Characteristics of the variation of galactic cosmic ray intensity observed at Guangzhou station on 1997 January 7–10

CHINESE ASTRONOM17 AND ASTROPHYSICS PERGAMON

Chinese Astronomy

and Astrophysics

27 (2003) 193-198

Characteristics of the Variation of Galactic Cosmic Ray Intensity Observed at Guangzhou Station on 1997 January 7-10" LE Gui-ming

YE Zong-hai XIAO Shao-yu

Center for Space Science

YU Shao-hua GONG Ju-hong

and Applied Research,

Chinese

Academy

LI Run-sen of Sciences, Beijing

100080

Abstract

Although

the CME event of 1997 Jan 7-10 caused

a magnetic

storm

of merely mediate intensity, it gave rise to very strong geophysical effects and modulated the observed intensity of Galactic cosmic rays. This paper describes and briefly analyses the characteristics of the variation of Galactic cosmic ray intensity in several directions as recorded with the multi- directional scintillation telescope

at Guangzhou

Key words:

Sun:

Cosmic

CME-cosmic

Ray Station. rays

1. INTRODUCTION Geomagnetic storms are disastrous space phenomena, and they can cause serious damages, Therefore, their prediction has become an important content of space physics. Research has shown that coronal mass ejection (CME) is the main troublemaker of non-periodic geomagnetic storms[1~21. However, accurate prediction of the magnitude of geomagnetic storms with a long time lead is a problem that is far from being resolved, and remains an ongoing and difficult task. Data on the disturbing and background solar winds may to certain degree provide clues on some aspects of the geomagnetic storms. Nowadays the monitoring of the solar wind is done by the ACE and other satellites. However, it should be noted that for the disturbing solar wind responsible for mediate (and more particularly, large) magnetic Received 2001-10-15; revised version 2002-08-25 * A translation of Chin. J. Space Sci. Vol. 22, No. 4, pp. 309-313,

2002

0275-1062/03/$-see front matter @ 2003 Elsevier Science B. V. All rights reserved. DOI: lO.l016/SO275-1062(03)00041-9

194

LE Gui-ming

et al. / Chinese

Astronomy

and Astrophysics

27 (2003)

193-198

storms, the time of propagation from the position of the ACE satellite to the earth is never longer than 30 minutes. Therefore, when predicting mediate or large magnetic storms with ACE data, the lead time is often too short. Now, Galactic cosmic rays pass through the heliopause and interplanetary space, enter the magnetosphere and finally arrive at earth’s surface. If the Galactic cosmic rays encounter a CME, the latter may exert some influence on the former. On passing across the CME, both the intensity and direction of the Galactic cosmic rays may vary, and this could be reflected in the intensity recorded at observing stations on earth’s surface. Because the velocity of Galactic cosmic rays is very high - much higher than that of CMEs, so it is possible to infer the disturbances in the interplanetary space from changes in intensity and anisotropy of the Galactic cosmic rays recorded on earth. In short, Galactic cosmic rays can be used to carry out long-range monitoring of CMEs. As pointed

out by Xue Shun-sheng

and Zhang

Gong-liang

I31 in 1989, the observation

of

anisotropy of cosmic rays is a powerful means of monitoring the interplanetary magnetic field, and can be used to predict disturbances in the space environment. The authors made a statistical

study

of the anisotropy

of cosmic rays in 1971-1975,

i.e. the related

isorotational

changes in the solar wind and the interplanetary magnetic field. Using Japanese data the authors calculated the daily average values of the magnetic field components of the solar wind and their correlation with the NS anisotropy of Galactic cosmic rays. A study of the modulation

