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QUASI-BIENNIAL VARIATIONS OF COSMIC-RAY INTENSITIES

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https://ntrs.nasa.gov/search.jsp?R=19660021061 2020-06-17T01:45:24+00:00Z

QUASI-BIENNIAL VARIATIONS OF COSMIC-RAY INTENSITIES"

Kaichi Maeda Gcddard Space F l ight Center

Greenbelt, Maryland

and

T. Suda Mete or ological Re search I n s t i t u t e

Tokyo, Japan

* To be submitted t o the Journal of Geophysical Research.

Quasi-Biennial Variations of Cosmic-Ray Intensities

Abstract

Owing to the atmospheric temperature effect on cosmic-rays, the

so-called quasi-biennial ( or 26-month) oscillation in the stratosphere

should be apparent in the hard component of cosmic-ray data at the

ground. As it is estimated theoretically, the cosmic-ray muon data

near the equator (Lae, 6'44 S) shows a significant biennial

variation, the period of which seems longer than 26 months, but

shorter than 30 months for 1954-1964.

analysis of Huancayo (12' S ) ion-chamber data (Maeda and Suda) is

a l s o elaborated by choosing the five highest counting days in eacn

The previous power spectrum

montn, indicating a significant peak at the period of 24 months.

similar analysis made for the ion-chamber data from Gcdhavn (69'5 N)

A

shows a very weak peak at 24 months for 1947-1959. It is concluded

from present analyses tnat biennial variations in cosmic-ray intensity

are predominantly of terrestrial origin, i.e., if the days of large

extraterrestrial modulation ( such as geomagnetic disturbed days) are

chosen no significant 26-month variation appears in cosmic-ray data.

- 2 -

Introduction

Since its discovery in the tropical stratospheric wind system

(Reed, 1960; Veryard and Ebdon, 1961), features of the biennial

variation, or the so-called 26-month oscillation in the earth's

atmosphere have been revealed t0.a great extent in the past few

years, except the theories to explain its origin and mechanisms

(Reed, 1962, 1963, 1964 and 1963; Reed and Rogers, 1962, Staley,

1963; Belmont and Dartt, 1964; Dartt and Belmont, 1964; Newell, 1964;

Kriester, 1964; Sparrow and Unthank, 1964; Wescott, 1964, etc.).

On the other hand, cosmic-ray intensities observed at the

earth' s surface are continuously modulated not only by the astro-

physical variations in outer space (particularly the magnetic field)

but also by the atmospheric variations.

of unstable components such as pions and muons, produced by incoming

primary cosmic-ray particles in the upper atmosphere, intensities

of cosmic-radiation at the ground change with variatlons of barometric

pressure and of the atmospheric temperatures (Jgnnosy, 1950;

Dauvillier, 1954; Heisenberg, 1963; Dorman, 1937). Therefore, the

cosmic-ray muon data, which are more commonly called cosmic-ray

meson data, or the hard component intensities measured at the ground

Due to the decay processes

c

- 3 -

.

and corrected fo r barametric effect , are very good indicators of

continuous atmospheric temperature var ia t ions, provided that information

about geomagnetic var ia t ions i s available.

can expect t he 26-month variations occurring i n the upper atmosphere

should be Tound a l s o i n the pressure-corrected cosmic-ray muon data.

This is already shown by means of power spectrum analysis of ion-

chamber data from Huancayo (geographic l a t . 12' s ) . s t a t ion i s not c lose enough t o the equator, t he result i s hardly

s igni f icant (as s h m i n Fig. 4 , Maeda and Suda, 1965). The purpose

of t he present paper is t o report another more s igni f icant detection

of quasi-biennial var ia t ions i n cosmic-ray data from t h e near-

equator s t a t i o n (Lae, 6'44' s) which w a s suggested i n the previous

paper, but not available a t t h a t t i m e .

