SOLAR INFLUENCES ON COSMIC RAYS - PNAS
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
SOLAR INFLUENCES ON COSMIC RAYS
BY ScoTr E. FORBUSH
CARNEGIE INSTITUTION OF WASHINGTON, DEPARTMENT OF TERRESTRIAL MAGNETISM,
WASHINGTON D.C.
Introduction.-Probably all the established variations with time of cosmic-ray
intensity are directly or indirectly due to solar influences. Ionization chambers,
because they are relatively simple and reliable, have proved valuable for con-
tinuous registration of cosmic-ray intensity over long periods of time. The longest
series of continuous observations with several identical instruments of this type is
that which has been obtained with the Carnegie Institution of Washington model
C Compton-Bennett meters.1 With these meters, and with the unselfish co-opera-
tion of several organizations, continuous data have been obtained for nearly two dec-
ades at each of the following places:2 Godhavn (Greenland), Cheltenham (Mary-
land), Huancayo (Peru), and Christchurch (New Zealand). Data for shorter
periods have been obtained from Teoloyucan and Ciudad Universitaria, Mexico,
D.F., and from Climax, Colorado. These ionization chambers are shielded by the
equivalent of 12 cm. Pb to screen out local gamma rays, the intensity of which may
vary with time.
Origin of Ai-Meson Component Measured in Ionization Chambers and Its Seasonal
Variation.-In ionization chambers the ionization is normally produced mainly by
,u-mesons, which have a mass of about 210 electron masses, unit electronic charge
(4), and a lifetime (at rest) of about 2 X 10-6 seconds. These very penetrating
,u-mesons arise from the decay of short-lived charged 7r-mesons (lifetime at rest
about 10-8 seconds), which in turn result from the interaction of high-energy pro-
tons of the primary cosmic-ray beam with atmospheric nuclei. There are other
types of reaction with different decay products which need not concern us here.
The maximum u-meson intensity occurs in the region where the pressure is roughly
100 mb., which normally is near an altitude of 16 km. or so. An increase in the
height of this pressure level lengthens the path for A-mesons, so that more of them
decay into not very penetrating electrons (and nutrinos) before reaching the instru-
ments at the ground. Thus the seasonal variation in height of the 100-mb. level re-
sults in a seasonal variation in cosmic-ray intensity as measured by ionization cham-
bers. The passage of meteorological "fronts" can also alter the height of the 100-
mb. level and consequently the apparent cosmic-ray intensity as recorded in ioni-
zation chambers (or with Geiger counters). The seasonal waves and meteorolog-
ical effects proved less interesting than troublesome, for they obscured for a while
the more interesting remaining changes in cosmic-ray intensity which are world-
wide3 and result directly or indirectly from solar influences. Since ionization
chambers are mainly sensitive to the ju-meson component, it is essential to remark
that normally most of the ionization in such detectors arises from primaries of rela-
tively high energy. For example, near sea level, the intensity in ionization cham-
bers near the geomagnetic pole is ordinarily only about 10 per cent greater than at
the equator. This means that about 90 per cent of the ionization results from
primaries which have enough momentum to arrive at the earth's geomagnetic
equator (about 15 Bev/c for protons). The primaries of lower momentum which
are not prevented by the earth's magnetic field from reaching the earth, say at or
Downloaded by guest on October 27, 2021
28VOL. 43, 1957 GEOPHYSICS: S. E. FORBUSH 29
north of geomagnetic latitude 500, are ineffective in producing penetrating u-
mesons.