of cosmic

rays by strong

magnetic

disturbances

was made

by Guo Wei-jie

and

Zhang Gong-liang 141. That paper was mainly devoted to the statistics of morphology and amplitude of the descent of cosmic rays caused by strong magnetic disturbances, but no study was made on the characteristics of the changes of cosmic ray intensity before the magnetic storms. In Ye Zhong-hai et al.% 151research on the variation of cosmic ray intensity and the K types of magnetic storms, the statistical study concentrated on the relation between the K-indices of various types and the morphology corresponding to the Forbush decrease as well as its amplitude. On 1997 Jan 6, at UT 1734, a CME occurred on the sun. The solar X-rays flux on Jan 5-6 is shown in Fig.l(a). As observed by the LASCO C2 coronagraph aboard satellite SOHO, it was a partial full-halo CME. On Jan 10, the distance between the satellite WIND and the earth was 85 R,.On Jan 10, at UT 0445, WIND observed the front part of the magnetic cloud (MC). Earlier, at UT 0053, the shock waves in front of the MC had arrived at the location of WIND. The velocity of the CME was about the typical value of 450 km/s. In the 20 hours beginning at 0445 UT of Jan 10, the earth was immersed in the CME. During this time, the B, component of the interplanetary magnetic field was almost always pointing in the south direction, and its intensity was 16 nT. Before the termination of the rear boundary of the MC, the direction of the magnetic field had gradually and at a uniform rate turned to point to the north. As illustrated by Fig.l(b), the CME event caused a geomagnetic storm. Moreover, all the three satellites, GOES, (the low earth orbit satellite) SAMPEX and POLAR detected an increase of electron flux in the radiation belt. This was an event of much concern. A large amount of research on this event was made abroad 16-*l. In our country a numerical simulation of the propagation of this CME event in the interplanetary of the changes in the space was run. In this paper we wish to present the characteristics Galactic cosmic rays recorded at the Guangzhou Station, as they encountered the CME in interplanetary space.

LE Gui-ming

et al. / Chinese

Astronomy

and Astrophysics

27 (2003)

195

193-198

10-7 [

i..~

A 10-s . Beijing: 1997-01-05-0000 UT 0

6

12

18 24 Time/h

30

WIND MFI KP Averages

(a) 36

42

RDAF

2400 UT Fig. 1

(a) Solar X-ray flux on 1997 Jan. 5-6, (b) B, component

2. OBSERVATIONAL

DATA

AND

of interplanetary

magnetic

field

ANALYSIS

The scintillation telescope in Guangzhou was built in September 1987. Its geographic location is 113”18’E and 23”6’N, its height above sea level is 21 m, and the vertical cut-off rigidity is 16 GV. The structure of the telescope is sketched in Fig.2. The parameters of the telescope are listed in Table 1. Table J_

Direction angle O0

Solid angle 58.6’

E W S N

32.2’ 32.2’= 32.2“ 32.2’

51.5O 51.5O 51.5O 51.5O

1

Parameters

of the telescope

No. of coincidence 12 6 6 10 10

units

Note A coincidence unit is a pair of upper asnd lower light guide boxes

The CME arrived at the earth on 1997 Jan 7-10, and gave rise to a geomagnetic storm of mediate intensity. Its Dst index time curve 191is shown in Fig.3 (a). The variations of the Galactic cosmic rays intensity in three directions are displayed in Figs.S(b), 3(c) and 3(d). As may be seen in Fig.3(b), the onset of the magnetic storm was the early morning of Jan. 10. From the solar wind data it can be inferred that the sudden commencement of the magnetic storm was at about 0100 UT of Jan. 10. Fig.3(b) shows a clear increase in the daily variation amplitude in the vertical direction in the second half of Jan. 8. This was about 28h earlier than the onset of the magnetic storm. Figs. 3(c) and 3(d) show that the effect of the CME was to cause some small increases in the cosmic ray intensities

196

LE Gui-ming

et al. / Chinese

T : 32.2ON

Fig. 2

in the south and north north/south anisotropy.

i"

Astronomy

and Astrophysics

27 (2003)

telescope of Guangzhou.

A sketch

193-198

NW 32.2'S

The multi-directional

directions;

