A Kalman filter technique to estimate relativistic electron lifetimes in the outer radiation belt

Data assimilation aims to smoothly blend incomplete and inaccurate observational data with dynamical information from a physical model, and has become an increasingly important tool in understanding and predicting meteorological, oceanographic and climate processes. As space-borne observations become more plentiful and space-physics models more sophisticated, dynamical processes in the radiation belts can be analyzed using advanced data assimilation methods. We use the Extended Kalman filter and observations from the Combined Release and Radiation Effects Satellite (CRRES) to estimate the lifetime of relativistic electrons during magnetic storms in the Earth's outer radiation belt. The model is a linear parabolic partial differential equation governing the phase-space density. This equation contains empirical coefficients that are not well-known and that we wish to estimate, along with the phase-space density itself. The assimilation method is first verified on model-simulated data, which allows us to reliably estimate the characteristic lifetime of the electrons. We then apply the methodology to CRRES measurements and show it to be useful in highlighting systematic differences between the parameter estimates for storms driven by coronal mass ejections (CMEs) and by corotating interaction regions (CIRs), respectively. These differences are attributed to the complex, competing effects of acceleration and loss processes during distinct physical regimes. The technique described herein may be applied next to constrain more sophisticated radiation belt and ring current models, as well as in other areas of magnetospheric physics.