Parallel and Perpendicular Diffusion of Energetic Particles in the Near-Sun Solar Wind Observed by Parker Solar Probe

We investigate energetic particle diffusion in the inner heliosphere ($\sim$0.06--0.3 AU) explored by Parker Solar Probe (PSP). Parallel ($\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-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 $\kappa_\parallel$ 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 rise accordingly in regions with relatively high turbulence levels $\delta B/B_0$. 
To validate our results, we estimate $\kappa_\parallel$ using upstream time-intensity profile of a solar energetic particle event observed by the PSP and compared it with theoretical values from different diffusion models. \textbf{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 the introduction of an ad hoc pitch-angle cutoff, may provide a more physically motivated description of energetic particle diffusion near the Sun.