Intense whistler-mode waves at foreshock transients: characteristics and regimes of wave-particle resonant interaction

TL;DR

This study statistically characterizes intense whistler-mode waves at Earth's foreshock transients, revealing their potential for nonlinear wave-particle interactions and electron acceleration in shock environments.

Contribution

It provides the first comprehensive statistical analysis of whistler waves at foreshock transients, highlighting their intensity, coherence, and nonlinear interaction regimes.

Findings

Many waves are intense and coherent enough for nonlinear interactions.

Background magnetic field gradients and wave amplitudes suggest phase trapping as a key acceleration mechanism.

Implications for electron acceleration in planetary and astrophysical shocks are discussed.

Abstract

Thermalization and heating of plasma flows at shocks result in unstable charged-particle distributions which generate a wide range of electromagnetic waves. These waves, in turn, can further accelerate and scatter energetic particles. Thus, the properties of the waves and their implication for wave-particle interactions are critically important for modeling energetic particle dynamics in shock environments. Whistler-mode waves, excited by the electron heat flux or a temperature anisotropy, arise naturally near shocks and foreshock transients. As a result, they can often interact with supra-thermal electrons. The low background magnetic field typical at the core of such transients and the large wave amplitudes may cause such interactions to enter the nonlinear regime. In this study, we present a statistical characterization of whistler-mode waves at foreshock transients around Earth bow shock, as they are observed under a wide range of upstream conditions. We find that a significant portion of them are sufficiently intense and coherent to warrant nonlinear treatment. Copious observations of background magnetic field gradients and intense whistler wave amplitudes suggest that phase trapping, a very effective mechanism for electron acceleration in inhomogeneous plasmas, may be the cause. We discuss the implications of our findings for electron acceleration in planetary and astrophysical shock environments.