Modeling the Effects of Drift Orbit Bifurcation on Radiation Belt Electrons

Drift Orbit Bifurcation (DOB) has been suggested to play a major role in the loss and transport of radiation belt electrons since it violates the second and third adiabatic invariants of particles. Results from our guiding-center test particle simulations using the Tsyganenko-1989c magnetic field model show that the DOB could affect a broad region of the outer radiation belt, which can penetrate inside the geosynchronous orbit at Kp ≥ 3, and its effects are more significant further away from Earth, at higher Kp, and for higher electron energies. Specifically, the short-term simulation results after one electron drift show both traditional and nontraditional DOB transport of electrons, with the nontraditional DOB, caused by a third minimum of the magnetic field strength near the equator, reported here for the first time. Moreover, our results show large ballistic jumps in the second invariant and radial distance for electrons at high equatorial pitch angles after one drift. In addition, long-term DOB transport coefficients of electrons over many drifts are calculated based on our simulation results. We find that the pitch angle and radial diffusion coefficients of electrons due to DOB could be comparable to or even larger than those caused by electron interactions with chorus and ULF waves, respectively. In sum, our results demonstrate that DOB could cause effective loss and transport of radiation belt electrons even in the absence of waves.