The delta 14C values in 42 rings of a white spruce grown in Mackenzie Delta was measured as a continuing effort of tracing the history of solar modulation of cosmic ray intensity. The delta 14C values in six rings were measured, in search of a 14C increase due to two large solar flares that occurred in 1942. The results are presented

Chen, T. M.

Dai, K. M.

Fan, C. Y.

Yun, S. X.

English

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371
SH 7.1-2
SOLAR MODULATION OF COSMIC RAY INTENSITY AND SOLAR FLARE
EVENTS INFERRED FROM l"C CONTENTS IN DATED TREE RINGS
C. Y. Fan
Department of Physics, University of Arizona, Tucson, AZ 85721, USA
T. M. Chen and S. X. Yun
Archaeological Section, History Department, Peking University
Beijing, China
K. M. Dai
Department of Physics, Nanjing University, Nanjing, China
We have measured the AI"C values in 42 rings (from 1824 to 1865
AD) of a white spruce grown in Mackenzie Delta (68°N, 130°W) as
our continuing effort of tracing the history of solar modulation of
cosmic ray intensity. We also measured the AI_C values in 6 rings
(from 1940 to 1945 AD), searching for a 1"C increase due to two
large solar flares that occurred in 1942 and were detected by
Forbush. The results will be presented and discussed.
1. Introduction. Carbon-14 nuclei in the atmosphere are produced by cosmic-
ray-generated neutrons in l_N(n,p)l_C reaction. After the production, these
nuclei react rapidly with ambient atmosphere to become 1"CO 2 molecules. The
concentration of the 1_C 2 molecules is mainly determined by the rate of
production and the rate of transfer, first to the troposphere from the stratosphere
where most of them are produced, and then from the troposphere to the deep-sea
sink reservoir. Since cosmic ray intensity is modulated by interplanetary
magnetic fields whose strength depends on the solar activity, it is expected that
the concentration is anti-correlated with sunspot numbers. While the long-term
change in AI_C in tree rings samples, which reflects the variation of the
atmospheric 1_C concentration, has been studied by many investigators and the
correlation established beyond any doubt (e.g., Stuiver and Quay, 1980, and
references therein), the question remains as to whether the amplitude of the
expected ll-year variation is large enough to be measurable.
There was some indication that Al_C values in grains and trees grown at high
lattitudes show 1&-year periodicity (Baxter and Farmer, 1973). To study the
" possible latitudinal effect, we searched and_ fortunately, obtained two sections of
dated white spruce grown near Campbell River in Mackenzie Delta, Canada (68°N,
130°W) from Dr. M. L. Parker of the University of British Columbia; one of the
o sections contains rings from 1510 to 1972 AO. Initial measurements of 27
samples, covering the period from 1866 to 1925 AD, indicated that the AI_C
values do exhibit a 10% o fluctuation, anticorrelated with sunspot numbers. The
results were reported at the 18th International Cosmic Ray Conference (Fan et
al., 1983a, 1983b). In this paper we shall report AI_C in the ring samples from
182/4 to 1865 AD. We also measured the AI_C in 1940 to 19b,5 rings, looking for
the signature of 1_C increase, due to two large solar flares that occurred in 1942
and were reported by Forbush (1946). The results will be discussed.
2. Experiment. The task of the measurements was divided between two
institutions; the 1824 to 1865 rings were measured at the Department of History
of Peking University, whereas the 1940 to 1945 rings were measured at the
https://ntrs.nasa.gov/search.jsp?R=19850026782 2020-03-20T16:50:32+00:00Z
372 SH 7.1-2
Department of Physics of Nanjing University. The equipment used was similar; a
liquid scintillator-photom ultiplier tube device, which were carefully
intercalibrated. Both systems used two photomuitipiier tubes in coincidence to
count 1_C decay electrons and a pulse height discrimination to reduce cross-talks
between the two tubes. There was, however, a slight difference between the two
systems. At Peking University, 5-10 cm of lead was used to reduce cosmic-ray-
induced background, whereas at Nanjing University, in addition to a lead shield, a
plastic scintillator-photomultiplier tube system was used as an anti-coincidence
shield. The result of the addition was a reduction of the baekground from 6 epm
to 2 cpm. An overall precision of both systems was about 4% o, including a pure
statistical uncertainty of -3'% o.
For each measurement, about l0 g of wood is needed. Because of the
narrowness of the rings, except the 1928 ring, we had to combine two, and
sometimes three, rings to obtain a sufficient specimen for one measurement.
After being treated with the routine HCP_-NaOH-HC)_ procedure to remove resin,
the samples were converted to CO 2 and then to benzene. For one measurement,
5 cc of synthesized benzene was used. The net count rate for a modern sample
was 39 cpm. For each sample, we measured 13C/lzC for fractionation correction.
3. Results on Modulation of Cosmic Ray Intensity. The experimental results are
expressed as A'_C values which are the relative deviations of the measured 1"C
