STUDY OF COSMIC RAY ANISOTROPY ALONGWITH INTERPLANETARY AND SOLAR WIND PLASMA PARAMETERS

COSMIC RAYS STUDY OF COSMIC RAY ANISOTROPY ALONGWITH INTERPLANETARY AND SOLAR WIND PLASMA PARAMETERS RAJESH KUMAR MISHRA 1, REKHA AGARWAL MISHRA 2 1 Computer and I.T. Section, Tropical Forest Research Institute, P.O. RFRC, Mandla Road, Jabalpur (M.P.) India 482 021 2 Department of Physics, Govt. Autonomous Model Science College, Jabalpur (M.P.) 482 001, India E-mail: rkm_30@yahoo.com or rajeshkmishra20@hotmail.com Received March 20, 2006 Using the cosmic ray intensity data recorded with ground-based neutron monitor at Deep River has investigated taking into account the associated interplanetary magnetic field and solar wind plasma data during 1981 94. A large number of days having abnormally high/low amplitudes for successive number of five or more days as compared to annual average amplitude of diurnal anisotropy have been taken as high/low amplitude anisotropic wave train events (HAE/LAE). The results clearly indicate that the time of maximum of diurnal variation significantly remains in the 18-Hr direction for majority of the HAE/LAE cases. The phase of enhanced diurnal anisotropy shows a remarkable systematic shift towards later hours as compared to the co-rotational direction for some of the HAE cases, whereas it shows a remarkable systematic shift towards earlier hours for some of the LAE cases as compared to the co-rotational direction. The occurrence of these high/low amplitude events is found to be independent of the nature of the IMF polarity. The high-speed solar wind streams (HSSWSs) do not play any significant role in causing these types of events. The source responsible for these unusual anisotropic wave trains in CR has been proposed. Key words: Anisotropy, high-speed solar wind streams and interplanetary magnetic field. 1. INTRODUCTION The long-term (solar cycle) variation of galactic cosmic ray intensity and its association with various solar, interplanetary and geophysical parameters; often have revealed contradictory results [1, 2, 3, 4, 5, 6, 7]. Many authors [3, 7, 8] have studied the relation between the solar activity parameters and the cosmic ray intensity. Balasubrahmanyan [9] has found a relation between the cosmic ray intensity and the geomagnetic activity. In all cases hysteresis effect between the different parameters and the cosmic ray intensity is clearly manifested in the dependence of cosmic ray intensity on the magnitude of the above parameters [10] Rom. Journ. Phys., Vol. 51, Nos. 9 10, P. 981 995, Bucharest, 2006

982 Rajesh Kumar Mishra, Rekha Agarwal Mishra 2 above parameters [10]. Xanthakis et al. [11] studied the cosmic ray modulation in the 20 th solar cycle, presented a more elaborate model. According to this model the modulated cosmic ray intensity that was measured by the groundbased stations is equal to the galactic cosmic ray intensity (un-modulated) at a finite distance corrected by a few appropriate solar and terrestrial activity indices, which cause the disturbances in interplanetary space. The systematic and significant deviations in the amplitude/phase of the diurnal/semi-diurnal anisotropy from the average values [12] are known to occur in association with strong geomagnetic activity [13]. Rao et al. [14] have shown that the enhanced diurnal variation of high amplitude events exhibits a maximum intensity in space around the anti-garden hose direction and a minimum intensity around the garden hose direction. Number of high amplitude events has been observed with a significant shift in the diurnal time of maximum to later hours or earlier hours [15]. Such days are of particular significance when occur during undisturbed solar/interplanetary conditions, as the superposed universal time effects are expected to be negligible. Mavromichalaki [13] observed that the large amplitude wave trains of cosmic ray intensity during June, July and August 1973. These events exhibit the same characteristics as the event of May 1973. During these days the phase of the enhanced diurnal anisotropy is shifted to a point earlier then either the corotation direction or the anti-garden-hose direction. The diurnal anisotropy is well understood in terms of a convective-diffusive mechanism [16]. Mavromichalaki [17, 18] has been observed that the enhanced diurnal variation was caused by a source around 1600 Hr or by a sink at about 0400 Hr. It was pointed out that this diurnal variation by the superposition of convection and field-aligned diffusion due to an enhanced density gradient of 8% AU 1. During the study of diurnal anisotropy of cosmic-ray intensity observed over the period 1970 1977 using the neutron monitor data of Athens and Deep River stations, Mavromichalaki [19] found that the phase of diurnal variation shows a remarkable systematic shift towards earlier hours than normally beginning in 1971. This phase shift continued until 1976, the solar activity minimum, except for a sudden shift to later hours for one year, in 1974, the secondary maximum of solar activity. It is observed that the behaviour of the diurnal phase has been consistent with the convective-diffusive mechanism, which relates the solar diurnal anisotropy of cosmic rays to the dynamics of the solar wind and of the interplanetary magnetic field. It once again confirmed the field-aligned direction of the diffusive vector independently of the interplanetary magnetic field polarity. It is also pointed out that the diurnal phase may follow in time the variation of the size of the polar coronal holes. All these are in agreement with the drift motions of cosmic-ray particles in the interplanetary magnetic field during this time period.

