27-day variation of galactic cosmic ray intensity by PAMELA experiment. Relationship with heliospheric parameters

Abstract

We analyzed the dynamics of the 27-day variation of galactic cosmic rays (GCR) using PAMELA data in the solar cycle 24 covering the period of 2006-2015. We studied the time line of the proton and helium spectra for different rigidities measured by PAMELA and compared with neutron monitor (NM) observations. We studied the relationship between the 27-day variations of GCR and heliospheric parameters: solar wind velocity and heliospheric magnetic field (HMF). We found that the 27-day variation observed by PAMELA was changing over the solar cycle 24. Near the solar minimum in the 2007-2008 it was observed the stationary 27-day variation with stable amplitude and time of maximum for almost ∼8 solar rotations. Solar wind velocity and Bx component of the HMF also showed the same clear 27-day periodicity. The recurrent changes of solar wind velocity were in strong antiphase with changes of GCR. In contrary in the rising phase of the solar cycle 24 the period of quasi-recurrent GCR variations was much more variable in time and in duration, in average 27-days, but also with the strong contamination of the higher harmonics. The antiphase relation between GCR and solar wind is not so clearly pronounced. Moreover, in the rising phase of the solar cycle 24 we established in all analyzed parameters some wave packages, periodically enhanced, taking place averagely each 1.5-2 years, suggesting the occurrence of the quasi biannual oscillations (QBOs).