Origin of the Nucleonic Component and Solar-Flare Effects.-The primary cosmic-
ray particles, protons or heavier nuclei, also generate a cascade of nucleons (protons
and neutrons) in the atmosphere; even relatively low-energy protons, with mo-
menta only great enough (a fewBev/c) to be allowed by the earth's field to reach the
atmosphere at geomagnetic latitude 50°, are very effective generators of the nu-
cleonic component. Most of the rare large increases in cosmic-ray intensity which
have been accompanied by solar flares or chromospheric eruptions are due to rela-
tively low-energy charged particles coming from the sun.4 The momenta of most
of these particles are too low, as shown by their large latitude effect, to result in
production of gt-mesons (through the ir -, jA decay). The large augmentation of
ionization observed in ionization chambers during these solar-flare effects results
from the nucleonic component, as was shown4 by the fact that the magnitude of the
augmentation increased with altitude at the same rate as that known for the nu-
cleonic component. During the solar flare of November 19, 1949, the total ioniza-
tion in a Compton-Bennett meter at Climax, Colorado, increased about 200 per
cent. At Climax, under 12 cm. Pb, probably no more than 10 per cent of the total
ionization is normally due to local radiation originating from the nucleonic com-
ponent. Assuming that this normal radiation was produced by particles in the
same energy band as that responsible for the increase of 200 per cent (at Climax)
in ionization which accompanied the solar flare of November 19, 1949, then it was
predicted4 that the rate of arrival of primary particles in that energy band must
have increased to at least 20 times the normal value. During the solar flare' of
February 23, 1956, increases of 20-fold and greater have been reported from detec-
tors sensitive only to the nucleonic component. Thus neutron detectors have
many advantages over the ionization chamber for measuring effects which arise
from variations in the intensity of the low-energy part of the cosmic-ray spectrum.
Magnetic-Storm Effects.-Figure 1 shows a decrease of daily means of cosmic-ray
intensity similar at three stations during the magnetic storm of April 25-30, 1937.
The bottom curve indicates the decrease and subsequent gradual recovery of daily
means of horizontal magnetic intensity, H, at Huancayo. It will be noted that
the rate of recovery of H toward normal is similar to that for cosmic-ray intensity.
Figure 2 shows three examples of decreases in daily means of cosmic-ray intensity
associated with decreases in horizontal magnetic intensity at Huancayo during the
period January 11-31, 1938, which contains what may for convenience be called
three separate magnetic storms. While the ratio of changes in cosmic-ray intensity
to those in horizontal magnetic intensity is relatively constant throughout any one
of these storms, it differs among the three storms. Not all storms are accompanied
by decreases in cosmic-ray intensity. The decrease in horizontal magnetic inten-
sity at Huancayo for the magnetic storm beginning August 21, 1937, was ac-
companied by no detectable decrease in cosmic-ray intensity in the Compton-Ben-
nett ionization chambers. Figure 3 depicts the daily means of cosmic-ray intensity
and of horizontal magnetic intensity, H, at Huancayo for 1946. The decrease of
several per cent in cosmic-ray intensity between February 3 and 6 occurred before
the major depression in H which took place on February 7. There was, however,
Downloaded by guest on October 27, 2021
on February 3 a. sudden magnetic commencement, although this was not im-20 J0 /0 20 Jo
-.0
1: V v IA MAY/7420
|H(TrENHAM, UNIrE0 S A MS
___ /(/ANCA~
EUAN,
REXICO
007{~~~~~~~~~~0
V PERU
1\§
V I , I I I
FIG. 1.-Daily means of cosmic-ray intensity and horizontal magnetic intensity, showing effect
of magnetic storm of April 25-30, 1937, on cosmic-ray intensity.
Downloaded by guest on October 27, 2021O-CHEL TENHAM, UNITED STA TES
X=HUANCAYO, PERU
O=OSTON, UN/TED STATES
FIG. 2.-Magnetic-storm effects on daily mean cosmic-ray intensity at Boston, United States,
Cheltenham, United States, and Huancayo, Peru, and on daily mean magnetic horizontal intensity
at Huancayo, Peru.
