A New Four-Component L*-dependent Model for Radial Diffusion based on
Solar Wind and Magnetospheric Drivers of ULF Waves
Abstract
Waves which couple to energetic electrons are particularly important in
space weather, as they drive rapid changes in the topology and intensity
of Earth’s outer radiation belt during geomagnetic storms. This includes
Ultra Low Frequency (ULF) waves that interact with electrons via radial
diffusion which can lead to electron dropouts and rapid acceleration and
inward transport of electrons during. In radiation belt simulations, the
strength of this interaction is specified by ULF wave radial diffusion
coefficients. In this paper we detail the development of new models of
electric and magnetic radial diffusion coefficients derived from in-situ
observations of the azimuthal electric field and compressional magnetic
field. The new models use L* as it accounts for adiabatic changes due to
the dynamic magnetic field coupled with an optimized set of four
components of solar wind and geomagnetic activity, Bz, V, Pdyn and
Sym-H, as independent variables (inputs). These independent variables
are known drivers of ULF waves and offer the ability to calculate
diffusion coefficients at a higher cadence then existing models based on
Kp. We investigate the performance of the new models by characterizing
the model residuals as a function of each independent variable and by
comparing to existing radial diffusion models during a quiet geomagnetic
period and through a geomagnetic storm. We find that the models
developed here perform well under varying levels of activity and have a
larger slope or steeper gradient as a function of L* as compared to
existing models (higher radial diffusion at higher L* values).