Observation of a New Type of Low Frequency Waves at Comet 67P/Churyumov-Gerasimenko

TL;DR

This study reports the discovery of a new type of low-frequency magnetic waves near comet 67P, characterized by large amplitude oscillations not explained by classical instabilities, suggesting a novel plasma process.

Contribution

The paper presents the first observation of a new low-frequency wave mode at comet 67P, proposing a cross-field current instability as its source, expanding understanding of cometary plasma environments.

Findings

Detection of large-amplitude, 40 mHz magnetic oscillations

Wave characteristics differ from classical cometary ion gyro-frequency waves

Proposed cross-field current instability as the wave source

Abstract

We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low activity state. Quasi-coherent, large-amplitude ($\delta B/B \sim 1$), compressional magnetic field oscillations at $\sim$ 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied comet-interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pick-up ion driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism.