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Time variations of the soft component of the cosmic radiation
Journal of Atmospherican(i Terrestrial Physics,1953,Vol. 3, pp. 295to 300. Pergamon Press Ltd.,London
Time variations of the soft component of the cosmic radiation D. I. DAWTON and H. ELLIOT The Physics
Laboratory,
The University,
(Received
11 June, 1953)
?!lanchester
ABSTRACT The time variations of the soft component stopped in 10 cm Pb and the meson int,ensityunder 10 cm Pb at sea level have been in\-estigatcd simultaneously. The intensity of the soft component is primarily a function of barometric pressure but there is in addition a small negative atmospheric temperature effect. The seasonal variation of the soft component has a smaller amplitude than that, for the mesons and this shown to be consistent with the observed temperature coefficients. On the other hand the diurnal variation for the soft component has about twice the amplitude of that for the mesons and this difference is discussed in relation to t,he diurnal variation in atmospheric temperature.
1.
INTR~DVCTION
During 1951 and 1952 an investigation of the t,ime rariaGons of the soft component of the cosmic ray fluxat sea level has been carried out in Manchester, the soft component being defined here as that part of the radiation which is stopped in 10 cm lead. The measurements were made by using t#wo similar counter arrays one containing no absorber and the other cont,aining 10 cm lead. The array with I no absorber measured the totSalflux whilst that with the lead measured the intensity 2 of the hard (meson) compound and the AAAT\AmmA difference obtained by substracting the vv--vvv rates gave a measure of the soft component 2, intensity. Using this arrangement the day ‘i-_-1 to day variations due to atmospheric 3. temperature and pressure changes together with the seasonal and diurnal variations have been investigated simultaneously for the soft component and for the meson intensity. 2.
EXPERIMENTAL
A ” Fig. 1. Counter arrangement used for simultaneous measurements of the total ionizing cosmic ray intensit,y and the hard component
10 cm lead.
penetrating
ARRANGEMENT
The count’er arrangement used is shown diagramatically in Fig. 1. Three counter t,rays wit,h dimensions 40 cm x 40 cm were placed vertically above each other the separation between each tray being 25 cm. The lead absorber was placed between the lower two trays and twofold coincidences were recorded between t,he top two trays (c12) and between the lower two trays (c.&. The coincidences cl2 t#herefore measured the total intensity and C,, the intensity of the hard componenL per hour respectively giving a soft per hour and -30,000 The rates were -40,000 During the greater part of the running time component rate of ~10.000 per hour. the arrangement was duplicated so as to give a check on the operation of t,he equipment. 295
D. I. Dswton and H. Elliot 3.
DAY
TO DAY
VARIATIONS IN THE INTENSITY
As a preliminary step in the analysis of the data two groups were selected each consisting of 30 days and, following DUPERIER (1949), partial correlation coefficients between the intensity and atmospheric pressure (r CB.u) and between the intensity and the height of several selected pressure levels (rCH.n) were computed for the total intensity and for the hard and soft components. The results are shown in Table 1. Table
1. Correlation
coepients
for
d$+wat selected atmospheric
pressure
leveb.
100mb
400mb
400 mb
Total Hard Soft
-0.53 -0-64 +0.01
-0.56 -0.67 -0.02
- 0.60 -0.69 -0.06
-0.49 -0,58 -0.01
TCB.I[
Total Hard Soft
-0-95 -0.89 -0.97
-0.95 -049 -0.97
-0.95 -0.90 -0.97
-0.96 -0.95 -0.98
.Qx!U3
Total Hard Soft
-0.53 -0.64 -0.07
-0.55 -0.66
-0.58 -0.67
-0.11
-0.14
-0.51 -0.59 -0.13
Total Hard Soft
-0-76 -0-58 -0.86
-0-82 -0.68 -0.88
-0.86 -0.76 -0.90
-0-93 -0.89 -0.95
Pressure
W!K.U Group
Group
level
200 mb
1
2
TCB.R
There are three points which are worthy of note. 1. None of the in~vidual values of r Cu.Rfor the soft component can be regarded as statisticaily sig~fioant. ~u~hermore when the analysis was extended to include the 900, 800 and 600 mb layers no significant correlation could be detected with these layers either. 2. The values of r,-.n,u for both the total and hard components increase up to the highest pressure level considered. 3. The values of rCu,s for the total and hard components increase with height of pressure level selected up to 200 mb and then appear to decrease again at 100 mb. It follows from (1) that the intensity of the soft component can at most be only slightly dependent on atmospheric temperature which controls the heights of the various pressure levels. Results (2) and (3) are in agreement with the findings of DUPERIER (1949) for the meson intensity under 25 cm lead and he has interpreted the fall off in rCHs from 200 mb to 100 mb as being due to the effect of atmospheric temperature in this region on the competing processes of decay and nuclear interaction for the 7~mesons. In view of these preliminary results partial correlations were next carried out for 8 groups each comprising data for 30 days using the barometric pressure (B), height of the 100 mb layer (H) and temperature at 100 mb (II) as the appropriate 296
Time variations of the soft component of the cosmic radiation
meteorological variables. The mean values obtained for the regression coefficients are shown in Table 2, the errors quoted are standard deviations estimated from the scatter of individual values for each group. DUPERIER’S values for the hard component under 25. cm lead are shown for comparison. Table 2. Absor@ion coeficicnts (&B.HT), Decay coeficiente (pCH.BT) and Temperature coe$kien.ts (BCT.BH)deduced from the day to day variations in the intensities. ?CB.HT % per cm. Hg
Total intensity Soft component Hard component under IOcm Pb Hard component under 25 cm Pb
@X.BT
/kT.BII
%perkm
%
per “C
-4.70
+ 0.18 $ 0.23
-3.02 -1.05
& 0.49 & 0.43
-1.67
* 0.16
-4.00
* 0.43
$0.056
zt 0.018
-1.05
f 0.16
-3.90
* 1.10
$0-123
* 0.024
-2.49
IL will be seen that the absorption
coefficient (/3cB.nT) for the intensity under by DUPERIER using 25 cm lead as would also appear to be consistent but the be expected. The decay coefficients (,!?CH.BT) cT,BH) obtained here is about half that found by temperature coefficient (#I DUPERIER. It is evident that the intensity of the soft component is primarily controlled by barometric pressure although there appears to be an indication of a small decay effect of --i*O5 per cent p& km and a small positive temperature effect of up to 0.04 per cent per “C cannot be excluded. The presence of a small decay effect is to be expected for two reasons. In the first place ~10 per cent of the radiation stopped in 10 cm lead consists of slow ,u mesons with momenta less than 200 MeV/c and secondly part of the soft component must arise from knock-on and decay electrons produced by the temperature dependent ,!J meson component. 10 cm lead is larger than that obtained
4.
THE
SEASONAL VARIATION
[ni\
In order to investigate the seasonal variation, -300 ._._. monthly mean values of the intensities were formed ‘\.; - _ - n,_ and corrected for variations in barometric pressure / using the appropriate absorption coefficients given ‘.,A *so in the previous section. The corrected values of the Fig. 2. The eeason variation for the total intensity and the hard and soft components total ionizing cosmic ray intensity, are shown plotted in Fig. 2 together with a plot of the hard and soft components and the height of the 100 mb pressure level. the monthly mean height of the 100 mb layer. This latter curve has been inverted in order to bring out clearly the relation between it and the cosmic ray intensity. It will be seen that in each case the cosmic rays have maximum intensity in winter and minimum intensity in summer. The variation o
297
--_
/-/’
D. I. Dawton and H. Elliot
for the meson intensity has an amplitude which is about 2.5 times that for the soft component. These results are in qu~~tative agreement with the generally accepted view that the seasonal variation of the meson intensity is largely due to variations in stmospheric temperature which control the height of the atmospheric layer in which the
00
02
04
06
00
IO
12
HOURS
Fig. 3.
14
16
Ia
20
22
24
I
GMT.
Solar daily variation for the total ionizing cosmic ray intensity.
p-mesons are formed. It has been shown in the previous section that the soft component is much less affected by the height of the 100 mb layer than is the meson component and consequently it would be expected that the seasonal variation in the intensity of the soft component should be correspondingly smaller as indeed it is. The monthly values of the hard and soft component intensities have been corrected using the absorption and decay coefficients given in the previous section. The corrected values show a barely sig~ficant residual variation of ~mp~tude
00
Fig. 4.
02
04
06
08
IO
I2
I4
HOURS G.M.T
16
1s
20
22
24
J
Solar daily variation for the hard component penetrating 10 cm lead.
