30TH INTERNATIONAL COSMIC RAY CONFERENCE

A Solar Activity Dependence of A Solar Wind Effecton Cosmic Ray Intensity Variations
H. KOJIMA1, Y. HAYASHI2, K. HAYASHI2, N. ITO2, S. KAWAKAMI2, T. MATSUYAMA2, M. MINAMINO2, T.
NONAKA2, T. OKUDA2, S. OGIO2, A. OSHIMA2, N.SHIMIZU2, S. K. GUPTA3, A. IYER3, P. JAGADEESAN3, A.
JAIN3, S. KARTHIKEYAN3, P. K. MOHANTY3, S. D. MORRIS3, P. K. NAYAK3, B. S. RAO3, K. C. RAVINDRAN3,K. SIVAPRASAD3, H. TANAKA3, S. C. TONWAR3, S. SHIBATA4, I. MORISHITA5
1
Nagoya Women’s University
2
Graduate School of Science, Osaka City University
3
Tata Institute of Fundamental Research
4
College of Engineering, Chubu University
5
Department of Information Management, Asahi University
hkojima@nagoya-wu.ac.jp
Abstract: We have investigated a solar activity dependence of a solar wind effect on cosmic ray intensity variations. The solar wind effect discussed here is concerned neither with transient origins
such as Forbush decreases nor with known solar modulations. The effect is represented by the regression coefficient of cosmic ray intensity variations with solar wind velocities. For our analysis, cosmic
ray intensity data recorded by the neutron monitor at Kiel and by the muon telescope at Nagoya during the period from 1963 to 2006 are used. The data recently observed by the muon telescopes(GRAPES) at Ooty in southern India are also used in the analysis. Solar wind velocities are referredto the OMNI 2 data set. To remove the transient effects, the days with more than 2% decrease of cosmic ray intensity compare to the average of the three preceding days and the days in the following recovery phase are rejected. Long term effects are also removed by taking differences from 27-day running average. For examining the solar activity dependence, we divide the corresponding period into 5
quiet terms and 4 active terms by referring the sunspot numbers. It is found that the regression coefficients of cosmic ray intensity variations with solar wind velocities are clearly dependent on the solaractivity. They are large in active term and small in quiet term, and the ratio of coefficients is about 1.8.

IntroductionAccording to the diffusion-convection theory, it isexpected that the cosmic ray intensities varies
with the solar wind velocities. If their relation isquantitatively clarified, it will become observationally possible to obtain the diffusion coefficient for the propagation of cosmic rays; that isneeded to describe the density of galactic cosmic
rays flown into the heliosphere from inter stellar
space.
Since the beginning of direct measurements ofsolar wind with satellites, there have been several
reports that describe the effect of solar wind on
the variations of the cosmic ray intensities, so
called the solar wind effect; i.e. the intensity ofcosmic rays decreases as the velocity of solarwind increases. Despite these investigations, there
are seldom quantitative analyses of the solar windeffect. The reason is as follows. Since the intensity of cosmic rays varies with several causes, itshows a very complicated time profile which iscomposed of many different variations with different time periods. For example, it contains along term intensity variation of eleven-year period, a transient variation like a Forbush decrease,or a short term variation caused by the displacement of an active region on solar surface faced tothe earth due to the solar rotation. Anisotropy ofcosmic ray flow due to the earth’s rotation orrevolution around the sun also causes anotherintensity variation of cosmic rays. In our observation, the intensity variation of the cosmic rays is

A SOLAR ACTIVITY DEPENDENCE

Analysis and Result
For our investigation, cosmic ray intensity data
observed by the neutron monitor at Kiel for 44years from 1963 to 2006 and by the muon telescope at Nagoya for 37 years from 1970 to 2006

are used. The data observed by the muon telescopes at Ooty for 6 years from 2000 to 2005 arealso used in the analysis. Solar wind velocities
and other plasma parameters of the interplanetary
magnetic field in the corresponding period arereferred to the OMNI 2 data set which is an archive of measurements obtained by various satellites.

