UCLA

Understanding the dynamics of energetic electron fluxes in Earth’s radiation belts remains one of the key challenges in Heliophysics. The two most studied mechanisms contributing to these dynamics are (1) wave–particle resonant interactions, which can accelerate and pitch-angle scatter electrons, and (2) radial diffusion driven by ultra-low-frequency (ULF) waves. Yet, observations often reveal behaviors that cannot be fully explained by these two processes. In particular, they necessitate considering radial transport mechanisms that go beyond classical diffusion. Radial transport can arise even in the absence of waves, solely due to the topology of magnetic field lines. This mechanism is known as Drift-Orbit Bifurcation (DOB), which occurs on the dayside, where solar wind compression splits the equatorial magnetic field minimum into two off-equatorial minima, violating the second adiabatic invariant and enabling radial transport. This mechanism has been investigated for magnetic field configurations exhibiting north–south symmetry relative to the geomagnetic equator and east–west symmetry relative to the noon meridian. Although asymmetric magnetic field configurations occur in nature as a rule rather than an exception, studies quantifying the radial transport associated with these asymmetries—either through case studies or statistical analyses—are still lacking.This thesis presents a quantitative investigation of the role of magnetospheric asymmetry in drift-orbit bifurcation. Specifically, it addresses three key questions: (1) the importance of east–west and north–south magnetic field asymmetries in the scattering and radial transport of electrons undergoing drift-orbit bifurcation, (2) the radial transport arising from this asymmetric drift-orbit bifurcation, and (3) the energy range of electrons for which the effects of such transport are comparable to ULF-driven diffusion. To address these questions, we combine (1) empirical magnetospheric magnetic field models driven by realistic solar wind conditions, (2) observations from the THEMIS spacecraft, and (3) the theoretical framework of adiabatic invariant destruction during separatrix crossings. Including realistic magnetospheric asymmetries, driven by a nonzero IMF By and dipole tilt, leads to larger jumps in adiabatic invariants and enhances radial transport driven by drift-orbit bifurcation. The developed theoretical framework explains these large jumps as the so-called geometric jumps of adiabatic invariants, known in the theory of Hamiltonian systems with an asymmetric separatrix in phase space. Using empirical magnetic field models, these jumps are quantified under various solar wind conditions. Approximating these jumps via a model (lower-dimensional) Hamiltonian system, we study its long-term dynamics and find distinctly fast, non-diffusive transport occurring on timescales comparable to or shorter than those driven by ULF waves for electrons with energies above 0.1–1 MeV. Combining THEMIS observations of dayside magnetopause–outer radiation belt crossings with guiding center simulations under realistic (asymmetric) magnetic field configurations, we show that these large jumps can explain isolated enhancements in energetic electron fluxes occurring near the magnetopause. These findings establish the critical role of magnetic field asymmetry in particle radial transport, advancing our understanding of the mechanisms controlling energetic electron dynamics in the dayside magnetosphere.