The Rapid Variability of Electric Field Waves within and near interplanetary shock ramps: STEREO Observations

TL;DR

This study uses STEREO spacecraft data to analyze the rapid variability of electric field waves near interplanetary shock ramps, revealing diverse wave modes and their spatial distribution around shocks.

Contribution

It provides the first observations of dispersive electrostatic waves and electron cyclotron drift instability driven waves at interplanetary shocks, expanding understanding of shock-associated wave phenomena.

Findings

High variability in wave frequency, amplitude, and mode observed.

All wave modes are more common downstream of shocks.

Large amplitude waves are bursty and observed in all regions.

Abstract

We present STEREO observations within 1500 proton gyroradii of 12 interplanetary shocks, with long-duration burst mode electric field acquisition by S/WAVES enabling observation of the evolution of waves throughout the entire ramp of interplanetary shocks. The shocks are low Mach number ($M_{f} \sim $1--5), quasi-perpendicular ($\theta_{Bn} \geq 45^{\circ}$), with beta ($\beta$) $\sim $0.2--1.8. High variability in frequency, amplitude, and wave mode is observed upstream, downstream, and in shock ramps. Observations in every region include ion acoustic-like waves, electron cyclotron drift instability driven waves, electrostatic solitary waves, and high frequency whistler mode waves. We also show for the first time the existence of "dispersive" electrostatic waves with frequencies in the ion acoustic range and the first observations of electron cyclotron drift instability (ECDI) driven waves at interplanetary shocks. Large amplitude waves are bursty and seen in all three regions with amplitudes from $\sim$5 to $>$ 200 mV/m. All wave modes are more commonly observed downstream of the shocks than upstream of them, usually within $\sim$ 63000 km ($\sim 1500 \rho_{gi}$) of the ramp.