Preprocessing
In order to reject some kinds of interference between several variations caused by the different
origins described in the previous section, the data
we used have been preprocessed as the followingway.
At first, we have scanned all the daily values of
the cosmic ray intensity. Then, all the day which
has a value with more than 2% decrease compared to the average of the three preceding days
has been rejected by regarding as an indication oftransient phenomena like a Forbush decrease. The
days in the following recovery phase beginning at
the day rejected just before have also been rejected from our analysis. We have adopted about
70% as the threshold of the recovery.
After above preprocess, we have removed theeffects of long term variations by using the 27day numerical high pass filter; i.e. by taking differences from 27-day running average of daily
values. Daily values and 27-day running averagesof the cosmic ray intensity recorded by the neutron monitor at Kiel are plotted in Figure 1.
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neutron intensity（％）

the mixture of these different phenomena. As forthe solar wind velocity, it is also known that there
exist several components of the variation such asa high speed flow accompanied by a solar flare, arecurrent variation with a period of around 27
days (13.5 days) corresponding to the solar rotation, or a long term variation with a period ofseveral years. Thus for the both of variations of
the cosmic ray intensity and the solar wind velocity, it is very difficult to separate each componentof variation with different cause in a simple way.
Then we are not always able to obtain an acceptable result from a simple correlation analysis
between these data of the cosmic ray intensity andthe solar wind velocity in an extended period.
Needless to say, it is even more difficult to study
the dependence of the solar wind effect on thesolar activity.
The cosmic ray intensity has been observed formore than sixty years. Since IGY (1958), theworld wide observational network of neutron
monitor stations has been developed, and almost
all the stations have still been continuing theobservation. As for the muon observation at thesurface of the earth, the observation using ionization chambers has started from sixty years ago
and the high accuracy observation using the large
scale telescope has started from late 1960’s. Amuon telescope with 36 m2 at Nagoya has been
continuing the observation from 1970 till now
and recently muon telescopes with larger area of560 m2 and with more fine angular resolution,
named GRAPES, have been working at Ooty insouthern India. As for the direct observation of
the solar wind in space, the continuous observation has also done by linking the data obtained by
the number of satellites since 1963.
In this paper, relations between the observed
cosmic ray data from Kiel neutron monitor which
is one of WDC stations, from Nagoya muon telescope and from GRAPES and the solar windvelocities are investigated. A solar activity dependence of the obtained solar wind effect is also
reported.

year

Figure 1: Time profile of daily values of the cosmic ray intensity (blue) and 27-day running averages (red).
For the corresponding data of the solar windvelocity, we have also applied the same numericalhigh pass filter. The same plots for the solar windvelocity are shown in Figure 2.
The preprocessed daily values of the cosmic rayintensities recorded by the neutron monitor atKiel and by the muon telescope at Nagoya are30TH INTERNATIONAL COSMIC RAY CONFERENCE 2007
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200yearFigure 2: Time profile of daily values of the solar
wind velocity (blue) and 27-day running averages(orange).

is coincide with an average of the maximum andthe minimum values at the nearest peak andtrough respectively. Then, an active term is defined as a term including a peak and a quite termis defined as a term including a trough. The time
profile of the smoothed sunspot numbers isshown in Figure 5. The active and quiet terms of
the solar activity defined as above are indicatedby colors in the same Figure; red and blue correspond to the active and quiet terms respectively
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SUNSPOTNO

shown along with the solar wind velocity in theFigure 3 and Figure 4 respectively. It can be seenfrom these figures that the long term variations
have clearly been removed from both the intensity of cosmic rays and the solar wind velocity.

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Figure 3: Time profile of the preprocessed daily
values of the cosmic ray intensity recorded by theneutron monitor at Kiel (blue) and the solar windvelocity (brown).

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Figure 4: Time profile of the preprocessed daily
values of the cosmic ray intensity recorded by themuon telescope at Nagoya (green) and the solar
wind velocity (brown).

