Direct observations of cross-scale energy transfer in space plasmas

TL;DR

This paper presents spacecraft measurements revealing how energy is transferred across scales in space plasmas, showing the role of wave interactions and resonances in ion and electron acceleration.

Contribution

It provides direct observational evidence of cross-scale energy transfer mechanisms involving ultra-low-frequency and whistler waves in space plasmas.

Findings

Ion acceleration from ultra-low-frequency waves destabilizes whistler waves.

Resonance between waves and particles facilitates energy transfer across scales.

Anomalous resonance plays a key role in wave coupling and energy dissipation.

Abstract

The collisionless plasmas in space and astrophysical environments are intrinsically multiscale in nature, behaving as conducting fluids at macroscales and kinetically at microscales comparable to ion- and/or electron-gyroradii. A fundamental question in understanding the plasma dynamics is how energy is transported and dissipated across different scales. Here, we present spacecraft measurements in the solar wind upstream of the terrestrial bow shock, in which the macroscale ultra-low-frequency waves and microscale whistler waves simultaneously resonate with the ions. The ion acceleration from ultra-low-frequency waves leads to velocity distributions unstable to the growth of whistler waves, which in turn resonate with the electrons to complete cross-scale energy transfer. These observations, consistent with numerical simulations in the occurrence of phase-bunched ion and electron distributions, also highlight the importance of anomalous resonance, a nonlinear modification of the classical cyclotron resonance, in the cross-scale wave coupling and energy transfer processes.