on the same nights.
C’orrelation coe&cients were evaluated for each month for which twenty or more pairs of values of the mean night-time indices were available, ant1 the results are shown in Table 2. It will be seen that high positive values of the correlation coetbcient occur mainly around August, September and October. This result is as expected, since in September upper transit occurs near midnight, and so the effective region of the ionosphere is t,hen close to the ionospheric observatory at Slough from which the spread-F data was obtained. The fall-off in correlation at other times indicates a patchiness in the distribution of the irregularities, with a scale of the order of 500 km (Fig. 2), and implies that the occurrence of spread-F echoes is uncorrelated at points separated by this distance. This is in agreement with the results of an earlier paper (BRIGGS, 11)68a). The importance of the results in Table 2 from the present point of view is that they indicate that the close, night by night, association between the occurrence of spread-F echoes and scintillations observed near upper transit is maintained throughout the whole of the solar cycle, in spite of the inverse trends of the mean values. This is rather surprising in view of the infrequent occurrence of spread-F echoes at sunspot maximum. ln order to investigate this point further, the values of the two indices were plotted against each other to produce scatter diagrams. These diagrams give more information about the form of the relationship between the two quantities than does the correlation coefficient alone. Scatter diagrams were plotted for each month, but as expected only those for August, September and October showed a significant relationship between the two indices. In order to reduce the number of diagrams to be reproduced, the results for these three months have been combined so that a single scatter plot is obtained for each year. These plots are shown in Fig. 10. The results used in constructing these diagrams were the same as those used for the calculations of the correlation coefficients of Table 2, viz., the mean values for 2100-0300 hours G.M.T. for each night. The diagrams of Fig. 10 show that at sunspot maximum there are many individual nights for which there are no spread-F echoes but nevertheless a considFor example, the plot for I958 shows that for 16 erable degree of scintillation. individual nights no spread-F echoes were observed at all in the period 2100-0300 hours G.M.T., and yet the scintillation index for the same period reached values as high as 3. By contrast, the plot for 1965 (sunspot minimum), shows no such occasions. The only possible explanation of these results appears to be that at sunspot maximum the irregularities are often situated at great heights, above the level of maximum ionization density of the F-layer, and are therefore unobservable by ground-based sounders. Even when spread-F echoes are visible it seems likely that the irregularities are more intense at greater heights, in order to account for the high values of scintillation index observed. I>irect confirmation of this hypothesis should be possible by the use of top-side sounders carried in satellites. Spread-F echoes have, in fact, been observed on top-side ionograms obtained by the Alouette satellite (PETRIE, 1963). However these observations were made at a time of low sunspot number, approaching the
Observations of radio star scintillations and spread-F 5-
1956
. . .
4 -
* .
5 .. .
.
. : l. * I .. . -. :; .
: .
21
echoes over E solar cycle
. % r3;.* .. . .* z .. 22:; *. z *. .o m m. I-.. .
4
.
. .
. *..
Spread-F
index
1958
5v . l
4-•
l. .
4-
.. .
%, .g
. I : v '. .z 3 . .:'
G=i
6 5 z2 E GJ
n* 3. ': s ::, 3:. . " 5 1 .', .: Z c2 , ,*. -. . .
...
0
I-
., : .*
1
I
I
Spreod
-F
2 index
.-. . I__ 1960
l
.
.
;:
.
l
.
.
l
0.
.
.. .
.
-
I
.=:
’ .
l
5
l
.E
. .
. .
l*:::.r
l
‘0 E*:.
. :. IE Spread-F
:'
l
s
.**.
0
I.
.
it 0 .E3 t
,
3
index
.
:
l .
-F
4
*
.
.
.
l
: ;:. . .; . 0 *4 :
x
I *.
Spread
.
.:
0
5
.
.*
. . . ... .
, 3
I959
“r
.
1 2 index
I I Spread-F
0
2
3 index
.
