Abstract

Diffusion-driven radiation belt models require multiple physics-based inputs to specify the radiation environment through which spacecraft travel, including diffusion coefficients. Even though event-specific coefficients are necessary for model accuracy, their routine integration in operational models has not yet been achieved. In fact, one of the key inputs, the radial diffusion coefficient, is still commonly determined by a Kp-driven parameterization. This work presents a method to determine continuous time series of time-varying radial diffusion coefficients. A theoretical model is developed in which electromagnetic radial diffusion is controlled by the magnetopause immediate time history. Specifically, radial diffusion is described as a function of the average, variance, and autocorrelation time of the geocentric stand-off distance to the subsolar point on the magnetopause. Because the magnitudes of these three magnetopause parameters vary with time and magnetic activity, so does radial diffusion. To a lesser extent, radial diffusion is also controlled by the drift frequency of the radiation belt population. Moreover, radial diffusion is quantified using a standard model in which the magnetopause is controlled by the solar wind. Although the resulting diffusion coefficients span several orders of magnitude per Kp index, the median magnitudes are remarkably similar to the ones provided by the standard Kp-driven statistical parameterization.