ABSTRACT. Introduction

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1 Characteristics of high speed solar wind streams in cosmic ray decreases Rekha Agarwal 1, Rajesh K. Mishra 2 and P. Shrivastava 3 1Department of Physics, Govt. Model Science College (Autonomous), Jabalpur (M.P.) , India 2Computer and IT Section, Tropical Forest Research Institute, P.O. RFRC, Mandla Road, Jabalpur (M.P.) , India 3 Tropical Forest Research Institute, P.O. RFRC, Mandla Road, Jabalpur (M.P.) , India rm_jbp@yahoo.co.in, rkm_30@yahoo.com ABSTRACT Two types of fast solar-wind streams are identified. Quiet streams, coming from near equatorial holes: stream interface in the initial phase followed by low proton density, quiet interplanetary magnetic field with constant polarity and proton temperature following the wind speed time profile. Perturbed streams, coming from active regions producing type-iv solar flares, for which the previous requirements are not satisfied. The high-speed solar wind streams lasting for several days are observed by satellites and spacecraft. These streams produce geomagnetic disturbances and changes in the level of cosmic ray intensity. High-speed plasma streams identified in the solar wind measurements can be separated into two categories: coronal-hole-associated streams and flare-generated streams. The influence of two types of high speed solar wind streamscoronal-hole and solar-flare-associated on cosmic ray intensity has been studies using the neutron monitor data of three different neutron monitoring stations having varying cut off rigidity threshold. Cosmic ray intensity data along with solar wind plasma and interplanetary magnetic field data were subjected to superposed epoch analysis with respect of these two types of high-speed solar wind streams. Effect in cosmic ray intensity influenced by coronal hole streams are much smaller than the typically Forbush like depressions. During these events the interplanetary magnetic field strength (B) and solar wind speed observed to increase significantly with some deviations. A significant correlation has been observed between disturbance storm time index (Dst) with geomagnetic activity index (Ap) and solar wind plasma temperature with plasma density. Superposed epoch analyses applied on solar-wind speed and neutron monitor intensity show that during the high-speed streams presumably coming from coronal holes the cosmic-ray intensity is depressed; the time behaviour of this depression follows the time profile of the wind speed. The perturbed streams are accompanied by Forbush decreases whose amplitude and time behaviour are not directly related to the speed increase. Keywords: cosmic ray, high speed solar wind streams, coronal hole, flare. Introduction Lot of studies indicates that the open magnetic flux from coronal holes may be a more important driver of cosmic-ray modulation than coronal mass ejections (CMEs), which originate from closed-field regions on the Sun. Earlier, it was thought that solar flares were responsible for major interplanetary particle events and geomagnetic storms.

2 However, recently we have seen an important paradigm shift such that now coronal mass ejections (CMEs), not flares, are considered the key causal link with solar activity. CMEs are plasma eruptions from the solar atmosphere involving previously closed field regions, which are expelled into the interplanetary medium. Such regions, and the shocks which they may generate, have pronounced effects on cosmic ray densities both locally and at some distance away. These energetic parti-cle effects can often be used to identify CMEs in the interplanetary medium, where they are usually called `ejecta'. When both the ejecta and shock effects are pre-sent the resulting cosmic ray event is called a `classical, two-step' Forbush decrease. Bieber and Evenson [1] noticed strong enhancements of the cosmic ray anisotropy before and during the January 1997 CME/magnetic cloud. From a multi-station analysis of neutron monitor data, they conclude that B n drift is a primary source of CME-related anisotropies for 5 GeV cosmic rays. Evolution of the cosmic ray density and density gradients is closely linked to magnetic properties of the ejecta, and provides information on the magnetic cloud and related features as they approach and pass Earth. Strong enhancement of the field-aligned anisot-ropy was observed primarily during the 9 hours prior to shock arrival condition of Earth. Cane et al. [2] reported a significant relationship between CMEs and cosmic ray variations. Shrivastava [3] argued that the coronal mass ejections in association with B- type solar flare might be the reason for the enhancement of geomagnetic field variation and CMEs indicate its better role in cosmic ray modulation. The intensity of galactic cosmic rays measured on Earth is related to the Sun's cycle of activity, which is well known by astronomers. The solar magnetic field flips every 11 years and the number of sunspots and 'coronal mass ejections' rises and falls twice in each complete 22-year cycle. The cosmic ray intensity on Earth also peaks twice every 22 years in time with the solar cycle. Cliver and Ling [4] have discovered a quirk in this pattern - and they believe that coronal mass ejections could be respon-sible for it. Cliver and Ling [4] propose that when cosmic rays im-pinge on the solar poles early in an 11-year cycle, they do not encounter CMEs. But cosmic rays do meet CMEs when they approach the equator at this time in the solar cycle. This means that the interaction of cosmic rays with the strong magnetic fields of CMEs affects the intensity of cosmic rays on Earth. There are many uncertainties inherent in predicting long-term trends from relatively short-term measurements, as Cliver and Ling point out. But the pattern is clearly evident from the data so far. Data Analysis Using the long-term plots of the cosmic ray intensity data as well as the amplitude observed from the cosmic ray pressure corrected hourly neutron monitor data using harmonic analysis the High amplitude wave train events have been selected on the basis of following criteria: 1. High amplitude wave train events of continuous days have been selected when the amplitude of diurnal anisot-ropy remains higher than 0.4% on each day of the event for at least five or more days. 2. In the selection of these types of events, special care has been taken, i.e. if there occurred any pre-forbush de-creases or post-forbush decrease before or after the event or if the event is in recovery phase or declining phase are not considered.

