27-Day variations of galactic cosmic rays and changes of solar and geomagnetic activities

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Adv. Space Res. Vol. 21, No. 3, pp. 619-624,2001 0 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/O] $20.00 + 0.00 PII: SO273-1177(01)00091-6

27-DAY VARIATIONS OF GALACTIC COSMIC RAYS AND CHANGES OF SOLAR AND GEOMAGNETIC ACTIVITIES M. V. Alania 1,2, D. G. Baranov3, M. I. Tyasto4, and E. S. Vemova4 I. Institute of Mathematics and Physics of University of Pod&e,

Siedlce, Poland 2. Institute of Geophysics, Georgian Academy of Sciences, Tbilisi, Georgia 3. A.F. Io#e Physico-Techn. Institute, Russian Academy of Sciences, St. Petersburg, Russia 4. IZMRAN, Russian Acad of Sciences, St. Petersburg Branch, St. Petersburg, Russia

ABSTRACT Neutron monitor, solar and geomagnetic activity data have been used to investigate variations of galactic cosmic ray intensity connected with the Sun’s rotation for different epochs of solar activity. Behaviors of longitudinal asymmetry of the sunspot distribution, coronal line emission and tilt angles of the heliospheric neutral sheet have been studied. A relationship of the temporal changes of the amplitudes of the 27-day variations of galactic cosmic rays and Kp index of geomagnetic activity with the tilt angles of the heliospheric neutral sheet have been investigated. It is shown that there are no any noticeable .relationship between the amplitudes of the 27-day variations of galactic cosmic rays and the tilt angles of the heliospheric neutral sheet for both of the qA>O and the qAO cycle than for the qA
INTRODUCTION Recently the features of the time-dependence of the amplitudes of the 27-day variation of galactic cosmic rays (GCR), which manifest themselves in the existence of two maxima in the course of the solar cycle, were established (Vemova et al., 1995). The anomalous decrease of the amplitude of the 27-day variations (Vemova et al., 1995; Alania et a1.,1999) of GCR was observed during high solar activity coinciding in time with the inversion (1990) of the Sun’s global magnetic field. This process is accompanied with a series of features in the behaviors of different parameters of the Sun, e.g., changes of the dipole magnetic field of the Sun and the longitudinal asymmetry of the sunspot distribution over the Sun’s disc and etc. A pronounced North-South (N-S) asymmetry is one of the main features of the Sun, which manifests itself, in particular, in the considerable difference between the processes of the inversion of the Sun’s global magnetic field in the Northern and the Southern hemispheres. At present, much attention is given to the problem of N-S asymmetry of the heliosphere (Smith et al., 2000). The data obtained by Ulysses revealed an asymmetry in GCR intensity with higher values in the Northern hemisphere, which was the first indication of the existence of such an asymmetry of GCR modulation. If a considerable N-S asymmetry does really exist in the heliospheric modulation parameters then it should be taken into account in new models for GCR modulation. It is therefore of interest to establish the connection between the reconstruction of the Sun’s global magnetic field with the change of solar activity distribution over the Sun’s disc, which may serve as the cause for the anomalous decrease of the amplitude of the 27-day variation in GCR. During the period (1990) between the first (1989) and the second (1991) maxima of the amplitude of the 27-day variation of GCR (Alania et al., 1999) the change of the polarity of the Sun’s global magnetic field took place. It is known that the conditions for GCR transport are essentially different for the qA>O and the qA
M. V! Alania et ol

