The role of drift orbit bifurcations in energization and loss of electrons in the outer radiation belt

Radiation levels in Earth's outer electron belt (L greater than or similar to 2.5) vary by orders of magnitude on the time scales ranging from minutes to days. Multiple acceleration and loss processes operate across the belt and compete in defining its global variability. One such process is the drift orbit bifurcation effect. Caused by coupling of the drift and bounce motions, it breaks the second adiabatic invariant of radiation belt electrons producing their transport in radius and pitch angle. In this paper we investigate implications of drift orbit bifurcations to the global state and variability of the outer electron belt. For this purpose we use three-dimensional test particle simulations of electron guiding center motion in a realistic magnetic field model. We show that even at most quiet solar wind conditions bifurcations affect a broad range of the belt penetrating inside the geosynchronous orbit. This has an important practical implication for the analysis of experimental particle data: since the third adiabatic invariant is undefined for bifurcating orbit, the electron phase space density cannot be expressed in terms of three adiabatic invariants. We show that long-term transport of electrons due to drift orbit bifurcations is a complex combination of large ballistic jumps and small-amplitude diffusion in the second invariant and radial location. To model long-term transport, we derive an empirical map of the second invariant and radial jumps at bifurcations. The map can also be implemented by other radiation belt models, which cannot directly account for the physics of drift orbit bifurcations. Drift orbit bifurcations can produce electron losses through the magnetopause escape and through pitch angle scattering into the atmospheric loss cone. Most electrons, however, can stay quasi-trapped in the bifurcation regions for very long time periods. The pitch angle and radial transport due to drift orbit bifurcations lead to their meandering back and forth across the region producing mixing and recirculation of particle populations with different initial conditions. We show that this recirculation can greatly amplify electron energization by radial diffusion. Compared to the diffusion alone, the combined action of radial diffusion and drift orbit bifurcations can double electron energization at each recirculation cycle. Our results suggest that drift orbit bifurcations can play an important role in the buildup of increased electron fluxes in the storm recovery phase.