Roles of Magnetospheric Convection on Nonlinear Drift Resonance Between Electrons and ULF Waves

In the Earth's inner magnetosphere, charged particles can be accelerated and transported by ultralow frequency (ULF) waves via drift resonance. We investigate the effects of magnetospheric convection on the nonlinear drift resonance process, which provides an inhomogeneity factor S to externally drive the pendulum equation that describes the particle motion in the ULF wave field. The S factor, defined as the ratio of the driving amplitude to the square of the pendulum trapping frequency, is found to vary with magnetic local time and, as a consequence, oscillates quasi‐periodically at the particle drift frequency ωd. To better understand the particle behavior governed by the driven pendulum equation, we carry out simulations to obtain the evolution of electron distribution functions in energy and L‐shell phase space. We find that resonant electrons can remain phase trapped by the low‐m ULF waves under strong convection electric field, whereas for high‐m ULF waves, the electrons trajectories can be significantly modified. More interestingly, the electron drift frequency ωd is close to the nonlinear trapping frequency ωtr for intermediate‐m ULF waves, which corresponds to chaotic motion of resonant electrons. These findings shed new light on the nature of coherent and diffusive particle transport in the inner magnetosphere. Plain Language Summary Ultralow frequency (ULF) waves are electromagnetic oscillations in Earth's inner magnetosphere with the frequency of a few millihertz. When the azimuthal phase velocity of the ULF waves matches the drift velocity of a charged particle, the particle experiences a constant wave electric field to have a net energy gain or loss. This process, referred to as the wave‐particle drift resonance, plays important roles in the acceleration and transport of magnetospheric particles. In this study, we analyze the role of large‐scale magnetospheric convection in the drift resonance process. We find that the nonlinear particle behavior can be described by a pendulum driven by a quasiperiodic external force controlled by the convection electric field. Therefore, the resonant particle behavior is largely determined by the ratio between the external driving frequency and the pendulum eigenfrequency, which in turn depends on the azimuthal wave number m. It is shown that magnetospheric convection can hardly affect the electron trapping motion in low‐m ULF waves, whereas for high‐m waves, the electron dynamics is largely controlled by th