620

27-day

variations

of GCR for Kiel neutron

monitor

for the period

May 1976-December 1997 (1642-1930 -Carrington rotations of the Sun). The amplitudes of the 27-day variations 2 2% generally are connected with the moderate and big Forbush decreases of GCR (Forbush effects with amplitudes higher than 5%), though for the amplitudes < 2% a certain role of the Forbush decreases still remains. A pure . rc separation of the 27-day variations 1976 1981 1986 1991 1996 YEAR of GCR on the background of the Forbush decreases is sufficiently difficult, however. Fig. la. Time profile of the amplitudes of the 27-day variations of GCR for Kiel neutron monitor for the period May 1976-December 1997. Changes of the amplitudes of the 27-day variations of GCR and of the Kp index of geomagnetic activity versus the tilt angles of the heliospheric neutral sheet (HNS) Distribution of the amplitudes of the 27-day variation of GCR for Kiel neutron monitor is presented versus the tilt angles of the HNS in Figure 1b for the period May 1976-December 1997 (1642-1930 Carrington rotations of the Sun). In Figure lb the distribution of the amplitudes of the 27 day variations of . . GCR for Kiel neutron monitor is divided . . in two parts by the solid line at the level . . . of 2%. The lower part (I 2%) corresponds to the amplitudes generally caused by heliolongitudinal the ’ :. . asymmetries of solar activity and solar . . . *y$‘: +.. wind, while the upper part (> 2%) is caused essentially by moderate and big Forbush decreases (see Figure la). It is 0 20 40 60 80 seen from Figure 1b that there is no any essential correlation between the Tilt angles (Degrees) amplitude of the 27-day variation of GCR and the tilt angles of HNS, neither Fig. lb. Distribution of the amplitudes of the 27-day variations of in the case when the role of Forbush GCR for Kiel neutron monitor versus the tilt angles of the HNS. decreases is not excluded (lower and upper parts of Figure lb taken together) nor in the case when‘the role of Forbush decreases is excluded (lower part of Figure lb). It seems that an expected from the theoretical modeling an azimuthal dependence of the intensity of cosmic rays on the waviness of the HNS (Kota and Jokipii, 1983; 1991; Hattingh and Burger, 1995; Hattingh et al., 1997) should be discussed in view of the above mentioned experimental results (Figure lb) for the energies of GCR to which neutron monitor is sensitive. 27-day wave in Kp index has been studied for a long time by different methods including harmonic analysis, e.g., Mursula and Zieger (1996) have used power spectrum method when studying quasiperiodic variations in the Kp index. 50 60 70 80 The amplitudes of the 27-day variation of the Kp 10 20 30 40 index of geomagnetic activity (in relative units) Tilt angles (Degrees) versus the tilt angles are presented in Figure 2 for May 1976-April 1998 (1642-1935 Carrington Fig. 2. Distribution of the amplitudes of the 27-day rotations). It is seen from Figure 2, that there is no variations of the Kp index versus the tilt angles of the HNS. any correlation between the amplitudes of the 27l

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y$=f* ; ,

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621

27-Day Variations of Galactic Cosmic Rays

day variation of the Kp index and the tilt angles of HNS. It seems, that changes of the tilt angles of the HNS do not manifest itself in the heliolongitudinal asymmetry of the heliosphere, e.g. in the solar wind speed and in the strength and fluctuations of the interplanetary magnetic field (IMF) and etc. The changes of the tilt angles are not pronounced not only in the region of the heliosphere, where the formation of the 27-day variation of GCR takes place (region of the heliosphere of a few or of tens astronomical unites around the Sun) Alania et al.( 1968), but also in the relatively limited region of the interplanetary space (close proximity to the Earth’s magnetosphere), which is responsible for the formation of the 27-day variation of geomagnetic activity. Changes of the amplitude of the 27-day variation of GCR in different qA>O and qA 0 cycle than during the qAO solar magnetic cycle, than that during the qAO cycle. In this paper we continue to study in more detail a behavior of the amplitudes of the 27-day variations of GCR for different neutron monitors. The amplitudes of the 27-day variations of GCR for Kiel, Rome, Tokyo and Huancayo neutron monitors have been calculated (using harmonic analysis) for the two solar magnetic cycles qA>O (1973-1978) and qAO and qA
of GCR for different Neutron Monitors

Station

Cut-off rigidity in GV

Kiel

2.29

0.67 k 0.10

0.54 5 0.10

Ratio of amplitudes of the 27-day variations of GCR for different qA >O and qA
Rome

6.32

0.45 * 0.05

0.35 f 0.05

1.30 + 0.25

Average amplitudes of the 27-day variation of GCR for different qA>O and qAO) 1982-1988 (qA
Tokyo

