Weak line discovered by Voyager 1 in the interstellar medium: Quasi-thermal noise produced by very few fast electrons

TL;DR

This paper explains the origin of a weak spectral line detected by Voyager 1 in the interstellar medium, attributing it to a small population of fast electrons producing plasma quasi-thermal noise, and discusses its implications for interstellar measurements.

Contribution

It provides a simple explanation for the weak line as caused by a tiny fraction of fast electrons and discusses how large fluctuations near the heliopause affect its detection.

Findings

The line is produced by a minute population of supra-thermal electrons.

Detection relies on long spectral averaging to overcome noise.

Large fluctuations near the heliopause obscure the line until some distance.

Abstract

A weak continuous line has been recently discovered onboard Voyager 1 in the interstellar medium, whose origin raised two major questions. First, how can this line be produced by plasma quasi-thermal noise on the Voyager short antenna? Second, why does this line emerge at some distance from the heliopause? We provide a simple answer to these questions, which elucidates the origin of this line. First, a minute quantity of supra-thermal electrons, as generally present in plasmas, whence the qualifier quasi-thermal, can produce a small plasma frequency peak on a short antenna, of amplitude independent of the concentration of these electrons; furthermore, the detection required long spectral averages, alleviating the smallness of the peak compared to the background. We therefore attribute the observed line to a minute proportion of fast electrons that contribute negligibly to the pressure. Second, we suggest that, up to some distance from the heliopause, the large compressive fluctuations ubiquitous in this region prevent the line to emerge from the statistical fluctuations of the receiver noise because it is blurred out by the averaging required for detection,especially in the presence of short-wavelength density fluctuations. These results open up novel perspectives for interstellar missions, by showing that a minute proportion of fast electrons may be sufficient to measure the density even with a relatively short antenna, because the quietness of the medium enables a large number of spectra to be averaged.