Foreshock Ultra-Low Frequency Waves at Mars: Consequence on the Particle Acceleration Mechanisms at the Martian Bow Shock

TL;DR

This study characterizes ultra-low frequency waves at Mars' foreshock, revealing their properties, likely generation mechanism, and implications for particle acceleration at the Martian bow shock.

Contribution

First statistical analysis of ULF waves at Mars' foreshock, identifying their polarization, propagation, and potential role in particle acceleration.

Findings

ULF waves are in the 0.008-0.086 Hz range with specific polarization.

Waves are consistent with fast magnetosonic waves generated by ion-ion instability.

Resonant backstreaming protons have velocities about 1.33 times the solar wind speed.

Abstract

Using Mars Atmosphere and Volatile EvolutioN Magnetometer observations, we report the first statistical study of ultra-low frequency (ULF) waves at the Martian foreshock. The analyzed foreshock ULF wave events are observed in the 0.008-0.086 Hz frequency range, with nearly circular and elliptical left-handed polarization in the spacecraft reference frame. These waves are propagated quasi-parallel to the ambient magnetic field, with a moderate wave amplitude. All these properties are consistent with fast magnetosonic waves, most likely generated through the ion-ion right-hand resonant instability. In addition, our results suggest that the associated resonant backstreaming protons' velocities parallel to the mean magnetic field in the solar wind reference frame is $1.33 \pm 0.40$ times the solar wind velocity. The similarity between our results and previous reports at other foreshocks may indicate the presence of a common acceleration process acting in planetary bow shocks and that is responsible for this particular backstreaming population.