Downloaded by guest on October 27, 2021Downloaded by guest on October 27, 2021
FIG. 3.-Daily means, cosmic-ray intensity, 1946. g = Godhavn, c = Cheltenham, cc =
Christchurch, h = Huancayo; H = horizontal magnetic intensity at Huancayo.VOL. 43, 1957 GEOPHYSICS: S. E. FORBUSH 33
mediately followed by a magnetic storm. This particular decrease in cosmic-ray
intensity, which began February 3, 1946, is unusual in that the cosmic-ray intensity
remained low for much of the remainder of the year. Incidentally, the large values
for July 25 at Godhavn (g) and at Cheltenham (c) result from the large increase as-
sociated with a solar flare on that date. No flare effect was registered at Huan-
cayo, since the charged particles from the sun had insufficient momenta for the
earth's magnetic field to allow their arrival at the equator. At Christchurch (cc)
the flare occurred during a period of a few days when the equipment was not op-
erating. In Figure 3 it is evident that the major changes in daily means of cosmic-
ray intensity are world-wide.3
Variability of Daily Means.-Figure 3 shows the variations in daily mean cosmic-
ray intensity for several stations for a year near maximum sunspot activity. The
curves for 1944 in Figure 4 indicate the variations of daily means for 1944 at sun-
spot minimum. The curve h 1944 for daily means of cosmic-ray intensity at Huan-
cayo exhibits much less variability than for 1946, and the same is true for the daily
means of magnetic horizontal intensity (H 1944) at Huancayo. Each of the curves
(g 1944, c 1944, and cc 1944) in Figure 8 exhibits greater variability, especially dur-
ing winter, than does h 1944. Moreover, most of this variability is uncorrelated
between different pairs of stations. Except at Huancayo, this variability is doubt-
less due to M-meson decay effects resulting from changes in the vertical distribution
of air mass with the passage of fronts. At Huancayo the variability of daily means
in some months of 1944 is not greatly in excess of the inherent "noise level" of the
instrument. Thus Huancayo is essentially free of the disturbing effects arising
from the passage of meteorological "fronts" which do not occur there. The stand-
ard deviation of daily means from monthly means (pooled for each year) of cosmic-
ray intensity at Huancayo is shown in Figure 5 for the period 1937-1955. This
variability is least in the years of sunspot minima, 1944 and 1954, and increases
near years of sunspot maxima. The variability is somewhat less when the five
magnetically disturbed days of each month are excluded. This shows that in
nearly every year significant cosmic-ray changes occur on at least some of the
magnetically disturbed days. In some years much of the variability arises from
the 27-day quasi-periodic variation.2
Disturbed minus Quiet-Day Difference.-For each month geomagneticians deter-
mine the five days when the earth's magnetic field is quiestest and the five days
when it is most disturbed. In Figure 6 the average difference in cosmic-ray inten-
sity for the disturbed less that for the quiet days for each month at Huancayo is
plotted against the corresponding difference for Cheltenham. These differences are
preponderantly negative and correlated between the two stations, showing that the
cosmic-ray intensity definitely tends to be less on magnetically disturbed than on
magnetically quiet days. The annual means for magnetically disturbed (60 per
year) less quiet days (60 per year) are shown in Figure 7 for cosmic-ray intensity at
each of three stations (and their average) and for magnetic horizontal intensity.
These differences are all negative and vary (algebraically) roughly with the sun-
spot cycle.
Twenty-seven-Day Variation.-Figure 8 (A) shows the variability of the 27-day
waves in magnetic activity (American magnetic character figure) and in cosmic-ray
intensity at Huancayo (B), for a sample of 34 solar rotations. The maxima of the
Downloaded by guest on October 27, 2021JAN FED MAR APR MAY JUN JUL AUB SEP OCT NOV OEe
'I
6, r4-l"44d ~~
1,94 3
"
2950 .A -A
-_ __ _
-AA.Al - .. .a [A A I -- 1
IC St'~~c 944 } t
t ^-~~~~~c 1944 --AA At H
90
4f 1FIG. 5.-Annual means: sunspot numbers and variability, cosmic-ray intensity at Huancayo.