3 Study of cosmic ray anisotropy 983 An investigation has been conducted by Ananth et al. [20] on the long-term changes in diurnal anisotropy of cosmic rays for the two solar cycles (20 and 21) during the period 1965 1990. They observed that the amplitude of the anisotropy is related to the characteristics of high and low amplitude days. The occurrence of high amplitude days are found to be positively correlated with the sunspot cycle while the low amplitude days are correlated negatively with the sunspot cycle. Further, the variability of the time of maximum of the anisotropy indicates that it is essentially composed of two components; one in the 1800 Hr (corotation) direction and the other, an additional component in the 1500 Hr direction (45 east of the S-N line) apparently caused by the reversal of the solar polar magnetic field. They also suggest that the direction of the anisotropy of high and low amplitude days contribute significantly to the long-term behaviour of the diurnal anisotropy as it produces an additional component of cosmic rays in the radial (1200 Hr) direction. Ananth et al. [21] suggested that the enhanced wave trains do not reveal any correlation with the solar or geomagnetic activity index and the direction of the anisotropy lies along the 1800 Hr (corotational) direction. But the low amplitude days show an inverse correlation with solar activity and have a time of maximum along the 1500 Hr direction. They also noticed that the slope of power spectrum density roughly characterized by power spectral index n' in the high frequency range 3.5 10 5 Hz to 8.3 10 4 Hz (time scales of 20 min. to 8 Hr) is different for the two classes of events. They suggested that different types of interplanetary magnetic field distributions produce the enhanced and low amplitude cosmic ray diurnal variations. Iucci et al. [22] and Shukla et al. [23] have noticed the close correspondence between the cosmic ray intensity decreases observed by high latitude neutron monitors and the increase in the solar wind speed during the period of highspeed streams, which probably originate in coronal holes. They have also shown that the high-speed streams produced by solar flares are accompanied by Forbush decreases whose amplitudes are not directly correlated with the increase in solar wind speed. These latter decreases are usually large and are dependent on the location of the solar flares. Venkatesan et al. [24] observed a difference in the rigidity spectrum of the short-term variation of cosmic ray intensity; which is attributed to the two types of high-speed solar wind streams of different solar origin, e.g., coronal hole and solar active regions. This difference in the rigidity spectrum of cosmic ray intensity has implications on the understanding of both the short- and long-term variations of cosmic ray intensity [5, 25]. They noticed that the small but significant cosmic ray intensity decreases with almost a flat rigidity spectral variation (exponent 0) are associated with a large number of high speed streams essentially predominant during the declining phase of the sunspot cycle. The number as well as the effect of large solar active centers is minimal during