(O-40 f 0.16) per cent with maximum intensity in January in each case. The day to day correlations hsve shown that there appears t,o be an appreciable upper air positive temperature effect and it was therefore to be expected that the residual variation wouId have s maximum in summer, when the t,emperatures in the upper atmosphere are highest. There appears to be a discrepancy here but in view of the uncertainties in absorption and decay coefficients it is of doubtfulsignificance. There is also the possibility that a small annual variation may be present due to geomagnetic activity or other unknown causes (FORBUSH 1938, 1939). 298
Time variations of the soft component 5.
of the cosmic radiation
THE SOLAR DAILY VARIATION
The diurnal variation has been computed for the total intensity and for the hard and soft components over a period of 20 months from May 1950 to December 1951. During the period when two independent sets were in operation the diurnal variations as measured by the two recorders showed good agreement and the two The final results are shown in Figs. sets of data for this period were combined. 3, 4 and 5 where curves have been fitted by means of Fourier analysis. The 24-hour waves for the hard and soft intensity are shown plotted on a harmonic dial in Fig. 6 after correcting for barometric pressure using the appropriate The error circles represent standard deviations estimated absorption coe~cients.
56h
Fig.
5. Solar daily variation of the soft component of the cosmic rays stopped in 10 cm lead. Fig. 6. Harmonic dial showing 24 hr waves for the hard and soft components after correcting for barometric pressure.
from the scatter of the harmonic coefficients when the data were subdi~lided into 30 independent groups. The phases of the 24-hour waves for the two components are very nearly the same but the amplitude for the soft component is appreciably greater than that for the hard. The reason for this difference in amplitude of the variation for the two components is not clear but it is of interest to note that it could be explained in terms of a diurnal variation in t,he height of the 100 mb pressure layer. Neteorological Oflice radio sonde observations appear to show the existence of such a variation but there is considerable doubt as to whether this is a radiation error in the instruments or a genuine variation in air temperature (KAY, 1951). If the radio sonde observations are correct the diurnal variation in the height of this layer has an amplitude of ~45 m, the maximum height occurring near noon. Because of p-meson decay this variation in the height of the 100 mb layer would reduce the amplitude of the observed diurnal variation of the mesons and 299
D. I. Dewton and If. Elliot: Time variations of the ecft component of the cosmic radiation
also, to a less extent, that of the soft component, If the daily variations for the mesons and the soft component are corrected for the height of the 100 mb layer using‘the decay coefficients of 4 per cent per km and 1 per cent per km the amplitudes become 0.33 per cent and 0.40 per Gent respectively. The times of maximum in each case are near noon and the difference in amplitude is no longer si~ifi~ant, According to the radio sonde data the diurnal variation in the height of the X00 mb pressure level is about twice as great in summer as in winter and if the vacation is real this difference should be detectable given su~ciently accurate measurements of the soft component daily variation. The amplitude of the variation in summer is ~55 m compared with ~26 m in winter and consequently the difference between the amplitudes of the daily variation of the soft and hard components should be 0.16 per cent in summer and 0.08 per cent in winter. The present data are not inconsistent with these figures but because of the rather large statist&l errors it is not possible to say with any certainty whether this interpretation of the difference in the diurnal variation for the two components is the correct one. 6.
CONCLUSIONS
The meson intensity under 10 cm lead shows a positive correlation with the temperature of the 100 mb layer which is in qualitative agreement with DUPERIER’S (1949) results under 25 cm lead but the temperature coefficient f+ 0.056 per cent per “C) is smaller than his value of + O-123 per cent per “C. The intensity of the soft component is primarily a function of barometric pressure but there is also a small negative atmospheric temper&ture effect which is to be expected since the soft component is at least in part secondary to the temperature dependent ,u-meson component and in addition some 10 per cent of the soft component stopped in 10 cm lead consists of slow y-mesons. The seasonal variation for the soft component is smaller than that for the mesons and this result can be explained in terms of the different negative temperature coefficients for the two components. The diurnal variation is appreciably larger for the soft component than for the mesons and it is possible that this difference may be due to a diurnal variation in the height of the 100 mb pressure level. A~~~owl~~rne~ts. We are indebted to Professor P. M. S. BLACEETT for providing the facilities for carrying out these experiments and for his helpful comments on this paper. One of us (D.I.D.) wishes to thank the Department of Scientific and Industrial Research for a maintenance grant. REFERENCES DUPERIER, A. FORBU~H, S. E. FORBUSH, s. E. KAY, R. R.