For t h i s reason, we

Since t h i s

Meteorological Ef fec ts on Cosmic -Ray Intens it ies

The var ia t ion of cosmic-ray in t ens i ty a t t he ground due t o the

atmospneric temperature var ia t ion is an accumulated e f f e c t of the

d i f f e r e n t i a l contribution from each layer i n the atmosphere, which

i s not only a f'unction of the alt i tude of each layer i n the

atmosphere, but a l s o a function of t he cut-off energy of observed

- 4 -

cosmic rays. The l a t t e r depends on the geomagnetic and geographic

locat ion of the observing s t a t ion and on the geometric condition of

the measuring instrument, such as the thickness of shield and the

type of cosmic-ray detector . These re la t ions are well-known both

experimentally and theore t ica l ly , and can be expressed by a simple

formula :

- - 'I - J ~ ( E 0 , x ) 6T (x ) dx I O 0

where IO, 6 1 are the mean and the deviat ion of cosmic-ray in t ens i ty

a t the atmospheric depth xo due t o the temperature var ia t ion 6T a t

the depth x, respectively.

Y(Eo,x) i s cal led t h e p a r t i a l temperature coeff ic ient , which

indicates the r e l a t ive var ia t ion of cosmic-ray muon in t ens i ty with

cut-off energy Eo a t the depth xo, due t o 1' C increase i n the

- 5 -

l ayer 6x a t x. The de ta i l s of these coeff ic ients as the function

of Eo and x, as well as comparisons with the experimental data

have been discussed by many workers (Maeda and Wada, 1954; Trefall,

1955; Wada and Kudo, 1956; Dorman, 1957; French and Chasson, 1 9 6 ;

Matthews, 1959; Wada, 1961; Carmichael e t a l . , 1963 ; e tc . ) . It

should be noted t h a t the atmospheric temperature e f f ec t on the

cosmic-ray in tens i ty consists essent ia l ly of two par ts ; one i s

posi t ive and due t o the change i n production r a t e of cosmic-ray

muons with temperature var ia t ions i n the upper atmosphere, cu(Eo,x) , and the other i s negative, corresponding t o the change of decay-rate

of muons i n the atmosphere, B(Eo,x).

where cu(Eo,x) and -B(Eo,x) are plotted against x, f o r Eo = 0.3, 10

and 40 GeV.

x. A s can be seen from these figures, the temperature coeff ic ient

i s mostly negative fo r usual cosmic-ray data, the cut-off energies

of which a re l e s s than the order of 0.5 Gev. On the other hand,

the posi t ive effect dominates a t high energies (par t icu lar ly above

the ground production l e v e l of cosmic-ray mesons, i .e. , above 200 mb

l e v e l ) , because decay-rates of muons produced with energies higher

than several Gev i n the atmosphere a r e prac t ica l ly negligible.

These a re shown i n Fig. 1,

Corresponding y(Eo,x) 's a re a l so shown i n Fig. 2 against

It i s known t h a t the phase of 26-month osc i l la t ion i n the upper

atmosphere d i f f e r s with height, sh i f t ing from higher a l t i t u d e down-

wards with a r a t e roughly of the order of 1 km/month. This is shown

- 6 -

i n the upper curves i n Figure 3 , i n which var ia t ions of the s t r a t o -

spheric temperature differences between 3' S and 28' N a r e plot ted

from data obtained during the period from 1931 t o 1961 a t four

d i f fe ren t l e v e l s above 100 mb (full l i n e s ) , and zonal winds a t

Balboa, Panama (8' N ) a r e a l s o shown by a dashed l i ne , whose scale

i s indicated on the r i g h t side with u n i t s m/sec (Reed, 1965).

Since the phase of 26-month o s c i l l a t i o n and the e f f e c t of

temperature var ia t ion of cosmic-ray in tens i ty a r e d i f fe ren t with

height, t h i s kind of information i s most sui table t o see the

corresponding var ia t ions i n cosmic-ray i n t e n s i t i e s a t t he ground.

By using the above-mentioned formula, we can see the amplitude

and phase of 26-month cosmic-ray var ia t ion f o r the corresponding

periods of years. The calculations a r e nade f o r three d i f fe ren t

cut-off energies, Eo = 0.3, 10 and 40 Gev, using y ( E o , x ) ' s , as shown

i n Fig. 3 (Maeda and Suda, 1965).

