The flux of galactic cosmic rays (GCRs) is isotropic in the interstellar space. However, in the heliosphere, the ram pressure of outward-moving solar wind convects the GCRs away from the Sun, thereby producing a density gradient in the radial direction. The diffusion of GCRs due to this gradient and scattering with the irregularities in the interplanetary magnetic field (IMF) induce variations in their flux that can be observed near the Earth. A framework for the diffusion-convection mechanism of GCR propagation developed by Parker and collaborators [Phys. Rev. 110, 1445 (1958); Planet. Space Sci. 13, 9 (1965); Astrophys. J. 772, 46 (2013); Space Sci. Rev. 78, 401 (1996); Astrophys. J. 234, 746 (1979); Astrophys. J. 361, 162 (1990); Space Sci. Rev. 176, 299 (2013)] offers a good description of this phenomenon. One of the outcomes of this framework is an anticorrelation of the variation in solar wind velocity (${\mathrm{V}}_{\mathrm{SW}}$) and the GCR flux. A second outcome of this gradient in the presence of IMF is the movement of GCRs perpendicular to the ecliptic plane called “Swinson flow.” Therefore, (i) the correlated variations of ${\mathrm{V}}_{\mathrm{SW}}$ and GCR flux and (ii) the GCR radial density gradient obtained from Swinson flow can each be used to independently measure the radial diffusion coefficient of GCRs in the inner heliosphere. In an earlier work [Phys. Rev. D 91, 121303(R) (2015)], the GCR flux was shown to be anticorrelated with ${\mathrm{V}}_{\mathrm{SW}}$ at $(-1.33\pm 0.07)\times {10}^{-3}\%(\mathrm{km}{\mathrm{s}}^{-1}{)}^{-1}$. This anticorrelation yields a radial diffusion coefficient $\kappa =0.97\times {10}^{19}{\mathrm{m}}^2{\mathrm{s}}^{-1}$ at 1 AU. In another work [Astropart. Phys. 62, 21 (2015)], the measurement of Swinson flow was used to obtain a GCR radial density gradient of $0.65{\mathrm{A}\mathrm{U}}^{-1}$ at a median rigidity of 77 GV. Here, we report a measurement of radial diffusion coefficient $\kappa =1.04\times {10}^{19}{\mathrm{m}}^2{\mathrm{s}}^{-1}$ at 1 AU from the above-mentioned density gradient, for a mean ${\mathrm{V}}_{\mathrm{SW}}$ of $450\mathrm{km}{\mathrm{s}}^{-1}$. Thus, these two distinct approaches essentially yielded similar values of the radial diffusion coefficient $\kappa ={10}^{19}{\mathrm{m}}^2{\mathrm{s}}^{-1}$ at 1 AU, characterizing the diffusion of GCRs at 77 GV. From this value of $\kappa$, the mean free path length for parallel diffusion ${\lambda }_{\parallel }$ was estimated to be 1.2 AU at 77 GV, consistent with earlier reports [Rev. Geophys. Space Phys. 20, 335 (1982); Astrophys. J. 420, 294 (1994); Astrophys. J. 604, 861 (2004)].

• Received 16 March 2018

DOI:https://doi.org/10.1103/PhysRevD.98.022004