1949 1938 1939 1951
300
Proc. Phys. Sot. A 62, 684 Phys. Rev. 64, 975 Rev. Mod. Phys. 11,168 Quarterly Jour. Roy. Met. Sot. ‘72, 427
Time variations of the soft component of the cosmic radiation D. I. DAWTON and H. ELLIOT The Physics
Laboratory,
The University,
(Received
11 June, 1953)
?!lanchester
ABSTRACT The time variations of the soft component stopped in 10 cm Pb and the meson int,ensityunder 10 cm Pb at sea level have been in\-estigatcd simultaneously. The intensity of the soft component is primarily a function of barometric pressure but there is in addition a small negative atmospheric temperature effect. The seasonal variation of the soft component has a smaller amplitude than that, for the mesons and this shown to be consistent with the observed temperature coefficients. On the other hand the diurnal variation for the soft component has about twice the amplitude of that for the mesons and this difference is discussed in relation to t,he diurnal variation in atmospheric temperature.
1.
INTR~DVCTION
During 1951 and 1952 an investigation of the t,ime rariaGons of the soft component of the cosmic ray fluxat sea level has been carried out in Manchester, the soft component being defined here as that part of the radiation which is stopped in 10 cm lead. The measurements were made by using t#wo similar counter arrays one containing no absorber and the other cont,aining 10 cm lead. The array with I no absorber measured the totSalflux whilst that with the lead measured the intensity 2 of the hard (meson) compound and the AAAT\AmmA difference obtained by substracting the vv--vvv rates gave a measure of the soft component 2, intensity. Using this arrangement the day ‘i-_-1 to day variations due to atmospheric 3. temperature and pressure changes together with the seasonal and diurnal variations have been investigated simultaneously for the soft component and for the meson intensity. 2.
EXPERIMENTAL
A ” Fig. 1. Counter arrangement used for simultaneous measurements of the total ionizing cosmic ray intensit,y and the hard component
10 cm lead.
penetrating
ARRANGEMENT
The count’er arrangement used is shown diagramatically in Fig. 1. Three counter t,rays wit,h dimensions 40 cm x 40 cm were placed vertically above each other the separation between each tray being 25 cm. The lead absorber was placed between the lower two trays and twofold coincidences were recorded between t,he top two trays (c12) and between the lower two trays (c.&. The coincidences cl2 t#herefore measured the total intensity and C,, the intensity of the hard componenL per hour respectively giving a soft per hour and -30,000 The rates were -40,000 During the greater part of the running time component rate of ~10.000 per hour. the arrangement was duplicated so as to give a check on the operation of t,he equipment. 295
D. I. Dswton and H. Elliot 3.
DAY
TO DAY
VARIATIONS IN THE INTENSITY
As a preliminary step in the analysis of the data two groups were selected each consisting of 30 days and, following DUPERIER (1949), partial correlation coefficients between the intensity and atmospheric pressure (r CB.u) and between the intensity and the height of several selected pressure levels (rCH.n) were computed for the total intensity and for the hard and soft components. The results are shown in Table 1. Table
1. Correlation
coepients
for
d$+wat selected atmospheric
pressure
leveb.
100mb
400mb
400 mb
Total Hard Soft
-0.53 -0-64 +0.01
-0.56 -0.67 -0.02
- 0.60 -0.69 -0.06
-0.49 -0,58 -0.01
TCB.I[
Total Hard Soft
-0-95 -0.89 -0.97
-0.95 -049 -0.97
-0.95 -0.90 -0.97
-0.96 -0.95 -0.98
.Qx!U3
Total Hard Soft
-0.53 -0.64 -0.07
-0.55 -0.66
-0.58 -0.67
-0.11
-0.14
-0.51 -0.59 -0.13
Total Hard Soft
-0-76 -0-58 -0.86
-0-82 -0.68 -0.88
-0.86 -0.76 -0.90
-0-93 -0.89 -0.95
Pressure
W!K.U Group
Group
level
200 mb
1
2
TCB.R
