Correlation analysis on the relation of the north-south (N-S) anisotropy of cosmic rays, observed by the Nagoya multidirectional meson telescope, with the interplanetary magnetic field (IMP) as well the solar wind velocity within solar Carrington rotation for the period 1971 to 1976 is discussed. It is found that the N-S anisotropy of cosmic rays correlates quite well with the Bx component of the IMF. The correlation coefficient is nearly equal to 0.8

Xao, S.

Xue, S.

Zhang, G.

English

NASA Technical Reports Server

27
SH 4.3-15
THE COROTATING VARIATION OF THE NORTH-SORTH
ANISOTROPY OF COSMIC RAYS
Xue Shunsheng Zhang Gongliang Xao Shaoyu.
(Institute of Space physics, Academia Sinica)
Correlation analysis on the relation of the
north-south (N-S) anlsotropy of cosmic rays,
observed by the Nagoya multi-dlrectlonal meson
telescope, with the IMF as well as the solar
wind velocity within solar Carrington rotation
for the period 1971-1976.
It is found that the N-S anisotropy of cosmic
rays correlates quite well with the Bx component
of the IMF. The correlation coefficient nearly
equalsto 0.8.
1. Introduction. Since the cosmic ray N-S anisotropy was
dlscovered, a number of studies about it have been publiahed.
The drift due to the radial gradient of cosmic ray density n
derives the N-S anisotropy in the presence of the IMF inten-
sity B, as interpreted by Swinson[l|. The correlation of the
cosmic ray anisotropic index GG( [4], [5]) with the daily
average direction of the IMF was studied by Yasue et al.[21,
and a good correlation coefficient (_0.75) was obtained. But
they did not give the quantitative analysis between the GG
and magnetic field intensity. Fujimoto et al.t3] pointed out
that the GG value decreased, as the IMF sector boundary
changes its polarity from toward to away the sun, and vice
verse. Munakata et al. [4]found that N-S anisotropy correlated
somewhat with the solar wind velocity, as well as the Bx
component of the IMF.
We made the correlation analysis between the N-S anisot-
ropy and solar flare, and found the flare of importance _- 2
with duration _--_1.5 hours could change significantly the
anisotropyt5_. The present paper studies the relation of
the corotating variation of cosmic ray N-S anisotropy with
solar wind velocity, as well as the parameters of the IMF
with the sun.
.?.Data and Method. The cosmic ray data used are the daily
average values taken from the multi-directlonal meson teles-
cope at Nagoya for the period 1971-197616]. The cosmic ray
N-S anlsotropie indexes A and AA are defined below,
. A=_N-S.)/2 11}A=(N-S)+(N-E)
Another parameter I is defined as
I=(N+S)I2 (3)
- Each term from formula (1) to (3) denotes a directional
component of the cosmic ray intensity having its central
direction of the viewing cone pointing toward the zenith
angle of 30° in the north (N), the south (S), and the east
(E)-directions, different from that adopted by Munakata et
al.. The solar wind velocity and the IMF data are taken from
https://ntrs.nasa.gov/search.jsp?R=19850026686 2020-03-20T16:45:57+00:00Z
28
SH 4.3-15
that compiled by King, J. H. [7].
Analysis of cosmic ray data are made by applying Chree
superposed epoch method with reference to their ordinal day
in a solar rotation[8]. An accurate Carrimgton rotation
Period is divided into 27 round days. It avoids not only the
drifting error caused by adopting Bartel's rotation, but
also the difficulty produced by decimal Carrlngton rotation.
Such a converted round day is called the "Carrington Day",
with length approximately equal to 1.01 calender days. After
taken the nearest whole number, the day at which the zero
Carrington longitude crossing the solar central meridian is
regard as the beginning day of each Carrington rotation.
In all figures of this paper the absissas represent the
Carrington ordinal day from the first day to the 27th day
corresponding the solar longitude from 360 ° to 0°. The
ordinates show the parameters concerned, i.e. A, AA and I,
as well as the solar wind velocity V, IMF total intensity
B, its components B_ and By and its azimuthal angle 5o in the
ecliptic plane. In order to making the figures more clear,
the values of the first three days are drawn repeatedly
after the 27th day
. Analysis and results• The distributions of cosmic ray
-S anlsotropy and IMF Bx component in the Carrington
rotation have been calculated. Simply shown are the results
for 1975 and 1974 in
o_o, Fig. I. In Fig. Ib,
_",2< ,_'__ two steady corotating
,4o< ,2_ structures of the IMF
"" '2_- can obviously be seen.
,_,-/_A Also noted are the
5_ ,_ __ vsriations of A and4, "' AA being similar to44 _ that of Bx component•
__ In other years, the
of I_-
_ steady corotating
'_ _ structures still exi-