11.61

0.33 f 0.02

0.29 + 0.03

1.14 f 0.17

Huancayo

13.45

2.29 6.32 11.61

0.32 + 0.03 Minimum epoch 1985-1987 (qA
1.09 + 0.18

Kiel Rome Tokyo Huancayo

0.35 + 0.03 Minimum epoch 19751977 (qA>O) 0.57 + 0.06 0.39 + 0.03 0.29 f 0.02

1.50 + 0.26 1.70 + 0.21 1.61 f 0.13

13.45

0.26 + 0.01

0.16 + 0.01

1.65 + 0.11

As it is seen from the Table 1, the average amplitudes of the 27-day variation of GCR for neutron monitors are slightly greater for the qA>O solar magnetic cycle (1973-1978) than for the qAO and qAO solar magnetic cycle than in the qA
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experimental findings of Richardson et al. (1999) and the modeling and experimental results of Alania et al. (2001). However, these experimental and modeling results need further careful investigation and interpretation. Temporal changes of the longitudinal asymmetry of the sunspot distribution over the Sun’s disc and the tilt angles of the HNS Certainly it is not able to observe the pure influence of the inversion of the global magnetic field of the Sun on the 27-day variation of GCR in natural conditions. During the inversion period of the global, magnetic field of the Sun a radical change in solar activity takes place. In particular, its distribution over the Sun’s disc changes significantly. Distribution of the magnetic fields over the solar disk controls the spatial characteristics of the solar wind streams taking magnetic field away from the Sun. The stable azimuthal asymmetry of the solar wind co-rotating with the Sun is the cause of the 27-day 1992 1988 1990 1994 1986 sg variation in GCR. That’s why it is of interest to consider the behavior of YEAR different solar activity characteristics at the period of the Sun’s global magnetic field inversion, which may affect the structure Fig. 3. Temporal changes of the longitudinal asymmetry (LA) of the heliosphere. It is most natural to of the sunspot distribution over the Sun’s disc for the Northern connect the appearance of the stable (LAN ) and Southern (LAs ) hemispheres and for the difference asymmetry of heliosphere with the ANS= (LAN -LAS) versus time. longitudinal asymmetry of solar activity distribution. The longitudinal asymmetry of the sunspot distribution over the Sun’s disc was estimated (quantitatively) by means of the polar diagram method (Vernov et al., 1979). Data of the Greenwich Royal Observatory (Greenwich, 1917-1954) and of the Pulkovo observatory (Pulkovo, 1955-1995) were used for the estimation of the sunspot distribution asymmetry. The values of longitudinal asymmetry (LA) were averaged over 13 Bartels rotations with a sliding shift of one rotation. All data in subsequent figures were treated in the same manner. An enormous decrease of the amnlitude of the 27-dav variation of GCR in 1990 falls’on the period of the radical reconstruction of solar activity distribution, as it is seen in Figure 3. In this Figure 3 the changes of the longitudinal asymmetry of the sunspot distribution for the Northern and the Southern hemispheres are shown. In the course of the solar asymmetries of two cycle the longitudinal hemispheres do not change synchronously. The longitudinal asymmetry of the Southern hemisphere is increasing more slowly than the asymmetry of the Northern hemisphere, reaching its maximum 1.5 years later. Near the maximum of solar activity the longitudinal asymmetries of the hemispheres LAN 1993 1995 1985 1987 1989 1991 and LAs change in anti-phase. The process of the change of the relative role of the Sun’s hemispheres is shown in Figure 3 by the time-dependence of the Fig. 4. Temporal changes of the tilt angles of the of the dtfference Ans = LAN - LAs. Domination HNS for the Northern (RN) and Southern (Rs) longitudinal asymmetry of the Northern hemisphere, hemispheres and for the sum (RN + Rs) versus time. which coincides with the first maximum of the 27day variation of GCR in 1989, is replaced by a rapid passing to domination of the Southern hemisphere. Different temporal changes of the tilt angles of the HNS for the Northern (RN) and Southern (Rs) hemispheres and as well for the sum (RN + Rs) are observed immediately before the Sun’s global magnetic field inversion. In