I -E IN r
-/ Iiqo
I I I
-/60 - 20 -80 -00 X -40 -20
FIG. 6.-Average difference for cosmic-ray intensity (AC) and for horizontal magnetic field
(AH) for five disturbed days less that for five quiet days in each month, April, 1937-December,
1946, at Huancayo.
O 0 20 ,--~--~--,
IiO
Z-
Z 0.2 a 30
IISUNSPO I_£:S MEAN FOR C-R |
NUMERS/n36
N
26
-24
,22
23
9
27
M11
/ 2
/7
/.
oo
C
0.0
A
t
(C) /211-
3 .4
f
l
I.6
2s
3.2
20
EC
.6
0.2 0
[
33
E26
A26O/CS
06
: 1
5
SCALE FOR TAME OF MAX/MUM
*
/8
.
I/N DAYS
*
i*
/6 /5
1
I
* "-
H~.
/4
|~~~~~~~2~
.0II
.-'
*
GEOPHYSICS: S. E. FORBUSH
27-day waves in cosmic-ray intensity have a statistically significant2 tendency to oc-
cur near the times of the minima of the 27-day waves in character figure. This is
shown in Figure 8 by the harmonic dials C and D obtained respectively by rotating
vectors in A to the vertical and by rotating vectors (derived from the correspond-
ing 27-day interval) in B through the same angle. If the phases of the 34 vectors in
D were random, then the probability that the magnitude of the average vector
\
.S
/
(A) /2
0
/3
61
8-24
90
2
:s*2/
111
\
26
t23
20
~/
/9
\
27
/
/8
27)
'125~
`24
_23~3:*
Q-
19 "
8
I
*
/7
I
2
~
I
*
3
I
/6
I
4
I
5
I
/5
I
4
6
SCALE FORP TAME OF MAX/MUM IN DAYS
*9
4o-.
.
/4
\
27
,
2~
.
/3
\.
PROC. N. A. S.
/2
N
(a)
7
/0_
/
4
6
vs
216715
Jaiv
17 /4 13
SCALES FOR AMPLITUDES0 0
CoSMIC-in vR INTENsITY IN PER cENT OFb rotAL ITvENtYr fv i
0u40ht
00 1.6 2.0
AM4ERICAN CftARACTER-FIGURE 0
0.0 0.2 0.4 06 4*10 /7 /6 IS /I4
FIG. 8.-Harmonic dials for departures from average of 27-day waves in American character
figure (A)beandabu
in cosmic-ray intensity at Huancayo, Peruiiarpoeueso
(B), computed from 34 rotations (inter-
valswould~~
of 27 days)~~~ 2j X June
beginning 13,18Terslso
1936; harmonic dials nFgr
(C) and (D) obtained respectively by
rotating vectors in (A) to vertical and by rotating vectors for corresponding intervals in (B)
through the same angle.
would equal or exceed the magnitude of the average vector actually obtained in D
would be about 2 X 1O-. The results of a similar procedure show in Figure 9
that the maxima of the 27-day waves in cosmic-ray intensity have a statistically
significant tendency to occur near the times of the maxima of the 27-day waves in
magnetic horizontal, H, intensity at Huancayo. Since low values of H occur when
magnetic activity is high, the results of Figures 8 and 9 are consistent. The mag-
netic-storm effects, the (magnetically) disturbed minus quiet-day differences, and
Downloaded by guest on October 27, 2021VOL. 43, 1957 GEOPHYSICS: S. E. FORBUSH 37
the 27-day variations in cosmic-ray intensity and in the horizontal component, H,
of the earth's magnetic field at the equator all indicate a significant tendency for de-
creased values of cosmic-ray intensity to occur with decreased values of H. The
latter decreases are known to arise from magnetic fields with sources outside the
earth. Thus the mechanism for most of these changes in cosmic-ray intensity is
closely connected with the mechanism responsible for the magnetic changes.