984 Rajesh Kumar Mishra, Rekha Agarwal Mishra 4 these periods. Which is consistent with the significant residual modulation of galactic cosmic ray intensity during years of minimum sunspot activity [5], when effects of solar polar coronal holes are more dominant [25, 26, 27]. It is also observed that during periods of sunspot maximum activity; these high-speed streams decline in number, but the ones present produce much larger decreases in cosmic ray intensity, the magnitude of which decreases with increasing particle rigidity (spectral exponent 1). In fact during high sunspot activity period, large Forbush decreases are observed during which the solar wind speed hardly exceeds 600 kms 1 and that too for much shorter periods. Nevertheless, their effects have been reported at large distances from the Sun [28] as well as in the long-term modulation of cosmic ray intensity [5]. Thus due to the time-varying rigidity-dependent effects on cosmic ray intensity which are attributed to two types of high speed solar wind streams emanating from two different solar sources, namely solar coronal holes and flare-associated solar active regions; the rigidity dependence of the long-term modulation of cosmic rays should vary with the phase of the solar cycle. Which is consistent with the results observed by Hatton [5] and Nagashima and Morishita [6, 7]. The solar flare generated high-speed solar wind streams are dominated during high solar activity period and produce large transient decrease in cosmic ray intensity [29]. After the identification of two types of solar wind plasma streams in 1988, several attempts have been made to show their influence on cosmic ray intensity on short-term basis [30]. More recently Shrivastava and Jaiswal [31] reported almost equal influence of FGS and CS solar wind streams on cosmic ray transient decreases for the period of 1991 to 1996, using the Oulu neutron monitors data. Shrivastava and Shukla [32] noticed that flare generated streams are more effective to produce cosmic ray decreases. SSC association with FGS enhanced in cosmic rays decreases. During the study of high-speed solar wind streams and cosmic ray intensity variation for the period 1991 to 1996, Shrivastava [33] observed that both the FGS and CS streams produce short-term transient decreases in cosmic ray intensity. It is also found that medium range (5 to 6 days duration) solar wind streams are found to be more effective in producing cosmic ray transient decreases. The average amplitude of diurnal and semi-diurnal anisotropy are found to be larger than normal during the initial phase of the stream while it is smaller as compared to the normal during the decreasing phase of the stream and phase is observed to remain almost constant [34], which infer that the diurnal as well as semi-diurnal variation of galactic cosmic ray intensity may be influenced by the solar polar coronal holes. The changes have also been observed in the amplitude and phase during the high-speed solar wind streams (HSSWS) coming from coronal holes [35, 36]. The diurnal variation might be influenced by the polarity of the magnetic field [37], so that the largest diurnal variation is observed during the days when the daily average magnetic field is directed outward from the Sun.

5 Study of cosmic ray anisotropy 985 The amplitude of the diurnal anisotropy is observed to be significantly large during the three types of clouds [38], in comparison of the amplitude observed on geomagnetically quiet days [39]. The phase has also observed to be shifted to earlier hours during these clouds in comparison of the phase on geomagnetically quiet days (QD). The behaviour of semi-diurnal anisotropy on LAE has been studied by Jadhav et al. [40], by comparing the average semidiurnal amplitude for each event with 27-day or annual average semi-diurnal amplitude. They observed no significant difference between the two wave trains. The semi-diurnal amplitude is observed as to be normal, which indicates that the diurnal and semi-diurnal anisotropies of daily variation are not related to each other for these LAE cases. An attempt has been made in this paper to investigate the probable reason causing the occurrence of these types of unusual events in CR intensity observed over the period 1981 94. 2. DATA ANALYSIS The pressure corrected data of Deep River Neutron monitor NM (cut off rigidity = 1.02 GV, Latitude = 46.1 N, Longitude = 282.5 E, Altitude = 145 M) has been subjected to Fourier Analysis for the period 1981 94 after applying the trend correction to have the amplitude (%) and phase (Hr) of the diurnal and semi-diurnal anisotropies of cosmic ray intensity for unusually low amplitude events. The amplitude of the diurnal anisotropy on an annual average basis is found to be 0.4%, which has been taken as reference line in order to select low amplitude events. The days having abnormally low amplitude for a successive number of five or more days have been selected as low amplitude anisotropic wave train events and having abnormally high amplitude for a successive number of five or more days have been selected as high amplitude anisotropic wave train events. The anisotropic wave train events are identified using the hourly plots of cosmic ray intensity recorded at ground based neutron monitoring station and selected twenty eight unusually low amplitude wave train events and 37 high amplitude anisotropic wave train events during the period 1981 94. Further, various features which are observed over the solar disk during the periods of events, have also been studied. 3. RESULTS AND DISCUSSION The amplitude (%) and phase (Hr) of each HAE has been plotted in Fig. 1. It is quite apparent from Fig. 1 that the phase of diurnal anisotropy has shifted to earlier hours in some of the events. During these events a negative magnetic cloud

