The long-term modulation of cosmic rays
0.400
0.500
0.380
0.400
E . 0.360
0.300
0 0 0.340
0.200 I
0.320
0.100
0.000
D 50 100 150 200 250 300
Frequency
0.300 0.200
0.250 0.300 0.350 0.400
Julian Dote (2446621.5+)
Figure 1. Power spectrum of the brightness of Hell X4,686 relative to X4,768 continuum for a 5-hour obsevation of 'y 2 Velorum.
Figure 2. Variation of the Hell X4,686 emission feature as compared to a sine curve with 1.26-hour period.
is a real variation in y 2 Velorum. This new discovery will aid in the study of the nature of the extended atmosphere of the Wolf-Rayet star system. This work was supported in part by National Science Foundation grants DPP 84-14128 and DPP 86-14550.
Chen, K-Y., J.P. Oliver, and F.B. Wood. 1986. Stellar photometry at the South Pole. Antarctic Journal of the U.S., 21(5), 281-282.
References
Sanyal, A., W. Weller, and S. Jeffers. 1974. Short-term spectral variability of -y 2 Velorum. Photometric observation. Astrophysical Journal, 187, L31-L33.
Chen, K-Y., J. Esper, J.D. McNeill, J.P. Oliver, G. Schneider, and F.B. Wood. 1986. An automated South Pole stellar telescope. In J. B. Hearnshaw and P.L. Cottrell (Eds.), Proceedings of the 118th Symposium of the International Astronomical Union.
The long-term modulation of cosmic rays JOHN W. BIEBER, JIASHENG CHEN, and MARTIN
A. POMERANTZ
Bartol Research Institute University of Delaware Newark, Delaware 19716
Various effects of solar activity on the cosmic-ray intensity have been observed for more than half a century (Pomerantz and Duggal 1974). One well-known consequence of this influence is the long-term variation. During the 11-year solar cycle, the peak of the galactic cosmic-ray intensity occurs within 1 year after sunspot minimum. Many investigations have been conducted over the years to determine the mechanisms whereby the Sun controls the cosmic-ray flux in the vicinity of the Earth. The role of fluctuations in the interplanetary magnetic field as an important factor in cosmic-ray modulation was recognized by Hedgecock (1975); however, attempts to establish correlations between cosmic-ray intensity and the interplanetary magnetic field magnetic-energy spectra were largely unsuccessful. In the present work, new analytical techniques have provided more exact determinations of the year-to-year variations 194
Chen, K-Y., J.P. Oliver, and F.B. Wood. 1987. Stellar photometry with the South Pole optical telescope. Antarctic Journal of the U.S., 22(5), 283-284.
Wood, F.B., and K-Y. Chen. 1985. A South Pole telescope. Antarctic Journal of the U.S., 20(5), 222-223.
in the properties of the interplanetary magnetic field. This study has revealed that the interplanetary magnetic field fluctuation parameters also display a solar-cycle variation, which is well correlated with the intensity of cosmic rays. The analysis used two data bases: the cosmic-ray intensity recorded by the Bartol neutron monitor at McMurdo Station, Antarctica, and the "Omnitape," which contains interplanetary magnetic field and plasma data from the National Space Science Data Center (Couzens and King 1986). The yearly mean neutron monitor count rate and the low-frequency magneticenergy spectral density were determined for each of 20 years, 1965-1984. The frequency range of magnetic-energy spectra between 5.8 x 10- 6 and 4.6 x 10 hertz corresponds to wavelengths that are responsible for the scattering of primary cosmic rays with neutron monitor energies (magnetic rigidity approximately 10 gigavolts). It is also consistent with reported values of the magnetic field correlation length (Matthaeus, Goldstein, and King 1986). The observed solar wind speed (obtained from the "Omnitape") was used to convert frequency spectra to wave-number spectra, and these spectra were then fitted to a power law in order to obtain the spectral amplitude. Figure 1 shows the amplitude of the low-frequency magnetic-energy spectra during the years 1965-1984. This quantity represents the magnetic-energy density for a wave number of 3 x 10-10permeter, which is representative of the fluctuations responsible for scattering 10-gigavolt cosmic rays. The yearly average cosmic-ray intensity, as represented by the McMurdo Station neutron ANTARCTIC JOURNAL
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65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 Year Figure 1. Magnetic-energy spectral amplitude in the years 19651984. Error bars are ± 1 o and are determined from the least-square fit. The arrows in the graphs indicate years when the Sun's north pole (N) and/or south pole (S) reversed polarity. (nT 2m denotes nanoteslas-squared meter.)
