Parallel and perpendicular diffusion of energetic particles in the near-Sun solar wind observed by Parker Solar Probe

Abstract

We investigate energetic particle diffusion in the inner heliosphere (approximately 0.06-0.3 AU) explored by Parker Solar Probe (PSP). Parallel (kappa_parallel) and perpendicular (kappa_perp) diffusion coefficients are calculated using second-order quasi-linear theory (SOQLT) and unified nonlinear transport (UNLT) theory, respectively. PSP's in situ measurements of magnetic turbulence spectra, including sub-Alfvenic solar wind, are decomposed into parallel and perpendicular wavenumber spectra via a composite two-component turbulence model. These spectra are then used to compute kappa_parallel and kappa_perp across energies ranging from sub-GeV to GeV. Our results reveal a strong energy and radial distance dependence in kappa_parallel. While kappa_perp remains much smaller, it can increase in regions with relatively high turbulence levels delta B / B0. To validate our results, we estimate kappa_parallel using the upstream time-intensity profile of a solar energetic particle event observed by PSP and compare it with theoretical values from different diffusion models. Our results suggest that the SOQLT-calculated parallel diffusion generally shows better agreement with SEP intensity-derived estimates than the classic QLT model. This indicates that the SOQLT framework, which incorporates resonance broadening and nonlinear corrections and does not require an ad hoc pitch-angle cutoff, may provide a more physically motivated description of energetic particle diffusion near the Sun.