DISTRIBUTION OF TENSOR ANISOTROPY OF COSMIC RAYS NEAR THE NEUTRAL CURRENT SHEET
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Abstract (English):
We analyze time profiles of isotropic intensity, components of vector and tensor anisotropies of cos-mic rays (CR) when Earth crosses the neutral sheet of the interplanetary magnetic field (IMF) in solar activity cycles 23–24. The moments of the crossings are de-termined from Wilcox Observatory synoptic charts and IMF data. Periods of Forbush decreases and ground level enhancements are excluded from the analysis. The events are analyzed for the epochs of positive and negative signs of the Sun’s general magnetic field. During each epoch, the crossings from the positive sector to the negative one and vice versa are separated. In total, 213 crossing events have been selected. The first two spherical harmonics of the angular CR-distribution are obtained using the global survey method. In each case, the average number of stations is equal to 32. The analysis shows that the temporal change of the isotropic component is caused by a magnetic mirror. For the first time, the zonal harmonics are reliably distinguished, and the existence of the antisymmetric diurnal CR-variation in a low energy range, which is oriented along IMF, is recognized. We compare our results with those obtained earlier.

Keywords:
cosmic rays, tensor anisotropy, neutral current sheet
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INTRODUCTION

The neutral current sheet (NCS) of the interplanetary magnetic field (IMF) has a specific corrugated structure as a relatively thin layer that separates opposite polarities of the Sun’s magnetic field [Svalgaard, 1972; Svalgaard, Wilcox, 1975]. NCS extends from the Sun to the heliopause boundaries, and its wave structure causes two- or four-sector IMF structure to form in the solar equatorial region [Hoeksema, Wilcox, Scherrer, 1983]. Thus, Earth when rotating around the Sun periodically experiences transitions between positive and negative IMF sectors.

Earlier [Krymsky et al., 2012], we have examined the behavior of angular CR distribution when  Earth crosses NCS, but as information on CR flux we used then only data from the neutron monitor station Moscow for January 1978 – January 2009. In the current study, the investigations are continued and expanded. We employ data from 32 stations of the worldwide network of neutron monitors and IMF and solar wind (SW) parameters from the OmniWeb database [http://omniweb.gsfc. nasa.gov/form/dx1.html]. The neutron monitor data are processed using the global survey method [Altukhov, Krymsky, Kuzmin, 1970]. We have thus obtained the components of the first two spherical harmonics of angular CR-distribution such as isotropic intensity, vector and tensor anisotropies.

References

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