Electrostatic steepening of whistler waves

TL;DR

This paper reports the observation of nonlinear steepening of whistler waves in the Earth's magnetosphere, revealing how wave coupling to electron-acoustic modes leads to harmonic generation and energy transfer among electrons.

Contribution

It introduces the first observational evidence of electrostatic steepening of whistler waves caused by coupling to electron-acoustic modes in a two-temperature plasma.

Findings

Harmonics of whistler waves observed in space plasma.

Steepening occurs within tens of milliseconds.

Energy transfer from resonant to lower energy electrons.

Abstract

We present surprising observations by the NASA Van Allen Probes spacecraft of whistler waves with substantial electric field power at harmonics of the whistler wave fundamental frequency. The wave power at harmonics is due to nonlinearly steepened whistler electrostatic field that becomes possible in the two-temperature electron plasma due to whistler wave coupling to the electron-acoustic mode. The simulation and analytical estimates show that the steepening takes a few tens of milliseconds. The hydrodynamic energy cascade to higher frequencies facilitates efficient energy transfer from cyclotron resonant electrons, driving the whistler waves, to lower energy electrons.