It i s found from previous calzidations t h a t the phase r e l a t i o n

between 26-month osc i l la t ion i n the upper atmospheric temperature

and t h a t of cosmic-ray in tens i ty a t the ground i s not simple, but

ra ther reversed a t low energies (Eo < 3.5 Gev) and a t high energies

(Eo >> 1 Gev). This r e s u l t s from the d i f fe ren t temperature e f f ec t s

a t low energies (negative) and a t high energies ( p o s i t i v e ) . The

former corresponds t o the usual hard component data such as those

observed by a n ion-chamber or by the so-called cubical meson telescope,

- 7 -

while the l a t t e r corresponds t o the underground cosmic-ray in t ens i t i e s .

These are shown by two dash-dot l ines i n the bottom of Fig. 3 f o r

Eo = 0.3 Gev (heavy l i n e ) and 10 Gev ( t h i n l i n e ) , respectively. It

should be noted t h a t although the posi t ive temperature e f f ec t

increases with increasing cut-off energy, there i s an upper l i m i t

(Maeda, 1960) and tha t because of i t s energy spectrum, cosmic-ray

in t ens i ty decreases rapidly with increasing cut-off energy, i . e . ,

with depth underground.*

A t any ra te , it is concluded t h a t i f continuous measurements

of cosmic-ray in tens i ty had been made a t the geographic equator f o r

more than one decade, the 26-month var ia t ion with amplitude of the

order of 0.0% or the maximum deviation of the order of 0.1% can

be detected even by ion-chamber data. I f the underground cosmic-

ray measurements had been made continuously fo r more than several

years near the geographic equator, the 26-month var ia t ion with

amplitude of more than 0.2% (which i s the order of magnitude observed

i n the diurnal var ia t ions of cosmic-ray in tens i ty) can a l so be found

i n these data with an anti-phase t o those of low energies (Maeda

and Sub , 1965).

* For example, r e l a t ive in tens i t ies with cut-off energies Eo = 0.3, 10 and 40 Gev are roughly 1 : 0.l : 0..005, respectively.

- 8 -

Quasi-Biennial Variations of Cosmic-Ray Intensities

By means of the power spectrum analysis applied for the ion-

chamber data from Huancayo, Peru (12' s, geographic) and from Cheltenham, Maryland (39' N, geographic) for the period of more than

20 years since 1937, the 26-month variations of cosmic-ray intensities

have been hardly shown, if only geomagnetically quiet days (5-Q

days in each month) are used (Maeda and Suda, 1965).

is somewhat elaborated by choosing 5-H days in each month as

shown in Fig. 4, where 5-H days means the five highest cosmic-ray

intensity (counting) days in each month.

rays are modulated by the change in solar emissions, their intensities

in general decrease. In other words, the highest counting days

correspond to the period when the effects of solar disturbances,

such as the Forbush effect, are eliminated, or at least minimum.

A similar analysis is also applied for the identical ion-chamber data

from Godhavn, Greenland (69'23' N geographic) for the period

extending from January 1947 to July 1959.

Fig. 5, where the scale of the ordinate is taken arbitrarily, but

is identical for all three curves. 5-Q and 5-H mean five quiet days

and five highest counting days in each month, respectively. Comparing

Figs. 4 and 3 , one can see that the biennial variation of cosmic-ray

intensity at high latitude is very small as compared with those

near the equator.

This result

When the galactic cosmic

The results are shown in

- 10 -

and zonal wind analysis (shown in the upper portion of Fig. 3) , are

shown by a full line in the bottom of Fig. 3, where a heavy dashed

line and dots represent the first and the second harmonics of this

curve.