Several attempts have been made to calculate the expected cosmic-ray changes in
magnetic storms, assuming that the external magnetic-storm field arises from a
hypothetical ring current concentric with the earth and in the plane of the geo-
magnetic equator (this would explain one of the main features of magnetic storms).
None of the results obtained has indicated that the decreases in cosmic-ray inten-
sity arise from magnetic effects of the ring current.
/ 2 3 4 I/ 2 3 2 3 4
SCALE FOR TIME OF 5 SCALE FOR TIME OF 5
MAX/MUM IN DAVS MAX/MUM IN DAKS
/9 / / 25 4 24 8
/25 4 8 23
25
20 /2 20
6234
/9 / 7 3 / / 7/ /5 /4 /3 /93 / 7/ / 4 /
23.9 amni ilo depature
FI.9.Hronicdilsfo deatue fro
fo averag
aveag of 22-a waenmgetchrzna
of2-a aenmantchrzna
intensity at Huancayo, Peru (A), computed from 34 rotations (intervals of 27 days) beginning
June 13, 1936; harmonic dials (B) and (C) obtained respectively by rotating vectors in (A) to
vertical and by rotating vectors for corresponding interval in (B) of Fig. 8 through the same angle.
Variation with Sunspot Cycle. Figure 10 showes the variation of annual means of
cosmic-ray intensity at four stations and a comparison with sunspot numbers of the
annual means averaged for all stations for the period 1937-1955. The latter curve
indicates that cosmic-ray intensity is higher near the sunspot minima. This 11-
year variation in cosmic-ray intensity at Huancayo, as shown in Figure 11, is about
the same for all days, for magnetically disturbed days (5 per month), and for mag-
netically quiet days (5 per month). This indicates that the 11-year variation is
not due directly to decreases in cosmic-ray intensity during magnetic storms, which
are more frequent near times of sunspot maxima. It has been suggested that con-
ducting solar streams (which give rise to magnetic storms) which carry "frozen-in"
magnetic fields away from the sun during sunspot maxima may pervade the solar
system to an extent which would reduce the flux of cosmic rays arriving at the earth
from outside the solar system. In connection with the 11-year variation, it should
be noted that Meyer and Simpson,6 using nucleonic detectors in jet aircraft at
30,000 feet altitude, found that the knee of their latitude curve moved northward
Downloaded by guest on October 27, 2021
30 between 1948 and 1951, showing that at the latter time additional low-rigidity38 GEOPHYSICS: S. E. FORBUSH PROC. N. A. S.
z
A-
hJ
v
wI
hi
0.
ooo
%
0
V)
I
Ix
z
nI
LI)
FIG. 10.-Annual means, cosmic-ray intensity, at four stations.
FIG. 1 1.-Annual means, cosmic-ray intensity, at Huancayo for all days, international magnetic
quiet days, and international disturbed days.
Downloaded by guest on October 27, 2021_~
VOL. 43, 1957 GEOPHYSICS: S. E. FORBUSH 39
40 -
.4
X
Q:
10
k
.Q461
94
IT
' ____ GODHAYN
--
--. /40
1~~~~~60
k
k-
.j14
Q:
W4. 40 - - 20
CHL CtENHMAAV
/00
X"Ik
ICLIMAX^
-4
IT DO
um4
SQ
20 500~
.0 20
| CHlSTC/HURCH
- 0 .0
I
/a 20 22 /O 20 22
NOVEMBER /949 NOVEMBER /949
FIG. 12.-Increase of cosmic-ray intensity, November 19, 1949.
Downloaded by guest on October 27, 202140 GEOPHYSICS: S. E. FORBUSH PROc. N. A. S.
III
80 I
I__
70
I
I
I
_f IM
60 I I
~ ~ II
w
I o CHELTENHAM
-J
*01 * GODHAVN
50 x HUANCAYO
w I A MEXICO
I °-o-0 CHRISTCHURCH
U-LI
4 0a
-V
La I
0 .. I
w I
° 30
co
20
20
,4. ..
