Optimisation of a Steady State Radial Diffusion Model to Derive Diffusion Coefficients for the Proton Radiation Belt

Abstract:

Proton flux measurements from the Proton Telescope instrument aboard the CRRES satellite are revisited, and used to drive a radial diffusion model of the inner proton belt at 1.1≤L≤1.7. Our model utilises a physics-based evaluation of the cosmic ray albedo neutron decay (CRAND) source, and coulomb collisional loss is driven by plasmaspheric electron density according to the Global Core Plasma Model. We drive our model using time-averaged data at L=1.7 to calculate steady state profiles of phase space density, and optimise our choice of radial diffusion coefficients based on four defining parameters, so as to minimise the difference between model and data. This is first performed for a quiet period when the belt can be assumed to represent steady state. Additionally, we investigate the viability of fitting steady state solutions to time averages taken during active periods that exhibit limited deviation from steady state, demonstrated by CRRES measurements following the 24th March 1991 storm. We discuss a way to make the optimisation process more reliable by excluding periods of variability in plasmaspheric density from any time average, applicable to other empirically modelled factors. Lastly, we compare our resultant diffusion coefficients to those derived via a similar process in previous work, as well as diffusion coefficients derived for electrons from ground and in situ observations. We find that higher diffusion coefficients are derived compared with previous work, but that our estimates may be increased by uncertainty in inner zone plasmaspheric density, which we identify as an important potential caveat for modelling.