27-Day

Variationsof GalacticCosmic Rays

623

Figure 4 are presented the temporal changes of the position (tilt angles) of the HNS for Northern and Southern hemispheres. It is seen from Figure 4 that there are observed the changes of the position of the HNS coinciding in time with the change of the dominating hemisphere. At the maximum of the solar cycle (1989) a noticeable N-S asymmetry of the position of HNS (tilt angles are greater in the Northern hemisphere than in the Southern hemisphere) was observed. Time profile of the Sun’s global magnetic field and of the intensity of the coronal green line The second maximum of the amplitude of the 27-day GCR variation falls on the period (199 1) of dominating of the longitudinal asymmetry of the Southern hemisphere. An analogous alternation of the relative role of the Sun’s hemispheres has been studied by different authors, e.g. by Vizoso and Ballester (1987). Sudden disappearances of solar prominence for the solar cycles 18-21 were considered. It was shown that at the period of the 10 9 high level of solar activity (during the inversion of $2 5the Sun’s global magnetic field) the change of 0 .s .9 4, domination of Sun’s hemispheres took place. In v Zs -5Figure 5 the time-dependence of the magnetic Zfv, field of the Sun considered as a star is presented E 0J 1;; !Zs (Stanford mean solar magnetic field). During the 3% period under consideration, when the radical -20 Z reconstruction of the sunspot distribution (from 1985 1987 1989 1991 1993 1995 1988 to 1991) was observed, the Sun’s global YEAR magnetic field preserved negative values. Before the inversion of the Sun’s global magnetic field the Fig. 5. Temporal changes of the magnetic field B of the Northern hemisphere was dominating. After the Sun (considered as a star) versus time. inversion was dominating the Southern hemisphere, but in both of cases the magnetic field of the hemisphere (dominating in the longitudinal asymmetry) had a negative polarity. Thus, the asymmetric structure of the heliosphere at the period of the maximum of the solar cycle (which evidently is responsible for the appearance of the 27-day variation of GCR), was determined mostly by the activity of one of the Sun’s hemispheres. The change of the dominating hemisphere and the disarrangement of the stable asymmetry of the heliosphere accompanying this process, probably account for 60 the anomalous decrease of the amplitude of the 27-day variation of GCR during 1990, when the 50 inversion of the Sun’s global magnetic field took 40 place. 30 Intensity of the coronal green line (h = 5003 ) 20 is one of the important parameters of solar 10 activity. It also shows considerable changes near 0 solar maximum (Figure 6). A sharp changes of the green coronal line intensity (N-S asymmetry) -10 5 coincide with the period of the inversion of the 1987 1989 1991 1993 1995 Sun’s global magnetic field. Thus, a complex YEAR changes of different parameters of solar activity, Sun’s magnetic field, tilt angles of the HNS and Fig. ‘6. Temporal changes of the intensity of the coronal green green line intensity of the Sun’s corona have been line for North (Nh) and South (Sh) hemispheres and of the revealed during the time reversal of the Sun’s difference (Nh-Sh) versus time. global magnetic field (1990), *when the anomalous decrease of the amplitudes of 27-day variation of GCR was observed. CONCLUSIONS 1.The amplitudes of the 27-day variation angles of the HNS for the qA>O and qAO solar magnetic cycle than that

of GCR and Kp index of geomagnetic activity do not depend on the tilt solar magnetic cycles. At the same time, the average amplitude of 27are about 1.5 times larger during the minima and near minima epochs for the qA
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M. V. Alania et al.

