Periodic Density Structures Around the Plasmaspheric Plume Boundary and Their Association With ULF Waves

Abstract

Here we report that the dynamics of the plasmaspheric plume can be affected by ULF waves based on observations from Van Allen Probes. Probe A observed periodic density structures with regular waveforms when traveling into the plume from its western boundary on 19 August 2013. The crests of the density structures were filled with intense hiss waves and low-energy $H^{+}$ that also appeared in the plume. The in-phase relationship between toroidal magnetic field perturbations and density structures in the northern hemisphere indicates that the plume boundary was stretched westward when the field lines were moving westward. These observations indicate that periodic density structures stem from the azimuthal oscillations of the plume boundary under the drive of the fundamental toroidal mode of ULF waves. ULF waves around the plume boundary exhibit the largest amplitude and out-of-phase polarization, and the polarization is changing with MLT across the plume boundary, demonstrating that field line resonance (FLR) occurred at the plume boundary. These observations are consistent with the theoretically predicted characteristics of 3-D FLR associated with the plamspheric plume.