986 Rajesh Kumar Mishra, Rekha Agarwal Mishra 6 Fig. 1. Amplitude and phase of the diurnal anisotropy for HAE of 24 29 Sept. 1983 and 4 8 Feb. 1993. without shock has noticed on Sep. 25 th at 0300 UT. Principal Magnetic storms on Feb. 7 th, 1993 at 0500 UT and Feb. 8 th, 1993 at 1900 UT has been observed during these events. However, for majority of HAEs plotted in Fig. 2 the phase of diurnal anisotropy remains in the co-rotational/18-hr direction. Which is in partial Fig. 2. Amplitude and phase of the diurnal anisotropy for HAE of 2 9 Sept. 1981, 17 21 Jul. 1983 and 2 7 Oct.1992.

7 Study of cosmic ray anisotropy 987 agreement with earlier findings [41, 42] where they noticed that the phase shifted towards later hours for some of the HAE, whereas it remains in the 18-Hr direction for majority of the HAE events during the period 1981 90. Principal Magnetic storm on Sep. 8 th, 1981 at 2141 has been observed during these events. Further, amplitude (%) and phase (Hr) of semi-diurnal anisotropy for HAE is plotted in Fig. 3. It is quite clear from Fig. 3 that the amplitude of the semi-diurnal anisotropy for each HAE remains statistically the same; whereas, the phase has shifted to later hours. Which is consistent with earlier findings [41, 42]. Three PMS on Mar. 20 th, 1994at 1600 UT, Mar. 21 st, 1994 at 0255 UT and Mar. 25 th, 1994 at 1517 UT has been observed during these events. Fig. 3. Amplitude and phase of the semi-diurnal anisotropy for HAE of 24 29 Sept. 1983 and 20 27 March 1994. The amplitude and phase of the diurnal anisotropy along with quiet days annual average values has been plotted in Fig. 4. It has been found that the amplitude of the diurnal anisotropy for each HAE is significantly large than the quite day annual average values throughout the period and the phase of the diurnal anisotropy has shifted to earlier hours as compared to the quiet day annual average values. Which significantly inconsistent with earlier findings [42] where they noticed that diurnal amplitude remains significantly large but the diurnal time of maximum shifts towards later hours as compared to the quiet day annual average values. The amplitude and phase of LAEs has been plotted in Fig. 5. It is quite apparent from these plots that for majority of the LAEs the phase of the diurnal anisotropy remains in the azimuthal or co-rotational direction. Two PMS on May 1 st, 1991 at 0100 UT and May 2 nd, 1991 at 0200 UT has been observed during these events. Whereas for some of the LAEs plotted in Fig. 6, the time of maximum has shifted to earlier hours. Which is in good agreement with earlier findings

9 Study of cosmic ray anisotropy 989 Fig. 6. Amplitude and phase of the diurnal anisotropy of LAE for the events 20 25 Apr. 1981, 17 23 Oct. 1992 and 4 8 Oct. 1994. [43, 42]. A positive magnetic cloud without shock has occurred on Apr.20 th, 1981 at 2300 UT during these events. Five PMS on Oct. 18 th, 1992 at 1200 UT, Oct. 19 th, 1992 at 2200 UT, Oct. 5 th, 1994 at 0800 UT, Oct. 6 th, 1994 at 1700 UT and Oct. 7 th, 1994 at 1300 UT has been observed. SSC has not been observed during these events. In Fig. 7 the amplitude and phase of the diurnal anisotropy for all the LAEs along with amplitude and phase of quiet day annual average have been plotted. It is observed that the amplitude of the diurnal anisotropy for majority of the LAE events attains significantly lower values as compared to the quiet day annual average amplitude throughout the period and the phase of the diurnal anisotropy has a tendency to shift towards earlier hours as compared to the quiet day annual average value for majority of the LAEs. Similar trends have been noticed by Kumar and Chauhan [43] and Kumar et al. [42] for the period 1981 90. Further, in Fig. 8 the amplitude and phase of the semi-diurnal anisotropy have been plotted. It is evident from these plots that amplitude of the semi-diurnal anisotropy remains statistically the same for all LAEs; whereas, the time of maximum is found to shifts to later hours. Similar trends have been observed by Jadhav et al. [40] for the period 1966 73 and Kumar and Chauhan [43] and Kumar et al. [42] for the period 1981 90. A PMS on Dec. 21 st, 1993 at 0100 UT has been observed.

