Ensemble Modeling of Long-term Radiation Belt Dynamics: Accounting for Variability in Radial Diffusion

Sang-Yun Lee^1,2*Weichao Tu³Greg Cunningham^4,5Misa Cowee⁴

• ¹Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, United States
• ²The Catholic University of America, Washington, United States
• ³University of Michigan, Ann Arbor, United States
• ⁴Los Alamos National Laboratory, Los Alamos, United States
• ⁵New Mexico Consortium, Los Alamos, United States

This study presents ensemble modeling of outer radiation belt dynamics observed by Van Allen Probes throughout 2017 using DREAM3D simulations. We utilize a recently developed empirical radial diffusion coefficient (𝐷𝐿𝐿) model with statistical distributions, employing two outer boundary (OB) conditions: Van Allen Probes data at 𝐿∗= 5.5 and GOES data at 𝐿∗= 6, for two magnetic moments 𝜇= 512 𝑘𝑒𝑉/𝐺 and 𝜇= 1237 𝑘𝑒𝑉/𝐺. The results demonstrate that OB location critically influences model performance. A model using Van Allen Probes OB at 𝐿∗= 5.5 better reproduces observations, particularly for high 𝜇 electrons. In this scenario, radial diffusion alone can largely explain the dynamics because the OB is located near the phase space density (PSD) peak region. Stronger 𝐷𝐿𝐿 outside the plasmasphere effectively captures inward/outward radial diffusion to/from the OB, while an intermediate 𝐷𝐿𝐿 around the 30th percentile best matches electron PSD inside the plasmasphere. With GOES OB at 𝐿∗= 6, not only does strong 𝐷𝐿𝐿 remain crucial for achieving good performance, but chorus heating also becomes critical. Model performance inside the plasmasphere is the best for the intermediate 𝐷𝐿𝐿 but consistently worse than that outside the plasmasphere, suggesting a need for improved 𝐷𝐿𝐿 and loss inputs in the plasmasphere.