Simulating the Ring Current Proton Dynamics in Response to Radial Diffusion by Ultra-Low-Frequency (ULF) Waves

Radial diffusion (RD) induced by ULF waves can contribute to particle acceleration and scattering. Past global simulations that incorporate RD often use dipole magnetic fields, which could not realistically reveal the role of RD. To better understand the effects of RD and identify whether a background magnetic field model matters in understanding the ring current dynamics in response to RD, we simulate a storm event with different magnetic configurations using a global kinetic ring current model. Results indicate that RD can effectively diffuse protons of hundreds of keV to inner regions (L similar to 3.5), especially in recovery phase. Comparisons with in-situ observations demonstrate that simulations with TS05 overall capture both the intensity and variations of proton fluxes with the aid of RD, whereas that with a dipole field significantly overestimates low-L region fluxes. This study implies adopting realistic magnetic fields is important for correctly interpreting the role of RD. Ultra-low-frequency (ULF) waves in the magnetosphere can scatter particles and diffuse them radially, called radial diffusion, resulting in particle acceleration and scattering and even precipitation down to the upper atmosphere. The interaction between ULF waves and particles is highly dependent on the strength of the magnetic field. This study quantified the role of ULF wave radial diffusions in the ring current dynamics using a global ring current model under different magnetic field configurations. Results indicate that radial diffusion could efficiently migrate energetic particles inward to L similar to 3.5, especially during storm recovery phase when the convection is weak. With a more realistic magnetic field configuration, distributions of energetic ring current particles agree much better with satellite observations than using a dipolar magnetic field. Adding the radial diffusion process in the simulation helps to accelerate particles and yield better data-model comparisons. Radial diffusions are able to effectively diffuse energetic (80 similar to 300 keV) ring current protons to L similar to 3.5 especially during recovery phase Simulations with a dipole field may overestimate the role of radial diffusion in low L regions, but underestimate in high L regions Adopting a more realistic magnetic field model is necessary to correctly interpret the role of radial diffusion