Discussion

The results shown in the previars section, particularly those

shown in Figures 3, 4, 5 and 6, indicate consistently that the

26-month variations in cosmic ray intensities are predominantly

of atmospheric origin. In other words, 26-month periodicity is

clearer in the geomagnetically quiet period than in geomagnetically

disturbed days.

the amplitude of the 26-month variation in the stratospheric

temperature field is largest at the geographic equator above the

100 mb level, which is of the order of 2' Cy and decreases with

latitude, but increases again slightly beyond 20 degrees of latitude,

indicating a minimum around 17 degrees in each hemisphere. It is

also indicated that the phase of 26-month oscillation is reversed

between these two regions, i.e., tropics and subtropics.

According to the latest investigation (Reed, 1963),

From the viewpoint of this present status of 26-month oscillations,

the location of Huancayo is rather close to the region of minimum

- 9 -

A s it w a s suggested i n the previous paper (Maeda and Suda, 19651,

t h e cosmic-ray data from Lae, New Guinea (6'44' S ) i s most promising

t o see the 26-month var ia t ion, because t h i s i s the data from the

s t a t i o n s c losest t o t h e geographic equator, where the var ia t ion i s

known t o be maximum.

The analysis is made as fo l lows: (i) a l l avai lable data, being

corrected by a coef f ic ien t -0.14%/mb obtained by the standard

s t a t i s t i c a l method, a r e folded by d i f f e ren t lengths of month

extending from 20 t o 30 months.

then divided by the number of folding, where the t o t a l avai lable

data consist of two periods, one from Ju ly 1957 t o October 1960,

and the other from September 1962 t o December 1964.

shown in Fig. 6, where 5-Q and 3 - H correspond t o t he data chosen

from f ive quiet (geomagnetically) days and f i v e high counting days

i n each month, respect ively. The bottom l i n e s stand fo r normalization

i n t o the same scale, taking the average value as 10%.

curves, one can see t h a t the period of b iennia l var ia t ion appearing

i n the cosmic-ray data from Lae i s somewhat longer than 26 months,

but l e s s than 30 months, f o r 1960-1964.

standard deviations (dispersions of each point i n v e r t i c a l s ca l e )

a re of the order of 0.1%.

(ii) The folded sum of the data i s

The result i s

From these

It should be noted t h a t t he

Finally, the r e s u l t s of 26-month folding of the 5 - H cosmic-ray

data from Huancayo, corresponding t o Reed' s s t ra tospher ic temperature

- 11 -

temperature var ia t ion, bu t within the region of t rop ic osc i l l a t ion

(not i n the subtropic) . I n t h i s respect, cosmic-ray data from

Makerere i n Kampala, E a s t Africa (0.33' N geographic, near sea l e v e l )

are more useful f o r the present analysis, though the da ta from Lae

have shown already a s igni f icant quasi-biennial var ia t ion as shown

i n Fig. 6. The difference between the theo re t i ca l ly estimated

cosmic-ray in t ens i ty var ia t ion and those obtained from data, as

shown a t the bottom of Fig. 3 , i s possibly due t o t h e following two

reasons:

Reed's and t h e present analysis, cosmic ray data a r e taken far from

Reed's w y s i s . (ii) Huancayo i s rather close t o the subtropic

boundary, while t heo re t i ca l estimations a re made f o r near-equator.

I n t h i s respect, fur ther analysis o f cosmic-ray data near t he equator

i s desirable .

(i) Though t he period of analysis i s the same f o r both

A s indicated by recent aerological observations, the quasi-

b i enn ia l var ia t ions a re pers i s ten t even i n the high la t i tudes ,

espec ia l ly i n the southern hemisphere, including the Antarctic (Funk

and Garnham, 1962; Angell and Korshover, 1964; Sparrow and Unthank,

1964 ; Reed, 1963). Since high energy cosmic-ray data, pa r t i cu la r ly

those measured underground, should be avai lable a t several places

i n t h e world, quasi-biennial var ia t ions i n cosmic-ray phenomena

s t i l l seem worthy of investigation.

