Theoretical Analysis of Radial Transport in the Inner Belt and the Slot Region

Recent measurements from the Van Allen Probes mission revealed unexpected behavior of the energetic and relativistic electron intensities in the Earth's inner radiation belt and the slot region. Over all geomagnetic storms spanned by the Van Allen Probes, relativistic electrons with the energies of ~1 MeV and higher never penetrated inside the energy-dependent boundary centered at L~3, whereas energetic electrons with the energies of 10s to 100s keV penetrated through the slot and filled out the inner belt during multiple of these storms. One of the fundamental processes that needs to be quantified in order to explain this unexpected behavior is the radial transport of electrons across their drift shells. Quantifying the radial transport rates inside L=3 on purely observational basis is difficult because the amplitudes of the ultra-low-frequency (ULF) electric fields that drive the transport cannot be resolved from the in situ measurements on fast moving platforms. Also, the empirical methods of computing the radial transport rates from the measurements of electron intensities inside L=3 are affected by the complexity of the Earth's inner magnetic field and other uncertainties. The goal of this paper is to investigate the properties of radial transport with the use of test-particle numerical simulations. Taking the amplitude of the ULF electric field as a free parameter we quantify the radial transport rates as a function of energy and L, and investigate at what values of energy and L and at what timescales the transport can be described as radial diffusion.