monitor count rate, is plotted in figure 2. The mean rigidity of response of this detector is about 10 gigavolts. Comparison of the two figures shows that lower cosmic-ray intensity corresponds to higher spectral amplitude with the exception of a few years. It is well known that certain solar phenomena, as well as the shape of the cosmic-ray intensity vs. time curves differ between successive solar cycles. The spectral amplitude during the first 10 years (1965-1974) and the second 10 years (1975-1984) also differs, supporting current ideas about the nature of the 20-year solar magnetic cycle. The regression plot in figure 3 reveals an anticorrelation between cosmic-ray intensity and low-frequency magnetic spectral amplitude (99 percent confidence level). It is noteworthy that the years with negative solar magnetic polarity (1965-1968 and 1981-1984) are grouped at a different level than years with positive solar polarity (1972-1979). A higher cosmicray intensity corresponds to positive polarity at fixed spectral amplitude. It remains to be determined whether the effects of helicity, drift, or other interplanetary magnetic field characteristics are involved in this phenomenon. This research was supported by National Science Foundation grants ATM 86-05124 and DPP 85-16501. References Couzens, D.A., and j.H. King. 1986. Interplanetary medium data book— Supplement 3, 1977-1985. Report NSSDC/WDC-A-R&S 86-04. Greenbelt, Maryland: National Aeronautics and Space Administration.
1988 REVIEW
65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84
Year Figure 2. McMurdo Station neutron monitor counting rate in the years 1965-1984. The arrows in the graphs indicate years when the Sun's north pole (N) and/or south pole (S) reversed polarity.
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Figure 3. Anticorrelation between neutron monitor counting rate and magnetic-energy spectral amplitude. Squares represent years of negative solar polarity, circles represent positive polarity, and triangles represent mixed polarity. (nT 2m denotes nanoteslassquared meter.)
Hedgecock, P.C. 1975. Measurements of the interplanetary magnetic
field in relation to the modulation of cosmic rays. Solar Physics, 42, 497-527. Matthaeus, W.H., M.L. Goldstein, and J.H. King. 1986. An interplanetary magnetic field ensemble at 1 AU. Journal of Geophysical Research, 91, 59-69. Pomerantz, M.A., and S.P. Duggal. 1974. The sun and cosmic rays. Reviews of Geophysics and Space Physics, 12, 343-361.
195
0.500
0.380
0.400
E . 0.360
0.300
0 0 0.340
0.200 I
0.320
0.100
0.000
D 50 100 150 200 250 300
Frequency
0.300 0.200
0.250 0.300 0.350 0.400
Julian Dote (2446621.5+)
Figure 1. Power spectrum of the brightness of Hell X4,686 relative to X4,768 continuum for a 5-hour obsevation of 'y 2 Velorum.
Figure 2. Variation of the Hell X4,686 emission feature as compared to a sine curve with 1.26-hour period.
is a real variation in y 2 Velorum. This new discovery will aid in the study of the nature of the extended atmosphere of the Wolf-Rayet star system. This work was supported in part by National Science Foundation grants DPP 84-14128 and DPP 86-14550.
Chen, K-Y., J.P. Oliver, and F.B. Wood. 1986. Stellar photometry at the South Pole. Antarctic Journal of the U.S., 21(5), 281-282.
References
Sanyal, A., W. Weller, and S. Jeffers. 1974. Short-term spectral variability of -y 2 Velorum. Photometric observation. Astrophysical Journal, 187, L31-L33.
Chen, K-Y., J. Esper, J.D. McNeill, J.P. Oliver, G. Schneider, and F.B. Wood. 1986. An automated South Pole stellar telescope. In J. B. Hearnshaw and P.L. Cottrell (Eds.), Proceedings of the 118th Symposium of the International Astronomical Union.
The long-term modulation of cosmic rays JOHN W. BIEBER, JIASHENG CHEN, and MARTIN
A. POMERANTZ
Bartol Research Institute University of Delaware Newark, Delaware 19716
Various effects of solar activity on the cosmic-ray intensity have been observed for more than half a century (Pomerantz and Duggal 1974). One well-known consequence of this influence is the long-term variation. During the 11-year solar cycle, the peak of the galactic cosmic-ray intensity occurs within 1 year after sunspot minimum. Many investigations have been conducted over the years to determine the mechanisms whereby the Sun controls the cosmic-ray flux in the vicinity of the Earth. The role of fluctuations in the interplanetary magnetic field as an important factor in cosmic-ray modulation was recognized by Hedgecock (1975); however, attempts to establish correlations between cosmic-ray intensity and the interplanetary magnetic field magnetic-energy spectra were largely unsuccessful. In the present work, new analytical techniques have provided more exact determinations of the year-to-year variations 194
Chen, K-Y., J.P. Oliver, and F.B. Wood. 1987. Stellar photometry with the South Pole optical telescope. Antarctic Journal of the U.S., 22(5), 283-284.