activities from the standard oxilic aeid activity of the U.S. National Bureau of
Standards, corrected for ages and isotope fractionations. For the sample prior to
1885, the correction for the Suess effect was not needed.
The data points, including 11 previously published values, are plotted in the
bottom panel of Fig. 1, along with other published AI_C values for comparison. To
show the correlation with solar activity, we plotted in the upper panel the sunspot
numbers with ordinates inverted. It is seen that, except for the 1922 cycle, with
a delay of about five years, the &i_c values are anti-correlated with sunspot
numbers; the cross-correlation coefficient is ~0.6. At present, we are baffled by
the reverse correlation in the 1822 cycle. Also, we feel uneasy about the five-
year delay, which is longer than what we had expected., We do not rule out a
possible error in the tree ring count.
/4. Results on l_C Produced by Solar Protons. Damon et al (1973), in their study
of the magnitude of the 11-year radiocarbon cycle, found that the 1"C content in
the 19/43 ring of their Arizona tree sample (32°N, 112°W) is exceptionally high.
They attributed this high activity to the 1_C production by two large solar flares,
on February 28 and March 7 of 1942, detected by Forbush (1946). A similar
increase was also observed by Burchuladze et al. (1980). Since our tree section
was from Mackenzie Delta, only about 10° from the geomagnetic pole, we expect o
to observe a larger 1_C activity in the tree rings than in the samples of Damon et
al.
In Fig. 2 we plotted the six AI_c values, together with the measurements by
Damon et al. (1973) and that by Stuver and Quay (1980), in the period from 1940
to 1954 AD. The high AI_C values in the Mackenzie wood, as compared with that
in the Washington wood of Stuiver and Quay and that in the Arizona wood of
Damon et al., can be explained by the fact that inthe near arctic circle, the air
is less contaminated by fossil fuel burning. Disregarding the difference in the
373
SH 7.1-2
01' _ ' IA I I I _ I I I J
_ 100
t50 / I 1 l I I 1 1 I 1 l
+20 i i i , i i i i i
-to
<_ -20 JF Thiswork
• StuiverandQuay(1981)
• Tonetel. (1979}
-30 Suess(t965)
I L 1 I I I I I L
1800 1820 1840 1860 1880 1900
YearA.D.
Fig. 1. Upper panel, sunspot numbers; Lower panel, AI_C.
general AI_C level, the increase in AI_C in the 1943 ring of the Mackenzie wood
agrees remarkably well with that in the Arizona wood. The puzzling fact is that
the WashingLon Stale sample does hal show any sign of increase. [f one assumes
thaL the increase in AI_C in the Mackenzie wood was indeed due to the two flares
in 1942, then the magnitude of the flares can be estimated as follows:
0 i I I / I I I I t I i I t I I
+-k-1- .'""'°"-113- A Damonetal. (1975)
• StuiverandQuayf198_)-
.e
o Burchulodze101.(t980)
"-t--ZO • • . p,
.a -3C A
o o o o o " A o _x ,_ AZ_ o
-4C - A A a
-50 I I I I I I I I I I I I _ _ it940 t942 1944 B46 t948 1950 t952 4954
Ye0rA.D.
Fig.2. Ai_c
374 SH 7.1-2
Ligenfelter and Ramaty (1970) calculated the 14C production rates during
the 1953-1954 solar mnimum and the 1957-1958 solar maximum period as 5.0 and
° 3.5 1_C nuelei/cm 2 see, respectively, in the 700-90 ° geomagnetic latitude range.
A 42% increase in the production rate from solar activity maximum to solar
activity minimum period. The eorresponding periods in the 19th century are 1866
to 1867 and 1870 to 1871. In these two periods, we found that there is a 20% o
increase in the AI_C value. That is, a a0% increase in the production rate in the
high latitude region results in a 20% o increase in the AI_C value. The A14C
increase in the 1943 ring of the Mackenzie wood is about 20%o. Therefore, the
1_C increase due to the two flares is equivalent to a 40% increase in the 1_C
production, or, 1.5 1_C nuciei/cm _ see. According to Lingenfelter and Ramaty
(1970), the number of 1_C nuclei produced by the 1956 solar flare is equivalent to
the one-year production by cosmic ray particles. Therefore, the combined effect
of the two flares in 1942 is about one-half of that of the 1956 flare. We are
continuing our search for larger events.
5. Acknowledqment. Dr. M. L. Parker of the University of British Columbia
supplied us with the two sections of dendrochronolgocally calibrated white spruce
that made this study possible. This project is partially supported by the
subcontract R 277664 from the University of Maryland.
6. References
Baxter, M. S. and 3. O. Farmer, Earth Planet. Sci. Lett. 20, 295 (1973).
IBurchu]adze, A. A. et al., Nature 287, 320 (19J30).
Damon, P. E., A. Long and E. I. Waliick, Earth Planet. Sci. Lett. 20,
500 (1973).
Fan, C. Y., T. M. Chen, S. ×. Yuu and K. M. Dai, Radiocarbon 25, No. 2,
205 (1983a); 18th [CRC, conference paper, vol. 3, 173 (1983b).
Forbush, S. E., Phys. Rev. 70, 771 (1946).
Lingenfelter, R. E. and R. Ramaty, Radiocarbon Variations and Absolute
Chronology, L U. Olsson, ed. (John Wiley and Sons, 1970), p. 513.
Stuiver, M. and P. D. Quay, Science 207, 11 (1980).
4'
Tans, P. P., et al., Nature 280, (1979).


Solar modulation of cosmic ray intensity and solar flare events inferred from (14)C contents in dated tree rings

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