2. During the maximum of solar activity (1990) when anomalous decrease of the amplitude of the 27-day variation of GCR is observed, the influence of the change of the sign of the Sun’s magnetic field on GCR intensity is essentially masked by the radical reconstruction of solar activity distribution. 3. At the maximum epoch of the 22nd solar cycle (when qAO ) significant changes in the N-S asymmetry of different parameters of the Sun and solar activity are observed. The dominating role of the Sun’s hemispheres changes from the Northern hemisphere at the maximum of solar activity to the Southern one at the period of the inversion of the Sun’s global magnetic field. It is noteworthy that the Sun’s hemisphere with the negative magnetic field polarity was the dominating one in the both periods, before and after the inversion of the Sun’s global magnetic field. Apparently, in this case the N-S asymmetry of the Sun plays a significant role and should be taken into account while developing a new model to explain the features of the 27-day variation of GCR for maxima epochs of solar activity. ACKNOWLEDGEMENTS Data of Kp-index, intensity of coronal green line and Stanford mean solar magnetic field have been taken from the National Geophysical Data Center database (ftp://ftp.ngdc.noaa.gov/), data of the tilt angles of the HNS of the Wilcox Solar Observatory from (http://quake.stanford.edu/-wso/) and data of neutron monitors from database ‘COSRAY-2000’, WDC-B2 of IZMIRAN, Russia. Authors thank Miss A. Gil for helping in preparing this paper. Special thanks to the referees for many useful comments and suggestions. REFERENCES Alania, M. V., E. S. Vernova, M. I. Tyasto, and D. G. Baranov, Features of 27-day variations of galactic cosmic rays and of solar activity, Adv. Space Res., 23, 471, 1999. Alania, M. V., L. Kh. Shatashvili, and L. I. Dorman, A study of the modulation region causing 27-day cosmic ray variations, Canadian Journal of Physics, 46, S970, 1968. Alania, M. V., E. S. Vemova, M. I. Tyasto, and D. G. Baranov, Experimental and modeling investigation of 27-day variation of galactic cosmic rays, Izvestia RAN, ser., Fiz. , 3, 200 1, in press. Greenwich Photo-Heliographic Results, Greenwich, 19 17- 1954. Hattingh, M., and R. A. Burger, Some properties of a fully three-dimensional drift model for the modulation of galactic cosmic rays, Proc. 241hInter. Cosmic Ray Conj, 4, 337, 1995. Hattingh, M., R. A. Burger, M. S. Potgieter, and L.J. Haasbroek, Cosmic ray latitudinal effects predicted by a threedimensional drift model, Adv. Space Res., 19, 893, 1997. Kota, J., and J. R. Jokipii, Effects of drift on the transport of cosmic rays. VI. A three-dimensional model including diffusion, Astrophys. J., 265,573, 1983. Kota, J., and J. R. Jokipii, The role of corotating interaction regions in cosmic ray modulation, Geophys. Res. Lett., l&1797,1991. Mursula, K., and B. Zieger, The 13.5-day periodicity in the Sun, solar wind, and geomagnetic activity: the last three solar cycles. J. Geophys. Res., 101,27077, 1996. Parker, E. N., The passage of energetic charged particles through interplanetary space, Planet. Space Sci., 13,9, 1965. Richardson, LG., G. Wibberenz, and H.V.Cane, 22-year dependence in the size of near ecliptic corotating cosmic ray depressions during five solar minima, J. Geophys. Res., 104, 12549, 1999. Smith, E. J., J. R. Jokipii, J. Kota, R. P. Lepping, and A. Szabo, Evidence of a N-S asymmetry in the heliosphere associated with a southward displacement of the heliospheric current sheet, A@ophys. J., 353, 1084,200O. Solar data, Nauka, Pulkovo, 1955-1995 (in Russian). Vemov, S. N., T. N. Charakhchyan, and G. A. Bazilevskaya, Anomalies of solar activity in 1964-1965, in the Proc. 16th Inter. Cosmic Ray Conj, Kyoto, 3,385, 1979. Vernova, E. S., M. I. Tyasto, D. G. Baranov, and M. S. Grigorian, The effect of solar activity longitudinal distribution of the northern and southern solar hemispheres on the 27-day galactic cosmic ray variation, Izvestia RAN, ser.Jiz., 59, 135, 1995. Vizoso, G., and J. L. Ballester, North-south asymmetry in sudden disappearances of solar prominence, Solar Phys., 112,317, 1987.