Persistent Plasma Waves in Interstellar Space Detected by Voyager 1

TL;DR

Voyager 1 has detected persistent, narrowband plasma waves in interstellar space, enabling continuous measurement of plasma density and turbulence over 10 au, revealing new insights into the interstellar medium.

Contribution

This study reports the first detection of persistent narrowband plasma waves in interstellar space, allowing steady plasma density measurements independent of shock events.

Findings

Detected steady plasma density measurements over 10 au

Identified au-scale density fluctuations indicating turbulence

Suggested possible mechanisms for narrowband emission

Abstract

In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 astronomical units (au) of interstellar space, revealing a large-scale density gradient and compressive turbulence outside the heliopause. Previous studies of the plasma density relied exclusively on the detection of discrete plasma oscillation events that are triggered ahead of shocks propagating outwards from the Sun, which were used to infer the plasma frequency and hence density. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 Plasma Wave System data that persists from 2017 onwards and enables a steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling time of 3 days, or 0.03 au. We find au-scale density fluctuations that trace turbulence in the interstellar medium between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and could be clarified by new findings from Voyager or a future interstellar mission. The persistence of the emission suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.