Wood, F.B., and K-Y. Chen. 1985. A South Pole telescope. Antarctic Journal of the U.S., 20(5), 222-223.
in the properties of the interplanetary magnetic field. This study has revealed that the interplanetary magnetic field fluctuation parameters also display a solar-cycle variation, which is well correlated with the intensity of cosmic rays. The analysis used two data bases: the cosmic-ray intensity recorded by the Bartol neutron monitor at McMurdo Station, Antarctica, and the "Omnitape," which contains interplanetary magnetic field and plasma data from the National Space Science Data Center (Couzens and King 1986). The yearly mean neutron monitor count rate and the low-frequency magneticenergy spectral density were determined for each of 20 years, 1965-1984. The frequency range of magnetic-energy spectra between 5.8 x 10- 6 and 4.6 x 10 hertz corresponds to wavelengths that are responsible for the scattering of primary cosmic rays with neutron monitor energies (magnetic rigidity approximately 10 gigavolts). It is also consistent with reported values of the magnetic field correlation length (Matthaeus, Goldstein, and King 1986). The observed solar wind speed (obtained from the "Omnitape") was used to convert frequency spectra to wave-number spectra, and these spectra were then fitted to a power law in order to obtain the spectral amplitude. Figure 1 shows the amplitude of the low-frequency magnetic-energy spectra during the years 1965-1984. This quantity represents the magnetic-energy density for a wave number of 3 x 10-10permeter, which is representative of the fluctuations responsible for scattering 10-gigavolt cosmic rays. The yearly average cosmic-ray intensity, as represented by the McMurdo Station neutron ANTARCTIC JOURNAL
E a,
D
, 10000
17 N,S
0 C.,
13
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65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 Year Figure 1. Magnetic-energy spectral amplitude in the years 19651984. Error bars are ± 1 o and are determined from the least-square fit. The arrows in the graphs indicate years when the Sun's north pole (N) and/or south pole (S) reversed polarity. (nT 2m denotes nanoteslas-squared meter.)
monitor count rate, is plotted in figure 2. The mean rigidity of response of this detector is about 10 gigavolts. Comparison of the two figures shows that lower cosmic-ray intensity corresponds to higher spectral amplitude with the exception of a few years. It is well known that certain solar phenomena, as well as the shape of the cosmic-ray intensity vs. time curves differ between successive solar cycles. The spectral amplitude during the first 10 years (1965-1974) and the second 10 years (1975-1984) also differs, supporting current ideas about the nature of the 20-year solar magnetic cycle. The regression plot in figure 3 reveals an anticorrelation between cosmic-ray intensity and low-frequency magnetic spectral amplitude (99 percent confidence level). It is noteworthy that the years with negative solar magnetic polarity (1965-1968 and 1981-1984) are grouped at a different level than years with positive solar polarity (1972-1979). A higher cosmicray intensity corresponds to positive polarity at fixed spectral amplitude. It remains to be determined whether the effects of helicity, drift, or other interplanetary magnetic field characteristics are involved in this phenomenon. This research was supported by National Science Foundation grants ATM 86-05124 and DPP 85-16501. References Couzens, D.A., and j.H. King. 1986. Interplanetary medium data book— Supplement 3, 1977-1985. Report NSSDC/WDC-A-R&S 86-04. Greenbelt, Maryland: National Aeronautics and Space Administration.
1988 REVIEW
65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84
Year Figure 2. McMurdo Station neutron monitor counting rate in the years 1965-1984. The arrows in the graphs indicate years when the Sun's north pole (N) and/or south pole (S) reversed polarity.
10000 a,
Year . 0 72.79 89 . 71 or SO
0 — 9500
D 08 6$ or 11-84
C
0 0
0
= 9000 C
0
E
C
0
8500
a,
Z
8000! 5
10
15
Spectral amplitude (nT5m)
Figure 3. Anticorrelation between neutron monitor counting rate and magnetic-energy spectral amplitude. Squares represent years of negative solar polarity, circles represent positive polarity, and triangles represent mixed polarity. (nT 2m denotes nanoteslassquared meter.)
Hedgecock, P.C. 1975. Measurements of the interplanetary magnetic
field in relation to the modulation of cosmic rays. Solar Physics, 42, 497-527. Matthaeus, W.H., M.L. Goldstein, and J.H. King. 1986. An interplanetary magnetic field ensemble at 1 AU. Journal of Geophysical Research, 91, 59-69. Pomerantz, M.A., and S.P. Duggal. 1974. The sun and cosmic rays. Reviews of Geophysics and Space Physics, 12, 343-361.
195
