Debye-scale electrostatic waves across quasi-perpendicular shocks

TL;DR

This study investigates the evolution of Debye-scale electrostatic waves across quasi-perpendicular collisionless shocks using spacecraft electric field measurements, revealing their consistent properties and potential generation mechanisms.

Contribution

It introduces a novel interferometry-based method to analyze wave evolution across shocks and shows the fundamental role of electrostatic waves regardless of shock parameters.

Findings

Most waves have frequencies below ion plasma frequency

Wavelengths are about 20 Debye lengths

Wave propagation is field-aligned upstream and downstream, oblique in the shock transition

Abstract

The evolution of the properties of short-scale electrostatic waves across collisionless shocks remains an open question. We use a method based on the interferometry of the electric field measured aboard the magnetospheric multiscale spacecraft to analyze the evolution of the properties of electrostatic waves across four quasi-perpendicular shocks, with $1.4 \leq M_A \leq 4.2$ and $66^\circ \leq \theta_{Bn} \leq 87^\circ$. Most of the analyzed wave bursts across all four shocks have a frequency in the plasma frame $f_{pl}$ lower than the ion plasma frequency $f_{pi}$ and a wavelength on the order of 20 Debye lengths $\lambda_D$. Their direction of propagation is predominantly field-aligned upstream and downstream of the bow shock, while it is highly oblique within the shock transition region, which might indicate a shift in their generation mechanism. The similarity in wave properties between the analyzed shocks, despite their different shock parameters, indicates the fundamental nature of electrostatic waves for the dynamics of collisionless shocks.