Time evolution of ULF waves driven by impulsive particle injection

Abstract:

Poloidal Alfven waves with high azimuthal wave number (m >> 1) are of great interest since they are capable of interacting with high-energy (ring current) particles via bounce-drift resonance. However, the behavior of these transient poloidal waves that are driven by wave-particle interaction has not been studied well. In this presentation, we have conducted three-dimensional MHD simulations in dipole geometry to investigate the time-dependent behavior of the local wave fields at various locations. We examine how the time evolution of the wave modes varies when local sources are assumed with different widths. Here, a localized impulsive wave source represents a particle injection in a limited region, and thus its width can be indicative of differential azimuthal states. Our results show that the poloidal mode remains for a longer period of time when the perturbation is imposed in a narrower region in local time, which is consistent with previous studies that the poloidal mode with higher m has longer lifetime than lower m. We also present the effect of the prolonged interaction during which the waves are excited by particles on lifetime of the poloidal mode. It is suggested that the temporal variation of the observed transient poloidal mode waves can imply how they interact with the high-energy particles.