Study Temporal Variations Of Cosmic Ray Anisotropy With Yangbajing ASγ Tibet-Ⅲ Array

The Earth is continuously bombarded by highly energetic particles-the cosmic rays(CRs).CRs are high-energy nuclei(mostly protons) from outer space.For nearly 100 years after their discovery,the origin and acceleration of the highest energy particles are still unknown.Most CRs are believed to be accelerated by supernova remnants (SNRs) in our Galaxy and continuously reach the Earth after propagating through the Galaxy and heliosphere.The intensity of CRs is nearly isotropic due to deflections of CRs in the Galactic magnetic field(GMF).However,extensive observations do show that there exists a slight anisotropy on the overall isotropic background.The research of CR anisotropy plays an important role because it has close relationship with the origin,acceleration and propagation of CRs.Until now,there are no convincing and widely accepted explanations for CR anisotropy.It may arise from several causes as follows.Firstly,it may result from the uneven distribution of CR sources such as SNRs and the process of CR propagation in the Galaxy.Secondly,the anisotropy can also be induced through both large-scale and local magnetic field configurations,possibly including effects of the heliosphere.In addition,an expected anisotropy is caused by the relative motion between the observer and the CR plasma,known as the Compton-Getting(CG) effect.From the analysis of numerous experiments,it can be seen that both the amplitude and the phase of the best-fit first harmonic vary with CR energy in a wide range from tens of GeV to PeV.Below several tens of GeV,solar modulation effects are most notable for CRs.CRs interact with the solar wind magnetic field,both an regular field and irregular field components,after entering the heliosphere.The spatial distribution of CRs can reflect the magnetic structure in the solar wind.With increasing energy, CRs become less sensitive to the solar modulation.In the multi-TeV range,the gyro radius of hundreds of AU becomes comparable to the spatial scale of the heliosphere in the nose direction toward the upstream of the interstellar medium flow.However,it is known that the heliosphere has a long heliotail,the modulation in the heliotail remains possible.Therefore,the large-scale sidereal anisotropy of CRs in this energy range gives us an important clue about the magnetic field structure of the heliosphere or the local interstellar space surrounding the heliosphere.The solar cycle shows a quasi-period of about 11 years and the global magnetic polarity reverses with a quasi-period of two solar activity cycles.Since CRs may be modulated by solar activities in the multi-TeV range as mentioned above,it might be expected that the sidereal anisotropy may follow the variation of solar cycle.Many experiments have been devoted to study the correlation between CR anisotropy and solar activity.There are contradictions among different observations and needs cross check from other experiments.Meanwhile,the solar anisotropy at 600 GV shows a clear 11-yr change related to the solar activity.It may give the the upper limiting energy of solar modulation in the heliosphere.The Tibet AS_γexperiment also reported the extra modulation which exceeds the expected CG effect at 4 TeV,suggesting the solar modulation effect possibly extending up to multi-TeV energies.In this thesis,the study on the temporal variation of the solar anisotropy at this energy can give useful information to understand the extra effect.The Tibet AS_γexperiment has been operating successfully at Yangbajing(90.522°E,30.102°N;4300 m above the sea level) in Tibet,China since 1990.The TibetⅢarray was completed in the late fall of 1999.Taking advantage of the large field of view and high count rates as well as the good angular resolution of the incident direction, the TibetⅢAir Shower Array provides currently the world's highest precision measurement of CR intensity in multi-TeV energy range.Using the equi-zenith principle, a developed analysis method can give two-dimensional structures of CR intensity. This analysis gives more information than simple 1D analysis in understanding the anisotropy.The observation period runs from 1999 November to 2008 December, covering more than a half of the 23rd solar activity cycle from the maximum to the minimum.Therefore,we can do more precise study on the anisotropy year by year in correlation with the solar cycle,not only the simple 1D profile as given by former experiments,but also the variations of 2D CR intensity maps in correlation with the solar activity.With previously developed method,when we analyze the anisotropy in each different period,the data have to be taken for a long enough time to avoid the mutual interference of the adjacent periods.We also have to do live time correction,considering the uneven event rate due to the discontinuous observational data taking.To study the CR anisotropy in a short term interval,we should look for a new effective method to avoid the mutual interference of different period.Using Lomb-Scargle Fourier transformation method with CR data recorded by the TibetⅢarray,Tibet AS_γexperiment has showed that except the well-known solar diurnal,sidereal diurnal and sidereal semi-diurnal modulations at a level of～10^-3,no other periodicity was found to have high enough significance from 1 hour to 2 years in the energy range from～3.0 TeV to～12.0 TeV.Based on this,we assume that at any moment t,the relative intensity of multi-TeV CRs at any given direction is modulated as a product of intensities in two different periods respectively.Different from the former method used in AS_γ, we can obtained the CR intensity variations both in the sidereal time and the solar time simultaneously using the relative short-time observations,and need not to do the live time correction due to the uneven observational data taking.In this thesis,we analyze temporal variations of solar anisotropy and sidereal anisotropy of multi-TeV CR intensity using the data of TibetⅢarray from 1999 November to 2008 December.Besides the results of temporal variations of 1D harmonics fit parameters,we gives the variations with time of 2D anisotropy structures. We found that both sidereal and solar anisotropy are stable during entire TibetⅢobservations. Note that at this point,we cannot investigate the influence of the polarity reversal of the global solar magnetic field on the sidereal anisotropy due to the lack of data before the current magnetic field reversal.We do not use the data of Tibet HD to perform the year-by-year analysis due to the limitation of the statistics,although it covers the period before the reversal.To study the influence of solar polarity reversal on anisotropy,we should keep on our continuous observations.The nature of the anisotropy and other unsolved problem need our further study.There are three innovative points in this thesis.Firstly,we can obtain the CR anisotropy using limited data which are taken from short term run phase of detectors. The method can avoid the mutual interference of the adjacent periods,and need not do live time correction used in former analysis.Secondly,it gives the proper 1D projection from 2D sky map,the obtained harmonic parameters can be compared with other experiments.And the property is verified by the corresponding Monte Carlo simulation.Third,we got the highest precision measurement on temporal variations of multi-TeV CR anisotropies both in solar time and sidereal time.Besides variations of the harmonics fits parameter,we can see the variations of the 2D CR intensity maps. And the deficiency of this work is:Limited to the statistics,we can not investigate the influence of the polarity reversal of the solar magnetic field.It needs our further continuous observations until covering the next reversal of solar polarity.