Page created by Rick Mcgee
                                                                    BY ScoTr E. FORBUSH
                                                                              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
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VOL. 43, 1957              GEOPHYSICS: S. E. FORBUSH                              29

                                           north of geomagnetic latitude 500, are ineffective in producing penetrating u-
                                              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,
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                                            on February 3 a. sudden magnetic commencement, although this was not im-
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                                            |H(TrENHAM, UNIrE0 S A MS

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                                                            V    PERU

                                                                           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.
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                                                                                         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.
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                                             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
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JAN        FED MAR APR MAY JUN JUL AUB SEP OCT NOV                                                   OEe

                                              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

                                                    4f 1
FIG. 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   ,--~--~--,
                                           Z 0.2 a 30
                                                      IISUNSPO I_£:S MEAN FOR C-R |





                                                            /   2




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                                                                                                   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


                                                                                                    (A) /2











                                                                                                                      19 "







                                                                                                                       SCALE FORP TAME OF MAX/MUM IN DAYS





                                                                                                                                                                       PROC. N. A. S.





                                                         17   /4 13
                                                                            SCALES FOR AMPLITUDES0                                                                                   0
                                            CoSMIC-in vR INTENsITY IN PER cENT OFb rotAL ITvENtYr fv                                                                                       i
                                                      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-
                                                 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-
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                                           netic-storm effects, the (magnetically) disturbed minus quiet-day differences, and
VOL. 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

                                               20                                                        /2    20
                                            /9 / 7                 3    /               /   7/   /5 /4    /3   /93   /     7/   /   4   /
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                                                                                                 aveag of  22-a waenmgetchrzna
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                                           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
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                                           30 between 1948 and 1951, showing that at the latter time additional low-rigidity
38                            GEOPHYSICS: S. E. FORBUSH                            PROC. N. A. S.








                                                             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.
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                                           VOL.     43, 1957                   GEOPHYSICS: S. E. FORBUSH                                         39

                                                   40   -

                                                        ' ____          GODHAYN

                                                                                                           --. /40

                                           W4.                                             40          -   -                               20

                                                                      CHL CtENHMAAV

                                                                                                 IT                                         DO
                                                                                            20                                             500~

                                                                                            .0                                         20
                                                                    | CHlSTC/HURCH
                                                            -                               0                  .0

                                                        /a               20           22                /O          20            22
                                                                NOVEMBER /949                                NOVEMBER /949
                                                                  FIG. 12.-Increase of cosmic-ray intensity, November 19, 1949.
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40                                   GEOPHYSICS: S. E. FORBUSH                                       PROc. N. A. S.


                                                  80                                I

                                                   _f   IM
                                                                 60                         I                                                                    I
                                                                    ~           ~            II
                                                                                                I                       o    CHELTENHAM
                                                                                 *01                                    *    GODHAVN
                                                   50                                                                   x    HUANCAYO
                                           w                                                        I                   A    MEXICO
                                                                                                    I               °-o-0    CHRISTCHURCH
                                                  4 0a
                                           La                                                           I

                                           0 ..                                                         I
                                           w                                                                I

                                           ° 30

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                                                                    1 X --&r
                                                                                -0-~0                                                   -   -

                                                             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.
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Vol. 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.
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