Quantification of ion scattering by solar-wind current sheets: Pitch-angle diffusion rates

The transport of energetic particles in the heliosphere is profoundly influenced by interactions with coherent structures in the turbulent magnetic field of the solar wind, particularly current sheets. While prior studies have largely relied on idealized turbulence models, this work quantifies the role of solar wind current sheets (quasi-1D plasma structures characterized by strong magnetic field gradients) in driving pitch-angle scattering. We present an analytical Hamiltonian framework coupled with test particle simulations, informed by observational data from the ARTEMIS and Wind missions, to model particle dynamics through current sheets with realistic parameters. Our results demonstrate that the scattering efficiency depends critically on the current sheet's shear angle, the relative magnitude of the magnetic field component directed along the normal to the current sheet surface, and the ratio of the particle gyroradius to the current sheet thickness. Large pitch-angle jumps, arising from nonadiabatic separatrix crossings in phase space, lead to rapid chaotization, whereas diffusive scattering broadens the pitch-angle distributions. Statistical analysis of solar wind current sheets at 1 AU reveals significant scattering rates for 100 keV to 1 MeV protons, with implications for particle transport mechanisms. The derived diffusion rates enable the inclusion of coherent structures into global transport models for a more accurate modeling of energetic particle dynamics in the heliosphere. These findings underscore the importance of current sheets in shaping energetic particle spatial distributions and provide practical methods for incorporating them in space and astrophysical plasmas.