0
3
Sp’read
-F:ndex
Fig. 10. Scatter diagrams showing the relation between scintillation index and spread-F index for the months August, September, October, when upper transit occurs at night.
minimum of the solar cycle. A crucial test of the above hypothesis can only be made at sunspot maximum, when it should be found that spread-J’ echoes are commonly observed on the top-side of the F-region even when none are observable on the bottom side. 4.3 The relation between daytime scintillation
and spread-P
echoes
As explained in Section 2.5, the daytime scintillations, which occur mainly near the maximum of the solar cycle, may be due to irregularities in the E-region. It is of some interest, however, to ask whether an explanation in terms of P-region irregularities is possible. At first sight such an explanation seems unlikely, since spread-P echoes are
usually thought of as a night-time phenomei~on. H o\ve\~t:r.at high latituth~s t,lw occurrence of spread-F echoes is different from that at lower laMrules (SIN~I.F:W13. l!lfiO; 8HIm4zAKr, l!t6'). 11 study of Fig. :! Of’ the tllikt north of 60” geomagnetic latitude spread-E’ clchoes occur by da?. as \\.ellits 1)~ iright. illso, during the day they do not occur south of‘ this latit,udc. There exists. in Incat, just the sudden increase at 60” geomagnrt~ic latitude which was postulated to explain the curves of Fig. S. Thus it seems very likely the day-time scintillations can be explained by irregularities iii the .I’-region. The analysis of SINGLETON applies to the ICY period, i.e. the maximum ot’ the solar cycle. The present results suggest that’ t,he irregularities which occur iii the F-region in the daytime, and the sharp increase at 60” geomagnetic latitude, exist only near the maximum of the solar cycle, since daytime scintillations do not occur at sunspot minimum. pil]Er
by
SINGI,E'I'OY
SllO\Vh
5. C:ONCLTJSlONS In the main, the conclusions reached are in agreement with existing knowledge, and serve to confirm results deduced from more limited samples of data. We shall not attempt to summarize these results here; the Figures throughout the paper However, some new results have emerged and these provide the best summary. should, perhaps, be emphasized. The study of the spread-F index near midnight (Section 3.2) has shown that the degree of irregularity of the ionization, AN, near the level of maximum ionization of it does not vary either with season the F-layer, is remarkably constant; or with the solar cycle. This suggests that many of the observed variations of the occurrence and intensity of spread-F echoes are secondary effects due to changes in the critical frequency of the F layer. Further, the results of Section 4.2 show that there must be irregularities above the maximum of the F layer, which this is particularly the case at are unobservable by ground-based sounders; sunspot maximum. For these reasons, the study of spread-F echoes seems to be an unreliable method for investigating the occurrence of irregularities in the ionization. The study of radio star or satellite scintillations is better, as the probing waves then traverse the whole thickness of the ionosphere. Even here, however, it must be remembered that if amplitude or intensity scintillations are observed, irregularities at different distances from the observing point are not equally effective: there is a “weighting function” which is different for radio stars and for satellites @RIGGS and PARRIN, 1963). The results have shown that the degree of irregularity of the ionosphere must increase with increasing latitude, since the variation of scintillation index with sidereal time is too large to be explained by the variation of zenith angle alone. At sunspot maximum a new phenomenon appears in the form of daytime scintillations. It has been shown that these are probably due to irregularities in the Fregion, and that the variation with latitude must be particularly large in this case. Finally, the very great variation in the scintillation index between sunspot maximum and sunspot minimum should by emphasized. This may be of importance in the planning of experiments in radio-astronomy, particularly those on lower frequencies where scintillations cause difficulties.
Observ&ions of mdio star scintilletions and spread-P echoes over a solar cycle
23
Acknowledgements-The observations of radio star scintillations were carried out when the author was at the Cavendish Laboratory, Cambridge, and would not have been possible without the co-operation of many colleagues who assisted in the running of the apparatus. I am indebted to the Director of Radio Research of the Department of Scientific and Industrial Research for the loan of ionograms for Slough for the period 1949-1960. I am grateful for the assistance received from the computing staffs at the Cavendish Laboratory and at the University of Adelaide in connection with the analysis of the large number of records, and the subsequent reduction of the data. REFERENCES BRINGS B. H. BRIGGS B. H. BRIGGS B. H. BRIGGS B. H. and PARKIN I. A. CHIVERS H. J. A. FORSYTH P. A. and PAULSON K. V. HEWISH A. HEY J. S., PARSONS S. J. and PHILLIPS J. W. JONES I. L. KOSTER J. R. KOSTER J. R. and WRIG~ R. W. PETRIE L. E. RYLE M. RYLE M. and HEWISH A. SHIMAZAKI T. SINGLETON D. G. SINQLETON D. G. SINGLETON D. G. SMITH F. G., LIYLZE C. G. and LOVELL A. C. B. SPENCER M. WILD J. P. and ROBERTS J. A.
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