Is Kp the Best Single Magnetic Activity Parameterization for Electromagnetic Radial Diffusion in the Outer Radiation Belt?

Specifying radial diffusion magnitude is one of the main requirements for most physics-based radiation belt models. Yet, radial diffusion quantification remains uncertain. The most commonly used parameterization for the logarithm of radial diffusion magnitude is a linear function of a magnetic index, Kp $\mathrm{K}\mathrm{p}$, with a coarse time resolution of 3 hours. This work presents alternate linear parameterizations of similar quality for the logarithm of radial diffusion magnitude, considering other magnetic indices and solar wind parameters. Using a publicly available time series for the logarithm of electromagnetic radial diffusion magnitude, we investigate linear relationships with magnetic indices such as Kp, Hp60, Hp30, AE, SymH, and Dst and solar wind parameters such as solar wind dynamic pressure, solar wind speed, and the north-south component of the interplanetary magnetic field. We find that Kp, Hp60, Hp30, and solar dynamical pressure yield the strongest linear correlation with the logarithm of radial diffusion magnitude. We also provide simple, energy-dependent, linear models of the logarithm of radial diffusion magnitude that best fit the time series, as well as quantifications of the root-mean square errors. This work contributes to improving the time resolution for radial diffusion parameterization and operational radiation belt models. In particular, it suggests that the new 30-min and 60-min geomagnetic indices Hpo (Hp30 and Hp60, respectively) could also be used in place of Kp in the most commonly used Kp-driven parameterizations for radiation belt radial diffusion, to improve the time resolution of the models.