the increases

run a similar

course,

changing

little

the

According to the standard theory of propagation, the anisotropy of Galactic cosmic rays has only four sources, namely, parallel diffusion, convection, vertical diffusion and the B x Vn drift. As mentioned above, the cut-off rigidity of cosmic rays at Guangzhou Station is 16 GV. So diffusion caused by convection can be completely ignored. As is known from the analysis of the data of interplanatary exploration [lo], the particles in interplanetary space are distributed mainly on the two sides of magnetic field lines, and not along them. The density gradient of the particles is perpendicular to the field lines. Hence diffusion parallel to the direction of the magnetic field lines can also be neglected. Thus, there could only be variations due to diffusion perpendicular to the field and due to the drift B x Vn. Nowadays the N/S anisotropy is generally recognized to be caused by the drift B x On. Therefore, from the fact that the difference was observed to be small it can be inferred that on Jan. 7-10 the gradient Vn of the interplanetary particles at the cut-off rigidity of about 16 GV was small. The fact that the present CME was inclined toward the southern hemisphere of the earth and that the Guangzhou Cosmic Hay Station is located in the northern hemisphere may be one of the reasons for the small anisotropy observed. The Guangzhou Station is located at a low latitude in the northern hemisphere. Hence the angle between the zenith direction and the ecliptic plane is not large. The change of cosmic ray intensity in the perpendicular direction is thus concerned with the distribution of velocity directions in the ecliptic plane. Among the particles in the interplanetary space, it is quite possible that the angle between the direction of their velocity vector and the magnetic field is small. The arrival of these particles at the observing station may make some contribution to the fluctuations of intensity in perpendicular direction. Besides, the B x Vn drift may also have some contribution to the fluctuations of intensity in the perpendicular direction. This is because the perpendicular direction of the Guangzhou Station at the time is not completely situated in ecliptic plane.

LE Gui-ming et al. / Chinese Astronomy

and Astrophysics

$ z

27 (2003)

193-198

197

0.000

2 d -0.005 Jan. 7

8

!z .o

6 ._ $

0.015

._3

9

10

Time

Time 0.020

;ii ._ a

0.010

>C

< .g aae

0.005

‘t; .o

+z

0.000

dY Se 5 s

-0.005

:.3 .s

P-CL

fZ -O.O’O Q .c -0.020 Jan. 7

8

Time Fig. 3 vertical

9

10

Time

Time variations of the Dst index (a), and of the Galactic cosmic rays intensity direction (b), in the south direction (c) and in the north direction (d)

3. CONCLUSIONS

AND

in the

DISCUSSION

From the above analysis, we have drawn the following conclusions. (1) The difference between the intensities in the south and north directions observed at Guangzhou Cosmic Ray Station is not large. This is related to the small Vn of interplanetary particles of about 16 GeV and to the CME being inclined toward the southern hemisphere. (2) The intensity of cosmic rays in the vertical direction observed at the Guangzhou Cosmic Ray Station showed an increase in the amplitude of daily variation. This phenomenon was caused by the B x Vn drift and the fact that the angle between the velocity vector of a part of the particles and the direction of the magnetic field is small. Galactic cosmic rays may be used for the long-range monitoring of CMEs that are “going” about

in interplanetary

space, and also for estimating

the inclination

of the CME’s

direction. This provides some clues for predicting magnetic storms with large lead time. The direction of the CME is an important factor affecting the intensity of the magnetic storms. Thus, the use of the variation in Galactic cosmic ray intensity may help us to judge the intensity of magnetic storms and to infer the status of the space environment.

198

LE Gui-ming

et al. / Chinese

Astronomy

and Astrophysics

27 (2003)

193-198

References Wang

Jia-long,

Gostling

Prog.

J.T.,

Xue Shun-sheng, Guo Wei-jie,

Webb

D.F.,

Villanate, Selesnick

9

Shi Yong,

10

Bieber

Zhang

Gong-liang,

Gong-liang,

Lu Xing-tang,

Zong Res.

R.S.,

Blake

Wei Feng-si,

J.W.,

Evensen

J.B., Feng

14, 8

Ejections,

Res.

Geophys.

J. Space

Qui-gang, Lett., Lett.,

Geophys.

Chin.

J. Space

99, 1997 Sci., 1989, 9, 155

Sci., 1995, 15, 295

Chin.

J. Space

Sci., 1987, 7, 306

25, 2469 25, 2593

Res.

Xue-shang,

P., Geophys.

Monograph

Xiao Shamyu,

Chin.

et al., Geophys. et al., Geophys.

8

1999,

Mass

Zhang

Ye Zong-hai,

Geophys.,

In: Coronal

Res.

Lett.,

25, 2593

Ye Zhan-yin, Lett.,

Chin.

25, 2955

J. Geophys.,

2001,

44, 303