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 (∼0.06–0.3 au) explored by Parker Solar Probe (PSP). Parallel (κ∥) and perpendicular (κ⊥) diffusion coefficients are calculated using second-order quasi-linear theory (SOQLT) and unified nonlinear transport theory, respectively. PSP’s in situ measurements of magnetic turbulence spectra, including sub-Alfvénic solar wind, are decomposed into parallel and perpendicular wavenumber spectra via a composite two-component turbulence model. These spectra are then used to compute κ∥ and κ⊥ across energies ranging from sub-GeV to GeV. Our results reveal a strong energy and radial distance dependence in κ∥. While κ⊥remains much smaller, it can rise accordingly in regions with relatively high turbulence levels δB/B0. To validate our results, we estimate κ∥ using an upstream time-intensity profile of a solar energetic particle event observed by the 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 solar energetic particle intensity-derived estimates than the classic quasi-linear theory model. This indicates that the SOQLT framework, which incorporates resonance broadening and nonlinear corrections and does not require the introduction of an ad hoc pitch-angle cutoff, may provide a more physically motivated description of energetic particle diffusion near the Sun.