Authors: Xiaohang Chen (University of Arizona), Joe Giacalone (University of Arizona), Fan Guo (Los Alamos National Lab) and Kristopher Klein (University of Arizona)

Diffusion coefficients of energetic charged particles in turbulent magnetic fields are a fundamental aspect of diffusive transport theory, however, remaining incompletely understood. In this work, we use the quasi-linear theory to evaluate the spatial variation of parallel diffusion coefficient in the inner heliosphere based on the magnetic field and plasma velocity measurements of Parker Solar Probe during Orbit 5-13. The parallel diffusion coefficient is estimated as a function of radial distance (from 13.3 solar radii to 0.8 AU) and the particle rigidity (from 100 keV to 1GeV). By fitting the results, we provide an empirical formula of parallel diffusion coefficient in the inner heliosphere which can be used in studying the transport and acceleration of solar energetic particles as well as the modulation of cosmic rays.