Diffuse ions produced by electromagnetic ion beam instabilities

TLDR

The study interprets ion distributions and hydromagnetic waves observed in the foreshock of Earth's bow shock and interplanetary shocks. The authors simulate the evolution of the electromagnetic ion beam instability driven by reflected ions backstreaming from Earth's bow shock into the foreshock. The simulation tracks the instability’s linear, quasi‑linear, and nonlinear stages, but does not explore additional acceleration mechanisms for diffuse ions. Linear and quasi‑linear phases match resonant parallel‑beam theory and a newly derived nonresonant antiparallel mode, while the quasi‑linear stage generates large‑amplitude transverse waves and intermediate ion distributions that transition into a nonlinear phase producing strongly compressive waves and diffuse ion populations.

Abstract

The evolution of the elecromagnetic ion beam instability driven by the reflected ion component backstreaming away from the earth's bow shock into the foreshock region is studied by means of computer simulation. The linear and quasi‐linear stages of the instability are found to be in good agreement with known results for the resonant mode propagating parallel to the beam along the magnetic field and with theory developed in this paper for the nonresonant mode, which propagates antiparallel to the beam direction. The quasi‐linear stage, which produces large amplitude δB ∼ B, sinusoidal transverse waves and “intermediate” ion distributions, is terminated by a nonlinear phase in which strongly nonlinear, compressive waves and “diffuse” ion distributions are produced. Additional processes by which the diffuse ions are accelerated to observed high energies are not addressed. The results are discussed in terms of the ion distributions and hydromagnetic waves observed in the foreshock of the earth's bow shock and of interplanetary shocks.