Radial Diffusion of Energetic Protons in the Earth's Inner Magnetosphere

Abstract:

Radial diffusion is one of the main source mechanisms of ring current ions and it could also contribute to the ring current loss by outward radial diffusion to the magnetopause. However, the relative contribution of radial diffusion to the dynamics of ring current ions has not been well quantified. Our work uses a 1D radial diffusion with charge exchange loss to simulate the long-term variations of ring current protons observed by Van Allen Probes to quantitatively study how much of the dynamics could be explained by radial diffusion. We first convert the observed proton flux over the period of November 2012 to September 2013 to phase space densities (PSD) as a function of adiabatic invariants (μ, K, L^*) using TS04 magnetic model. Then the PSD variation at L*=5.5 is used as our outer boundary condition. Our model uses empirical formulae of radial diffusion coefficient and charge exchange lifetime as inputs but including a free parameter in each formula to account for its uncertainties. The free parameter values are determined by best fitting the model results to PSD data at given μ and K values. The simulation results show that our model generally captures the transport and acceleration of ring current protons at μ=30,50,80 MeV/G and K=0.11G G^1/2R_E, which demonstrates that radial diffusion is the dominant source of >75 keV protons in the ring current. The observed fast decay of PSD at lower μ and slow decay at higher μ is also well captured by the charge exchange loss in the model. However, fast dropouts of PSD are observed over a wide range of L*. The large-L* dropout at L*>4 is reproduced by the model with outward radial diffusion to the reduced outer boundary. But the smaller-L* loss (on the time scale of hours) is too fast to be explained by charge exchange, which could potentially be due to other loss mechanisms including EMIC wave scattering and field line curvature scattering. These loss mechanisms will be included in our model in the future.