11 Study of cosmic ray anisotropy 991 For individual HAE/LAE cases, the interplanetary magnetic field (IMF) and solar wind parameter (SWP) have also studied. The amplitude and time of maximum of the diurnal anisotropy for each HAE/LAE case along with the variation in the associate values of the z-component of the interplanetary magnetic field, i.e., Bz have been depicted in Fig. 9 and Fig. 10. It is quite observable from these Figs that the amplitude of diurnal anisotropy is evenly aligned for both positive and negative polarity of IMF for all HAEs. The amplitude of diurnal anisotropy for both the polarity is higher and phase shifts towards earlier hours as compared to the co-rotational values for most of the HAEs. Which shows that the occurrence of HAE events is independent of nature of Bz. It is noticed that for positive or away polarity of IMF, the amplitude is high and phase shifts to early hours; whereas, for negative or towards polarity of IMF the amplitude is lower and phase shifts to early hours as compared to co-rotational value during 1967 68 [44, 45]. The trends we noticed in this study for HAE reveals that for both away and towards polarity days the time of maximum for diurnal anisotropy shifts to later hours or it remains in the 18-Hr direction with an exception for one event. An enhanced mean amplitude of diurnal anisotropy correlates with positively directed sectors while the amplitude of the diurnal anisotropy seems to decrease during sector boundaries [46], which Fig. 9. Amplitude and phase of the diurnal anisotropy for each HAE along with the variation in associated value of Bz.

992 Rajesh Kumar Mishra, Rekha Agarwal Mishra 12 Fig. 10. Amplitude and phase of the diurnal anisotropy for each LAE along with the variation in associated value of Bz. significantly differs with our findings; that is the occurrence of high amplitude anisotropic wave train events is independent of nature of IMF polarity. Further for LAE events the amplitude of the diurnal anisotropy for positively directed IMF (+Bz) is significantly large for most of the events; whereas, the amplitude remains significantly low for negatively directed IMF ( Bz) for most of the LAE events. The time of maximum of the diurnal anisotropy for both positive and negative polarity of Bz has a tendency to shift towards earlier hours as compared to co-rotational value for the each LAE events. Which is well consistent with the earlier trends reported by Hashim and Bercovitch [44] and Kananen et al. [45], for the period 1967 68 i.e. for positive or away polarity of IMF, the amplitude is high and phase shifts to early hours; whereas, for negative or towards polarity of IMF the amplitude is lower and phase shifts to early hours as compared to co-rotational value. The frequency histogram of solar wind velocity for each HAE/LAE has been plotted in Figs. 11a and b. It is observable from Figs. 11a and b that the majority of the HAE/LAE events have occurred when the solar wind velocity lies in the interval 400 500 km/s i.e., being nearly average. Usually, the velocity of high-speed solar wind streams (HSSWSs) is 700 km/s [35]. Therefore, it may infer from these figs that HAE/LAE events are not caused either by the HSSWS or by the sources on the Sun responsible for producing the HSSWS such as polar coronal holes (PCH) etc. Thus, we may conclude that the HSSWS do not serve

