Nonlinear evolution of EMIC waves in a uniform magnetic field: 2. Test‐particle scattering

Abstract

The trajectories of individual proton test particles are computed as they traverse a temporally and spatially evolving electromagnetic ion cyclotron (EMIC) wavefield. This EMIC wavefield is simulated using a hybrid code, which is described in a companion paper, with parameters representative of those found in the dawnside outer magnetosphere (L ∼ 9). The scattering of a representative group of hot protons (E0 = 4 keV, α0 = 60°) and cool protons (E0 = 30 eV, α0 = 20°) is analyzed in configuration space, velocity space, E, α, and phase. Results indicate that the hot protons are scattered by the EMIC wave in a manner consistent with resonant wave‐particle interactions, that they are constrained to move along single‐wave characteristic curves, and that the particles experience moderate phase bunching. On the other hand, the cool protons are not scattered by the EMIC wave resonantly and experience strong phase bunching, which results in large positive advective motion in both E and α, as well as large relative diffusion. The phase bunching and heating of the cool proton population is consistent with observations of cool, heavy‐ion heating in space, and the phasing of the particle v⊥ and EMIC wave's E field is consistent with a previously described mechanism known as electric phase bunching, which results in efficient energization of cool particles.