There are three points which are worthy of note. 1. None of the in~vidual values of r Cu.Rfor the soft component can be regarded as statisticaily sig~fioant. ~u~hermore when the analysis was extended to include the 900, 800 and 600 mb layers no significant correlation could be detected with these layers either. 2. The values of r,-.n,u for both the total and hard components increase up to the highest pressure level considered. 3. The values of rCu,s for the total and hard components increase with height of pressure level selected up to 200 mb and then appear to decrease again at 100 mb. It follows from (1) that the intensity of the soft component can at most be only slightly dependent on atmospheric temperature which controls the heights of the various pressure levels. Results (2) and (3) are in agreement with the findings of DUPERIER (1949) for the meson intensity under 25 cm lead and he has interpreted the fall off in rCHs from 200 mb to 100 mb as being due to the effect of atmospheric temperature in this region on the competing processes of decay and nuclear interaction for the 7~mesons. In view of these preliminary results partial correlations were next carried out for 8 groups each comprising data for 30 days using the barometric pressure (B), height of the 100 mb layer (H) and temperature at 100 mb (II) as the appropriate 296
Time variations of the soft component of the cosmic radiation
meteorological variables. The mean values obtained for the regression coefficients are shown in Table 2, the errors quoted are standard deviations estimated from the scatter of individual values for each group. DUPERIER’S values for the hard component under 25. cm lead are shown for comparison. Table 2. Absor@ion coeficicnts (&B.HT), Decay coeficiente (pCH.BT) and Temperature coe$kien.ts (BCT.BH)deduced from the day to day variations in the intensities. ?CB.HT % per cm. Hg
Total intensity Soft component Hard component under IOcm Pb Hard component under 25 cm Pb
@X.BT
/kT.BII
%perkm
%
per “C
-4.70
+ 0.18 $ 0.23
-3.02 -1.05
& 0.49 & 0.43
-1.67
* 0.16
-4.00
* 0.43
$0.056
zt 0.018
-1.05
f 0.16
-3.90
* 1.10
$0-123
* 0.024
-2.49
IL will be seen that the absorption
coefficient (/3cB.nT) for the intensity under by DUPERIER using 25 cm lead as would also appear to be consistent but the be expected. The decay coefficients (,!?CH.BT) cT,BH) obtained here is about half that found by temperature coefficient (#I DUPERIER. It is evident that the intensity of the soft component is primarily controlled by barometric pressure although there appears to be an indication of a small decay effect of --i*O5 per cent p& km and a small positive temperature effect of up to 0.04 per cent per “C cannot be excluded. The presence of a small decay effect is to be expected for two reasons. In the first place ~10 per cent of the radiation stopped in 10 cm lead consists of slow ,u mesons with momenta less than 200 MeV/c and secondly part of the soft component must arise from knock-on and decay electrons produced by the temperature dependent ,!J meson component. 10 cm lead is larger than that obtained
4.
THE
SEASONAL VARIATION
[ni\
In order to investigate the seasonal variation, -300 ._._. monthly mean values of the intensities were formed ‘\.; - _ - n,_ and corrected for variations in barometric pressure / using the appropriate absorption coefficients given ‘.,A *so in the previous section. The corrected values of the Fig. 2. The eeason variation for the total intensity and the hard and soft components total ionizing cosmic ray intensity, are shown plotted in Fig. 2 together with a plot of the hard and soft components and the height of the 100 mb pressure level. the monthly mean height of the 100 mb layer. This latter curve has been inverted in order to bring out clearly the relation between it and the cosmic ray intensity. It will be seen that in each case the cosmic rays have maximum intensity in winter and minimum intensity in summer. The variation o
297
--_
/-/’
D. I. Dawton and H. Elliot
for the meson intensity has an amplitude which is about 2.5 times that for the soft component. These results are in qu~~tative agreement with the generally accepted view that the seasonal variation of the meson intensity is largely due to variations in stmospheric temperature which control the height of the atmospheric layer in which the
00
02
04
06
00
IO
12
HOURS
Fig. 3.
14
16
Ia
20
22
24
I
GMT.
Solar daily variation for the total ionizing cosmic ray intensity.
p-mesons are formed. It has been shown in the previous section that the soft component is much less affected by the height of the 100 mb layer than is the meson component and consequently it would be expected that the seasonal variation in the intensity of the soft component should be correspondingly smaller as indeed it is. The monthly values of the hard and soft component intensities have been corrected using the absorption and decay coefficients given in the previous section. The corrected values show a barely sig~ficant residual variation of ~mp~tude
00
Fig. 4.
02
04
06
08
IO
I2
I4
HOURS G.M.T
16
1s
20
22
24
J
Solar daily variation for the hard component penetrating 10 cm lead.