- 12 -

Finally, it s h m l d be e~~phaoizzd. that si:izz the G S G ~ C ~ G ~f

cosmic-ray variations are terrestrial as well as extraterrestrial,

they are separable, as shown in Figures 5 and 6.

the present analysis show, however, the quasi-biennial variations

in cosmic-ray data are predominantly of terrestrial origin. This

conclusion seems to be consistent with the results of spectrum

analysis of the quiet day geomagnetic variations at Huancayo (12' S

geographic), Alibag (19' N geographic) and Apia (14' S geographic)

given by Stacey and Wescott (1962).

however, to check the consistency with other phenomena which have

been discussed recently by many authors (Shapiro and Ward, 1962;

Hope, 1963; Wescott, 1964; Newell, 1964 a.b; Linden, 1964; Reed,

1965, etc.).

Investigations of

Further analysis seems necessary,

._

- 13 -

Acknowledgement

The cosmic-ray data used in the present analysis are obtained

through the courtesy of Dr. S. E. Forbush at the Carnegie

Institution of Washington, D. C., to whom we are very gratefUl. We

wish also to express our appreciation to Prof. R. T. Reed at the

University of Washington, who has kindly advised us to use a part

of his latest results as well as his early findings.

- 14 -

Figure Captions

Figure 1

Figure 2

Figure 3

Coefficients of the partial temperature e f f e c t

( i n Lg/g cm-' O C ) for cosmic-ray in t ens i ty a t sea

l e v e l with cut-off energies, Eo = 0.3, 10 and 40

Gev, p lo t ted against atmospheric depth x ( i n

g an-*).

pos i t ive (production) e f f ec t , cr(E0,x) and negative

(decay) effect , -fl(Eo,x), respect ively.

Nl l ines and dashed l i n e s stand f o r

Composite coeff ic ient of t he partial temperature

e f fec ts i n l i n e a r scale, y(E0,x) i n '%/g 'C,

derived from Fig. 1.

The 26-month var ia t ions of t rop ica l s t ra tospher ic

temperatures (AT)* a t Canton Island (3' S) and

zonal wind at Balboa (8' N) given by Reed (1965)

fo r the period from 191 t o 1961. The full l i n e

~~ ~~

* The temperature difference between Canton Island and f i v e subtropical Since the var ia t ion a t the la t te r s t a t i o n s (average l a t i t u d e 27' N ) .

i s very small as compared t o the one at the equator, AT can be regarded as the va r i a t ion a t Canton Island.

- 13 -

Figure 4

Figure 5

in the lower portion is a similar expression of

the ion-chamber data from Huancayo (12' S) based

on 5-Q days in each month. Dashed line and dots

are the first and the second harmonics of the

full-line.

Dashed lines are theoretical estimations correspond-

ing to AT-curves in the upper portion of the

figure computed by coefficients Shawn in Fig. 2,

where heavy dash-dot and thin dash-dot lines

correspond to the cut-off energies, Eo = 0.3 Gev

and 10 Gev, respectively.

Power spectrum (Periodogram) of Huancayo (12' S )

ion-chamber data based on 5-H days in each month

for the period from 1937 to 1961.

scale is arbitrary and T in the horizontal scale

is in months.

The vertical

Power spectrum of Godhavn (69O23' N) ion-chamber

data for the period from 1947 to 1939.

3-H stand for the data of five quiet days and

five high counting days in each month.

5-Q and

Figure 6

- 16 -

The folded-average curves of cosmic-ray muon data

from Lae (6'44' S), where horizontal scale

indicates the length of folding and v e r t i c a l scale

is r e l a t ive cosmic-ray in t ens i ty corrected fo r

barametric e f fec t . The meaning of 5-Q and 5-H

m e the same as Fig. 5 . The bottom curves

correspond t o the r e l a t ive var ia t ion with a mean

value normalized t o lo@.

\

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41; Wada, M.> a.nd s. Kum, 4. statistical i2vestigatior- f o r the atmospheric temperature effect on Cosmic-ray intensity, J. Sci. Res. Inst., (Tokyo), 50, 1 - 9, 1956.

42. Trefall, H., On the positive temperature effect in the Cosmic radiation and the Mu - e decay, Proc. Phys. SOC. 68A, 893 - 904, 1955 '

43. Danvillier, A * , Les Rayons Cosmignes, Dunod, Paris, 1954.

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