10
0
-
I*
1 X --&r
'-a-I
-0-~0 - -
L
i
3 4 5 6 7 8 9
GMT HOURS
FIG. 13.-Cosmic-ray intensity following solar flare at 0330 g.m.t., Feb. 23, 1956.
Downloaded by guest on October 27, 2021Vol. 43, 1957 GENETICS: S. E. FORBUSH 41
particles were reaching this level in the atmosphere. In addition, they found that
the total cosmic-ray intensity was 13 per cent greater in 1951 than in 1948. Neher
and Stern,7 using high-altitude balloon-borne ionization chambers in the summers
of 1951 and 1954, found, at the latter time, new low-energy particles arriving at
geomagnetic latitudes north of 580. Assuming that the new particles were pro-
tons, they calculated that there was no magnetic cutoff of primary protons above
150 Mev in 1954, whereas the cutoff for protons in 1951 was estimated at 800 Mev.
Solar-Flare Effects.-During nearly two decades of continuous operation of
Compton-Bennett ionization chambers, five4 6 unusually large increases in cosmic-
ray intensity have been recorded. Each of these followed within an hour (in all
but one case, within a quarter-hour) the onset of a solar flare or radio fadeout. One
of these increases is shown in Figure 3, another in Figure 12, and one in Figure 13.
In four of the five increases no increase occurred at Huancayo (at the geomagnetic
equator). The extra large increase at Climax relative to that at Cheltenham in
Figure 12 was due mainly4 to the high altitude of Climax relative to Cheltenham
(3,428 meters). In the second section of this review ("Origin of the Nucleonic
Component and the Solar-Flare Effect") the arguments were presented for the con-
clusion that the solar-flare increases were due to the nucleonic component gen-
erated by a great increase in the flux of charged particles in the lower-energy part
of the cosmic-ray spectrum. Figure 13 shows the recent increase of February 23,
1956, as measured at several stations with Compton-Bennett ionization chambers.
This is the first occasion on which a solar-flare increase has been observed at the
equator and shows that charged particles with momenta of at least 15 Bev/c were
involved. Schluter8 and Firor9 have shown that if the particles responsible for
the solar-flare increases come from the sun, then the observed intensities should
be much greater in certain "impact zones" which depend on the geomagnetic lat-
itude and local geomagnetic time. For example, in Figure 13 the large difference
in the intensity at Cheltenham and Christchurch (both at about the same geo-
magnetic latitude, except for sign) is due to the location of these stations relative to
the impact zones.
It thus seems reasonably certain that the solar-flare increases in cosmic-ray in-
tensity are due to charged particles from the sun which are accelerated by some
mechanism closely associated with the flare. The remaining variations of inten-
sity are doubtless perturbations imposed by the magneto-hydrodynamical state of
the solar system upon a steady influx of cosmic-ray particles coming from outside
the solar system. The mechanism which effects these perturbations of intensity is
not clearly understood.
1 A. H. Compton, E. 0. Wollan, and R. D. Bennett, Rev. Sci. Indtr., 5, 415, 1934.
2 Scott E. Forbush, J. Geophys. Research, 59, 525, 1954.
3 Scott E. Forbush, Phys. Rev., 54, 975, 1938.
4Scott E. Forbush, M. Schein, and T. B. Stinchcomb, Phys. Rev., 79, 501, 1950.
5Scott E. Forbush, J. Geophys. Research, 61, 155, 1956.
6 Peter Meyer and J. A. Simpson, Phys. Rev., 99, 1517, 1955.
7H. V. Neher and E. A. Stern, Phys. Rev., 98, 845, 1955.
a A. Schiuter, Z. Naturforech., 6a, 613, 1951.
9 J. Firor, Phys. Rev., 94, 1017, 1954.
Downloaded by guest on October 27, 2021You can also read