3 On the basis of above selection criteria we have selected thirty-eight high amplitude wave train events during the period The hourly cosmic ray intensity data for Deep River, Goose Bay, Inuvik neutron monitoring station have been investigated in the present study... Results and Discussion Cosmic-ray (CR) observations date back to the first half of the 20th century. Together with these observations an effort started to study the CR modulation and define the parameters that affect it (e.g., Forbush, 1958; Nagashima and Morishita, 1980a; Xanthakis, Mavromichalaki, and Petropoulos, 1981). Initially all data were restricted to ground-based measurements, since only after the 1960s were spacecraft able to provide us with data from interplanetary space.with the launch of PAMELA in 2006 and the planned launch of AMS-2 the situation will be greatly improved, as these space-borne detectors will directly and routinely measure the CR spectrum in a wide energy range. However, a major part of the data used in research originates from the worldwide neutron monitor network, since the Earth provides us with the highest accuracy cosmic-ray detector (see Belov, 2000). Only on the Earth have CR observations been carried out at the same distance from the Sun and within a narrow heliolatitude range for more than fifty years, covering six solar activity cycles and three solar magnetic cycles (Belov, 2000). The year 1974 is an interesting epoch with a remarkable 27-day recurrence in the occurrence of the HSSs. These HSSs arise from long-lived coronal holes which are located in both hemispheres of the sun and show the solar equatorial extension for this period. Coincidentally, en-hanced diurnal wave trains of the cosmic ray intensity variations also persist with a 27-day recurrence, as ob-served for example by the NM at Deep River and by the MU at Nagoya (vertical; Pm 60 GV) and shown in Fig. 1. In the figure, the daily intensity variations of the hourly means, taken as deviations from the 24- hur running averages, are dis-played. Iucci et al. (1983) and Dorman et al. (1984) investigated these diurnal wave trains with the presence of the HSS during 1974 using NM data at Deep River, and Munakata et al. (1987, referred to Paper I) followed them. They showed that the modulation of the diurnal anisotropy in space by the HSS is appreciable and significant; the amplitudes are enhanced, the phases are also modulated and are invariant in 18 hr direction, and the rigidity spectrum exhibits slightly positive. Swinson et al. (1980) carried out another kind of analysis of enhanced diurnal waves trains for 1974 using the MU data at Nagoya. However, the role of the HSS in cosmic ray pp c 2003 by Universal Academy Press, Inc modulation, specifically in terms of the daily variation of cosmic ray intensity variation, has not yet fully understood. The occurrence of two types of HSSWS during high amplitude days for the period has been plotted elsewhere. It is clearly seen from these plots that number of corotating streams is greater than the number of flare-generated streams and also indicates the tendency for larger duration in corotating streams for HAE. To study the effect of these streams on high amplitude days, we have adopted the Chree analysis of superposed epoch for days 5 to +5 and plotted (not shown here) as a percent deviation of cosmic ray in-tensity data along with statistical error bars (I)) for the period during HAE. Deviation for each event is obtained from the overall average of 11 days.

