On the Propagating Ultralow-frequency Waves and their Influences over Plasmaspheric Charged Particles

Abstract:

In Earth's magnetosphere, the behavior of charged particles in response to ultralow-frequency (ULF) waves have long been understood in the framework of their resonant interactions with standing hydromagnetic waves. It has been argued that during the initial wave cycles, hydromagnetic waves should propagate along magnetic field lines before the reflected waves are superimposed to establish a standing structure. However, little has been investigated on particle behavior within these propagating waves. Here we use a simple model of ULF wave evolution from propagating to standing waves, to investigate the response of low-energy electrons immediately after the waves are excited by the impact of an interplanetary shock. We find that a spacecraft in the off-equatorial region would observe clear dispersion signatures in both pitch angle and energy spectra of electron fluxes, and the flux modulation can appear prior to the observations of electromagnetic wave perturbations. These predicted signatures, resulting from a Landau-type resonance between propagating waves and field-aligned streaming particles, are consistent with observations from Cluster spacecraft over an ULF wave event on 25 September 2001. Our identification of such signatures, therefore, provides a new understanding of ULF wave evolution and wave-particle interactions in the inner magnetosphere.