Properties and Implications of Radial Transport in the Outer Electron Belt

Earth's outer radiation belt extends above approximately 3.5 Re and is populated by relativistic electrons trapped in the geomagnetic field. Radiation levels across the belt can vary by multiple orders of magnitude on the time scales ranging from minutes to days. One of the basic processes leading to global variability of radiation levels in the belt is radial transport of electrons across their drift shell. The inward radial diffusion followed by "adiabatic" acceleration was the first mechanism put forward to explain creation of the outer belt. This paper reviews the results of recent analysis of basic properties of radial transport and discusses their implications to the global state of the belt. We will focus on stochastic transport which traditionally is referred to as radial diffusion. Stochastic radial transport is driven by interactions of the gradient curvature motion of the electron guiding center with ULF waves. Long-term electron motion can become stochastic due to non-linearity of electron interaction with the waves as well as to the random nature of their solar-wind driver. In spite of the underlying stochasticity the radial diffusion limit is not fully attainable in the outer radiation belt. This is attributed to the fact that phase correlations in electron motion do not have time to decay due to finite size of the system. As a result collective motion of the outer belt electrons can exhibit large deviations from radial diffusion. We will also discuss how the electron belt is affected by drift orbit bifurcations (DOBs). In a day-side compressed geomagnetic field electron orbits around Earth can exhibit bifurcations which violate their second adiabatic invariant and produce complex non-diffusive radial transport. Consequently, the third invariant is undefined for the bifurcating orbits, which means that electron motion can no not be analyzed in terms of adiabatic invariants. Even during quiet solar wind conditions DOBs affect a broad region of the belt penetrating inside the geosynchronous orbit. The pitch-angle and radial transport due to DOBs leads to their meandering back and forth across the region, producing mixing and recirculation of particle populations with different initial conditions at the time rates comparable or higher to standard radial transport estimates. Electron recirculation can greatly amplify the efficiency of acceleration by radial transport: the combined action of radial transport and DOBs can produce a factor of two increase in electron energization at each recirculation cycle.