4 Epoch day (zero day) correspond to the starting days of high-speed solar wind streams. During coro-tating streams the de-crease in cosmic ray intensity starts from 4 day and reaches to minimum on 1 day i.e. before one day of the onset of HSSWS. It starts increasing from 1 day to +1 day and then de-creases upto +5 day. However during flaregenerated streams the intensity significantly in-creases 4 day and reaches to it s maximum on +1 day i.e. after one day of the onset of stream and then de-creases up to +5 day. Thus we observed that significant deviations are observed in cosmic ray intensity during HAE events for both types of solar wind streams. Shrivastava and Jaiswal [11] and Shrivastava [12] re-ported almost equal influence of flare generated and coronal hole associated solar wind streams on cosmic ray transient decreases. Badruddin [13] studied the two classes, coronal hole and solar flare associated streams along with the observed heliospheric plasma and field parameters of these streams such as speed, field strength and its variance in a system-atic manner in order to see their effects in cosmic ray modulation. He found that flare associated streams are much more effective in modulation than streams from coronal holes. However, the possibility that solar wind structures during two types of streams might be different, the field variance appears to be the most critical parameter responsible for this difference in their effectiveness in modulation. Sabbah [14] studied the behavior of cosmic rays ob-served by three stations during a time of high-speed so-lar-wind events. These stations cover the median rigidity range GV. The influence of the interplanetary magnetic field associated with HSSW has also been studied. They reported that both the cosmic-ray intensity and geomagnetic activity are enhanced by coronal-mass-ejection events. They argued that IMF magnitude and fluctuation are responsible for the depression of cosmic-ray intensity during HSSW events and this depression is rigidity dependent. Low-energy cosmic rays suffer more intensity depression. The rigidity spectrum of the cosmic-ray intensity decreases is dependent upon the phase of the solar cycle. Conclusion In the present study we noticed that on the onset of both types of streams the cosmic ray intensity reaches to its minimum during high amplitude events and then in-creases statistically. The two types of solar wind streams (Corotating and Flare generated) produce significant deviations in cosmic ray intensity during high amplitude anisotropic wave train events. References [1] N. Iucci, M. Parisi, M. Storini and G. Vil loresi, Nuovo Cimento, 2C, 421, [2] J.P. Shukla, A.K., Shukla, R.L. Singh, and S.P. Agrawal, Ind. J. Rad. And Space Phys., 8, 230, [3] R. S. Yadav, N. K. Sharma, and Badruddin, Solar Phys., 151, 393, [4] King, Interplanetary Medium Data Book-Supplement 2, NSSD/WDC-A, Goddard Space Flight Centre, Green-belt, MA, [5] J. King, Interplanetary Medium Data Book-Supplement 3, NSSD/WDC-A, Goddard Space Flight Centre, Greenbelt, MA, 1986a.

5 [6] J. King, Interplanetary Medium Data Book-Supplement 3A, NSSD/WDC-A, Goddard Space Flight Centre, Greenbelt, MA, 1986b. [7] J. King, Interplanetary Medium Data Book-Supplement 4, NSSD/WDC-A, Goddard Space Flight Centre, Greenbelt, MA, [8] J. King, and N. E. Papitashvili, Interplane-tary Medium Data Book, God-dard Space Flight Centre, Greenbelt, Mary-land, [9] H. Mavromichalaki, A. Vassilaki, and E. Mar-matsouri, Solar Phys., 115, 345, [10] R. Agarwal, and R. K. Mishra, J. Phys. Stu., 2 (4), 99, [11] P. K. Shrivastava, and K. L. Jaiswal, Solar Phys., 214, 195, [12] P. K. Shrivastava, 28th Int. Cosmic Ray Conf., Tsukaba, 3, 3731, [13] Badruddin, Astrophys. and Space Sci., 246, 2, 171, [14] Sabbah, Can. J. Phys./Rev. can. phys., 78, 4, 293, 2000.

LOW/HIGH AMPLITUDE ANISOTROPIC WAVE TRAIN EVENTS IN COSMIC RAY INTENSITY AS AN EFFECT OF INTERPLANATERY TURBULANCES

28th International Cosmic Ray Conference 3999 LOW/HIGH AMPLITUDE ANISOTROPIC WAVE TRAIN EVENTS IN COSMIC RAY INTENSITY AS AN EFFECT OF INTERPLANATERY TURBULANCES Rajesh K. Mishra 1 and Rekha Agarwal Mishra