Whistler Waves in the foot of Quasi-Perpendicular Super-Critical Shocks

TL;DR

This study uses MMS spacecraft data to analyze whistler waves near super-critical shocks, revealing their oblique propagation, wavelength, and resonance with reflected ions, which suggests wave-particle interactions.

Contribution

First detailed measurement of whistler wave vectors and their relation to ion distributions at super-critical shocks using 4-spacecraft timing.

Findings

Whistler waves are oblique with wave-normal angles of 20°-42°.

Wavelengths are around 100 km, near the ion inertial length.

Reflected ion beams resonate with the observed whistler waves.

Abstract

Whistler waves are thought to play an essential role in the dynamics of collisionless shocks. We use the magnetospheric multiscale (MMS) spacecraft to study whistler waves around the lower hybrid frequency, upstream of 11 quasi-perpendicular super-critical shocks. We apply the 4-spacecraft timing method to unambiguously determine the wave vector $\mathbf{k}$ of whistler waves. We find that the waves are oblique to the background magnetic field with a wave-normal angle between $20^{\circ}$ and $42^{\circ}$, a wavelength around 100 km which is close to the ion inertial length. We also find that $\mathbf{k}$ is predominantly in the same plane as the magnetic field and the normal to the shock. By combining this precise knowledge of $\mathbf{k}$ with high-resolution measurements of the 3D ion velocity distribution we show that a reflected ion beam is in resonance with the waves, opening up the possibility for wave-particle interaction between the reflected ions and the observed whistlers. The linear stability analysis of a system mimicking the observed distribution, suggests that such a system can produce the observed waves.