(O-40 f 0.16) per cent with maximum intensity in January in each case. The day to day correlations hsve shown that there appears t,o be an appreciable upper air positive temperature effect and it was therefore to be expected that the residual variation wouId have s maximum in summer, when the t,emperatures in the upper atmosphere are highest. There appears to be a discrepancy here but in view of the uncertainties in absorption and decay coefficients it is of doubtfulsignificance. There is also the possibility that a small annual variation may be present due to geomagnetic activity or other unknown causes (FORBUSH 1938, 1939). 298
Time variations of the soft component 5.
of the cosmic radiation
THE SOLAR DAILY VARIATION
The diurnal variation has been computed for the total intensity and for the hard and soft components over a period of 20 months from May 1950 to December 1951. During the period when two independent sets were in operation the diurnal variations as measured by the two recorders showed good agreement and the two The final results are shown in Figs. sets of data for this period were combined. 3, 4 and 5 where curves have been fitted by means of Fourier analysis. The 24-hour waves for the hard and soft intensity are shown plotted on a harmonic dial in Fig. 6 after correcting for barometric pressure using the appropriate The error circles represent standard deviations estimated absorption coe~cients.
56h
Fig.
5. Solar daily variation of the soft component of the cosmic rays stopped in 10 cm lead. Fig. 6. Harmonic dial showing 24 hr waves for the hard and soft components after correcting for barometric pressure.
from the scatter of the harmonic coefficients when the data were subdi~lided into 30 independent groups. The phases of the 24-hour waves for the two components are very nearly the same but the amplitude for the soft component is appreciably greater than that for the hard. The reason for this difference in amplitude of the variation for the two components is not clear but it is of interest to note that it could be explained in terms of a diurnal variation in t,he height of the 100 mb pressure layer. Neteorological Oflice radio sonde observations appear to show the existence of such a variation but there is considerable doubt as to whether this is a radiation error in the instruments or a genuine variation in air temperature (KAY, 1951). If the radio sonde observations are correct the diurnal variation in the height of this layer has an amplitude of ~45 m, the maximum height occurring near noon. Because of p-meson decay this variation in the height of the 100 mb layer would reduce the amplitude of the observed diurnal variation of the mesons and 299
D. I. Dewton and If. Elliot: Time variations of the ecft component of the cosmic radiation
also, to a less extent, that of the soft component, If the daily variations for the mesons and the soft component are corrected for the height of the 100 mb layer using‘the decay coefficients of 4 per cent per km and 1 per cent per km the amplitudes become 0.33 per cent and 0.40 per Gent respectively. The times of maximum in each case are near noon and the difference in amplitude is no longer si~ifi~ant, According to the radio sonde data the diurnal variation in the height of the X00 mb pressure level is about twice as great in summer as in winter and if the vacation is real this difference should be detectable given su~ciently accurate measurements of the soft component daily variation. The amplitude of the variation in summer is ~55 m compared with ~26 m in winter and consequently the difference between the amplitudes of the daily variation of the soft and hard components should be 0.16 per cent in summer and 0.08 per cent in winter. The present data are not inconsistent with these figures but because of the rather large statist&l errors it is not possible to say with any certainty whether this interpretation of the difference in the diurnal variation for the two components is the correct one. 6.
CONCLUSIONS
The meson intensity under 10 cm lead shows a positive correlation with the temperature of the 100 mb layer which is in qualitative agreement with DUPERIER’S (1949) results under 25 cm lead but the temperature coefficient f+ 0.056 per cent per “C) is smaller than his value of + O-123 per cent per “C. The intensity of the soft component is primarily a function of barometric pressure but there is also a small negative atmospheric temper&ture effect which is to be expected since the soft component is at least in part secondary to the temperature dependent ,u-meson component and in addition some 10 per cent of the soft component stopped in 10 cm lead consists of slow y-mesons. The seasonal variation for the soft component is smaller than that for the mesons and this result can be explained in terms of the different negative temperature coefficients for the two components. The diurnal variation is appreciably larger for the soft component than for the mesons and it is possible that this difference may be due to a diurnal variation in the height of the 100 mb pressure level. A~~~owl~~rne~ts. We are indebted to Professor P. M. S. BLACEETT for providing the facilities for carrying out these experiments and for his helpful comments on this paper. One of us (D.I.D.) wishes to thank the Department of Scientific and Industrial Research for a maintenance grant. REFERENCES DUPERIER, A. FORBU~H, S. E. FORBUSH, s. E. KAY, R. R.
1949 1938 1939 1951
300
Proc. Phys. Sot. A 62, 684 Phys. Rev. 64, 975 Rev. Mod. Phys. 11,168 Quarterly Jour. Roy. Met. Sot. ‘72, 427