., .. st, but not as clear
°, "'-_/-_ii_ as in 1974 Because
c _/-_/_ _ o_ _] the correlation coef-
_' ficient between the
.at
_._,._%.____ _.___ i.,,_.,l,__.w._.,.,.,,._5 _ J5 2o _ 3 A and AA is almost
,o, o_ ,u _ closed to I, only the
_,q., c_,o,o,,._. o,,_o,,,A,AAon_,_. AA is discussed in
following.
The single and bi-correlation between AA and Bx and By
can be expressed as the following formulas,
AA=ao2 +amy By
AA=ao+a i_Bx +ai2 By
Table I summarizes the yearly coefficients in formula. (4) to
(6) and respective correlation coefficients rx, ry and rxy,
as well as the I and AA. It is striking that AA correlates
quite well with the Bx component with a high correlation
29
SH 4.3-15
coefficient rx 0.8 The ry is negative, its magnitude is a
bit less than rx. The bi-correlation coefficient rxy is a
little higher than rx. The rx obtained here is much higher
than that by Munakata et al.. This is because the present
correlation coefficients are calculated on a yearly beses,
while the previous ones using five year data altogether,
and the change of the AA following the solar cycle reduced
Table 1. Values of various coefficients, as well
as the I and AA during 1971-1975
,,. u, J , .. i
year ao_ alx rx aoz aly ry ao a_ a_z rxy I AA
1971 14 4.2 0.86 14 -4.1 -0.80 14 3.0 -I .4 0.87 1705 3
1972 58 4.2 0.71 39 -4.1 -0.63 59 5.I -1.7 0.74 1557 38
1973 69 4.0 0.73 70 -4.3 -0.69 69 3.4 -0.8 0.73 1400 64
1974 90 3.4 0.88 92 -3.6 -0.88 91 1.8 -1.6 0.88 1294 91
1975 114 3.3 0.82 113 -3.3 -0.78 114 3.5 -0.3 0.82 1187 97
the statistical accuracy. Meanwhile the superposed epoch
method has filtered the short term variation and raised also
the statistical accuracy. The coefficients alx, aly, a,J and
ai2 do not change very much, but ao, , ao2 and ao increase
approximately from 14 to 114 with the de creasing solar
activity during 1971-1975. This fact illustrates that the
variation of N-S anisotropy is due to not only Bxvn, but
other factor, for example, the variation solar general
magnetic field.
We have also calculated the distributions of cosmic ray
N-S anlsotropy, solar wind velocity and the IMF parameters
in the Carrington
_ 1971 °°" 197q rotation for this pe-3C I I(
_c ,o, riod. For simplicity
,o _ .Tonly the results forc 2
., o 1971 and 1974 are
-_"__ /_ _y_ -_ -2 shown in Fig.2. Table
2
4
o 2 summarizes the cor-2
relation coefficients
/k_.j__. _ of the AA with the V,
.lOG_
-,00' __ _ _d B reBpectively.o- o, The magnitude of rv,IOO° K)O'
. /, /_/_ /,r_ and rB are much
'° _ lower than those of
,o _k_/ V _ _ Bx and By. Hence itis the best to study
. ___ _ ................ L the correlation of AA
_oj ooy tb) ooy with IMF component.
Fi(j. 2. Corolalin@ varlQlions o| AA, 8x, 8y, EI,_t' a_cI V.
It is well known that the solar activity weekens from
1971-1974, the solar modulation of cosmic rays inereases,
and the omnidirectional intensity of cosmic rays increases
3O
SH 4.3-15
Table 2. The rv, r_ and every year. The present results
rB for 1971 and 1974 show the AA follows a similar
trend, however, it is unexpected
Year rv r9 r B that the I decreases from year
......... to year, quite different from
1971 -0.35 -0.61 0.29 the variation of omnidirectional
1974 0.21 -0.75 0.02 intensity.
4. Conclusion. The cosmic ray N-S anlsotropy correlates
quite well with the IMF components Bx and By with a correla-
tion coefficient nearly equal to 0.8, a positive one for
the Bx component and negative for the By. The bi-correlation
coefficient with both Bx and By is almost closed to 0.8. The
variation of the regressive coefficients illustrates that
the cosmic ray N-S anlsotropy, in addition to the gradlemt
drift effect, is controlled by some kinds of long term
var£ayion of the solar cycle.
5. AcknowledKements. The authors are grateful to Prof. K.
Nagashima for providing the valuable Nagoya multi-d/rectional
meson telescope data.
References
1, Swlnson, D.B., (1969), J. Geophys. Res., 74, 5591
2. Yasue, S., Morl, S. and Munakata, Y., (19_-_), 17th ICRC,
Parls, 4, 88
3. _uJimoto, K., KoJima, H., Murakami, K. and Nagashima, K.,
(1981); 17th ICRC, Paris, 4, 72
4. Munakata, Y., Mori, S. and Nageshima, K., (1979), 16th
ICRC, Kyoto, 4, 131
5. Xao, S., Xue, S. and Zhang, G., (1985), to be published
6. Sekido, Y. et al., Report of Cosmic-Ray Leboratory,
Multi-Directional Cosmic-Ray Intensltles, No. I
Nagashima, K. et al., Report of Cosmic-Ray Laboratory,
Multl-Directlonal Cosmic-Ray Intensltles, No. 3
7. King, J, H., Interplanetary Data Book, NSSDC/WDC-A-R & S,
77----04, 1977
8. Zhang, G., Yaug, G. and Lu, C., (1985), Acta Astrophys.
Sinica, Vol. 5, No. 2, 140
t


The corotating variation of the north-south anisotropy of cosmic rays

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