Ring Current Ion Acceleration Due to Drift-Bounce Resonance With ULF Waves

Abstract:

Ultra-low-frequency (ULF) waves have a strong influence on charged particle dynamics in Earth’s inner magnetosphere. The energy dispersion evident in many particle measurements from spacecraft such as Cluster and the Van Allen Probes suggest ULF wave poloidal electric fields can modulate the flux of energetic ions and electrons in a narrow range of energy. In this paper, we use test particle simulations and a newly developed analytic model of poloidal-mode ULF waves to analyze modulations of energetic ion fluxes produced by ULF waves. These modulations typically have periods of one hundred to several hundred seconds that are the same as the associated observed ULF waves. Satellite and ground signatures of these waves show they are standing waves, which taken together with the particle measurements, leads to the conclusion that resonant wave-particle interactions are taking place. Simulations presented reproduce observed ion differential flux modulations and their amplitude distribution, which in the case of drift-resonance (N=0) peaks at pitch-angles less than 90-degrees because the wave electric field is a maximum off the equator. Test-particle simulation are used to examine the energy and pitch angle dependence of energetic ions in particular events. The advantage of our approach is that it does not impose wave and particle characteristics that bias results obtained. We show, in particular, that predictions of the Southwood-Kivelson theory of drift-bounce resonance emerge naturally from test particle simulations.