Nonlinear Theory and Simulation of Toroidal ULF Wave and Particle Interaction in a Pure Dipole Magnetic Field

Abstract:

We examine the drift-resonant particle dynamics for toroidal ultralow frequency (ULF) waves in a pure dipole background geomagnetic field. We confirm that the resonant condition originally believed to apply only for poloidal ULF waves, mω_d=ω, also applies for toroidal waves. The resulting particle motion can be described by a modified pendulum equation with solutions depending sensitively on the wave number m. For high-m toroidal waves, the resonant islands become asymmetric to perturb the particle trajectories within each potential well and consequently increase the trapping widths in both energy and L-shell. To compare the effects of toroidal and poloidal waves, we carry out test-particle simulations to show the evolution of distribution functions for electrons interacting with either wave. We find that poloidal waves accelerate electrons faster than toroidal waves with a low wave number, while toroidal waves with intermediate or high wave numbers can play a dominant role. These findings highlight the importance of toroidal ULF waves in magnetospheric particle dynamics.