Division by the solar activity
To investigate the dependence of the solar windeffect on the solar activity, we have divided all
the preprocessed data into 4 active and 5 quiet
terms defined as follows. At first, the daily valuesof the sunspot numbers have smoothed by taking
a 365-day and a 27-day running averages. And
next, we have determined every boundary day
between the active and the quite terms as the day
when the smoothed value of the sunspot number2006

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Figure 5: Time profile of the smoothed sunspot
numbers after taking the 365-day and 27-day
running averages and the definition of the active(red) and quiet (blue) terms of the solar activity.

Solar wind effect
All the data preprocessed as described before are
divided into the terms thus defined to construct
the data sets (9 sets for the neutron in 1963 to2006 and 8 sets for the muon in 1970 to 2006).
Then, for each data set, we have done the regression analyses separately to derive the regression
coefficient in each solar activity term. The regression coefficients thus obtained are thought torepresent the solar wind effect on cosmic rayintensity variations in the different solar activity
terms. They are plotted along with the averagevalues of the sunspot number in each term inFigure 6.
It is evident from Figure 6 that there is a clear
difference between the regression coefficients, orthe solar wind effects, in the active terms and inthe quiet terms for both neutron and muon. From
the regression analysis using all the data combined in the active terms, the regression coefficient is (-0.00360+/-0.00018) %/(km/s) for neutron and (-0.00140+/-0.00011) %/(km/s) for muon.
From that in the quite terms, on the other hand, itis (-0.00200+/-0.00010) %/(km/s) for neutron andA SOLAR ACTIVITY DEPENDENCE(-0.00080+/-0.00007) %/(km/s) for muon. Then it
is found that the ratio (active/quiet) of coefficients
is about 1.8 for both components of cosmic rays.
140-0.0045-0.0040sunspot.NO

100-0.0035-0.003080-0.0025
60-0.0020-0.001540-0.0010

20-0.0005

0solar wind effect(%/km/s)-0.0050

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Acknowledgements
The authors express our thanks to the staff of theneutron monitor station at University of Kiel andthe staff of Solar-Terrestrial Environment Laboratory of Nagoya University for providing the cosmic ray data used in this analysis. The staffs ofSpace Physics Data Facility (SPDF) and National
Space Science Data Center (NSSDC) are also
acknowledged for allowing us to use the data inOMNI 2 data set.

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0.0000

References

year/month

Figure 6: Variations of the regression coefficients
between cosmic ray intensities and solar windvelocities (blue: neutron vs. velocity and green:Nagoya muon vs. velocity, brown: GRAPESmuon vs. velocity) on the solar activities plottedalong with the average of the sunspot numbers(red) in each term.

Conclusion
Although it was often pointed out that the intensity of cosmic rays varies with some transientvariations of solar wind velocity such as the highspeed flow associated with a solar flare, there
have been seldom quantitative analyses of thesolar wind effect on the intensity of cosmic rays.
In this paper, as the result of the analysis of data
observed in an extended period of 44 years orfour solar cycles, it is clearly shown that there arerather steady negative correlation between theintensity of cosmic rays and the solar wind velocity, so called the solar wind effect, besides thetransient variations described above.
By the examination of the solar activity dependences using the data divided into 4 active and 5
quiet terms by referring the sunspot numbers for
44 years, it is found that the regression coefficients of cosmic ray intensity variations withsolar wind velocities are clearly dependent on thesolar activity. They are large in active term andsmall in quiet term, and the ratio of coefficients isabout 1.8.

[1] Y. Munakata and K. Nagashima. Correlation
Analysis between Cosmic Ray Intensity Variation
and Interplanetary Plasma Parameters, 16th Int.
Conf. Cosmic Rays, Kyoto, 3, 530, 1979
[2] K. Fujimoto, H. Kojima and K. Murakami.
Cosmic Ray Intensity Variations and Solar Wind
Velocity, 18th Int. Conf. Cosmic Rays, Bangalore,
3, 267, 1983
[3] K. Fujimoto, H. Kojima and K. Murakami.
The Solar Wind Effect on Cosmic Rays and theSolar Activity, 19th Int. Conf. Cosmic Rays, SanDiego, 3, 262, 1985
[4] H. Kojima et al. The Change in Cosmic Rays
Intensity Variation with the Solar Wind Velocity,
published in this conference