33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013 THE ASTROPARTICLE PHYSICS CONFERENCE Diffusion coefficient and radial gradient of galactic cosmic rays

Abstract: We present the temporal changes of the diffusion coefficient K of galactic cosmic rays (GCRs) at the Earth orbit calculated based on the experimental data using two different methods. The first approach is based on the Parker convection-diffusion approximation of GCR modulation [1]: i.e. K ∼Vr=dI where dI is the variation of the GCR intensity measured by neutron monitors (NM),V is the solar wind velocity and r is the radial distance. The second approach is based on the interplanetary magnetic field (IMF) data. It was suggested that parallel mean free path l ∥ can be expressed in terms of B as follows [2]-[4]: l ∥ µ B 5 3 dB2, where dB is the standard deviation. In our calculations we used an approximately equivalent expression l ∥ µ BdB. Using data of the product l∥Ñrn of the parallel mean free path l ∥ and radial gradient Ñrn of GCR calculated based on the GCR anisotropy data (Ahluwalia et al., this conference ICRC 2013, poster ID: 487 [5]), we estimate the temporal changes of the radial gradient Ñrn of GCR at the Earth orbit. We show that the radial gradient Ñrn exhibits a strong solar cycle dependence (11-year variation) and a weak solar magnetic cycle dependence (22-year variation), being in agreement with the previous other calculations and with PIONEER/VOYAGER observations