13 Study of cosmic ray anisotropy 993 Fig. 11. Frequency histogram of solar wind velocity for all (a) HAE and (b) LAE during 1981 94. an important role in causing the HAEs/LAEs, which is consistent with earlier findings [35] and inconsistent with the earlier results that the solar diurnal amplitude is enhanced during the HSSWSs coming from coronal holes [47, 48]. Ahluwalia and Riker [49] have found that the annual mean amplitude appears to have large values during the epoch of HSSWS. The amplitude of the diurnal anisotropy in free space is found to enhance by 0.15 % along 17 18 Hrs LT in HSSWS while it is diminished by 0.1 % along 17 18 Hrs LT, in low speed solar wind streams [50]. Further, for HAE/LAE the second peak shown in Fig. 11a and b where SWV is in the range of 450 500 Km/sec is attributed to the days when the polarity of IMF is not very well defined i.e., either it is positive or negative or mixed polarity days. According to Ahluwalia and Riker [51] there is no relation between solar wind speed and diurnal variation in high rigidity region. The modulation of solar diurnal anisotropy is weakly or less dependent on the solar wind velocity [35]. No significant difference has been found between the variation of diurnal vectors in high-speed days and the days, when, the speed is normal. 4. CONCLUSION The following conclusions may be drawn on the basis of above findings: The results clearly indicate that the amplitude persists continually high/low for at least five or more number of days and the phase of the diurnal

994 Rajesh Kumar Mishra, Rekha Agarwal Mishra 14 anisotropy remains in the co-rotational direction for majority of the HAE/LAE. However, it shifts towards earlier hours for some of the HAE/LAE cases The amplitude of semi-diurnal anisotropy remains statistically invariant, whereas, phase has shifted to later hours for both types of events. The amplitude of diurnal anisotropy remains significantly high for HAEs and low for LAEs; whereas the time of maximum shifts towards earlier hours for both type of events as compared to the quite day annual average values. The occurrence of HAE/LAE is dominant when solar wind velocity is being average or moderate, which shows that these are not due to HSSWS. The nature of IMF polarity does not serve any significant role in causing HAE/LAE events. These observations indicate that the HAE/LAE could be the consequence of the accumulated effects of the multiple sources causing such an unusual behaviour of diurnal anisotropy in CR intensity. During such wave trains these sources affect the anisotropy such that the amplitude remains persistently high/low and the phase retains the direction of corotation; whereas, on the other hand, the amplitude remains high/low but the phase shifts to early hours for some of the HAE/LAE cases. Nevertheless, it is obvious that these sources act in such a way that either the resultant anisotropy remains in the co-rotational direction or two sources act oppositely. Thus, we may conclude that the interplanetary transients responsible for unusual behaviour of cosmic ray modulation do not reach the Earth and the possible source(s) to cause the HAE/LAE may be due to the intense solar activity on the backside of the visible solar disk. Acknowledgements. The authors are indebted to various experimental groups, in particular, Prof. Margret D. Wilson, Prof. K. Nagashima, Miss. Aoi Inoue and Prof. J. H. King for providing the data. REFERENCES 1. S. E. Forbush, J. Geophys. Res., 63, 651, 1958. 2. U. R. Rao, Space Sci. Rev., 12, 719, 1972. 3. M. A. Pomerantz and S. P. Duggal, Rev. Geophys. Space Phys., 12, 343 (1974). 4. H. Moraal, Space Sci. Rev., 19, 845, 1976. 5. C. J. Hatton, Solar Phys., 66, 159, 1980. 6. K. Nagashima and I. Morishita, Planet. Space Sci., 28, 177, 1980a. 7. K. Nagashima and I. Morishita, Planet. Space Sci., 28, 195, 1980b. 8. J. Xanthakis, in C. Macris (ed.), Physics of the Solar Corona, D. Reidel Publ. Co., Dordrecht, Holland, 1971. 9. V. K. Balasubrahmanyan, Solar Phys., 7, 39, 1969. 10. H. Mavromichalaki and B. Petropoulos, Astrophys. Space Sci., 106, 61, 1984.

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Fig. 4. Amplitude and phase of the diurnal anisotropy for each HAE along with quiet day annual average values during 1981 94. Fig. 5. Amplitude and phase of the diurnal anisotropy of LAE for the events 13 18 June 1985, 13 18 Jan 1991 and 1 5 May 1991.

Fig. 7. Amplitude and Phase of diurnal anisotropy for LAE alongwith quiet day annual average values during 1981 94. Fig. 8. Amplitude and phase of the semi-diurnal anisotropy of LAE for the events 13 18 June 1985, 13 18 Jan 1991and 21 25 Dec. 1993.